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X-engine according to US Patent 1889583 from 1928[1]
Symmetrical X-Engine (90°/90°/90°/90°)

An X engine is a piston engine with four banks of cylinders around a common crankshaft, such that the cylinders form an "X" shape when viewed front-on.

The advantage of an X engine is that it is shorter than a V engine of the same number of cylinders,[2] however the drawbacks are greater weight and complexity as compared to a radial engine. Therefore, the configuration has been rarely used.

Several of the X engine designs were based on combining two V engines.

Examples

[edit]

Four types of X engines are known to have reached production. In 1939–1942 Rolls-Royce Vulture, a 42 L (2,563 cu in) X-24 aircraft engine which was built using two Rolls-Royce Peregrine V12 engines.[3] The Rolls-Royce Vulture was briefly used in the Avro Manchester heavy bomber, before engine failures caused it to be replaced by the Avro Lancaster (powered by the Rolls-Royce Merlin V12 engine).

General Motors also produced X engines for US naval ships during the Second World War. The 16-184 engine was installed in several hundred "subchaser" boats from 1941 onwards, where they were typically used in pairs. It was considered a successful design and a few have survived to the present day. From 1944 the similar 16-338 engine was produced for submarines, where four were used in each of the first four members of the USN Tang class. These proved unreliable in service and were subsequently replaced by three Fairbanks-Morse engines in each boat.[4]

The other production X engine is the ChTZ Uraltrac 12N360 X-12 engine, first produced in 2015, and used in the Russian Armata tank platform.[5]

Several prototype 24-cylinder X engines for military aircraft were developed during World War II, including the Daimler-Benz DB 604, Rolls-Royce Exe and Isotta Fraschini Zeta R.C. 24/60, along with the 16-cylinder Napier Cub.[citation needed]

Other prototype X engines include a 1920s Ford X-8 automotive engine, which was investigated during the development process of the Ford Flathead V8 engine.[6][7] During the 1960s, Honda is said to have experimented with an X-32 engine configuration for their Formula One racing efforts, but abandoned the design as being too complex and unreliable.[citation needed] From 2006 to 2010, the Revetec X4v1 and Revetec x4v2 X-4 experimental petrol engines were developed by an engine research company,[8][9] followed in 2013 by the Revetec X4-D1 experimental petrol engine.[10]

See also

[edit]

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

X engine

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from Grokipedia
An X engine is a piston engine configuration featuring four banks of cylinders arranged around a common crankshaft to form an "X" shape when viewed from the front, typically consisting of two opposed V-shaped cylinder blocks joined at their apexes. This layout emerged in the early primarily for applications, where high power density and compact size were critical; the first known was the Napier "Cub," a 16-cylinder developed in 1919 that produced 1,000 horsepower from a 60-liter displacement and powered the Avro Aldershot . During , X engines saw limited but notable use, such as the , an X24 formed by coupling two Peregrine V12s, which delivered around 1,760 horsepower and was employed in the bomber before being phased out due to reliability issues. In marine and diesel contexts, ' 16-184, a 16-cylinder X engine generating 1,200 horsepower and 3,500 pound-feet of torque, powered U.S. Navy sub-chasers. Experimental automotive adaptations included Ford's X8 prototype from the , an 8-cylinder design with 107 cubic inches displacement yielding 35 horsepower, intended as a space-efficient alternative to the Flathead V8 but ultimately not produced due to complexities. Other wartime efforts, like the canceled Daimler-Benz DB 604—a 24-cylinder planned for 2,500 horsepower in German fighters—highlighted the configuration's potential but also its engineering challenges, including vibration and cooling difficulties. In contemporary applications, the X layout persists in specialized heavy-duty roles; for instance, the Russian ChTZ-Uraltrak 12N360 A-85-3A, an X12 turbocharged diesel introduced around 2015, produces 1,500 horsepower at 2,000 rpm with 60-degree vertical and 120-degree horizontal bank angles, powering the . Despite advantages in balance and power output, X engines remain uncommon compared to V or inline configurations owing to increased mechanical complexity, higher production costs, and the dominance of more straightforward designs in modern propulsion systems. Note that the term "X engine" can also refer to unrelated modern rotary designs, such as LiquidPiston's pistonless , a multi-fuel rotary using a High Hybrid Cycle for up to 30% greater and 10 times smaller size than equivalent diesels, targeted at and UAV applications since the company's founding in the early .

