Wankel engine

The first DKM Wankel engine designed by Felix Wankel, the DKM 54 (Drehkolbenmotor), at the Deutsches Museum in Bonn, Germany: the rotor and its housing spin

The first KKM Wankel Engine designed by Hanns Dieter Paschke, the NSU KKM 57P (Kreiskolbenmotor), at Autovision und Forum, Germany: the rotor housing is static.

In 1951, NSU Motorenwerke AG in Germany began development of the engine, with two models being built. The first, the DKM motor, was developed by Felix Wankel. The second, the KKM motor, was not developed by Felix Wankel but by Hanns Dieter Paschke. The Paschke engine was adopted as the basis of the modern Wankel engine, yet named after Felix Wankel.[11]

The basis of the DKM type of motor was that both the rotor and the housing spun around on separate axes. The DKM motor reached higher revolutions per minute (up to 17,000 rpm) and was more naturally balanced. However, the engine needed to be stripped to change the spark plugs and contained more parts. The KKM engine was simpler, having a fixed housing.

The first working prototype, DKM 54, produced 21 hp (16 kW) and ran on February 1, 1957, at the NSU research and development department Versuchsabteilung TX.[1][12]

The KKM 57 (the Wankel rotary engine, Kreiskolbenmotor) was constructed by NSU engineer Hanns Dieter Paschke in 1957 without the knowledge of Felix Wankel, who later remarked, "you have turned my race horse into a plow mare".[13]

In 1960, NSU, the firm that employed the two inventors, and US firm Curtiss-Wright, signed a joint agreement. NSU was to concentrate on low- and medium-powered Wankel engine development, with Curtiss-Wright developing high-powered engines, including aircraft engines of which Curtiss-Wright had decades of experience designing and producing.[14] Curtiss-Wright recruited Max Bentele to head their design team.

Many manufacturers signed license agreements for development, attracted by the smoothness, quiet running, and reliability emanating from the uncomplicated design. Among them were Alfa Romeo, American Motors, Citroën, Ford, General Motors, Mazda, Mercedes-Benz, Nissan, Porsche, Rolls-Royce, Suzuki, and Toyota.[1] In the United States in 1959, under license from NSU, Curtiss-Wright pioneered improvements in the basic engine design. In Britain, in the 1960s, Rolls Royce's Motor Car Division pioneered a two-stage diesel version of the Wankel engine.[15]

Citroën did much research, producing the M35, GS Birotor, and RE-2 helicopter, using engines produced by Comotor, a joint venture of Citroën and NSU. General Motors seemed to have concluded the Wankel engine was slightly more expensive to build than an equivalent reciprocating engine. General Motors claimed to have solved the fuel-economy issue, but failed in obtaining in a concomitant way to acceptable exhaust emissions. Mercedes-Benz fitted a Wankel engine in their C111 concept car.

Deere & Company designed a version that was capable of using a variety of fuels. The design was proposed as the power source for United States Marine Corps combat vehicles and other equipment in the late 1980s.[16]

In 1961, the Soviet research organization of NATI, NAMI, and VNIImotoprom commenced development creating experimental engines with different technologies.[17] Soviet automobile manufacturer AvtoVAZ also experimented in Wankel engine design without a license, introducing a limited number of engines in some cars.[18]

By mid-September 1967, even Wankel model engines became available through the German Graupner aeromodeling products firm, made for them by O.S. Engines of Japan.

Despite much research and development throughout the world, only Mazda has produced Wankel engines in large quantities.

In Britain, Norton Motorcycles developed a Wankel rotary engine for motorcycles, based on the Sachs air-cooled rotor Wankel that powered the DKW/Hercules W-2000 motorcycle. This two-rotor engine was included in the Commander and F1. Norton improved on the Sachs's air cooling, introducing a plenum chamber. Suzuki also made a production motorcycle powered by a Wankel engine, the RE-5, using ferroTiC alloy apex seals and an NSU rotor in a successful attempt to prolong the engine's life.

