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62-453: The Rolls-Royce Crecy was a British experimental two-stroke , 90-degree, V12 , liquid-cooled aero-engine of 1,593.4 cu.in (26.11 L ) capacity , featuring sleeve valves and direct petrol injection . Initially intended for a high-speed "sprint" interceptor fighter, the Crecy was later seen as an economical high-altitude long-range powerplant. Developed between 1941 and 1946, it was among

124-544: A North American P-51 Mustang for engine installation trials. This prompted a series of studies for a Crecy version and the Mustang turned out a more suitable mount than the Spitfire. However these studies were not taken further. As the possibility of a flight-worthy engine approached, on 28 March 1943 Hawker Henley L3385 was delivered to Hucknall for fitting with a Crecy. However the engine never became available and

186-415: A power-valve system . The valves are normally in or around the exhaust ports. They work in one of two ways; either they alter the exhaust port by closing off the top part of the port, which alters port timing, such as Rotax R.A.V.E, Yamaha YPVS, Honda RC-Valve, Kawasaki K.I.P.S., Cagiva C.T.S., or Suzuki AETC systems, or by altering the volume of the exhaust, which changes the resonant frequency of

248-421: A rotary valve is that it enables the two-stroke engine's intake timing to be asymmetrical, which is not possible with piston-port type engines. The piston-port type engine's intake timing opens and closes before and after top dead center at the same crank angle, making it symmetrical, whereas the rotary valve allows the opening to begin and close earlier. Rotary valve engines can be tailored to deliver power over

310-482: A Crecy engine and compared this to a Griffon 61 -powered variant. The report stated that the Crecy's maximum power output would be too much for the Spitfire airframe but that a derated version would have considerable performance gains over the Griffon-powered fighter. Studies on the de Havilland Mosquito also showed it to raise complex problems with Crecy installation. In 1942 Rolls-Royce Hucknall received

372-421: A DKW design that proved reasonably successful employing loop charging. The original SAAB 92 had a two-cylinder engine of comparatively low efficiency. At cruising speed, reflected-wave, exhaust-port blocking occurred at too low a frequency. Using the asymmetrical three-port exhaust manifold employed in the identical DKW engine improved fuel economy. The 750-cc standard engine produced 36 to 42 hp, depending on

434-448: A crossflow engine) is always best and support is good. In some engines, the small end is offset to reduce thrust in the intended rotational direction and the forward face of the piston has been made thinner and lighter to compensate, but when running backward, this weaker forward face suffers increased mechanical stress it was not designed to resist. This can be avoided by the use of crossheads and also using thrust bearings to isolate

496-630: A higher power-to-weight ratio than a four-stroke engine, since their power stroke occurs twice as often. Two-stroke engines can also have fewer moving parts , and thus be cheaper to manufacture and weigh less. In countries and regions with stringent emissions regulation, two-stroke engines have been phased out in automotive and motorcycle uses. In regions where regulations are less stringent, small displacement two-stroke engines remain popular in mopeds and motorcycles. They are also used in power tools such as chainsaws and leaf blowers . The first commercial two-stroke engine involving cylinder compression

558-470: A patent in 1880 in Germany. The first truly practical two-stroke engine is attributed to Yorkshireman Alfred Angas Scott , who started producing twin-cylinder water-cooled motorcycles in 1908. Two-stroke gasoline engines with electrical spark ignition are particularly useful in lightweight or portable applications such as chainsaws and motorcycles. However, when weight and size are not an issue,

620-449: A scavenging function. The units run in pairs, with the lower half of one piston charging an adjacent combustion chamber. The upper section of the piston still relies on total-loss lubrication, but the other engine parts are sump lubricated with cleanliness and reliability benefits. The mass of the piston is only about 20% more than a loop-scavenged engine's piston because skirt thicknesses can be less. Many modern two-stroke engines employ