Design and Configuration

Cylinder Arrangement

An is a piston engine featuring four banks of cylinders arranged around a common , forming an "X" shape when viewed from the front. This configuration positions two banks above the crankshaft and two below, typically in a symmetrical or mirrored layout to achieve balance and compactness. While most designs are symmetric, early examples like the Napier Cub featured asymmetric bank angles. The geometric layout often consists of two pairs of V-shaped banks, with the upper and lower Vs mirrored horizontally or vertically relative to the axis. Bank angles vary by design but commonly include 60° or 90° for the individual Vs, resulting in wider angles (such as 120°) between opposing banks to accommodate the X formation. For instance, the X-2775 used 60° angles for its upper and lower banks, creating 120° separations on the sides. This arrangement allows for a narrow frontal profile, beneficial in applications like where drag reduction is critical. Common cylinder counts in X engines include 12 cylinders (three per bank), 16 cylinders (four per bank), and 24 cylinders (six per bank). An early X engine design, patented in 1928, featured 24 cylinders with six per bank in V-formations on inclined supporting walls around a prismatic crankcase. Examples include the Russian CTZ-Uraltrak 12N360 (an X-12 with 60° and 120° bank angles) and the Porsche Type 203 Sla 16 (an X-16 diesel). Fundamentally, an can be viewed as two V engines joined to share a single , which shortens the overall engine length compared to operating them as separate units while maintaining similar displacement and power potential. This shared integration enables a more compact package without sacrificing the multi-bank advantages of V configurations.

Crankshaft and Valvetrain

The crankshaft in an X engine is a single, elongated component shared by all four cylinder banks, designed to accommodate the complex arrangement while maintaining balance and structural integrity. Typically constructed from forged steel or alloy, it features multiple throws—often grouped into fewer crankpins with master and articulated connecting rods to connect the pistons from opposing banks. For instance, the Daimler-Benz DB 604 employed a single crankshaft with six crankpins spaced at 0°, 120°, 240°, 240°, 120°, and 0°, where each crankpin utilized one master rod and three articulated rods to serve four cylinders, enabling efficient power transfer across the banks. Similarly, the Rolls-Royce Vulture used a hollow, six-throw crankshaft supported by seven main bearings within a two-piece aluminum crankcase split horizontally at the centerline, with throws arranged to align the banks in an X configuration. This design often incorporates 90° or 180° phasing between adjacent banks to optimize balance and reduce torsional vibrations, though the elongated nature demands robust main bearings and precise alignment to prevent flexing under high loads. The in X engines generally adopts an overhead (OHV) layout per bank, with two and two exhaust per to support high airflow in aviation applications. Camshafts are typically single overhead types (SOHC), one per bank, driven by bevel gears, vertical shafts, or inclined drives from the to ensure precise timing across the multi-bank setup. In the X-2775, for example, each bank's SOHC was synchronized via inclined shafts from the , featuring innovative valve springs—seven small helical units per —that rotated the approximately once every 40 revolutions to prevent wear and promote even cooling. The similarly used SOHC per bank with sodium-cooled exhaust , but synchronization posed challenges due to the separated banks, requiring reinforced driveshafts to maintain under varying loads. Firing order in X engines is engineered for even distribution to minimize vibrations from the offset banks, typically sequencing cylinders at regular intervals—such as every 30° of for a 24-cylinder setup—to achieve smooth operation. A representative sequence for an X-24, as seen in designs like the DB 604, follows an evenly spaced pattern (e.g., 1-13-5-17-9-21-3-15-7-19-11-23-2-14-6-18-10-22-4-16-8-20-12-24) that alternates between banks for balance. Due to inherent uneven forces from the X layout, some configurations incorporate auxiliary balance shafts to counteract secondary vibrations, particularly in high-power prototypes where firing impulses could induce torsional oscillations along the long . Lubrication and cooling systems are integrated to address the extended and multi-bank geometry, with galleries drilled along the 's length to supply bearings and connecting rods via a hollow core. In the Packard X-2775, circulated through hollow and passages, extending to exhaust stems for additional cooling, while the dry-sump prevented under high g-forces. Coolant passages are routed through multi-bank jackets, often with balance tubes to equalize pressure across banks, as implemented in the to mitigate overheating from uneven flow. These features ensure reliable operation but require careful design to avoid starvation in or localized hot spots.

