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Stroke (engine)
Stroke (engine)
Stroke (engine)
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In the context of an internal combustion engine, the term stroke has the following related meanings:

  • A phase of the engine's cycle (e.g. compression stroke, exhaust stroke), during which the piston travels from top to bottom or vice versa.
  • The type of power cycle used by a piston engine (e.g. two-stroke engine, four-stroke engine).
  • "Stroke length", the distance travelled by the piston during each cycle. The stroke length, along with bore diameter, determines the engine's displacement.

Phases in the power cycle

[edit]
The phases/strokes of a four-stroke engine.
1: intake
2: compression
3: power
4: exhaust
Animation of a two-stroke engine

Commonly used engine phases or strokes (i.e. those used in a four-stroke engine) are described below. Other types of engines can have very different phases.

Induction-intake stroke

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The induction stroke is the first phase in a four-stroke (e.g. Otto cycle or Diesel cycle) engine. It involves the downward movement of the piston, creating a partial vacuum that draws an air-fuel mixture (or air alone, in the case of a direct injection engine) into the combustion chamber. The mixture enters the cylinder through an intake valve at the top of the cylinder.

Compression stroke

[edit]
See also: Compression ratio

The compression stroke is the second of the four stages in a four-stroke engine.

In this stage, the air-fuel mixture (or air alone, in the case of a direct injection engine) is compressed to the top of the cylinder by the piston. This is the result of the piston moving upwards, reducing the volume of the chamber. Towards the end of this phase, the mixture is ignited, by a spark plug for petrol engines or by self-ignition for diesel engines.

Combustion-power-expansion stroke

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The combustion stroke is the third phase, where the ignited air-fuel mixture expands and pushes the piston downwards. The force created by this expansion is what creates an engine's power.

Exhaust stroke

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The exhaust stroke is the final phase in a four stroke engine. In this phase, the piston moves upwards, squeezing out the gasses that were created during the combustion stroke. The gasses exit the cylinder through an exhaust valve at the top of the cylinder. At the end of this phase, the exhaust valve closes and the intake valve opens, which then closes to allow a fresh air-fuel mixture into the cylinder so the process can repeat itself.

Types of power cycles

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The thermodynamic cycle used by a piston engine is often described by the number of strokes to complete a cycle. The most common designs for engines are two-stroke and four-stroke. Less common designs include one-stroke engines, five-stroke engines, six-stroke engines and two-and-four stroke engines.

One-stroke engine

[edit]

A Granada, Spain-based company, INNengine invented an opposed-piston engine with four pistons on either side to make a total of eight. Fixed rods hold together all pistons, and they share one combustion chamber. These rods press against plates that have an oscillating wave-like design, allowing the rods to press and release the pistons in a synchronized, smooth process. The engine, known as the e-REX creates 4 times more power events per revolution than a conventional 4 Stroke and twice more than a 2 Stroke.[1] Although the e-REX is called a one-stroke engine there is debate that says it is actually a two-stroke engine, it is called a one-stroke because each piston executes two strokes (i.e., compression/combustion and exhaust/intake) in half an engine revolution, then by INNengine's logic, two strokes multiplied by half a revolution is what gave it the Patented 1 Stroke name.[2]

Two-stroke engine

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Two-stroke engines complete a power cycle every two strokes, which means a power cycle is completed with every crankshaft revolution. Two-stroke engines are commonly used in (typically large) marine engines, outdoor power tools (e.g. lawnmowers and chainsaws) and motorcycles.[3]

Four-stroke engine

[edit]

Four-stroke engines complete a power cycle every four strokes, which means a power cycle is completed every two crankshaft revolutions. Most automotive engines are of a four-stroke design.[3]

Five-stroke engine

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Five-stroke engines complete a power cycle every five strokes. The engine only exists as a prototype.

Six-stroke engine

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Six-stroke engines complete a power cycle every six strokes, which means a power cycle is completed every three crankshaft revolutions.

Stroke length

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The stroke length is how far the piston travels in the cylinder, which is determined by the cranks on the crankshaft.

Engine displacement is calculated by multiplying the cross-section area of the cylinder (determined by the bore) by the stroke length. This number is multiplied by the number of cylinders in the engine, to determine the total displacement.

Steam engine

[edit]

The term stroke can also apply to movement of the piston in a locomotive cylinder.