Mazda and NSU signed a study contract to develop the Wankel engine in 1961 and competed to bring the first Wankel-powered automobile to market. Although Mazda produced an experimental Wankel that year, NSU was first with a Wankel automobile for sale, the sporty NSU Spider in 1964; Mazda countered with a display of two- and four-rotor Wankel engines at that year's Tokyo Motor Show.[1] In 1967, NSU began production of a Wankel-engined luxury car, the Ro 80.[19] NSU had not produced reliable apex seals on the rotor, though, unlike Mazda and Curtiss-Wright. NSU had problems with apex seals' wear, poor shaft lubrication, and poor fuel economy, leading to frequent engine failures, not solved until 1972, which led to large warranty costs curtailing further NSU Wankel engine development. This premature release of the new Wankel engine gave a poor reputation for all makes, and even when these issues were solved in the last engines produced by NSU in the second half of the '70s, sales did not recover.[1] Audi, after the takeover of NSU, built, in 1979, a new KKM 871 engine with side intake ports, a 750-cc chamber, 170 hp (130 kW) at 6,500 rpm, and 220 Nm (162 ft-lb) at 3,500 rpm. The engine was installed in an Audi 100 hull named "Audi 200", but was not mass-produced.

Mazda's first Wankel engine, precursor to the 10A, at the Mazda Museum in Hiroshima, Japan

Mercedes-Benz C111 was fitted with a four-rotor Wankel engine

Mazda, however, claimed to have solved the apex seal problem, operating test engines at high speed for 300 hours without failure.[1] After years of development, Mazda's first Wankel-engine car was the 1967 Cosmo 110S. The company followed with a number of Wankel ("rotary" in the company's terminology) vehicles, including a bus and a pickup truck. Customers often cited the cars' smoothness of operation. However, Mazda chose a method to comply with hydrocarbon emission standards that, while less expensive to produce, increased fuel consumption. Unfortunately for Mazda, this was introduced immediately prior to a sharp rise in fuel prices. Curtiss-Wright produced the RC2-60 engine, which was comparable to a V8 engine in performance and fuel consumption. Unlike NSU, Curtiss-Wright had solved the rotor sealing issue with seals lasting 100,000 miles (160,000 km) by 1966.[20]

Mazda later abandoned the Wankel in most of their automotive designs, continuing to use the engine in their sports car range only, producing the RX-7 until August 2002. The company normally used two-rotor designs. A more advanced twin-turbo three-rotor engine was fitted in the 1991 Eunos Cosmo sports car. In 2003, Mazda introduced the Renesis engine fitted in the RX-8. The Renesis engine relocated the ports for exhaust from the periphery of the rotary housing to the sides, allowing for larger overall ports, better airflow, and further power gains. Some early Wankel engines also had side exhaust ports, the concept being abandoned because of carbon buildup in ports and the sides of the rotor. The Renesis engine solved the problem by using a keystone scraper side seal, and approached the thermal distortion difficulties by adding some parts made of ceramics.[21] The Renesis is capable of 238 hp (177 kW) with improved fuel economy, reliability, and lower emissions than previous Mazda rotary engines,[22] all from a nominal 1.3-L displacement, but this was not enough to meet more stringent emissions standards. Mazda ended production of their Wankel engine in 2012 after the engine failed to meet the more stringent Euro 5 emission standards, leaving no automotive company selling a Wankel-powered vehicle.[23] The company is continuing development of the next generation of Wankel engines, the SkyActiv-R. Mazda states that the SkyActiv-R solves the three key issues with previous rotary engines: fuel economy, emissions, and reliability.[24] Mazda and Toyota announced they combined to produce a range extending rotary engine for vehicles.[25][26]

This 1972 GM rotary engine cutaway shows twin rotors.

American Motors Corporation (AMC), the smallest U.S. automaker, was so convinced "... that the rotary engine will play an important role as a powerplant for cars and trucks of the future ...", that the chairman, Roy D. Chapin Jr., signed an agreement in February 1973 after a year's negotiations, to build Wankel engines for both passenger cars and military vehicles, as well as the right to sell any rotary engines it produced to other companies.[27][28] AMC's president, William Luneburg, did not expect dramatic development through to 1980, but Gerald C. Meyers, AMC's vice president of the engineering product group, suggested that AMC should buy the engines from Curtiss-Wright before developing its own Wankel engines, and predicted a total transition to rotary power by 1984.[29] Plans called for the engine to be used in the AMC Pacer, but development was pushed back.[30][31] American Motors designed the unique Pacer around the engine. By 1974, AMC had decided to purchase the General Motors (GM) Wankel instead of building an engine in-house.[32] Both GM and AMC confirmed the relationship would be beneficial in marketing the new engine, with AMC claiming that the GM Wankel achieved good fuel economy.[33] GM's engines had not reached production, though, when the Pacer was launched onto the market. The 1973 oil crisis played a part in frustrating the use of the Wankel engine. Rising fuel prices and talk about proposed US emission standards legislation also added to concerns.

