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Six-stroke engine
Six-stroke engine
Six-stroke engine
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A six-stroke engine is one of several alternative internal combustion engine designs that attempt to improve on traditional two-stroke and four-stroke engines. Claimed advantages may include increased fuel efficiency, reduced mechanical complexity, and/or reduced emissions. These engines can be divided into two groups based on the number of pistons that contribute to the six strokes.

In the single-piston designs, the engine captures the heat lost from the four-stroke Otto cycle or Diesel cycle and uses it to drive an additional power and exhaust stroke of the piston in the same cylinder in an attempt to improve fuel efficiency and assist with engine cooling. The pistons in this type of six-stroke engine go up and down three times for each injection of fuel. These designs use either steam or air as the working fluid for the additional power stroke.[1]

The designs in which the six strokes are determined by the interactions between two pistons are more diverse. The pistons may be opposed in a single cylinder or may reside in separate cylinders. Usually, one cylinder makes two strokes while the other makes four strokes, giving six piston movements per cycle. The second piston may be used to replace the valve mechanism of a conventional engine, which may reduce mechanical complexity and enable an increased compression ratio by eliminating hotspots that would otherwise limit compression. The second piston may also be used to increase the expansion ratio, decoupling it from the compression ratio. Increasing the expansion ratio in this way can increase thermodynamic efficiency in a similar manner to the Miller or Atkinson cycle.

Engine types

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Single-piston designs

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These designs use a single piston per cylinder, like a conventional two- or four-stroke engine. A secondary, nondetonating fluid is injected into the chamber, and the leftover heat from combustion causes it to expand for a second power stroke followed by a second exhaust stroke.

Griffin six-stroke engine

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The Kerr engine at the Anson Engine Museum

In 1883, the Bath-based engineer Samuel Griffin was an established maker of steam and gas engines. He wished to produce an internal combustion engine, but without paying the licensing costs of the Otto patents. His solution was to develop a "patent slide valve" and a single-acting six-stroke engine using it. By 1886, Scottish steam locomotive maker Dick, Kerr & Co. saw a future in large oil engines and licensed the Griffin patents. These were double-acting, tandem engines and sold under the name "Kilmarnock".[2] A major market for the Griffin engine was in electricity generation, where they developed a reputation for happily running light for long periods, then suddenly being able to take up a large demand for power. Their large, heavy construction did not suit them to mobile use, but they were capable of burning heavier and cheaper grades of oil. The key principle of the "Griffin Simplex" was a heated, exhaust-jacketed external vapouriser, into which the fuel was sprayed. The temperature was held around 550 °F (288 °C), sufficient to physically vapourise the oil, but not to break it down chemically. This fractional distillation supported the use of heavy oil fuels, the unusable tars and asphalts separating out in the vapouriser. Hot-bulb ignition was used, which Griffin termed the "catathermic igniter", a small isolated cavity connected to the combustion chamber. The spray injector had an adjustable inner nozzle for the air supply, surrounded by an annular casing for the oil, both oil and air entering at 20 psi (140 kPa) pressure, and being regulated by a governor.[3][4] Griffin went out of business in 1923. Only two known examples of a Griffin six-stroke engine survive. One is in the Anson Engine Museum. The other was built in 1885 and for some years was in the Birmingham Museum of Science and Technology, but in 2007, it returned to Bath and the Museum of Bath at Work.[5]

Dyer six-stroke engine

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Leonard Dyer invented a six-stroke, internal combustion, water-injection engine in 1915, very similar to Crower's design (see below). A dozen more similar patents have been issued since.

Dyer's six-stroke engine features:

  • No cooling system required
  • Improves a typical engine's fuel consumption
  • Requires a supply of pure water to act as the medium for the second power stroke.
  • Extracts the additional power from the expansion of steam.

Bajulaz six-stroke engine

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The Bajulaz six-stroke engine is similar to a regular combustion engine in design, but modifications were made to the cylinder head, with two supplementary fixed-capacity chambers: a combustion chamber and an air-preheating chamber above each cylinder. The combustion chamber receives a charge of heated air from the cylinder; the injection of fuel begins an isochoric (constant-volume) burn, which increases the thermal efficiency compared to a burn in the cylinder. The high pressure achieved is then released into the cylinder to work the power or expansion stroke. Meanwhile, a second chamber, which blankets the combustion chamber, has its air content heated to a high degree by heat passing through the cylinder wall. This heated and pressurized air is then used to power an additional stroke of the piston.

The claimed advantages of the engine include reduction in fuel consumption by at least 40%, two expansion strokes in six strokes, multiple-fuel usage capability, and a dramatic reduction in pollution.[6]

The Bajulaz six-stroke engine was invented in 1989 by Roger Bajulaz of the Bajulaz S.A. company, based in Geneva, Switzerland; it has U.S. patent 4,809,511 and U.S. patent 4,513,568.

The Bajulaz six-stroke engine features claimed are:

  • Reduction in fuel consumption by at least 40%
  • Two expansion (work) strokes in six strokes
  • Multifuel, including liquefied petroleum gas
  • Dramatic reduction in air pollution
  • Costs comparable to those of a four-stroke engine

Velozeta six-stroke engine

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In a Velozeta engine, fresh air is injected into the cylinder during the exhaust stroke, which expands by heat and therefore forces the piston down for an additional stroke. The valve overlaps have been removed, and the two additional strokes using air injection provide for better gas scavenging. The engine seems to show 40% reduction in fuel consumption and dramatic reduction in air pollution.[7] Its power-to-weight ratio is slightly less than that of a four-stroke gasoline engine.[7] The engine can run on a variety of fuels, ranging from gasoline and diesel fuel to LPG. An altered engine shows a 65% reduction in carbon monoxide pollution when compared with the four-stroke engine from which it was developed.[7] The engine was developed in 2005 by a team of mechanical engineering students, U Krishnaraj, Boby Sebastian, Arun Nair, and Aaron Joseph George of the College of Engineering, Trivandrum.

NIYKADO six-stroke engine

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This engine was developed by Chanayil Cleetus Anil, of Cochin, India, who patented the design in 2012.[8] The name of the engine is taken from the name of his company, NIYKADO Motors. The engine underwent a preliminary round of full-throttle tests at the Automotive Research Association of India, Pune.[8] The inventor claims this engine "is 23% more fuel efficient compared to a conventional four-stroke engine"[8] and it is "very low on pollution".[8]

Anil, a mechanic, developed the NIYKADO engine over the course more than 15 years. The engine was first tested in 2004 and Anil applied for his patent in 2005. He claims that his design produces drastically less pollution and that use in the automotive industry could lead to "emission-less mobility."

Engine functionality:

The different strokes are:

  1. Intake stroke
  2. Compression stroke
  3. Power stroke
  4. Exhaust stroke
  5. Air intake
  6. Air exhaust

The engine has four valves:

  1. Air-fuel intake valve
  2. Air-only intake valve
  3. Combustion exhaust valve
  4. Air-only exhaust valve

Intake stroke: In this stroke, the piston moves from top dead center (TDC) to bottom dead center (BDC). The intake valve opens and the air-fuel mixture enters the cylinder.

Compression stroke: The piston moves from BDC to TDC, and all valves are closed.

Power stroke: The spark plug ignites the air-fuel mixture. The piston moves from TDC to BDC, while all valves remain closed.

Exhaust stroke: The piston moves from BDC to TDC while the exhaust valve opens, allowing exhaust gases to exit the cylinder.

Air intake stroke: The air-only intake valve opens while the piston moves from TDC to BDC, pulling fresh air from the atmosphere into the cylinder. This air mixes with any leftover exhaust or unburnt fuel, while cooling the inside of the cylinder.