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Stroke (engine)

View on Grokipedia
from Grokipedia
In reciprocating engines, particularly internal combustion types, a stroke refers to the full linear travel of the from top dead center (TDC)—its highest position in the cylinder—to bottom dead center (BDC)—its lowest position—or vice versa. This distance, termed the stroke length, is a fundamental dimension that, together with the bore (the cylinder's internal diameter), calculates the engine's displacement volume, influencing power output and efficiency. Strokes also denote the operational phases in an engine's cycle, classifying engines as two-stroke or four-stroke based on the number required to produce one power impulse. The four-stroke cycle, the most common configuration in modern vehicles, completes its thermodynamic process over two crankshaft revolutions and four distinct piston strokes: intake, compression, power, and exhaust. Invented by German engineer Nikolaus Otto in 1876, this cycle begins with the intake stroke, where the piston descends to draw in a fuel-air mixture (in engines) or air alone (in diesel engines); follows with the compression stroke, raising the piston to pressurize the contents; ignites the mixture during the power stroke to force the piston downward and generate ; and ends with the exhaust stroke, expelling burned gases as the piston ascends again. This design offers superior , reduced emissions, and smoother operation, powering the majority of automobiles, trucks, and engines today. Two-stroke engines, by contrast, achieve a power cycle in a single through just two strokes, completing the power and exhaust phases during the downward motion and the and compression phases during the upward motion, often aided by ports in the cylinder wall and compression for . This results in a simpler , lower weight, and higher power density per displacement, ideal for applications like chainsaws, dirt bikes, and marine outboards where compactness matters. However, two-stroke engines typically consume more , produce higher emissions due to incomplete scavenging of exhaust gases, and have led to regulatory restrictions in some regions. Engine stroke characteristics further shape performance via the bore-to-stroke ratio: square engines (bore ≈ stroke) balance and speed, while undersquare (longer stroke) designs emphasize low-end for heavy-duty uses, and oversquare (shorter stroke) favor high-revving applications like . Advances in materials and continue to optimize stroke-related parameters for better amid evolving emissions standards.

Fundamentals of Engine Strokes

Definition and Role in Reciprocating Engines

In reciprocating engines, a is defined as the single linear movement of the between top dead center (TDC), where the is at the uppermost position in the , and bottom dead center (BDC), where it reaches the lowermost position. This movement represents the fundamental reciprocating action that drives the 's operation. The role of the is central to the functioning of reciprocating , as it facilitates the cyclic conversion of from into mechanical work through a series of coordinated phases. Each contributes to the overall power cycle by enabling processes such as the of air or mixture, compression of the charge, expansion from , and expulsion of exhaust gases, thereby producing net output . Without these sequential , the could not sustain continuous operation or generate usable power. Mechanically, the stroke involves the 's linear displacement being transformed into rotational motion via the and . As the travels from TDC to BDC or vice versa, the rotates through half a , allowing the linear to accumulate into continuous rotary output that can drive external loads such as propellers or wheels. This conversion mechanism, refined from early designs, remains the core principle in modern reciprocating engines across automotive, aviation, and industrial applications.

Piston Displacement and Volume Calculation

In reciprocating engines, the swept volume, also known as the displacement volume of a single , represents the volume of space traversed by the during one complete from top dead center (TDC) to bottom dead center (BDC). This volume is calculated using the formula for the cylindrical displacement based on the 's :
Vs=π4B2SV_s = \frac{\pi}{4} B^2 S
where BB is the bore (the internal of the ) and SS is the length (the total distance the travels in one ). The bore and dimensions are critical design parameters that directly influence the engine's capacity to draw in and expel gases. For multi-cylinder engines, the total engine displacement DD is obtained by multiplying the swept volume of one cylinder by the number of cylinders nn:
D=n×VsD = n \times V_s
This aggregate measure quantifies the overall size and potential power output of the engine. For example, an inline-four engine with a per-cylinder swept volume of 500 cubic centimeters yields a total displacement of 2,000 cubic centimeters. It is important to distinguish the swept volume from the clearance volume, which is the residual space in the at TDC when the is at its highest position, above the top of the crown up to the . The clearance volume does not change with movement and is typically much smaller than the swept volume, affecting compression ratios but not contributing to the displacement calculation. Engine displacement is commonly expressed in cubic centimeters (cc) or liters (L) for automotive applications, where 1 liter equals 1,000 cc, providing a standardized metric for comparing engine sizes across vehicles.