By 1974, GM R&D had not succeeded in producing a Wankel engine meeting both the emission requirements and good fuel economy, leading a decision by the company to cancel the project. Because of that decision, the R&D team only partly released the results of its most recent research, which claimed to have solved the fuel-economy problem, as well as building reliable engines with a lifespan above 530,000 miles (850,000 km). Those findings were not taken into account when the cancellation order was issued. The ending of GM's Wankel project required AMC to reconfigure the Pacer to house its venerable AMC straight-6 engine driving the rear wheels.[34]

In 1974, the Soviet Union created a special engine-design bureau, which in 1978, designed an engine designated as VAZ-311 fitted into a VAZ-2101 car.[35] In 1980, the company commenced delivery of the VAZ-411 twin-rotor Wankel engine in VAZ-2106 and Lada cars, with about 200 being manufactured. Most of the production went to the security services.[36][37] The next models were the VAZ-4132 and VAZ-415. A rotary version of the Samara was sold to Russian public from 1997. Aviadvigatel, the Soviet aircraft-engine design bureau, is known to have produced Wankel engines with electronic injection for fixed-wing aircraft and helicopters, though little specific information has surfaced.

Ford conducted research in Wankel engines, resulting in patents granted: GB 1460229, 1974, method for fabricating housings; US 3833321 1974, side plates coating; US 3890069, 1975, housing coating; CA 1030743, 1978: Housings alignment; CA 1045553, 1979, reed-valve assembly. In 1972, Henry Ford II stated that the rotary probably would not replace the piston in "my lifetime".[38]

Increasing the displacement and power of a rotary engine by adding more rotors to a basic design is simple, but a limit may exist in the number of rotors, because power output is channeled through the last rotor shaft, with all the stresses of the whole engine present at that point. For engines with more than two rotors, coupling two bi-rotor sets by a serrate coupling (such as a Hirth joint) between the two rotor sets has been tested successfully.

Research in the United Kingdom under the SPARCS (Self-Pressurising-Air Rotor Cooling System) project, found that idle stability and economy was obtained by supplying an ignitable mix to only one rotor in a multi-rotor engine in a forced-air cooled rotor, similar to the Norton air-cooled designs.

The Wankel engine's drawbacks of inadequate lubrication and cooling in ambient temperatures, short engine lifespan, high emissions and low fuel efficiencies were tackled by Norton rotary engine specialist David Garside, who developed three patented systems in 2016.[49][50]

• SPARCS
• Compact-SPARCS
• CREEV (Compound Rotary Engine for Electric Vehicles)

SPARCS and Compact-SPARCS provides superior heat rejection and efficient thermal balancing to optimise lubrication. A problem with rotary engines is that the engine housing has permanently cool and hot surfaces when running. It also generates excessive heat inside the engine which breaks down lubricating oil quickly. The SPARCS system reduces this wide differential in heat temperatures in the metal of the engine housing, and also cooling the rotor from inside the body of the engine. This results in reduced engine wear prolonging engine life. As described in Unmanned Systems Technology Magazine, "SPARCS uses a sealed rotor cooling circuit consisting of a circulating centrifugal fan and a heat exchanger to reject the heat. This is self-pressurised by capturing the blow-by past the rotor side gas seals from the working chambers."[51][52] CREEV is an ‘exhaust reactor’, containing a shaft & rotor inside, of a different shape to a Wankel rotor. The reactor, located in the exhaust stream outside of the engine's combustion chamber, consumes unburnt exhaust products without using a second ignition system before directing burnt gasses into the exhaust pipe. Horse power is given to the reactors shaft. Lower emissions and improved fuel efficiency are achieved. All three patents are currently licensed to UK-based engineers, AIE (UK) Ltd.[53][54][55][56][57]

Unlike a piston engine, in which the cylinder is heated by the combustion process and then cooled by the incoming charge, Wankel rotor housings are constantly heated on one side and cooled on the other, leading to high local temperatures and unequal thermal expansion. While this places great demands on the materials used, the simplicity of the Wankel makes it easier to use alternative materials, such as exotic alloys and ceramics. With water cooling in a radial or axial flow direction, and the hot water from the hot bow heating the cold bow, the thermal expansion remains tolerable. Top engine temperature has been reduced to 129 °C (264 °F), with a maximum temperature difference between engine parts of 18 °C (32 °F) by the use of heat pipes around the housing and in side plates as a cooling means.[40]