682-658: A similar system. Traditional flywheel magnetos (using contact-breaker points, but no external coil) worked equally well in reverse because the cam controlling the points is symmetrical, breaking contact before top dead center equally well whether running forward or backward. Reed-valve engines run backward just as well as piston-controlled porting, though rotary valve engines have asymmetrical inlet timing and do not run very well. Serious disadvantages exist for running many engines backward under load for any length of time, and some of these reasons are general, applying equally to both two-stroke and four-stroke engines. This disadvantage

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744-518: A turbocharger. Crankcase-compression two-stroke engines, such as common small gasoline-powered engines, are lubricated by a petroil mixture in a total-loss system . Oil is mixed in with their petrol fuel beforehand, in a fuel-to-oil ratio of around 32:1. This oil then forms emissions, either by being burned in the engine or as droplets in the exhaust, historically resulting in more exhaust emissions, particularly hydrocarbons, than four-stroke engines of comparable power output. The combined opening time of

806-400: A wider speed range or higher power over a narrower speed range than either a piston-port or reed-valve engine. Where a portion of the rotary valve is a portion of the crankcase itself, of particular importance, no wear should be allowed to take place. In a cross-flow engine, the transfer and exhaust ports are on opposite sides of the cylinder, and a deflector on the top of the piston directs

868-413: Is accepted in most cases where cost, weight, and size are major considerations. The problem comes about because in "forward" running, the major thrust face of the piston is on the back face of the cylinder, which in a two-stroke particularly, is the coolest and best-lubricated part. The forward face of the piston in a trunk engine is less well-suited to be the major thrust face, since it covers and uncovers

930-422: Is attributed to Scottish engineer Dugald Clerk , who patented his design in 1881. However, unlike most later two-stroke engines, his had a separate charging cylinder. The crankcase -scavenged engine, employing the area below the piston as a charging pump, is generally credited to Englishman Joseph Day . On 31 December 1879, German inventor Karl Benz produced a two-stroke gas engine, for which he received

992-597: Is being phased out. Honda , for instance, ceased selling two-stroke off-road motorcycles in the United States in 2007, after abandoning road-going models considerably earlier. Due to their high power-to-weight ratio and ability to be used in any orientation, two-stroke engines are common in handheld outdoor power tools including leaf blowers , chainsaws , and string trimmers . Two-stroke diesel engines are found mostly in large industrial and marine applications, as well as some trucks and heavy machinery. Although

1054-574: Is common in on-road, off-road, and stationary two-stroke engines ( Detroit Diesel ), certain small marine two-stroke engines ( Gray Marine Motor Company , which adapted the Detroit Diesel Series 71 for marine use ), certain railroad two-stroke diesel locomotives ( Electro-Motive Diesel ) and large marine two-stroke main propulsion engines ( Wärtsilä ). Ported types are represented by the opposed piston design in which two pistons are in each cylinder, working in opposite directions such as

1116-432: Is that the two-stroke's crankcase is sealed and forms part of the induction process in gasoline and hot-bulb engines . Diesel two-strokes often add a Roots blower or piston pump for scavenging . The reed valve is a simple but highly effective form of check valve commonly fitted in the intake tract of the piston-controlled port. It allows asymmetric intake of the fuel charge, improving power and economy, while widening

1178-409: Is under every circumstance more efficient than cross-flow scavenging. In a uniflow engine, the mixture, or "charge air" in the case of a diesel, enters at one end of the cylinder controlled by the piston and the exhaust exits at the other end controlled by an exhaust valve or piston. The scavenging gas-flow is, therefore, in one direction only, hence the name uniflow. The design using exhaust valve(s)

1240-637: The Aeronautical Research Committee (ARC), was a proponent of a high-powered "sprint" engine for fighter aircraft and had foreseen the need for such a powerplant as early as 1935 with the threat of German air power looming. It has been suggested that Tizard influenced his personal friend Harry Ricardo to develop what eventually became the Crecy. The idea was officially discussed for the first time at an engine sub-committee meeting in December 1935. "The Chairman remarked that if it