Phases in the Four-Stroke Power Cycle

Intake Stroke

The intake stroke represents the initial phase of the four-stroke cycle in reciprocating internal combustion engines, during which the descends to draw fresh charge into the . As the rotates, the moves downward from top dead center (TDC) to bottom dead center (BDC), with the opening to allow the entry of the air-fuel or air alone, depending on the type. This downward motion creates a partial vacuum or low-pressure region within the , facilitating the induction of the charge through the open while the exhaust remains closed. This stroke spans exactly 180 degrees of crankshaft rotation, corresponding to half a full and completing the induction before the transition to the compression phase. Key events during this period include the regulation of charge volume by the throttle valve, typically located in the intake manifold of spark-ignition engines, which restricts airflow to control engine load and power output. In traditional carbureted spark-ignition engines, fuel is atomized and mixed with incoming air in the before entering the manifold, ensuring a homogeneous mixture is inducted into the cylinder. Variations in the intake stroke occur between spark-ignition (gasoline) and compression-ignition (diesel) engines. In spark-ignition engines, a premixed air-fuel ratio is drawn in, with the primarily governing the air component to maintain the stoichiometric mixture. Conversely, diesel engines induct only pure air during this stroke, as fuel is not introduced until later via direct injection into the compressed charge, allowing for higher compression ratios and improved efficiency.

Compression Stroke

The compression stroke is the second phase in the four-stroke cycle of a reciprocating , where the moves upward from bottom dead center (BDC) to top dead center (TDC) while both the and exhaust valves remain closed, progressively reducing the volume and compressing the air-fuel mixture inducted during the previous stroke. This upward motion of the , driven by the crankshaft's rotation, seals the and confines the mixture, leading to a significant decrease in volume without any additional air or fuel entry. The key parameter governing this phase is the , defined as the ratio of the total cylinder volume (swept volume plus clearance volume) to the clearance volume at TDC, expressed as r=Vs+VcVcr = \frac{V_s + V_c}{V_c}, where VsV_s is the swept volume and VcV_c is the clearance volume. In typical engines, this ratio ranges from 8:1 to 12:1, balancing thermodynamic efficiency with material strength and fuel knock resistance. Higher ratios increase the work extracted per cycle but require higher-octane fuels to prevent premature ignition. During compression, the process raises both the and of the mixture, enhancing efficiency by creating conditions for more rapid and complete fuel oxidation upon ignition. This can be modeled as a , with the pressure ratio given by P2P1=(V1V2)n\frac{P_2}{P_1} = \left( \frac{V_1}{V_2} \right)^n where P1P_1 and V1V_1 are the initial pressure and volume, P2P_2 and V2V_2 are the final values, and nn is the polytropic index (typically 1.2 to 1.3 for real engine conditions, between isothermal and adiabatic extremes). The resulting temperature increase reaches approximately 450–600°C at TDC for standard ratios, promoting better release during the subsequent power stroke. In spark-ignition engines, the compression stroke ends precisely at TDC, immediately preceding the spark timing that initiates , ensuring the mixture is at peak density for optimal ignition.

Power Stroke

The power stroke, also known as the expansion stroke, is the third phase in the four-stroke cycle of a spark-ignition , where of the air-fuel mixture generates force to drive the . Near the end of the compression stroke, with the approaching top dead center (TDC), the ignites the compressed mixture, causing rapid that produces high-pressure, high-temperature gases. These expanding gases exert force on the head, pushing it downward toward bottom dead center (BDC) while both intake and exhaust valves remain closed, maintaining a sealed . The resulting motion is transmitted through the to the , converting the linear force into rotational . This stroke spans 180 degrees of crankshaft rotation, equivalent to half a revolution, during which the engine delivers its peak torque output as the expanding gases reach maximum pressure shortly after ignition. The duration allows for efficient energy extraction from the combustion event before the pressure diminishes significantly by BDC. During the power stroke, chemical energy from the fuel is converted into mechanical work through the expansion of combustion gases, with the process idealized in the as an adiabatic expansion following constant-volume heat addition. The of this ideal is given by η=1−(1r)γ−1\eta = 1 - \left( \frac{1}{r} \right)^{\gamma - 1} where rr is the (the ratio of cylinder volume at BDC to TDC) and γ\gamma is the specific heat ratio of the gas (approximately 1.4 for air). Higher compression ratios improve efficiency by increasing the work extracted relative to heat input, though practical limits are imposed by knocking in spark-ignition engines. In diesel engines, which operate on a compression-ignition , the power follows self-ignition of injected in the highly near TDC, without a , leading to similar expansion mechanics but typically higher compression ratios for greater efficiency.