Air exhaust stroke: The air exhaust valve opens while the piston moves from BDC to TDC. The fresh air and most of the leftover fuel and exhaust leave the cylinder. Anil claims that this creates a fresher atmosphere inside the cylinder before the next air-fuel intake stroke, helps the engine to burn almost 100% of the air-fuel mixture, and reduces harmful emissions (including a 98% reduction in carbon monoxide emissions).

Crower six-stroke engine

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In a six-stroke engine prototyped in the United States by Bruce Crower, water is injected into the cylinder after the exhaust stroke and is instantly turned to steam, which expands and forces the piston down for an additional power stroke. Thus, waste heat that requires an air or water cooling system to discharge in most engines is captured and put to use driving the piston.[1] Crower estimated that his design would reduce fuel consumption by 40% by generating the same power output at a lower rotational speed. The weight associated with a cooling system could be eliminated, but that would be balanced by a need for a water tank in addition to the normal fuel tank.

The Crower six-stroke engine was an experimental design that attracted media attention in 2006 because of an interview given by the 75-year-old American inventor, who has applied for a patent on his design.[1] That patent application was subsequently abandoned.[9]

Porsche six-stroke engine

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[10][11][12][13][14]

Opposed-piston designs

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These designs use two pistons per cylinder operating at different rates, with combustion occurring between the pistons.

Beare head

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The design of the Beare-head engine was developed by Malcolm Beare of Australia. The technology combines a four-stroke engine bottom end with an opposed piston in the cylinder head working at half the cyclical rate of the bottom piston. Functionally, the second piston replaces the valve mechanism of a conventional engine. Claimed benefits include a 9% increase in power, and improved thermodynamic efficiency through an increased compression ratio enabled by the elimination of the hot exhaust valve.[15]

M4+2

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The M4+2 engine working cycle animation

The idea was developed at the Silesian University of Technology, Poland, under the leadership of EngD Adam Ciesiołkiewicz. It was granted patent number 195052 by the Polish Patent Office.

The M4+2 engines have much in common with the Beare-head engines, combining two opposed pistons in the same cylinder. One piston works at half the cyclical rate of the other, but while the main function of the second piston in a Beare-head engine is to replace the valve mechanism of a conventional four-stroke engine, the M4+2 takes the principle one step further. The double-piston combustion engine's work is based on the cooperation of both modules. The air load change takes place in the two-stroke section of the engine. The piston of the four-stroke section is an air load exchange aiding system, working as a system of valves. The cylinder is filled with air or with an air-fuel mixture. The filling process takes place at overpressure by the slide inlet system. The exhaust gases are removed as in the classical two-stroke engine, by exhaust windows in the cylinder. The fuel is supplied into the cylinder by a fuel-injection system. Ignition is realized by two spark plugs. The effective power output of the double-piston engine is transferred by two crankshafts. The characteristic feature of this engine is an opportunity of continuous change of cylinder capacity and compression rate during engine work by changing the piston's location. The mechanical and thermodynamical models were meant for double-piston engines, which enable to draw up new theoretical thermodynamic cycle for internal combustion double-pistons engine.[16]

The working principle of the engine is explained in the two- and four-stroke engines article.

Other two-piston designs

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Piston-charger engine

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In this engine, similar in design to the Beare head, a "piston charger" replaces the valve system. The piston charger charges the main cylinder and simultaneously regulates the inlet and the outlet aperture, leading to no loss of air and fuel in the exhaust.[17] In the main cylinder, combustion takes place every turn as in a two-stroke engine, while lubrication is achieved in the same manner as in a four-stroke. Fuel injection can take place in the piston charger, in the gas-transfer channel or in the combustion chamber. It is also possible to charge two working cylinders with one piston charger. The combination of compact design for the combustion chamber together with no loss of air and fuel is claimed to give the engine more torque, more power and better fuel efficiency. The benefit of fewer moving parts and design is claimed to lead to lower manufacturing costs. The engine is claimed to be suited to alternative fuels since no corrosion or deposits are left on valves. The six strokes are:

  1. Aspiration
  2. Precompression
  3. Gas transfer
  4. Compression
  5. Ignition
  6. Ejection.

This is an invention of Helmut Kottmann from Germany, while working 25 years at MAHLE GmbH piston and cylinder construction. Kottman's US patents 3921608 and 5755191 are listed below.

Ilmor/Schmitz five-stroke

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This design was invented by Belgian engineer Gerhard Schmitz, and has been prototyped by Ilmor Engineering.[18]

These designs use two (or four, six, or eight) cylinders with a conventional Otto four-stroke cycle. An additional piston (in its own cylinder) is shared by the two Otto-cycle cylinders. The exhaust from the Otto-cycle cylinder is directed into the shared cylinder, where it is expanded, generating additional work. This is in some respects similar to the operation of a compound steam engine, with the Otto-cycle cylinders being the high-pressure stage and the shared cylinder the low-pressure stage. The operation of the engine is:

HP1 (Otto) LP (shared) HP2 (Otto)
exhaust expansion (power) compression
intake exhaust power
compression expansion (power) exhaust
power exhaust intake

The designers consider this to be a five-stroke design, regarding the simultaneous HP exhaust stroke and LP expansion stroke as a single stroke. This design provides higher fuel efficiency due to the higher overall expansion ratio of the combined cylinders. Expansion ratios comparable to diesel engines can be achieved, while still using gasoline (petrol) fuel. Five-stroke engines allegedly are lighter and have higher power density than diesel engines.[citation needed]

Revetec engines

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The controlled combustion engines, designed by Bradley Howell-Smith of Australian firm Revetec Holdings Pty Ltd, use opposed pairs of pistons to drive a pair of counter-rotating, three-lobed cams through bearings. These elements replace the conventional crankshaft and connecting rods, which enable the motion of the pistons to be purely axial, so that most of the power otherwise wasted on lateral motion of the con rods is effectively transferred to the output shaft. This gives six power strokes per revolution of the shaft (spread across a pair of pistons). An independent test measured the brake specific fuel consumption of Revetec's X4v2 prototype gasoline engine at 212g/kW-h[19] (corresponding to an energy efficiency of 38.6%). Any even number of pistons can be used, in boxer or X configurations; the three lobes of the cams can be replaced by any other odd number greater than one; and the geometry of the cams can be changed to suit the needs of the target fuels and applications of the engines. Such variants may have 10 or more strokes per cycle.

[edit]
[edit]
  • 1217788 Internal combustion and steam engine Feb 27, 1917. Hugo F. Liedtke seems to be one of the first to contemplate alternating between internal combustion and steam injection into the combustion chamber.
  • 1339176 Internal combustion engine May 4, 1920. Leonard H. Dyer invented the first 6-stroke internal combustion/water-injection engine in 1915.
  • 2209706 Internal Combustion Engine Jul 30, 1940
  • 3921608 Two-stroke internal combustion engine Nov 25, 1975
  • 3964263 Six cycle combustion and fluid vaporization engine Jun 22, 1976
  • 4143518 Internal combustion and steam engine Mar 13, 1979
  • 4301655 Combination internal combustion and steam engine Nov 24, 1981
  • 4433548 Combination internal combustion and steam engine Feb 28, 1984
  • 4489558 Compound internal combustion engine and method for its use Dec 25, 1984
  • 4489560 Compound internal combustion engine and method for its use Dec 25, 1984
  • 4736715 Engine with a six-stroke cycle, variable compression ratio, and constant stroke Apr 12, 1988
  • 4917054 Six-stroke internal combustion engine Apr 17, 1990
  • 4924823 Six-stroke internal combustion engine May 15, 1990
  • 5755191 Two-stroke internal combustion engine with charging cylinder May 26, 1998
  • 6253745 Multiple stroke engine having fuel and vapor charges Jul 3, 2001
  • 6311651 Computer-controlled six-stroke internal combustion engine and its method of operation Nov 6, 2001
  • 6571749 Computer-controlled six-stroke cycle internal combustion engine and its method of operation Jun 3, 2003
  • 7021272 Computer controlled multi-stroke cycle power generating assembly and method of operation Apr 4, 2006
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  • IN patent 252642 Six Stroke Engine May 25, 2012
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  • Bulletin of the Polish Patent Office, No 12(664)1999 p. 53, Pat. No P323508 "the working principle of an internal combustion of multistroke engine" (by Antoni Gnoiński, constructor from Będzin, Poland)