History and Development

Early Concepts (1920s–1930s)

The X engine configuration emerged in the late 1910s and 1920s as an innovative approach to delivering high in a compact package, surpassing the length limitations of inline and V-type engines prevalent at the time. The earliest known prototype was the Napier Cub, a 16-cylinder X engine developed in 1919 that produced 1,000 horsepower from a 60-liter displacement and powered the Avro . filed the earliest known patent for an X-8 design in 1920 (US Patent 1,639,333), describing four banks of two cylinders each arranged around a shared to create a shorter overall engine suitable for automotive use. This concept addressed the growing demand for more efficient powerplants amid the automotive industry's expansion following . Ford pursued the X-8 as an experimental from 1920 to 1927, constructing prototypes with two 90-degree V-4 sections mated side-by-side on a common , producing approximately 80 horsepower in a unit measuring just 18 inches long. Intended as a successor to the Model T's inline four-cylinder , the design promised superior compactness and smoothness but was ultimately shelved due to intricate challenges, including vibration issues and complex mechanics that proved too costly to refine. In the 1930s, applications drove further experimentation with X configurations, as engineers sought shorter powerplants to integrate into streamlined fuselages for improved and performance in monoplanes. British efforts included early Rolls-Royce proposals for X-type engines in the mid-1930s, aimed at providing over 2,000 horsepower to rival the power and reliability of radial engines like the while maintaining a reduced frontal area. Similarly, German designers at Daimler-Benz initiated the DB 604 project in 1939, an X-24 with four banks of six cylinders designed from scratch for high-speed applications, emphasizing liquid cooling and a narrow profile to fit within tight nacelles. These interwar initiatives reflected a broader push for compact, high-output engines to enhance fighter and designs amid evolving tactical requirements. Economic constraints from the curtailed widespread adoption, restricting most X engine work to prototypes and halting production due to high development costs and uncertain markets. Despite these limitations, the era's experiments laid foundational concepts for multi-bank arrangements, influencing later wartime advancements.

World War II Era and Beyond

During , the demand for compact, high-power propulsion systems in and marine applications spurred significant experimentation with X engine configurations, as nations sought to maximize output within constrained and hull designs. Germany's Daimler-Benz pursued the DB 604, an experimental liquid-cooled X-24 engine initiated in the early , featuring four banks of six cylinders arranged in a rhomboid layout around a single , targeted for advanced fighters and the bomber; prototypes achieved 2,313 hp at 3,200 rpm but faced development delays and were ultimately canceled before entering production. In Britain, Rolls-Royce developed the (also known as ), an air-cooled X-24 sleeve-valve engine from the late through the mid-1940s, rated at 1,200 hp for naval like Fairey designs; despite promising bench tests, it progressed only to limited ground running and was abandoned in favor of more reliable V-12 alternatives. Following the war, X engines declined rapidly in aviation due to the dominance of and a shift toward simpler, more manufacturable inline and V-type designs, rendering complex multi-bank layouts economically unviable. Marine applications provided isolated exceptions, notably the General Motors 16-338, a 16-cylinder diesel with a four-layer vertical quasi-radial () arrangement of horizontal cylinders around a central vertical , introduced in 1944 for U.S. Navy submarines; it delivered 1,000 hp at 1,600 rpm and powered vessels like the Tang-class through the early 1950s before being phased out for more conventional diesels. By the late , X configurations saw sporadic automotive experimentation, exemplified by the Revetec X4 series, an opposed-piston design developed from 2006 to 2013; prototypes, including the X4v2 tested in 2008, demonstrated enhanced and in a compact four-cylinder layout, with independent evaluations confirming up to 30% better economy than comparable conventional engines. A notable 21st-century revival emerged in military applications with the ChTZ 12N360, a four-stroke X-12 diesel engine produced by Russia's Chelyabinsk Tractor Plant starting in 2015 for the T-14 Armata main battle tank; its 34.6-liter displacement and multi-bank compactness enable 1,500 hp output while fitting within a low-profile hull, addressing modern requirements for power density in armored vehicles.