Among the alloys cited for Wankel housing use are A-132, Inconel 625, and 356 treated to T6 hardness. Several materials have been used for plating the housing working surface, Nikasil being one. Citroën, Mercedes-Benz, Ford, A P Grazen and others applied for patents in this field. For the apex seals, the choice of materials has evolved along with the experience gained, from carbon alloys, to steel, ferrotic, and other materials. The combination between housing plating and apex and side seals materials was determined experimentally, to obtain the best duration of both seals and housing cover. For the shaft, steel alloys with little deformation on load are preferred, the use of Maraging steel has been proposed for this.

Leaded gasoline fuel was the predominant type available in the first years of the Wankel engine's development. Lead is a solid lubricant, and leaded gasoline is designed to reduce the wearing of seal and housings. The first engines had the oil supply calculated with consideration of gasoline's lubricating qualities. As leaded gasoline was being phased out, Wankel engines needed an increased mix of oil in the gasoline to provide lubrication to critical engine parts. Experienced users advise, even in engines with electronic fuel injection, adding at least 1% of oil directly to gasoline as a safety measure in case the pump supplying oil to combustion chamber related parts failed or sucked in air. Lead tetraethyl (TEL) burns in the engine to form carbon dioxide, lead oxide and water. Since the lead oxide would deposit in the combustion chamber, TEL is used together with ethyl bromide or ethylene chloride, which convert the lead oxide into lead bromide or lead chloride, which evaporate more easily. [58] A SAE paper by David Garside extensively described Norton's choices of materials and cooling fins.

Several approaches involving solid lubricants were tested, and even the addition of LiquiMoly (containing MoS₂), at the rate of 1 cc (1 mL) per liter of fuel, is advised. Many engineers agree that the addition of oil to gasoline as in old two-stroke engines is a safer approach for engine reliability than an oil pump injecting into the intake system or directly to the parts requiring lubrication. A combined oil-in-fuel plus oil metering pump is always possible.[59]

Early engine designs had a high incidence of sealing loss, both between the rotor and the housing and also between the various pieces making up the housing. Also, in earlier model Wankel engines, carbon particles could become trapped between the seal and the casing, jamming the engine and requiring a partial rebuild. It was common for very early Mazda engines to require rebuilding after 50,000 miles (80,000 km). Further sealing problems arose from the uneven thermal distribution within the housings causing distortion and loss of sealing and compression. This thermal distortion also caused uneven wear between the apex seal and the rotor housing, evident on higher mileage engines. The problem was exacerbated when the engine was stressed before reaching operating temperature. However, Mazda rotary engines solved these initial problems. Current engines have nearly 100 seal-related parts.[1]

The problem of clearance for hot rotor apexes passing between the axially closer side housings in the cooler intake lobe areas was dealt with by using an axial rotor pilot radially inboard of the oils seals, plus improved inertia oil cooling of the rotor interior (C-W US 3261542, C. Jones, 5/8/63, US 3176915, M. Bentele, C. Jones. A.H. Raye. 7/2/62), and slightly "crowned" apex seals (different height in the center and in the extremes of seal).

The Wankel engine has problems in fuel efficiency and emissions when burning gasoline. Gasoline mixtures are slow to ignite, have a slow flame propagation speed and a higher quenching distance on the compression cycle of 2 mm compared to hydrogen's 0.6 mm. Combined, these factors waste fuel that would have created power, reducing efficiency. The gap between the rotor and the engine housing is too narrow for gasoline on the compression cycle, but sufficiently wide for hydrogen. The narrow gap is needed to create compression. When the engine uses gasoline, leftover gasoline is ejected into the atmosphere through the exhaust. This is not a problem when using hydrogen fuel, as all the fuel mixture in the combustion chamber is burnt which gives nearly no emissions and raises fuel efficiency by 23%.[60][61]