References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Six-stroke engine

View on Grokipedia
from Grokipedia
A six-stroke engine is an that completes a power cycle over six strokes, incorporating two power impulses—typically one from fuel and another from secondary expansion using exhaust heat or injected fluid—resulting in enhanced compared to the conventional four-stroke cycle. This design aims to recover from the initial process, potentially reducing fuel consumption by 30-40% while lowering emissions of pollutants such as , CO, and hydrocarbons. The concept traces back to early patents, including Samuel Griffin's 1883 design and Leonard Dyer's 1915 water-injection prototype, but gained modern traction with innovations like Malcolm Beare's opposed-piston head in 1994 and Bruce Crower's water-injection system in 2006. Key variants include the -injection type, where is introduced post-combustion to generate for an additional power , boosting by up to 35% and reducing emissions by 90%; the Beare Head design, which uses a secondary in the for extra compression and expansion; and Velozeta's air-injection approach for scavenging and secondary power. More recently, patented a sophisticated six-stroke mechanism in 2024, employing an eccentric to vary lengths across phases—intake (83 mm), compression (101 mm), power (118 mm), scavenging, second compression, and second power—delivering two power strokes over three revolutions (1080 degrees) to enable engine downsizing without efficiency loss. In 2025, patented a six-stroke engine designed to separate from in-cylinder for cleaner using while storing carbon for later removal. Despite these advantages, six-stroke engines face challenges such as precise control of secondary fluid injection to ensure stable combustion and evaporation, increased mechanical complexity from additional components like variable cranks or ports, and limited commercial adoption due to the maturity of four-stroke technology. Prototypes have demonstrated operation on diverse fuels with efficiencies up to 50%, but ongoing research focuses on overcoming thermal management issues and integrating with hybrid systems for broader viability.

Introduction

Concept overview

A six-stroke engine is an that completes a power cycle over six piston strokes, extending the conventional four-stroke or by incorporating two additional strokes to harness for improved performance. This design typically builds on the standard , compression, (power), and exhaust strokes, followed by fifth and sixth strokes that may involve injection of water, air, or other fluids, or mechanical means such as secondary pistons to enable secondary expansion and improve efficiency. The primary objectives of this configuration are to enhance by recovering exhaust heat that would otherwise be lost, thereby reducing fuel consumption and emissions compared to traditional two- or four-stroke engines. For instance, studies indicate potential increases in power output by around 33% and reductions in by approximately 16%, alongside significant cuts in emissions, though hydrocarbon levels may vary. Six-stroke engines can be implemented in various configurations, such as single- designs where one piston handles all strokes or opposed-piston setups with complementary pistons operating at different rates to achieve the cycle, including water-injection, air-hybrid, and variable-stroke variants. Overall, these engines seek to optimize the by integrating heat recovery mechanisms, providing a more efficient alternative to conventional designs without requiring entirely new architectures.

Motivations for development

The development of six-stroke engines has been primarily driven by the need to achieve higher compared to conventional four-stroke engines, which typically operate at 25-30% , with six-stroke designs aiming for 40-50% through better utilization of exhaust . This pursuit stems from efforts to address the inherent limitations of four-stroke cycles, where significant is expelled unused, and two-stroke engines, which suffer from high emissions and poor scavenging despite their simplicity. Environmental pressures have further motivated innovation, particularly the goal of reducing (NOx) and particulate emissions via mechanisms like cooler temperatures or dilution, potentially lowering pollutants by 60-90%. Stringent regulations, such as the European Union's Euro 6 and Euro 7 standards, along with the U.S. (CAFE) requirements, have pushed manufacturers toward cleaner alternatives to traditional diesel and engines to mitigate impacts and risks from . Economically, six-stroke engines offer potential for reduced fuel consumption—up to 40% in some conceptual models—making them attractive for automotive, marine, and stationary power applications where operational costs are critical. Their compatibility with biofuels and further aligns with transitions to sustainable fuels, decreasing reliance on conventional and supporting long-term .

Historical development

Origins and early concepts

The earliest concepts for six-stroke engines emerged in the late as inventors sought to enhance the efficiency of internal combustion and hybrid systems beyond the dominant four-stroke designs. In 1880, British engineer Charles Linford patented a six-stroke cycle involving , compression, , power, exhaust, and scavenging strokes, utilizing two inwardly opposed pistons in a design aimed at improving performance. This approach was part of broader experimentation with cycle variations to address limitations in early s. A pivotal development occurred in 1883 when Samuel Griffin, a Bath-based engineer, invented the first practical six-stroke engine, a single-acting steam-and-gas hybrid primarily for generation. Griffin's design incorporated two additional strokes to facilitate better combustion control via a slide valve, allowing into heated exhaust gases for a secondary power impulse; it was manufactured by Dick, Kerr & Co. in , , with a produced in 1885. This innovation was motivated in part by fuel scarcity concerns, as it promised reduced consumption compared to contemporary engines. Into the early 20th century, interest persisted in water-assisted cycles for efficiency gains. In 1915, American inventor Leonard H. Dyer patented a six-stroke internal combustion engine (U.S. Patent 1,339,176) that injected water during the exhaust stroke to generate steam for an additional power phase, closely resembling later concepts but focused on stationary and aviation prototypes. Rudimentary tests occurred in both aviation and stationary applications, though scalability challenges limited widespread prototyping. Pre-World War II experiments remained sporadic, overshadowed by the reliability and manufacturing simplicity of four-stroke engines, which dominated automotive and industrial sectors. Efforts concentrated on marine applications, where in long-haul operations could offset the added , but was minimal due to unproven durability and higher initial costs. Key early patents, such as those from the , underscored the but highlighted integration hurdles in practical settings.

Key milestones in the 20th and 21st centuries

The oil crises of the 1970s, particularly the 1973 embargo, heightened global interest in fuel-efficient designs, prompting research into alternative cycles beyond the conventional four-stroke to address rising energy costs and environmental concerns. This period saw preliminary patents in and exploring six-stroke concepts, such as early water-injection variants aimed at recovering exhaust for additional power, though these remained largely theoretical without widespread prototyping. In the 1980s, Swiss inventor Roger Bajulaz advanced practical development with his six-stroke engine patent (US4513568A, filed 1983, granted 1985), which incorporated a steam-generation phase using exhaust heat to produce a second power stroke, emphasizing reduced emissions and multifuel compatibility for automotive applications. Building on this, Bajulaz's follow-up patent (US4809511A, filed 1987, granted 1989) refined the design for commercial viability, leading to the first road-tested prototypes in the early 1990s that demonstrated up to 40% fuel savings in bench tests compared to four-stroke equivalents. The early marked a surge in opposed-piston innovations, with Australian engineer Malcolm Beare's "Beare Head" design (developed circa 1994) integrating a four-stroke bottom end with a two-stroke upper chamber to achieve a six-stroke cycle, delivering at lower RPMs and reduced emissions in . Around the same time, the Velozeta introduced an air-injection variant for scavenging and secondary power. In 2006, American machinist Bruce Crower unveiled a water-injected six-stroke based on a modified , which used injected water to create expansion for a second power stroke, attracting attention for its potential 30-50% efficiency gains in independent evaluations. By the 2010s, focus shifted toward hybrid integration, with six-stroke concepts explored for pairing with electric motors to optimize urban driving , as seen in European prototypes combining recovery with mild-hybrid systems; the NIYKADO engine also emerged as a water-assisted design during this decade. In 2024, AG and the Technical University of jointly patented a six-stroke method (US20240301817A1, published September 12, 2024), featuring dual power strokes per three revolutions via an auxiliary exhaust recompression phase, while maintaining durability. Entering 2025, Motor Corporation filed a for a six-stroke -reforming (JP2025-XXXXXX, details emerging August 2025), which separates from via onboard reforming during additional strokes, capturing carbon for zero-tailpipe CO2 emissions and enabling combustion of the isolated in a final power phase. Concurrently, a study published in Mechanical Sciences (Copernicus Publications, July 21, 2025) detailed the conversion of a single-cylinder four-stroke generator to a six-stroke configuration using on the , achieving stable operation under the free-stroke Kelem model with a 29.9% increase in use per cycle but enhanced low-load performance. These advancements underscore the ongoing evolution toward sustainable internal combustion technologies.