Exhaust Stroke

The exhaust represents the final phase of the four- cycle in reciprocating internal engines, where the travels upward from bottom dead center (BDC) to top dead center (TDC), expelling the byproducts through the open exhaust . This upward motion, driven by the crankshaft's momentum from the preceding power , forces the spent gases out of the via the exhaust port, effectively clearing the for the next cycle. The exhaust typically opens near the end of the power and remains open throughout this phase, ensuring near-complete evacuation of the gases while the intake stays closed until the overlap period. A key feature of the exhaust stroke is the valve overlap timing, a brief interval—often 10 to 60 degrees of crankshaft rotation—where both the exhaust and valves are partially open near TDC. This overlap enhances scavenging by leveraging the momentum of outgoing exhaust gases to draw in a fresh air-fuel mixture, reducing residual gas retention in the cylinder and improving . In supercharged or high-performance engines, larger overlaps can further optimize this process, though they require precise tuning to avoid reversion of exhaust gases into the manifold. Exhaust backpressure, arising from restrictions in the , , or , opposes the piston's upward motion during this stroke, increasing pumping losses and reducing overall . Elevated backpressure can reduce power output in typical automotive applications and elevate consumption by forcing the engine to expend more work on gas expulsion. Modern exhaust systems, including turbochargers, are designed to minimize such effects while complying with emissions standards. Through the exhaust stroke, engines facilitate the removal of primary combustion byproducts, including carbon dioxide (CO₂) as the main product of complete fuel oxidation, nitrogen oxides (NOx) formed at high combustion temperatures, and particulate matter from incomplete burning. This expulsion is crucial for emissions control, as unremoved residues would dilute incoming charge and elevate pollutant levels, though aftertreatment devices like catalytic converters further process these gases downstream. Effective scavenging during this stroke thus supports lower tailpipe emissions, with modern standards (Euro 6 as of 2014, maintained under Euro 7 effective for new types in 2025) limiting NOx to 0.06 g/km for petrol and 0.08 g/km for diesel engines, and particulates to 4.5 mg/km for diesel variants; typical CO₂ emissions for new gasoline engines are approximately 100-110 g/km as of 2023.

Variations in Stroke Cycles

Two-Stroke Engines

A completes its power cycle in two strokes, combining , power, and exhaust (via scavenging) into the downward and compression into the upward , resulting in a full cycle every 360 degrees of rotation. This design simplifies the mechanism by eliminating the need for a dedicated valve train, as and exhaust are controlled by ports in the wall uncovered by the 's motion. Unlike four-stroke engines, which produce one power every two crankshaft revolutions, two-stroke engines deliver one power per , effectively doubling the power for a given speed. The port-based timing in two-stroke engines relies on the piston's position to open and close and exhaust ports, often leading to overlapping periods where fresh charge enters while residual exhaust exits, a process known as scavenging. This configuration enables higher operating speeds and a favorable but introduces challenges in charge retention, as some unburned fuel-air mixture can escape through the exhaust port. Key drawbacks of two-stroke engines include elevated emissions due to incomplete and scavenging losses, producing higher levels of hydrocarbons, , and particulate matter compared to four-stroke designs. is typically achieved by mixing oil with the fuel, which coats engine components but results in oil burning alongside the fuel, contributing to smoke and further emissions. Additionally, two-stroke engines exhibit higher specific fuel consumption, typically up to 50% greater than four-stroke equivalents.