1302-464: The Junkers Jumo 205 and Napier Deltic . The once-popular split-single design falls into this class, being effectively a folded uniflow. With advanced-angle exhaust timing, uniflow engines can be supercharged with a crankshaft-driven blower, either piston or Roots-type. The piston of this engine is "top-hat"-shaped; the upper section forms the regular cylinder, and the lower section performs

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1364-486: The crankshaft . The Crecy sleeve valves were of similar construction but differed in their operation compared to the rotary sleeve valve design that was pioneered by Roy Fedden , and used successfully for the first time in an aircraft engine, the Bristol Perseus , in 1932. Supercharging was used to force the charge into the cylinder, rather than crankcase compression, as on most two-stroke engines. This allowed

1426-512: The expansion chamber , such as the Suzuki SAEC and Honda V-TACS system. The result is an engine with better low-speed power without sacrificing high-speed power. However, as power valves are in the hot gas flow, they need regular maintenance to perform well. Direct injection has considerable advantages in two-stroke engines. In carburetted two-strokes, a major problem is a portion of the fuel/air mixture going directly out, unburned, through

1488-479: The oil reservoir does not depend on gravity. A number of mainstream automobile manufacturers have used two-stroke engines in the past, including the Swedish Saab , German manufacturers DKW , Auto-Union , VEB Sachsenring Automobilwerke Zwickau , VEB Automobilwerk Eisenach , and VEB Fahrzeug- und Jagdwaffenwerk , and Polish manufacturers FSO and FSM . The Japanese manufacturers Suzuki and Subaru did

1550-528: The 1960s due in no small way to the increased power afforded by loop scavenging. An additional benefit of loop scavenging was the piston could be made nearly flat or slightly domed, which allowed the piston to be appreciably lighter and stronger, and consequently to tolerate higher engine speeds. The "flat top" piston also has better thermal properties and is less prone to uneven heating, expansion, piston seizures, dimensional changes, and compression losses. SAAB built 750- and 850-cc three-cylinder engines based on

1612-407: The 1960s, especially for motorcycles, but for smaller or slower engines using direct injection, the deflector piston can still be an acceptable approach. This method of scavenging uses carefully shaped and positioned transfer ports to direct the flow of fresh mixture toward the combustion chamber as it enters the cylinder. The fuel/air mixture strikes the cylinder head , then follows the curvature of

1674-485: The Chief Designer. Bore was 5.1 in (129.5 mm), stroke 6.5 in (165.1 mm), compression ratio 7:1 and weight 1,900 lb (862 kg). The firing angle was 30 degrees BTDC , and 15 lbf/in² (100 kPa ) supercharger boost was typical. In bench-testing it produced 1,400 horsepower (1,000 kW), but there were problems with vibration and the cooling of the pistons and sleeves. The thrust produced by

1736-523: The Crecy itself, in conjunction with the Ricardo company , the decision had been taken by the Air Ministry to revert to a more conventional spark-ignition layout, although still retaining fuel injection. The Crecy has been described as one of the most advanced two-stroke aero engines ever built. The first complete V12 engine was built in 1941, designed by a team led by Harry Wood with Eddie Gass as

1798-452: The German inventor of an early form in the mid-1920s, it became widely adopted in Germany country during the 1930s and spread further afield after World War II . Loop scavenging is the most common type of fuel/air mixture transfer used on modern two-stroke engines. Suzuki was one of the first manufacturers outside of Europe to adopt loop-scavenged, two-stroke engines. This operational feature

1860-508: The V12 engines with a further 8,600 hours of testing on the V-twins. The fate of the six Crecy engines remains unknown. The Crecy proved a unique exercise and Rolls-Royce did not develop any other two-stroke aero engines, the whole concept of advanced piston engines at that time being overtaken by the advent of the practical jet engine . In the summer of 1941, Supermarine Spitfire Mk II P7674

1922-443: The aircraft remained at Hucknall until it was scrapped on 11 September 1945. From 1943 a number of postwar transport projects were considered, taking advantage of the Crecy's unique characteristics for land, sea and air applications. None got further than the drawing-board. Data from The Rolls-Royce Crecy. Related development Comparable engines Related lists Two-stroke engine Two-stroke engines often have