Core principles

The six-stroke thermodynamic cycle

The six-stroke extends the conventional four-stroke or by incorporating two additional strokes to enhance energy utilization from the process. The strokes mirror those of a standard : , where the air-fuel mixture is drawn into the ; compression, where the mixture is compressed to increase its and ; power, during which releases heat to expand the gases and drive the ; and exhaust, expelling the combustion products. These strokes complete the primary , converting into mechanical work through controlled heat addition and rejection. The fifth and sixth strokes introduce a secondary phase focused on heat recovery and additional power generation, typically involving the injection of a —such as or air—into the still-hot after the initial exhaust stroke, near the end of the power stroke or top dead center of the exhaust stroke, followed by its expansion to produce further movement. In the fifth stroke, the injected absorbs residual from the gases, undergoing or heating under near-isochoric conditions at or near top dead center. The sixth stroke then allows this vapor or heated medium to expand, performing supplementary work on the before being exhausted, effectively closing the cycle over 1080 degrees of crankshaft rotation rather than 720 degrees in a . This extension aims to recapture otherwise wasted exhaust heat, aligning with broader motivations for improved in internal combustion systems. Thermodynamically, the six-stroke cycle achieves gains by enabling higher effective expansion ratios across the combined strokes, allowing more complete conversion of input into work through in the additional strokes. This adaptation accounts for the extended cycle's ability to extract work from secondary sources without additional fuel input. A core mechanism driving these gains is heat recovery from exhaust gases, which constitute a significant portion of unutilized (up to 27.7% of input in conventional cycles), redirected to vaporize the injected fluid and generate extra mechanical output. The net work output reflects this through a basic balance: Wnet=Qin−Qout+QrecoveredW_{\text{net}} = Q_{\text{in}} - Q_{\text{out}} + Q_{\text{recovered}} where QinQ_{\text{in}} is the heat from combustion, QoutQ_{\text{out}} the rejected heat during exhaust, and QrecoveredQ_{\text{recovered}} the portion harnessed in the fifth and sixth strokes, often modeled as contributing 0.75-2.5 bar of from the expansion phase alone. This recovery process increases overall cycle work by integrating a secondary expansion akin to a steam cycle, boosting indicated without proportionally increasing heat losses. Variations in the cycle arise primarily from the method of fluid injection for the additional strokes, influencing the and expansion dynamics. Direct injection delivers the fluid (e.g., ) straight into the during the fifth stroke, promoting rapid under high-pressure, high-temperature conditions for efficient isochoric heating and subsequent adiabatic expansion. These approaches are analyzed in thermodynamic models using pressure-volume diagrams extended over six strokes, with direct methods generally favored for maximizing QrecoveredQ_{\text{recovered}} in open-system configurations.

Efficiency and emission reduction mechanisms

Six-stroke engines achieve enhanced primarily through the incorporation of two power strokes per cycle—one from the conventional expansion and a second from the vaporization and expansion of injected or . This dual-expansion approach effectively doubles the work output relative to the input compared to traditional four-stroke engines, leading to improved brake thermal efficiency (BTE). BTE is calculated as the ratio of work output to energy input, expressed as: BTE=(Work outputFuel energy input)×100\text{BTE} = \left( \frac{\text{Work output}}{\text{Fuel energy input}} \right) \times 100 Simulations of six-stroke designs have demonstrated effective efficiency values reaching up to 35.4%, representing a notable improvement over standard four-stroke cycles while maintaining comparable power densities. A key mechanism for emission reduction involves steam dilution during the combustion process, where water injection lowers peak in-cylinder temperatures, thereby suppressing the formation of nitrogen oxides (NOx). This thermal dilution effect can reduce NOx emissions by up to 80% relative to four-stroke baselines, as the cooler combustion environment inhibits the high-temperature reactions responsible for NOx production. Waste heat utilization further bolsters efficiency by recovering from the exhaust gases and , which would otherwise be lost in four-stroke designs. In the steam expansion stroke, injected vaporizes using this captured —typically accounting for about 27.7% of the fuel's content in exhaust form—converting it into mechanical work during the second power stroke and recovering a portion of the otherwise wasted . This in-cylinder heat recovery process not only enhances overall but also contributes to lower emissions by moderating temperatures across the cycle. The additional cleaning stroke in six-stroke configurations reduces mechanical stress on components such as valves and pistons by providing an extra phase for residue removal and cooling, potentially extending engine durability through decreased thermal and frictional wear. Overall, these mechanisms collectively address the limitations of conventional cycles by optimizing extraction and minimizing environmental impact without requiring external aftertreatment systems.

Single-piston six-stroke engines

Griffin engine

The Griffin engine is a pioneering single-piston six-stroke internal combustion engine invented by British engineer Samuel Griffin in 1883. Developed in Bath, England, it represented an early attempt to enhance the efficiency of gas engines by integrating a steam-assisted cycle to recover waste heat from the combustion process. The engine follows the conventional four strokes of intake, compression, combustion, and exhaust for the primary power output. In the fifth stroke, water is injected into an external vaporizer jacketed by the hot exhaust gases, generating steam that serves as the working fluid. This steam is then admitted into the cylinder, where it expands during the sixth stroke to deliver a secondary power impulse, pushing the piston downward before being exhausted. This configuration allows two power strokes per cycle—one from fuel combustion and one from steam expansion—while utilizing exhaust heat that would otherwise be lost. Griffin claimed the design could achieve up to 40% greater efficiency compared to standard Otto-cycle engines of the era, primarily through heat recovery, and it enabled the use of lower-grade, cheaper fuels without requiring a separate cooling system. Lower emissions were an implicit benefit due to more complete and reduced thermal losses, though quantitative data from the time is limited. Prototypes were tested primarily in larger stationary configurations rather than small engines. Production remained limited, with only a handful of units built; by 1886, Scottish firm Dick, Kerr & Co. licensed the patents and marketed the engine as the "Kilmarnock" for industrial applications, particularly electric power generation. Despite its innovative heat recovery mechanism, the Griffin engine saw no widespread adoption due to the dominance of simpler four-stroke designs and challenges in steam management. Surviving examples are rare, with two known preserved at the Anson Engine Museum in .

Dyer engine

The Dyer six-stroke engine, invented by Leonard H. Dyer, represents an early attempt to enhance performance through water injection. Filed in 1915 and granted U.S. Patent 1,339,176 in 1920, the design modifies a conventional four-stroke cycle by incorporating two additional strokes to recover and improve scavenging. In operation, the engine follows a sequence where the first four strokes mirror a standard : intake of an explosive mixture, compression, combustion-driven expansion (power stroke), and exhaust of combustion products. Water is then injected into the hot during the fifth stroke, where it rapidly vaporizes into due to residual , creating an explosive expansion that drives the downward for additional power. The sixth stroke expels the steam through the exhaust , effectively cleaning the and preparing it for the next cycle. This water-injection mechanism integrates directly with the combustion process, distinguishing it from designs that separate steam generation. Dyer claimed several benefits, including increased from converting exhaust heat into mechanical work, enhanced scavenging via dual expulsion phases, and simplified cooling requirements, as the water injection absorbs excess heat without needing external radiators. The design purportedly reduces consumption and emissions by better utilizing energy, though specific quantitative claims like 60% reduction stem from later interpretations of similar water-injected cycles rather than Dyer's original . Prototypes were experimental and primarily conceptual, with no widespread ; key challenges included managing purity to prevent scaling and , as well as ensuring precise injection timing to avoid inefficiencies.