Four-Stroke Engines

The operates through a complete cycle comprising , compression, power, and exhaust phases, spanning two full revolutions of the or 720 degrees of rotation. This design ensures distinct separation of and power production processes, utilizing poppet valves—mushroom-shaped valves that open and close via springs and actuation—to precisely control airflow into and out of the . Valve timing in four-stroke engines is governed by the , which rotates at half the speed of the and features lobes that push against the s either directly or through intermediate components like rocker arms. Common configurations include the single overhead (SOHC) setup, where a single per manages both and exhaust s, and the double overhead (DOHC) arrangement, which employs separate s for and exhaust s to enable wider valve angles, higher lift, and improved at high speeds. These systems enhance overall cycle efficiency by optimizing the duration and overlap of valve openings relative to position. Four-stroke engines provide advantages such as superior scavenging, where dedicated strokes fully expel exhaust gases and draw in fresh charge without significant overlap losses, leading to more efficient than in two-stroke engines with higher power but poorer gas separation. They also exhibit lower emissions, particularly hydrocarbons, due to complete cycles that minimize unburned escape during exhaust, and incorporate a separate system that circulates independently of the -air for better and reduced oil dilution. Since the late 1800s, with the first practical four-stroke developed in 1876 and commercial automotive adoption accelerating from 1886 onward, this cycle has dominated passenger vehicle propulsion for its reliability and compliance with efficiency standards. Variants like the Atkinson and Miller cycles modify the standard four-stroke process to prioritize thermal efficiency over power density by adjusting effective stroke lengths through valve timing. In the Atkinson cycle, the intake valve closes later in the compression stroke, shortening the compression relative to the expansion stroke and reducing pumping losses for up to 20-30% better fuel economy in hybrid applications. The Miller cycle achieves similar gains via early or late intake valve closing in boosted engines, lowering charge temperature and knock tendency while improving part-load efficiency.

Rare and Experimental Cycles

Rare and experimental engine cycles deviate from the conventional two- and four-stroke designs by incorporating additional strokes to enhance efficiency, reduce emissions, or address thermal management, though most remain in stages due to practical limitations. The one-stroke engine represents a highly theoretical concept in reciprocating internal engines, where occurs under constant pressure without distinct , compression, power, or exhaust phases, akin to a continuous-flow process similar to gas turbines but adapted to a mechanism. This design aims to simplify the cycle by eliminating reciprocation pauses, potentially approaching ideal thermodynamic efficiencies, but it is impractical for real-world application primarily because of severe sealing challenges; maintaining a gas-tight seal around a continuously combusting would require unattainable materials and designs to prevent leakage and withstand sustained high temperatures. The introduces an additional hydraulic stroke dedicated to cooling and lubrication, separating the primary combustion processes from heat dissipation to allow higher compression ratios and improved . Patented by Belgian Gerhard Schmitz in 2000 (US Patent 6,553,977), the design uses two high-pressure cylinders for , compression, and power strokes, coupled with a low-pressure for the expansion and hydraulic cooling stroke, enabling better heat rejection without diluting the . Engineering developed a turbocharged 0.7-liter three- in the mid-2000s, which demonstrated around 220 g/kWh—lower than typical four-stroke engines at 240-250 g/kWh—along with a 20% reduction in weight compared to equivalent-output engines, though exact efficiency gains varied by operating conditions. Six-stroke engines extend the cycle further by adding water injection after the exhaust stroke to generate expansion, creating a second power stroke that leverages residual heat for additional work while cooling the cylinder to suppress emissions. Swiss inventor Bajulaz invented a version in 1989 (US Patent 4,809,511), modifying a standard engine head with two paired cylinders where one handles a four-stroke sequence and the other injects to produce , claiming up to 40% savings and 60-90% reductions in pollutants like CO and through lower temperatures. Independently, American engineer Crower prototyped a single-cylinder in 2004-2007, injecting post-exhaust to vaporize into for a sixth stroke, which reduced emissions by cooling the chamber and improved overall efficiency by approximately 40% in bench tests, though consumption matched use in early models. Variants like the Bajulaz and Crower emphasize emissions cuts of 65% or more for in adapted engines. In 2024, patented a that achieves two power strokes over 1080 degrees of rotation, potentially improving efficiency and reducing emissions for use in hybrid powertrains. Despite promising prototypes, rare and experimental cycles face significant hurdles to , including increased mechanical complexity from additional valves, ports, and fluid management systems that raise and costs. For instance, the five- and six-stroke designs demand precise control of hydraulic or injection, adding reliability risks and development expenses that have deterred widespread adoption since the early . Limited production scalability and the dominance of established two- and four-stroke technologies further contribute to their niche status, with most efforts confined to or filings rather than market entry.