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1984-423: The bore diameter for reasonable piston ring life. Beyond this, the piston rings bulge into the exhaust port and wear quickly. A maximum 70% of bore width is possible in racing engines, where rings are changed every few races. Intake duration is between 120 and 160°. Transfer port time is set at a minimum of 26°. The strong, low-pressure pulse of a racing two-stroke expansion chamber can drop the pressure to -7 psi when

2046-459: The combustion chamber, and then is deflected downward. This not only prevents the fuel/air mixture from traveling directly out the exhaust port, but also creates a swirling turbulence which improves combustion efficiency , power, and economy. Usually, a piston deflector is not required, so this approach has a distinct advantage over the cross-flow scheme (above). Often referred to as "Schnuerle" (or "Schnürle") loop scavenging after Adolf Schnürle,

2108-453: The crankshaft commonly spins in the same axis and direction as do the wheels i.e. "forward". Some of the considerations discussed here apply to four-stroke engines (which cannot reverse their direction of rotation without considerable modification), almost all of which spin forward, too. It is also useful to note that the "front" and "back" faces of the piston are - respectively - the exhaust port and intake port sides of it, and are not to do with

2170-415: The cycle's potential for high thermodynamic efficiency makes it ideal for diesel compression ignition engines operating in large, weight-insensitive applications, such as marine propulsion , railway locomotives , and electricity generation . In a two-stroke engine, the exhaust gases transfer less heat to the cooling system than a four-stroke, which means more energy to drive the piston, and if present,

2232-400: The engine from end loads. Large two-stroke ship diesels are sometimes made to be reversible. Like four-stroke ship engines (some of which are also reversible), they use mechanically operated valves, so require additional camshaft mechanisms. These engines use crossheads to eliminate sidethrust on the piston and isolate the under-piston space from the crankcase. On top of other considerations,

2294-507: The exceptionally loud two-stroke exhaust was estimated as being equivalent to a 30% increase in power at the propeller on top of the rated output of the engine. The power of the engine was interesting in its own right, but the additional exhaust thrust at high speed could have made it a useful stop gap between engines such as the Rolls-Royce Merlin and anticipated jet engines. Serial numbers were even, because Rolls-Royce practice

2356-415: The exhaust port in the cylinder, the hottest part of the engine, where piston lubrication is at its most marginal. The front face of the piston is also more vulnerable since the exhaust port, the largest in the engine, is in the front wall of the cylinder. Piston skirts and rings risk being extruded into this port, so having them pressing hardest on the opposite wall (where there are only the transfer ports in

2418-409: The exhaust port, and direct injection effectively eliminates this problem. Two systems are in use: low-pressure air-assisted injection and high-pressure injection. Since the fuel does not pass through the crankcase, a separate source of lubrication is needed. For the purpose of this discussion, it is convenient to think in motorcycle terms, where the exhaust pipe faces into the cooling air stream, and

2480-413: The fresh intake charge into the upper part of the cylinder, pushing the residual exhaust gas down the other side of the deflector and out the exhaust port. The deflector increases the piston's weight and exposed surface area, and the fact that it makes piston cooling and achieving an effective combustion chamber shape more difficult is why this design has been largely superseded by uniflow scavenging after

2542-443: The intake and exhaust ports in some two-stroke designs can also allow some amount of unburned fuel vapors to exit in the exhaust stream. The high combustion temperatures of small, air-cooled engines may also produce NO x emissions. Two-stroke gasoline engines are preferred when mechanical simplicity, light weight, and high power-to-weight ratio are design priorities. By mixing oil with fuel, they can operate in any orientation as

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2604-475: The model year. The Monte Carlo Rally variant, 750-cc (with a filled crankshaft for higher base compression), generated 65 hp. An 850-cc version was available in the 1966 SAAB Sport (a standard trim model in comparison to the deluxe trim of the Monte Carlo). Base compression comprises a portion of the overall compression ratio of a two-stroke engine. Work published at SAE in 2012 points that loop scavenging