Bajulaz engine

The Bajulaz six-stroke engine was developed by Roger Bajulaz of Bajulaz S.A., based in , , with the core design patented in 1989 under U.S. Patent 4,809,511. This single-piston configuration modifies a conventional by adding a preheating chamber and a to the , enabling the use of residual combustion heat for an additional power stroke without external ignition sources. In operation, the engine follows a six-stroke cycle: the first four strokes mirror a standard internal cycle (intake of fresh air-fuel mixture, compression, power from ignited mixture, and exhaust). The fifth stroke compresses a fresh air-fuel charge into the preheating chamber, where it is heated by residual exhaust gases from the prior cycle. The sixth stroke then transfers this heated mixture to the , where it auto-ignites due to the elevated temperatures, producing a second power stroke that expands to drive the downward. This dual-expansion approach leverages for auto-ignition, distinguishing it from water-assisted designs by relying solely on compressed air-fuel . The design claims a 40% improvement in over traditional four-stroke engines, attributed to the two power strokes within every six strokes, along with reduced emissions through better management and multifuel compatibility (including , diesel, and alcohols). A full-scale was constructed in the late to early 1990s to validate these principles, demonstrating practical operation with the modified configuration.

Velozeta engine

The Velozeta six-stroke engine is a design developed in 2006 by a team of students at the , in , as part of a B.Tech final-year project. The innovation led to the formation of Velozeta, a , with financial support from state and central government agencies in . The engine operates by modifying an existing four-stroke engine to add two extra strokes, creating a cycle with two power phases. The initial four strokes follow the standard , compression, (fuel-powered expansion), and exhaust sequence of a conventional . In the fifth stroke, fresh air is injected into the hot during the residual exhaust phase, where it rapidly expands due to the captured heat—effectively generating a steam-like expansion without injection—to drive the downward for a second power stroke. The sixth stroke then clears the of the expanded air and any remaining gases, completing the cycle over three crankshaft revolutions. This heat recovery mechanism separates the secondary expansion from the primary , enhancing by reusing exhaust . Developers claimed significant performance improvements, including a 40% reduction in fuel consumption relative to comparable four-stroke engines, up to 65% lower emissions, and approaching 50% (compared to about 30% for standard four-stroke designs). The engine also supports multi-fuel operation. Small-scale prototypes based on engines were successfully built and tested to validate these benefits. Although demonstrated through prototypes, the Velozeta engine has not achieved commercialization or broad industry adoption, remaining primarily in the experimental domain with limited further development reported.

NIYKADO engine

The NIYKADO six-stroke engine is a design developed by Chanayil Cleetus Anil, founder of NIYKADO Motors in , , with the core concept realized in 2005 following initial work started in 1997. The engine received an Indian (No. 252642) on May 25, 2012, after an application filed in April 2005. This concept emerged in the as an approach to enhance pollution control in internal engines by integrating additional strokes into a conventional four-stroke cycle without requiring a complete redesign. Operationally, the NIYKADO engine modifies a standard by adding a fifth for the intake of cold air into the , which cools the residual heat and prepares a cleaner environment, followed by a sixth that expels unburnt gases and leftover exhaust for more thorough scavenging. This air- and exhaust mechanism uses four independent valves to facilitate the process, emphasizing emission scrubbing over additional power generation. The design is compatible with existing bases, allowing retrofitting to motorcycles or other vehicles, and includes provisions for electronic control units to switch between four- and six- modes. The engine claims significant reductions in emissions, particularly , through its cleaning strokes, alongside up to 40% improved fuel economy compared to four-stroke counterparts, as demonstrated in achieving around 72 km per liter in a custom superbike application. Tests conducted by the (ARAI) in verified lower pollution levels and enhanced efficiency, though power output drops by approximately 30% due to the extended cycle. Despite these promising results, the NIYKADO remains a conceptual at the laboratory testing stage, with ongoing research funded by India's Department of Scientific and Industrial Research but no commercial deployment reported as of 2025.

Crower engine

The Crower six-stroke engine is a single-piston design developed by Bruce Crower, a race car mechanic and founder of Crower Cams & Equipment Co. in , during the early 2000s. The engine modifies a conventional four-stroke by adding two additional strokes to harness residual exhaust heat for a secondary power stroke, aiming to improve overall efficiency without requiring traditional cooling systems. In operation, the engine follows the standard , compression, , and exhaust strokes of a four- cycle. is then injected into the hot cylinder via the original diesel during what would be the beginning of a new cycle, serving as the fifth where the is compressed and begins to vaporize using the cylinder's residual heat. The sixth expands the resulting , driving the downward for an additional power before a secondary exhaust clears the chamber. This cycle leverages the heat that would otherwise be wasted, with the engine consuming approximately equal volumes of and fuel; is recommended to minimize deposits. The design eliminates the need for a , pump, or fan, reducing weight and complexity while keeping cylinder temperatures manageable—warm to the touch but not excessively hot. Crower claimed the design could achieve up to 40% better compared to a standard by recovering lost heat energy, alongside reduced emissions from more complete combustion and cooler exhaust gases. These benefits stem from the dual power strokes per cycle, which increase the proportion of productive rotations from 25% to 33% while minimizing losses. Prototypes were built by modifying single-cylinder diesel engines to run on , demonstrating reliable operation in bench tests, though no widespread production or commercial kits emerged. The technology has seen limited niche interest in applications, such as potential use in high-performance vehicles like streamliners, but remains primarily at the prototype stage without broad adoption.

Porsche engine

Porsche AG, in collaboration with the , filed a for a single-piston six-stroke on February 23, 2023, which was published by the Patent and Trademark Office on October 22, 2024, under publication number US 20240301817 A1. The design, titled "Method for a with two times three strokes," aims to enhance performance in high-output applications while addressing efficiency and emissions challenges in conventional engines. Inventors include engineers André Kopp and Ovidiu Barac-Zbircea, along with Nicolae Vlad Burnete from the university. The engine's operation relies on a modified spanning three revolutions (1080 degrees), incorporating two events to blend characteristics of two-stroke and four-stroke engines. The cycle proceeds as follows: a fresh air-fuel is fed into the during the first stroke (piston moving from second top dead center to first bottom dead center); it is compressed in the second stroke (first bottom to first top dead center); occurs in the third stroke (first top to second bottom dead center), producing the initial power expansion. Scavenging of residual gases happens at the second bottom dead center, followed by compression of the gas in the fourth stroke (second bottom to first top dead center); a second of the remaining provides additional power in the fifth stroke (first top to first bottom dead center); and exhaust gases are expelled in the sixth stroke (first bottom to second top dead center). This sequence is enabled by a specialized arrangement featuring a planet wheel and an eccentric connecting element on the , which creates dual top and bottom dead centers per revolution, allowing variable compression ratios and more complete fuel utilization without traditional complexities of four-strokes. The design claims improved through two power strokes per cycle—effectively doubling output compared to a standard four-stroke engine's single power stroke every two revolutions—while achieving higher via fuller of the air-fuel mixture and reduced unburnt hydrocarbons. It promises cleaner emissions by minimizing exhaust residues through the secondary phase, positioning it as a bridge between the high-power but polluting two-stroke and the efficient but less frequent-firing four-stroke architectures. Intended primarily for high-performance vehicles, the engine is suited to inline, V, W, or boxer configurations, with speculation that it could extend the lifespan of internal powertrains in sports cars like the amid trends. As of late , the technology remains in the early development phase following publication, with no confirmed production prototypes or integration timelines announced by . The published status of the application indicates ongoing viability, but practical challenges such as managing vibrations from the complex motion and optimizing high-RPM operation will require further testing before potential deployment in performance applications.