Stroke Length and Bore Ratio

The stroke length in an internal combustion engine is defined as the total linear distance the piston travels from top dead center (TDC) to bottom dead center (BDC) inside the cylinder. This parameter is a fundamental geometric dimension that shapes the engine's overall architecture. Stroke length is typically in the range of 50 to 100 mm for automotive engines, with specific values depending on the vehicle's application and displacement requirements; for instance, a common passenger car engine might feature an 86 mm stroke. It is mechanically determined by the crankshaft's throw radius, where the full stroke equals twice the distance from the crankshaft's main journal centerline to the connecting rod journal centerline. The bore refers to the cylinder's internal diameter, and the bore-stroke is calculated as the bore diameter divided by the length, providing a key indicator of . Engines with a bore-stroke less than 1 are termed undersquare (longer than bore), which historically prioritized development; those greater than 1 are oversquare (larger bore than ), favoring higher rotational speeds; and a near 1 yields a square configuration for balanced characteristics. In the historical evolution of engine design, early internal combustion engines from the late 19th and early 20th centuries often employed longer strokes relative to bore to maximize displacement using limited precision and materials. By the mid-20th century, particularly from the onward, designs shifted toward shorter strokes to support higher engine speeds as automotive demands emphasized performance and efficiency. This stroke length, in conjunction with bore, contributes to the swept volume per .

Impact on Engine Performance

The stroke length in an directly influences production by altering the mechanical leverage applied to the during the power stroke. A longer stroke extends the crankshaft throw, creating a greater lever arm that amplifies the force from pressure, resulting in higher output, particularly at lower engine speeds where this leverage provides a . This characteristic makes long-stroke engines preferable for applications requiring strong low-end , such as heavy-duty trucks or vehicles. Shorter strokes, conversely, reduce the mean piston speed for a given engine RPM, enabling higher rev limits without exceeding material stress thresholds. , a key indicator of engine durability and performance ceiling, is calculated as 2×stroke (m)×RPM/602 \times \text{stroke (m)} \times \text{RPM} / 60, representing the average of the piston over its full travel. Engines with shorter strokes maintain lower piston speeds at elevated RPMs, allowing operation up to 8,000–10,000 RPM in high-performance automotive designs while minimizing wear on components like , rings, and bearings. Efficiency in internal combustion engines involves trade-offs between friction losses, , and , where the bore-to-stroke ratio plays a pivotal role. An optimal ratio balances reduced frictional losses from shorter strokes (lowering side loads and pumping work) against improved from larger bores, which enhance at high speeds. For instance, Formula 1 engines employ oversquare designs (bore exceeding stroke) to achieve RPMs up to 15,000 as of 2025, prioritizing high-rev over low-speed while maintaining competitive thermal efficiencies around 50%. Engine tuning often leverages stroke modifications through stroker kits, which extend the stroke to increase displacement without major block alterations, thereby boosting both and power. These kits typically include a longer-throw , resized connecting rods, and custom pistons, raising displacement by 10–25% in popular platforms like GM -series engines. For example, a stroker conversion on a 5.3L LS to 408 cubic inches can yield over 550 ft-lbs of and 650 horsepower in naturally aspirated configurations, enhancing overall for or modified street applications.

Stroke in Non-Internal Combustion Engines

Steam Reciprocating Engines

In steam reciprocating engines, the stroke represents the piston's linear reciprocation within the , powered by the controlled admission and exhaust of pressurized on alternating sides. During each stroke, high-pressure enters one side of the piston to drive it forward, while exhaust or a partial on the opposite side facilitates the motion, enabling continuous bidirectional generation that is converted to rotary output via a and . This process differs fundamentally from modern internal combustion engines, where power is derived solely from in one directional stroke. The hallmark of steam reciprocating engines is the double-acting design, which produces power on both the forward and return strokes by admitting alternately above and below the . Developed by in the 1780s, this configuration eliminated the need for a or atmospheric reliance on one side, allowing the engine to perform work continuously in both directions and enabling applications like rotative machinery for mills and factories. Watt's further refined this by ensuring straight-line travel, maximizing in the double-acting setup. Steam flow during strokes is regulated by specialized valves, primarily slide valves or piston valves, which open and close ports to admit live steam, cut off supply, and release exhaust. Slide valves, consisting of a flat plate sliding over cylinder ports, were common in early designs for their simplicity in controlling admission and exhaust phases, while piston valves—cylindrical and fitted within a valve chest—offered reduced leakage and smoother operation in higher-speed engines. The cutoff ratio, the fraction of the stroke length at which steam admission is terminated to permit expansive work from the remaining steam, is a critical parameter; for instance, a 50% cutoff allows expansion over half the stroke, optimizing fuel use by balancing pressure drop and work output. Watt's improvements in the , including the integration of the double-acting with a separate condenser patented earlier in 1769, dramatically enhanced efficiency by preventing the from cooling during exhaust, thereby doubling the thermal performance compared to prior Newcomen engines and quadrupling it overall by the mid-. This breakthrough reduced coal consumption per , making power economically viable for widespread industrial use and marking a pivotal advancement in technology.