Applications

Aviation Uses

The X-24 engine configuration was adapted for aviation primarily to meet the demands of fighters and bombers requiring high power output within a compact length, offering a favorable power-to-weight ratio for aerial applications. This layout allowed for inline mounting in aircraft fuselages, facilitating streamlined designs while incorporating superchargers to enhance performance at altitude. A notable example is the British , an X-24 engine producing 1,760 hp, which powered the twin-engined introduced in the early 1940s. The Manchester, with two engines, entered RAF service in November 1940 and flew 1,269 sorties before its phase-out in June 1942, but installation challenges included complex access to spark plugs due to manifold and mount placements, complicating maintenance. Supercharging on the Vulture supported operations up to 19,200 feet, though the aircraft fell short of design goals like a 275 mph top speed. In , the Daimler-Benz DB 604 X-24 prototype, rated at around 2,660 hp at with a two-stage for high-altitude performance, was developed in 1939 for the program to power fast medium bombers such as the and prototypes. Intended for inline fuselage installations in these aircraft, the DB 604 underwent bench testing and limited flight trials in a , but development ceased in 1942 without entering production due to program shifts favoring other engines like the Junkers Jumo 222. Service performance of X-24 engines in aviation was limited by reliability issues; the Vulture, for instance, experienced frequent bearing failures from overheating and lubrication problems, contributing to the Manchester's replacement by more dependable four-engined bombers like the Avro Lancaster. Post-war, X-24 configurations saw no significant aviation revivals, as the advent of turbojet and turboprop engines rendered such complex piston designs obsolete for aircraft propulsion.

Marine and Ground Vehicle Uses

The X-16 configuration of the General Motors 16-184 , introduced in 1941, powered 110-foot subchasers (SC-class) during , delivering 1,200 horsepower in a compact vertical arrangement suitable for small naval vessels requiring high-speed propulsion. In submarine applications, the GM 16-338, an X-16 "" engine developed around 1944, equipped early Tang-class boats with four units producing a combined 4,000 shaft horsepower, enabling efficient diesel-electric operation in confined hull spaces. These engines were prized for their low profile and reduced length compared to traditional layouts, facilitating streamlined designs, though reliability issues led to their replacement by opposed-piston Fairbanks-Morse engines in later Tang-class vessels and post-war fleets. Ground vehicle adoption of X engines has been limited, with the ChTZ 12N360 serving as a notable exception in the Russian since its 2015 introduction; this X-12 outputs 1,500 horsepower while maintaining a low height of approximately 32 inches (820 mm), allowing for a reduced turret ring diameter and enhanced crew protection through a lower overall profile. As of 2025, production of the remains limited, with fewer than 100 units delivered, due to high costs and technical challenges including engine overheating. The design's stacked cylinder banks minimize engine bay intrusion, supporting the Armata's unmanned turret configuration. Adaptations for marine and ground use emphasize vibration management, as the X layout's offset banks can generate torsional oscillations; in naval installations like the 16-338, specialized mounts and balancing shafts were incorporated to mitigate hull resonance, while the 12N360 employs advanced counterweights and elastomeric isolators to dampen track-induced s in tracked vehicles. Liquid cooling systems are standard across these applications, with the 16-184 and 16-338 using seawater-compatible heat exchangers for submerged operations, and the 12N360 featuring a water-cooled block to handle dusty combat environments without clogging. By the , X engines largely phased out of marine and ground production in favor of inline and V configurations, driven by the former's higher maintenance demands from complex valvetrains and shared stresses, as evidenced by the U.S. Navy's shift to simpler radial diesels post-Tang class.