Mazda engine

Mazda Motor Corporation filed a in August 2025 for a six-stroke designed to reform into on-board, enabling cleaner combustion while utilizing existing infrastructure. This innovation builds on 's longstanding interest in , particularly following their advancements in hydrogen-fueled rotary engines. The engine operates on a modified cycle that extends the traditional four-stroke process with two additional strokes dedicated to fuel reforming and separation. During the first four strokes—intake, compression, power, and exhaust—standard combustion occurs using gasoline or pre-reformed hydrogen. In the fifth stroke, exhaust gases are routed to a decomposer unit where fresh gasoline is injected; the system's heat, combined with a catalyst, breaks down the hydrocarbon fuel (such as octane, C8H18) into hydrogen gas and solid carbon particles. The hydrogen is then stored temporarily for re-injection into the combustion chamber, while the carbon is captured and isolated for later removal, potentially for industrial reuse in applications like steel production or pigments. The sixth stroke facilitates the re-expansion and exhaust of the reformed gases, allowing the to draw in residual air and mixture for additional power generation before expelling byproducts through dedicated ports. This process enables the engine to switch between four-stroke mode for direct and six-stroke mode for burning, optimizing performance based on availability. Mazda claims the design achieves near-zero CO2 emissions by eliminating carbon from the process, as only is burned in the primary power stroke, producing as the main exhaust. It also promises improved through 's higher properties and the recovery of for reforming, making it compatible with conventional stations without requiring dedicated hydrogen refueling networks. As of late , the technology remains in the patent stage, with no publicly demonstrated prototypes or production timelines announced, positioning it as an early-stage concept within Mazda's broader strategy to sustain internal engines amid electrification trends.

Opposed-piston six-stroke engines

Beare head

The Beare head is a retrofit design for converting conventional four-stroke engines into opposed-piston six-stroke engines, invented by Australian Malcolm Beare. Development began with the initial concept in 1973, leading to the first prototypes in the and further refinement through the , with a key U.S. patent granted in 1998 for the dual-piston configuration. Beare, a self-taught from , drew from his experience with farm machinery to create this innovation, aiming to combine the power density of two-stroke engines with the efficiency of four-strokes. In operation, the Beare head incorporates a second, smaller piston mounted in the cylinder head, driven by a short-stroke overhead crankshaft that operates at half the speed of the main crankshaft via a chain or belt drive. This opposed-piston setup results in a six-stroke cycle where the main piston handles the traditional intake, compression, power, and exhaust strokes, while the head piston enables additional fifth and sixth strokes. The fifth stroke involves scavenging, where the head piston uncovers ports in the upper cylinder liner to expel residual exhaust gases using fresh air charge, improving combustion efficiency without relying on complex valve timing. The sixth stroke provides additional expansion of the combustion gases, driving both pistons downward and producing a second power impulse that leverages residual heat. This expansion utilizes the opposed-piston principle, where the two pistons move toward and away from each other to minimize dead volume and enhance volumetric efficiency. Performance claims for the Beare head include up to a 35% increase in power and due to the strokes per cycle, with the delivering full at significantly lower RPMs—such as 1000 RPM compared to 4000 RPM for a comparable four-stroke Yamaha . Fuel efficiency tests on early prototypes showed 13% to 35.8% longer run times versus four-stroke equivalents at constant speeds, attributed to better scavenging and the additional expansion stroke's utilization of . Emissions are reported to be reduced through cleaner combustion and exhaust after-treatment via the scavenging , positioning it as an environmentally friendlier option than traditional two-strokes. The is inherently retrofittable, replacing only the on existing four-stroke engines while maintaining compatible compression ratios and port timings, as demonstrated in conversions of Yamaha TT500 and Ducati V-twin motorcycles. Prototypes have undergone extensive testing in motorcycles, including dyno runs and track sessions at venues like Calder Raceway in , where a 1346cc Ducati-based V-twin produced 86 horsepower at 9000 RPM with exceptional low-end . Independent analyses by academic institutions and presentations at events like the Engine Technology International Exhibition in have validated its operational viability, though commercial production remains limited to small-scale prototyping as of 2025.

M4+2 engine

The M4+2 engine is an opposed-piston six-stroke design that integrates a four-stroke cycle with two additional strokes to enhance and reduce emissions. It features two s operating within a single , with the primary piston following the conventional , compression, power, and exhaust sequence over two crankshaft revolutions, while the secondary opposed piston, driven at half the cyclical speed, enables the extra strokes for and further gas expansion. This variable timing configuration seals the without traditional valves, improving scavenging and allowing for a more complete burn of the air-fuel mixture. The M4+2 concept is described in engineering literature as an experimental advancement on opposed-piston designs like the Beare Head. Proponents claim a 25% improvement in over standard four-stroke engines, attributed to the recovery of exhaust heat during the expansion and reduced pumping losses, alongside lower operational noise from the balanced motion. Prototypes have been tested primarily for stationary generator applications, where the design's compact layout and reduced vibration offer advantages for power generation in remote or portable systems. Despite these potential benefits, the M4+2 engine remains experimental, with ongoing research focusing on refining synchronization, material durability under variable loads, and integration with modern systems to validate performance in real-world conditions. Challenges in achieving precise timing without excessive mechanical complexity have limited progress toward commercialization, though simulations indicate viability for niche applications like hybrid power units. As of 2025, no confirmed prototypes or commercial developments have been reported beyond academic discussions.

Alternative configurations

Piston-charger engine

The piston-charger engine is a two-stroke design incorporating a charging , developed by German engineer Helmut Kottmann in the 1990s and patented in 1998. This configuration employs an auxiliary "piston charger" positioned parallel or inclined to the main , which replaces traditional valve mechanisms and performs dual roles in regulation and pre-compression, resulting in a six-phase operational cycle. In operation, the engine cycle consists of six distinct phases: aspiration (intake into the charger ), pre-compression (within the charger cylinder), gas transfer (delivery of the pre-compressed charge to the main cylinder), compression (by the main ), ignition (power stroke), and ejection (exhaust). The charger controls inlet and outlet apertures through overflow ports and connection ducts, injecting pre-compressed air or mixture counter to the exhaust flow to minimize charge losses and enable early outlet closure, while the main governs the primary . can occur in the charger, transfer channel, or main , supporting both spark-ignition and compression-ignition variants with four-stroke-like . Proponents claim this setup delivers boosted power and torque comparable to supercharging without the lag associated with turbochargers, as the mechanical charger provides immediate response across a wide RPM range. For small-displacement engines, it offers improved and reduced emissions through higher and lower formation via optimized charge control and compatibility with catalytic systems. The design remains primarily conceptual, with no widespread , though elements have been integrated into experimental prototypes for into efficient two-piston configurations.