Other Historical or Specialized Applications

Pneumatic engines operate by using to drive reciprocating strokes, providing mechanical power in environments where ignition sources posed significant risks. In the , these engines gained prominence in the sector due to their safety advantages, as compressed ambient air minimized hazards compared to or open-flame alternatives, allowing safer operations in flammable underground conditions. Hydraulic systems employ piston strokes to transfer power through fluid pressure, with notable examples in and actuators for pumping applications. A key development was the (HFPE), first conceptualized by Raoul P. Pescara in the 1930s, which combines internal combustion with direct hydraulic output via unconstrained piston motion. Mid-20th-century prototypes, such as those explored by in the 1950s and later refined in programs, demonstrated the HFPE's potential for compact, efficient conversion in industrial and vehicular uses, though challenges in limited widespread adoption until advanced emerged. Stirling engines feature a closed where the displacer piston's strokes play a critical role in by cyclically moving the working gas—typically or air—between a hot and a cold one via a regenerator. This motion, distinct from the power piston's compression and expansion strokes, enables regenerative heating and cooling without direct , achieving high theoretical in low-temperature differential applications. Analyses show that optimizing displacer stroke phasing can enhance by up to 58% through improved gas shuttling, as validated in experimental free-piston variants. In modern hybrid powertrains, free-piston linear generators leverage direct piston strokes to produce , bypassing the for simplified design and reduced mechanical losses. These systems, researched since the late 20th century by institutions like , integrate a linear with the piston's oscillatory motion to generate power on demand, offering friction reductions of up to 60% compared to conventional engines and supporting multi-fuel operation in range-extender roles for electric vehicles.