Notable Examples

Production Engines

The was one of the few X engines to achieve significant production during , serving primarily in British aviation applications. This X-24 liquid-cooled inline featured a displacement of 42.47 liters and delivered 1,760 horsepower at 3,000 rpm in its service configuration. Production ran from 1940 to March 1942, with 538 units manufactured at Rolls-Royce's facilities. The powered the heavy bomber, which entered RAF service in November 1940 with two engines per aircraft, but reliability issues including failures and overheating led to its withdrawal by mid-1942 after around 200 were built. Early prototypes of the , derived from the , also tested engines before transitioning to more dependable powerplants. In the marine sector, General Motors' Electro-Motive Division produced the 16-184 as a compact for U.S. Navy vessels during . The had a displacement of 48.2 liters, a bore of 152 mm, and a stroke of 165 mm, generating 1,200 horsepower at 1,800 rpm. Introduced in 1941, approximately 544 units of the 16-184A variant were built, with 506 installed in pairs aboard 253 SC-1-class 110-foot submarine chasers for anti-submarine duties. Its vertical "pancake" layout, stacking four banks of four cylinders around a central , allowed a low profile and high suitable for small patrol boats, contributing to their top speeds of up to 22 knots. The engine's design influenced later marine diesels, though many chasers were decommissioned post-war in the late 1940s. A follow-on to the 16-184, the General Motors 16-338 was an X-16 diesel developed for submarine propulsion in the post-World War II era. Sharing the same 48.2-liter displacement, bore, and stroke as its predecessor, it produced around 1,000 horsepower per unit at 1,600 rpm in generator sets. Production occurred from 1944 through the 1950s by the , with dozens of units constructed despite persistent development challenges. Four 16-338 engines equipped the first four Tang-class submarines (USS Tang, Trigger, , and ), commissioned in the early 1950s, serving as the main diesel engines for surface propulsion and electrical generation until reliability problems like excessive vibration and oil leaks prompted their replacement with Fairbanks-Morse units by 1958. Remaining installations served into the on other diesel-electric submarines before full decommissioning of the class in the late . The ChTZ 12N360 represents a modern X engine entering production for Russian armored vehicles. This X-12 four-stroke diesel, also designated A-85-3A, has a displacement of 34.6 liters and outputs 1,500 horsepower at 2,000 rpm, with potential for up to 2,000 horsepower in boosted modes. Produced by the Chelyabinsk Tractor Plant since around 2015, it powers the T-14 Armata main battle tank and related platforms like the T-15 infantry fighting vehicle, enabling high mobility with a power-to-weight ratio exceeding 25 hp/ton. The engine's compact X configuration, measuring 813 mm long, 1,300 mm wide, and 820 mm high at 1,550 kg, integrates into the Armata's unmanned turret design for enhanced survivability. However, production has been severely limited, with only around 20 units built as of 2025 and reports of discontinuation in favor of upgrading existing tank designs due to high costs and military priorities.

Prototypes and Experimental Designs

The Daimler-Benz DB 604 was an experimental liquid-cooled X-24 aircraft engine developed in Germany during the early 1940s as part of the Luftwaffe's Bomber B program for long-range heavy bombers, such as the Junkers Ju 288, Dornier Do 317, and Focke-Wulf Fw 191. Featuring four banks of six cylinders arranged at 90-degree angles, the engine had a displacement of approximately 46.4 liters in its initial form, with an enlarged variant reaching 49.9 liters. It targeted a power output of around 2,500 horsepower (1,864 kW) at takeoff, with the prototype achieving 2,313 hp (1,725 kW) during its first run in late 1939 and later versions reaching up to 2,660 hp (1,984 kW) at sea level. Development emphasized high-altitude performance, with the enlarged model projected to deliver 3,450 hp (2,575 kW) at 11,000 meters. However, the project was canceled in September 1942 due to wartime fuel shortages, the engine's mechanical complexity, and prioritization of the competing Junkers Jumo 222 radial engine, preventing any production or aircraft integration. In Britain, the (also known as ) represented an innovative air-cooled X-12 sleeve-valve engine designed in the late 1930s for naval aircraft, particularly projects like the . Comprising four banks of three cylinders each at 90-degree intervals, it displaced 22.1 liters and was tested from 1936 onward, achieving up to 1,200 hp (895 kW) at 4,200 rpm during bench runs, with ambitions for 1,500 hp (1,119 kW) in advanced configurations. The full engine debuted in September 1936, completing a 40-hour endurance test by late 1937; a four-cylinder test unit ran in 1938, and it powered a testbed aircraft on its first flight in November 1938. Despite promising results in and , development halted in and ceased entirely by 1941, as resources shifted to the more versatile and Griffon V-12 engines amid demands, leaving the Exe as an unproduced prototype. More recently, the Revetec X4 series from Australian firm Revetec Holdings introduced a novel opposed-piston X-4 configuration using counter-rotating tri-lobed cams instead of a traditional to drive pistons, aiming to enhance and reduce weight for automotive and applications. The Version 2 prototype, tested in 2008, featured a 2.4-liter displacement with a conventional overhead valve head, dual spark plugs per , and a 10:1 , producing up to 87 horsepower (65 kW) and 130 lb-ft (176 Nm) of at 3,500 rpm while achieving brake specific fuel consumption as low as 212 g/kWh () under part-load conditions. Development spanned the , with prototypes emphasizing delivery across a broad RPM range and lower emissions compared to conventional flat-four engines of similar size. Despite independent validation by Orbital confirming superior fuel economy and reduced side-loading on walls, the design did not advance to , remaining in low-volume experimental use due to challenges in scaling for broader commercial viability.