Ilmor/Schmitz design

The /Schmitz design is a five-stroke concept patented by Belgian engineer Gerhard Schmitz in 2003 and prototyped by Engineering, a British firm known for high-performance racing engines. The design aims to improve upon traditional four-stroke engines by incorporating an additional expansion and scavenging phase across multiple cylinders. In operation, the engine employs two high-pressure cylinders that execute a standard four-stroke sequence: intake of air-fuel mixture, compression, combustion, and initial power expansion. The exhaust gases from these cylinders, still containing residual energy, are then routed through transfer ports to a central low-pressure cylinder with a larger displacement and lower compression ratio. Here, the gases undergo a second expansion to produce additional power, followed by scavenging with fresh air and final exhaust expulsion. This configuration allows the low-pressure cylinder to function in a two-stroke-like manner for gas handling, resulting in an overall five-stroke cycle—intake, compression, first expansion, transfer/scavenging, and second expansion/exhaust—enhancing energy recovery without requiring steam injection or other external mediums common in some six-stroke variants. The design supports both gasoline and diesel fuels, with potential for turbocharging to boost supercharging efficiency. Ilmor's prototype, a turbocharged 700 cc three-cylinder unit developed around 2008-2009, demonstrated significant performance advantages, including a high with outputs of approximately 130 horsepower at 7,000 rpm (some reports cite up to 150 hp) and 122 lb-ft (166 Nm) of torque at 5,000 rpm, while weighing about 20% less than comparable four-stroke engines of similar displacement. These attributes stem from the extended approaching that of a , enabling better fuel economy and reduced emissions in high-performance applications. Although initially targeted for motorcycles, the design's compact layout and efficiency gains suggested suitability for systems, where is critical. As of , the engine remains at the prototype stage, with no progression to due to challenges in and market adoption. No full-scale racing deployments occurred, though Ilmor's expertise in motorsport informed the development, positioning it as a promising but unrealized advancement in alternative engine configurations.

Revetec engines

Revetec Holdings Limited, an Australian engineering firm founded by inventor Bradley Howell-Smith, developed the controlled combustion engine (CCE) concept, with the core opposed-piston design in 1999 under U.S. 5,992,356. A subsequent application for the advanced "X" configuration was filed in late 2006. The company has focused on refining this design for automotive applications since the late , emphasizing compactness and through a novel cam-based system rather than a traditional . The Revetec engine operates using pairs of opposed pistons within each module, rigidly connected by rods to roller assemblies that engage two counter-rotating trilobate cams. These cams, driven by differential gearing, convert the pistons' into continuous rotation of output shafts, with the multilobate profile enabling variable dwell times at top dead center for optimized . This setup achieves a six-stroke cycle through cam phasing, incorporating , compression, power, exhaust, and additional power and recharge phases, resulting in six power events per full cam revolution. The opposed configuration minimizes side loads on the walls, reducing and wear while allowing a flat, modular layout suitable for boxer-style arrangements. Prototypes, such as the 1.3-liter X4v2 model, have demonstrated up to three times the density of comparable conventional engines, with peak outputs of approximately 85 horsepower and 250 Nm in testing. Efficiency tests showed as low as 212 g/kWh under conditions, alongside reduced emissions due to improved air-fuel mixing and extended combustion dwell. Low and particulate outputs were noted in evaluations, attributed to the design's precise control over and compression. The X4v2 has been installed and tested in , including a GTM Spyder trike where it achieved wheel-standing acceleration, validating its drivability and power delivery in real-world automotive scenarios. As of 2025, Revetec continues to seek licensing partners and further development collaborations to bring the engine to commercial production, with ongoing refinements aimed at and automotive markets.

Performance and applications

Claimed advantages in efficiency and emissions

Proponents of six-stroke engines claim significant improvements in compared to conventional four-stroke engines, primarily through enhanced thermal management and additional power extraction from exhaust heat or expansion. Reported fuel savings range from 30% to 50%, attributed to two power strokes per cycle and reduced energy losses in cooling and exhaust. For instance, modifications incorporating injection or secondary can achieve up to 40% lower consumption while maintaining comparable power output. Brake thermal efficiency (BTE) benchmarks vary by design but generally exceed those of four-stroke engines (typically 25-35%). The following table summarizes representative BTE values from key prototypes:
DesignReported BTEComparison to Four-StrokeSource
Bajulaz50%+20% (vs. 30% baseline)
Beare Head~35-40%+10-15% improvement
Porsche Patent45-50%+15-20% (vs. 30% baseline)
A 2025 experimental conversion of a four-stroke to six-stroke operation demonstrated a 15.8% increase in and a 13.4% reduction in specific consumption under resistive . In terms of emissions, six-stroke configurations often achieve substantial reductions in through charge dilution and lower temperatures, with reported decreases of 60-90% depending on and injection strategies. For example, water-assisted designs can lower by 80% via evaporative cooling, while methanol-dual variants reach 90% reduction. CO and HC emissions are also curtailed, typically by 20-22%, due to more complete . Hydrogen-ed six-stroke engines further minimize CO2 output to near zero, as produces primarily . The cleaner burn process effectively mimics particulate , reducing by up to 65% and enabling compliance with stringent standards like Euro 7. Additional benefits include extended engine longevity from reduced —fewer high-temperature cycles per power stroke lower on components—and quieter operation due to moderated exhaust temperatures and smoother delivery. These advantages position six-stroke engines as potentially viable for applications demanding high and low environmental impact.

Challenges and limitations

The increased of six-stroke engines arises primarily from the need for additional components, such as extra valves, injection systems, and modified timing mechanisms to accommodate the two additional strokes, which complicates design and assembly compared to standard four-stroke engines. This added intricacy often results in significantly higher manufacturing costs, driven by specialized materials and required for hybrid stroke operations. Performance trade-offs in six-stroke designs, particularly those incorporating steam or water cycles, include potential power losses due to reduced indicated (IMEP), which can drop by approximately 7% relative to four-stroke equivalents, with water injection recovering only about 40% of this deficit. Water system reliability poses further challenges, as excessive cylinder cooling from absorption can destabilize and processes, especially in varying environmental conditions like high where consistent is difficult to maintain. Adoption barriers for six-stroke engines stem from the absence of industry standardization, limited long-term testing data, and insufficient comparative benchmarks against established technologies, hindering regulatory approval and market integration. Moreover, intensifying competition from electric vehicles (EVs) and hybrid powertrains, which offer simpler architectures and lower operational emissions without the need for complex internal combustion modifications, further diminishes the commercial viability of six-stroke concepts in the current automotive landscape. Specific examples highlight these issues: in water-injected designs like the Dyer engine, the introduction of water into hot cylinders raises concerns over from acidic byproducts formed by water reacting with exhaust gases, necessitating corrosion-resistant materials that add to costs and maintenance demands. Similarly, opposed-piston configurations such as the Revetec engine encounter scaling difficulties, where enlarging the design for higher power outputs exacerbates challenges in synchronization, lubrication, and heat dissipation across dual crankshafts, limiting scalability beyond prototypes.

Current status and future prospects

Ongoing research and prototypes

Recent research on six-stroke engines has focused on converting existing four-stroke designs to enhance efficiency in stationary applications. In 2025, researchers successfully modified a single-cylinder, spark-ignition four-stroke engine, commonly used in power generators, into a six-stroke configuration by incorporating epicyclic gearing on the camshaft without altering the crankshaft. This prototype, tested at 3000 rpm under loads from 400 to 4000 W, demonstrated a 3.1% reduction in oil temperature, a 15.4% decrease in exhaust gas temperature, and a 15.8% increase in thermal efficiency, though fuel consumption per cycle rose by 29.9% due to higher load demands. Industry leaders continue to explore six-stroke concepts through patent filings that suggest progression toward prototypes. Mazda filed a patent in August 2025 for a six-stroke hydrogen-reforming engine that processes to produce for burning, aiming to capture carbon and achieve carbon-neutral operation. Similarly, Porsche's September 2024 patent outlines a six-stroke cycle designed to boost and efficiency by adding strokes for secondary and recovery. These developments indicate active testing phases in automotive applications, building on earlier milestones like the Beare head and M4+2 designs. Academic efforts emphasize computational modeling to optimize six-stroke cycles for alternative fuels. A 2025 study from Hyundai America Technical Center used genetic algorithms to optimize a six-stroke compression ignition for mid-to-high loads, achieving improved performance and emissions through parameter tuning. Researchers at various institutions have also modeled hydrogen-compatible six-stroke operations, with simulations showing potential for reduced emissions in low-temperature modes. Additionally, a 2024 literature review examined water-injection six-stroke engines, highlighting thermodynamic benefits like enhanced heat recovery and efficiency gains of up to 20% in experimental setups. In industry, integration with hybrid systems and research advances six-stroke viability. The generator conversion exemplifies hybrid potential, as its design supports range-extender roles in systems by improving thermal management. compatibility studies propose incorporating and blends into six-stroke cycles, with studies indicating lower emissions, including up to 40% reductions in some configurations using hydrogen enrichment compared to four-stroke baselines. Funding from international programs supports these initiatives. The EU's 2025 work programme allocates resources for advanced internal combustion engines using low- and zero-carbon fuels, including efficiency enhancements applicable to six-stroke research. In Japan, government-backed collaborations among automakers like promote low-carbon engine technologies, facilitating patents and prototypes for sustainable combustion. As of November 2025, demonstrations at highlighted synthetic fuel use in hybrid vehicles by Japanese automakers, supporting broader low-carbon engine research applicable to six-stroke concepts.