2666-480: The most advanced two-stroke aero-engines ever built. The engine never reached flight trials and the project was cancelled in December 1945, overtaken by the progress of jet engine development. The engine was named after the Battle of Crécy , after Rolls-Royce chose battles as the theme for naming their two-stroke aero engines. Rolls-Royce did not develop any other engines of this type. Sir Henry Tizard , Chairman of

2728-399: The oil pump of a modern two-stroke may not work in reverse, in which case the engine suffers oil starvation within a short time. Running a motorcycle engine backward is relatively easy to initiate, and in rare cases, can be triggered by a back-fire. It is not advisable. Model airplane engines with reed valves can be mounted in either tractor or pusher configuration without needing to change

2790-487: The other - the inlet pipe having passage to the crankcase only when the two cutouts coincide. The crankshaft itself may form one of the members, as in most glow-plug model engines. In another version, the crank disc is arranged to be a close-clearance fit in the crankcase, and is provided with a cutout that lines up with an inlet passage in the crankcase wall at the appropriate time, as in Vespa motor scooters. The advantage of

2852-434: The piston covering and uncovering the ports as it moves up and down in the cylinder. In the 1970s, Yamaha worked out some basic principles for this system. They found that, in general, widening an exhaust port increases the power by the same amount as raising the port, but the power band does not narrow as it does when the port is raised. However, a mechanical limit exists to the width of a single exhaust port, at about 62% of

2914-440: The piston is at bottom dead center, and the transfer ports nearly wide open. One of the reasons for high fuel consumption in two-strokes is that some of the incoming pressurized fuel-air mixture is forced across the top of the piston, where it has a cooling action, and straight out the exhaust pipe. An expansion chamber with a strong reverse pulse stops this outgoing flow. A fundamental difference from typical four-stroke engines

2976-550: The power band. Such valves are widely used in motorcycle, ATV, and marine outboard engines. The intake pathway is opened and closed by a rotating member. A familiar type sometimes seen on small motorcycles is a slotted disk attached to the crankshaft , which covers and uncovers an opening in the end of the crankcase, allowing charge to enter during one portion of the cycle (called a disc valve). Another form of rotary inlet valve used on two-stroke engines employs two cylindrical members with suitable cutouts arranged to rotate one within

3038-466: The principles remain the same, the mechanical details of various two-stroke engines differ depending on the type. The design types vary according to the method of introducing the charge to the cylinder, the method of scavenging the cylinder (exchanging burnt exhaust for fresh mixture) and the method of exhausting the cylinder. Piston port is the simplest of the designs and the most common in small two-stroke engines. All functions are controlled solely by

3100-480: The project. Crecy s/n 10 achieved 1,798 horsepower (1,341 kW) on 21 December 1944 which after adjustment for the inclusion of an exhaust turbine would have equated to 2,500 horsepower (1,900 kW). Subsequent single-cylinder tests carried out on the Ricardo E65 engine achieved the equivalent of 5,000 brake horsepower (3,700 kW) for the complete engine. By June 1945 a total of 1,060 hours had been run on

3162-821: The same in the 1970s. Production of two-stroke cars ended in the 1980s in the West, due to increasingly stringent regulation of air pollution . Eastern Bloc countries continued until around 1991, with the Trabant and Wartburg in East Germany. Two-stroke engines are still found in a variety of small propulsion applications, such as outboard motors , small on- and off-road motorcycles , mopeds , motor scooters , motorized bicycles , tuk-tuks , snowmobiles , go-karts , RC cars , ultralight and model airplanes. Particularly in developed countries, pollution regulations have meant that their use for many of these applications

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3224-438: The spark plugs reduced detonation , allowing higher compression ratios or supercharger boost. Supercharger throttling was used as well to achieve idling. The supercharger throttles were novel vortex types, varying the effective angle of attack of the impeller blades from 60 to 30 degrees. This reduced the power required to drive the supercharger when throttled, and hence fuel consumption at cruising power. Later testing involved