Advantages and Disadvantages

Performance Benefits

The X engine configuration provides significant performance advantages through its inherently compact design, enabling high power output in space-constrained environments such as fuselages and marine vessels. By arranging four banks in an X shape around a common , the engine achieves a shorter overall than equivalent V or inline configurations with the same number of cylinders, facilitating easier integration into tight installations. For example, the X-24 measured 87.2 inches (2,214 mm) in length, allowing for more efficient packaging compared to longer multi-cylinder alternatives. This compactness contributes to a superior power-to-length ratio, enhancing the engine's suitability for applications requiring minimal axial space. The standard Vulture, with a displacement of 42.47 liters, delivered 1,800 horsepower at takeoff, yielding approximately 42 hp per liter; the Vulture V variant achieved up to 1,995 horsepower with an increased displacement of ~50 liters. This form factor was better adapted for fuselage mounting than contemporary radial engines of similar output. X engines also exhibit high , often exceeding 1,000 horsepower in and marine uses, thanks to the multi-bank firing sequence that supports elevated rotational speeds. The balanced design promotes smoother operation by minimizing vibrations, as seen in the X-2775 X-24, which produced 1,250 horsepower in a 77.5-inch (1,969 mm) length while weighing just 1,402 pounds (636 kg). In aircraft applications, the reduced engine length shortens the propeller shaft, enabling a more aerodynamic nose profile and improved overall aircraft efficiency.

Engineering Challenges

X-engines, with their four cylinder banks arranged in an X configuration around a shared crankshaft, imposed substantial weight penalties compared to conventional radial or V-type engines, owing to the reinforced framing and additional structural elements needed for stability. The , for example, tipped the scales at 2,450 pounds dry, roughly 50 percent heavier than the V-12's 1,640 pounds, while delivering comparable power in the 1,700 horsepower range. The design's complexity further exacerbated reliability concerns, as synchronizing valvetrains and drives across multiple banks increased the number of components prone to failure. In the Vulture, big-end bearing problems arose from inadequate lubrication, crankcase misalignment, and overheating, resulting in frequent catastrophic failures that grounded aircraft throughout the 1940s. Maintenance proved particularly arduous due to limited access to inner cylinder banks, buried within the compact layout, which demanded extensive disassembly for inspections and repairs. Early X-engines like the Vulture also generated excessive vibrations from uneven firing orders, requiring supplementary balancers to mitigate torsional stresses and prevent fatigue. Beyond wartime applications, the elevated manufacturing costs stemming from precision machining of intricate parts curtailed scalability, rendering X-engines rare in postwar production. Contemporary efforts, such as the X-12 diesel powering Russia's T-14 Armata tank, continue to grapple with supply chain constraints, including sanctions-induced component shortages that hinder mass production and reliability. As of 2025, production of the T-14 remains limited to low volumes, with fewer than 100 units fielded, due to ongoing sanctions and resource allocation to proven designs.

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