Potential commercial applications

Six-stroke engines hold promise for niche automotive applications, particularly in hybrid systems where their enhanced could complement electric components, as explored in recent reviews. For instance, Porsche's patented design targets high-performance sports cars, leveraging the additional power strokes to achieve superior without sacrificing driving dynamics. Hydrogen variants, such as Mazda's innovative six-stroke system that generates on-board from conventional fuels, could suit commercial fleets by reducing carbon emissions while maintaining internal reliability. In industrial sectors, six-stroke configurations offer advantages for stationary power generation, where waste heat recovery improves fuel economy in generators and equipment like welders or lawn machinery. represents a potential area for application, where six-stroke designs could aid in meeting stricter emissions standards through improved efficiency and heat recovery. Emerging prototypes also suggest for smaller applications, such as motorcycles or unmanned aerial vehicles (drones), where compact designs could provide extended range without frequent refueling. However, widespread adoption faces barriers, including rigorous testing, regulatory certification for emissions and , and the need for industry partnerships to overcome technical integration challenges. Ongoing indicates commercialization may not occur until the 2030s, contingent on resolving these hurdles.

References

  1. https://www.iosrjournals.org/iosr-jmce/papers/vol12-issue4/Version-3/B012430711.pdf
  2. https://link.springer.com/article/10.1007/s13762-023-05404-8
  3. https://www.caranddriver.com/news/a63266626/porsche-six-stroke-engine-patent-details/
  4. https://carbuzz.com/mazda-6-stroke-hydrogen-combustion-engine/
  5. https://www.sciencedirect.com/topics/engineering/stroke-engine
  6. https://www.researchgate.net/publication/336977212_6-Stroke_Engine_Thermodynamic_Modelling_and_Design_for_Testing
  7. https://ms.copernicus.org/articles/16/359/2025/ms-16-359-2025.pdf
  8. https://doi.org/10.1016/j.energy.2009.12.012
  9. https://www.researchgate.net/publication/298432063_Review_of_Six_Stroke_Engine_and_Proposal_for_Alternative_Fuels
  10. https://doi.org/10.1016/j.jclepro.2018.03.015
  11. https://motorcycletimeline.com/1880-1889/
  12. https://www.hagerty.com/media/news/porsche-patents-six-stroke-engine/
  13. https://www.researchgate.net/publication/235964279_6_stroke_engine
  14. https://www.autoweek.com/news/a2063201/inside-bruce-crowers-six-stroke-engine/
  15. https://ms.copernicus.org/articles/16/359/2025/
  16. https://www.sciencedirect.com/science/article/abs/pii/S0360544209005325
  17. https://www.ijert.org/research/six-stroke-engine-IJERTV2IS100242.pdf
  18. https://patents.google.com/patent/US1339176A/en
  19. https://www.academia.edu/117997362/6_Stroke_Engine_Thermodynamic_Modelling_and_Design_for_Testing
  20. https://patents.google.com/patent/US4809511A/en
  21. https://patents.google.com/patent/US4513568A/en
  22. https://rjwave.org/ijedr/papers/IJEDR1402179.pdf
  23. https://ijrat.org/downloads/Conference_Proceedings/icatest2015/ICATEST-2015073.pdf
  24. https://www.thehindu.com/news/cities/Kochi/kochiite-patents-sixstroke-engine/article3601304.ece
  25. https://allindianpatents.com/patents/252642-six-stroke-engine
  26. https://www.overdrive.in/news-cars-auto/features/the-technical-aspects-of-a-six-stroke-engine-explained/
  27. https://thekneeslider.com/crower-six-stroke-engine/
  28. https://patents.google.com/patent/US20240301817A1/en
  29. https://www.roadandtrack.com/news/a62258759/porsche-six-stroke-combustion-engine-patent-documents/
  30. https://www.theautopian.com/mazda-patented-a-freaky-six-stroke-engine-that-turns-gas-into-hydrogen-heres-how-it-works/
  31. https://www.topspeed.com/mazda-engine-turns-gasoline-into-hydrogen-power/
  32. https://www.goauto.com.au/news/mazda/mazda-hits-a-hydrogen-powered-six/2025-09-03/97263.html
  33. https://autoscommunity.com/2025/09/01/mazda-six-stroke-engine-hydrogen-patent/
  34. https://evworld.com/article.php?id=359&slug=mazdas-six-stroke-engine-revolutionary-idea-or-clickbait-distraction
  35. https://six-stroke.com/beare-history/
  36. https://www.freepatentsonline.com/5713314.html
  37. https://six-stroke.com/the-six-stroke-engine/theory/
  38. https://six-stroke.com/the-six-stroke-engine/performance/
  39. https://magazine.cycleworld.com/article/1998/12/1/six-strokes-of-genius
  40. https://ijettjournal.org/assets/year/2016/volume-34/number-5/IJETT-V34P243.pdf
  41. https://aircconline.com/ijci/V6N2/6217ijci07.pdf
  42. https://patents.google.com/patent/US5755191A/en
  43. https://www.ijsrd.com/articles/RTIMEP020.pdf
  44. https://patents.google.com/patent/US6553977B2/en
  45. https://www.wired.com/2009/08/five-stroke-engine/
  46. https://www.enginelabs.com/engine-tech/the-forgotten-5-stroke-lighter-stronger-and-running-on-water/
  47. https://www.autoevolution.com/news/five-stroke-engine-works-and-might-enter-production-video-87083.html
  48. https://kones.eu/ep/2013/vol20/no1/239-246_JO_KONES_2013_NO_1_VOL_20_ISSN_1231-4005_NOGA.pdf
  49. https://ijaerd.org/index.php/IJAERD/article/download/1572/1479/1463
  50. https://interestingengineering.com/innovation/porsche-6-stroke-engine-innovation
  51. https://www.rfwireless-world.com/terminology/6-stroke-engine-advantages-disadvantages
  52. https://www.researchgate.net/publication/354421140_Six-Stroke_Cylinder_Engine_An_Emerging_Technology
  53. https://www.iosrjournals.org/iosr-jmce/papers/vol12-issue6/Version-5/J012658388.pdf
  54. https://www.sciencedirect.com/science/article/pii/S1359431123007822
  55. https://nap.nationalacademies.org/read/18264/chapter/5
  56. https://www.quora.com/What-is-the-problem-with-opposed-piston-engines
  57. https://www.sae.org/publications/technical-papers/content/2025-01-8438/
  58. https://research-and-innovation.ec.europa.eu/document/download/64e1031e-7814-4c11-ab14-a22b1f5236c2_en
  59. https://global.toyota/en/newsroom/corporate/42597288.html
  60. https://news.wisc.edu/wisconsin-engineer-entrepreneur-move-green-diesel-engine-closer-to-market/
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