3286-520: The top or bottom of the piston. Regular gasoline two-stroke engines can run backward for short periods and under light load with little problem, and this has been used to provide a reversing facility in microcars , such as the Messerschmitt KR200 , that lacked reverse gearing. Where the vehicle has electric starting, the motor is turned off and restarted backward by turning the key in the opposite direction. Two-stroke golf carts have used

3348-420: The use of a conventional lubrication system, instead of the total-loss type found in many two-stroke engines. Stratified charge was used: the fuel was injected into a bulb-like extension of the combustion chamber where the twin spark plugs ignited the rich mixture. Operable air-fuel ratios of from 15 to 23:1 were available to govern the power produced between maximum and 60%. The rich mixture maintained near

3410-470: The use of an exhaust turbine which was a half-scale version of that used in the Whittle W.1 turbojet , the first British jet engine to fly. Unlike a conventional turbocharger the turbine was coupled to the engine's accessory driveshaft and acted as a power recovery device . It was thought that using the turbine would lower fuel consumption allowing the engine to be used in larger transport aircraft. This

3472-539: The worth of further research into a two-stroke sleeve-valved design. Both these engines had initially been converted to diesel sleeve-valved operation with a lower power output than the original design being noted along with increased mechanical failures, although one converted Kestrel was subsequently used successfully by Captain George Eyston in a land-speed record car named Speed of the Wind . The second engine

3534-525: Was confirmed during testing, but failures due to severe overheating and drive shaft fractures were experienced. The following table summarises the test running programme, hours run, and highlights some of the failures experienced. Data from: The progress of jet engine development overtook that of the Crecy and replaced the need for this engine. As a result, work on the project ceased in December 1945 at which point only six complete examples had been built, however an additional eight V-twins were built during

3596-404: Was delivered to Hucknall and fitted with a Crecy mock-up to enable cowling drawings and system details to be designed. It was planned for the first production Spitfire Mk III to be delivered to Hucknall in early 1942 for fitting of an airworthy Crecy, but this never took place. A Royal Aircraft Establishment report (No. E.3932) of March 1942 estimated the performance of the Spitfire fitted with

3658-414: Was further converted to petrol injection which then gave a marked power increase over the standard Kestrel. Single-cylinder development began in 1937 under project engineer Harry Wood using a test unit designed by Ricardo. The Crecy was originally conceived as a compression ignition engine and Rolls-Royce had previously converted a Kestrel engine to run on Diesel. By the time they started development of

3720-636: Was the desire of the Air Ministry to develop a type of sprint engine for home defence....there was the question as to how far fuel consumption could be disregarded. Mr Ricardo had raised this point in a recent conversation by enquiring whether a high fuel consumption might not be permissible under certain circumstances, for if so, an investigation of the possibilities of the two-stroke petrol engine appeared to be attractive." Previous experience gained between 1927 and 1930 using two converted Rolls-Royce Kestrel engines through an Air Ministry contract had proven

3782-473: Was to have even numbers for engines rotating clockwise when viewed from the front. The reciprocating sleeve valves were open-ended, rather than sealing in a junk head . The open end uncovered the exhaust ports high in the cylinder wall at the bottom of the sleeves' stroke, leaving the ports cut into the sleeve to handle the incoming charge only. The sleeves had a stroke of 30% of the piston travel at 1.950 in (49.5 mm) and operated 15 degrees in advance of

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3844-541: Was used in conjunction with the expansion chamber exhaust developed by German motorcycle manufacturer, MZ, and Walter Kaaden. Loop scavenging, disc valves, and expansion chambers worked in a highly coordinated way to significantly increase the power output of two-stroke engines, particularly from the Japanese manufacturers Suzuki, Yamaha, and Kawasaki. Suzuki and Yamaha enjoyed success in Grand Prix motorcycle racing in

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