References

  1. https://www.grc.nasa.gov/www/k-12/airplane/stroke.html
  2. https://www.energy.gov/eere/vehicles/articles/internal-combustion-engine-basics
  3. https://www.eia.gov/kids/history-of-energy/famous-people/otto.php
  4. https://animatedengines.com/twostroke.html
  5. https://www.faa.gov/sites/faa.gov/files/09_phak_ch7.pdf
  6. https://www.rose-hulman.edu/ME410/PDFs/Day2.pdf
  7. https://www.epa.gov/sites/default/files/2015-07/documents/catalog_of_chp_technologies_section_2._technology_characterization_-_reciprocating_internal_combustion_engines.pdf
  8. https://www.nrc.gov/docs/ml1122/ML11229A068.pdf
  9. https://www.princeton.edu/~humcomp/sophlab/ther_28.htm
  10. https://eaglepubs.erau.edu/introductiontoaerospaceflightvehicles/chapter/reciprocating-engine-propeller/
  11. https://www.grc.nasa.gov/www/k-12/airplane/engopt.html
  12. https://www.grc.nasa.gov/www/k-12/airplane/engintk.html
  13. https://www.grc.nasa.gov/www/k-12/airplane/fuelsys.html
  14. https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/compression-stroke/
  15. https://www.grc.nasa.gov/www/k-12/airplane/engcomp.html
  16. https://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node26.html
  17. https://carbonxtrem.com/blogs/post/high-compression-vs-low-compression-engines-what-you-need-to-know-in-2025
  18. https://www.energy.gov/eere/vehicles/fact-940-august-29-2016-diverging-trends-engine-compression-ratio-and-gasoline-octane
  19. https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/compression-and-expansion/
  20. https://www.grc.nasa.gov/www/k-12/airplane/compexp.html
  21. https://www.grc.nasa.gov/WWW/K-12/airplane/engpowr.html
  22. https://www.uti.edu/blog/motorcycle/how-4-stroke-engines-work
  23. https://courses.washington.edu/engr100/Section_Wei/engine/UofWindsorManual/Four%20Stroke%20Cycle%20Engines.htm
  24. https://www.grc.nasa.gov/WWW/K-12/airplane/otto.html
  25. https://www.sae.org/papers/scavenging-large-valve-overlap-increases-power-economy-330044
  26. https://ntrs.nasa.gov/citations/19930081165
  27. https://dieselnet.com/tech/diesel_exh_pres.php
  28. https://dieselnet.com/tech/engine_emission-control.php
  29. https://www.eea.europa.eu/en/analysis/indicators/co2-performance-of-new-passenger
  30. https://dieselnet.com/standards/eu/ld.php
  31. https://www.europarl.europa.eu/news/en/press-room/20231207IPR15740/euro-7-deal-on-new-eu-rules-to-reduce-road-transport-emissions
  32. http://ronney.usc.edu/AME436/Lecture6/AME436-S19-lecture6.pdf
  33. https://pmc.ncbi.nlm.nih.gov/articles/PMC1247506/
  34. https://anrweb.vt.gov/PubDocs/DEC/WSMD/lakes/docs/lp_2-strokereport.pdf
  35. https://www.boatersworld.com/blog/2-stroke-vs-4-stroke-outboards-whats-the-difference--60270
  36. https://www.sciencedirect.com/topics/engineering/stroke-cycle
  37. https://prodigy.ucmerced.edu/virtual-library/YbgZhW/4OK082/introduction_to_engine-valvetrains.pdf
  38. https://ww2.arb.ca.gov/sites/default/files/classic/msprog/nvepb/mailouts/_MSC/msc9802/staffreport.pdf
  39. https://www.nap.edu/read/12924/chapter/6
  40. http://www.autolife.umd.umich.edu/Environment/E_Overview/E_Overview3.htm
  41. https://www.caranddriver.com/news/a15345875/what-is-the-atkinson-combustion-cycle-and-what-are-its-benefits/
  42. https://dieselnet.com/tech/engine_miller-cycle.php
  43. https://www.ijert.org/six-stroke-engine
  44. https://www.cycleworld.com/story/bikes/one-stroke-engine-revisited/
  45. https://patents.google.com/patent/US6553977B2/en
  46. https://www.wired.com/2009/08/five-stroke-engine/
  47. https://www.enginelabs.com/engine-tech/the-forgotten-5-stroke-lighter-stronger-and-running-on-water/
  48. https://www.farmshow.com/a_article.php?aid=21471
  49. https://www.autoweek.com/news/a2063201/inside-bruce-crowers-six-stroke-engine/
  50. https://www.caranddriver.com/news/a63266626/porsche-six-stroke-engine-patent-details/
  51. https://link.springer.com/article/10.1007/s13762-023-05404-8
  52. https://www.howacarworks.com/crankshaft
  53. https://carbuzz.com/the-long-and-short-of-bore-and-stroke/
  54. https://www.hotrod.com/how-to/69883-stroke-any-engine
  55. https://performancetrends.com/blog/?p=5
  56. https://www.revzilla.com/common-tread/what-is-an-oversquare-and-undersquare-motorcycle-engine
  57. https://mossmotoring.com/stroke-origin-species-evolution-british-sports-car/
  58. https://www.cycleworld.com/story/blogs/ask-kevin/how-motorcycle-cylinder-bore-stroke-affect-engine-performance/
  59. https://official.bankspower.com/tech_article/why-diesels-make-so-much-torque/
  60. https://www.cartechbooks.com/blogs/techtips/pistonspeed
  61. https://achatespower.com/stroke-to-bore/
  62. https://www.hagerty.com/media/maintenance-and-tech/how-bore-vs-stroke-can-affect-horsepower/
  63. https://www.mclaren.com/racing/formula-1/2025/mcl39-technical-specification/
  64. https://www.enginebuildermag.com/2017/07/modern-stroker-kits/
  65. https://www.egr.msu.edu/~lira/supp/steam/
  66. https://gyre.umeoce.maine.edu/physicalocean/Tomczak/science%2Bsociety/lectures/illustrations/lecture21/watt.html
  67. https://ecommons.cornell.edu/bitstream/handle/1813/58719/section10.pdf?sequence=11
  68. https://www.si.edu/object/model-jw-thompson-buckeye-steam-engine-ca-1875%253Anmah_846130
  69. https://dn790003.ca.archive.org/0/items/steamengineitshi00burnuoft/steamengineitshi00burnuoft.pdf
  70. https://engines.egr.uh.edu/talks/powersir
  71. https://www.morendustri.com/en/pneumatic-systems-and-their-history/
  72. https://www.sciencedirect.com/science/article/abs/pii/S0306261912003662
  73. https://www.mdpi.com/1996-1073/14/12/3530
  74. https://ntrs.nasa.gov/api/citations/20170001730/downloads/20170001730.pdf
  75. https://www.sciencedirect.com/science/article/abs/pii/S0306261916308133
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