The shape of the Wankel combustion chamber is more resistant to preignition operating on lower-octane rating gasoline than a comparable piston engine.[62] The combustion chamber shape may also lead to incomplete combustion of the air-fuel charge using gasoline fuel. This would result in a larger amount of unburned hydrocarbons released into the exhaust. The exhaust is, however, relatively low in NOx emissions, because combustion temperatures are lower than in other engines, and also because of exhaust gas recirculation (EGR) in early engines. Sir Harry Ricardo showed in the 1920s that for every 1% increase in the proportion of exhaust gas in the admission mix, there is a 7 °C reduction in flame temperature. This allowed Mazda to meet the United States Clean Air Act of 1970 in 1973, with a simple and inexpensive "thermal reactor", which was an enlarged chamber in the exhaust manifold. By decreasing the air-fuel ratio, unburned hydrocarbons (HC) in the exhaust would support combustion in the thermal reactor. Piston-engine cars required expensive catalytic converters to deal with both unburned hydrocarbons and NOx emissions.

This inexpensive solution increased fuel consumption. Sales of rotary engine cars suffered because of the oil crisis of 1973 raising the price of gasoline leading to lowering of sales. Toyota discovered that injection of air into the exhaust port zone improved fuel economy reducing emissions. The best results were obtained with holes in the side plates; doing it in the exhaust duct had no noticeable influence.[47] The use of a three-stage catalysts, with air supplied in the middle, as for two-stroke piston engines, also proved beneficial meeting emissions regulations.[63]

Mazda had improved the fuel efficiency of the thermal reactor system by 40% with the RX-7 introduction in 1978. However, Mazda eventually shifted to the catalytic converter system.[6] According to the Curtiss-Wright research, the factor that controls the amount of unburned hydrocarbon in the exhaust is the rotor surface temperature, with higher temperatures producing less hydrocarbon.[64] Curtiss-Wright showed also that the rotor can be widened, keeping the rest of engine's architecture unchanged, thus reducing friction losses and increasing displacement and power output. The limiting factor for this widening was mechanical, especially shaft deflection at high rotative speeds.[65] Quenching is the dominant source of hydrocarbon at high speeds, and leakage at low speeds.[66]

Automobile Wankel rotary engines are capable of high-speed operation. However, it was shown that an early opening of the intake port, longer intake ducts, and a greater rotor eccentricity can increase torque at lower rpm. The shape and positioning of the recess in the rotor, which forms most of the combustion chamber, influences emissions and fuel economy. The results in terms of fuel economy and exhaust emissions varies depending on the shape of the combustion recess which is determined by the placement of spark plugs per chamber of an individual engine.[67]

Mazda's RX-8 car with the Renesis engine met California State fuel economy requirements, including California's low emissions vehicle (LEV) standards. This was achieved by a number of innovations. The exhaust ports, which in earlier Mazda rotaries were located in the rotor housings, were moved to the sides of the combustion chamber. This solved the problem of the earlier ash buildup in the engine, and thermal distortion problems of side intake and exhaust ports. A scraper seal was added in the rotor sides, and some ceramic parts were used in the engine. This approach allowed Mazda to eliminate overlap between intake and exhaust port openings, while simultaneously increasing the exhaust port area. The side port trapped the unburned fuel in the chamber, decreased the oil consumption, and improved the combustion stability in the low-speed and light load range. The HC emissions from the side exhaust port Wankel engine are 35–50% less than those from the peripheral exhaust port Wankel engine, because of near zero intake and exhaust port opening overlap. Peripheral ported rotary engines have a better mean effective pressure, especially at high rpm and with a rectangular shaped intake port.[68][69][70] However, the RX-8 was not improved to meet Euro 5 emission regulations and was discontinued in 2012.[71]

Mazda is still continuing development of next-generation of Wankel engines. The company is researching engine laser ignition, which eliminates conventional spark plugs, direct fuel injection, sparkless HCCI ignition and SPCCI ignition. These lead to greater rotor eccentricity (equating to a longer stroke in a reciprocating engine), with improved elasticity and low revolutions-per-minute torque. Research by T. Kohno proved that installing a glow-plug in the combustion chamber improved part load and low revolutions per minute fuel economy by 7%.[72] These innovations promise to improve fuel consumption and emissions.[73]

To improve fuel efficiency further, Mazda is looking at using the Wankel as a range-extender in series-hybrid cars, announcing a prototype, the Mazda2 EV, for press evaluation in November 2013. This configuration improves fuel efficiency and emissions. As a further advantage, running a Wankel engine at a constant speed gives greater engine life. Keeping to a near constant, or narrow band, of revolutions eliminates, or vastly reduces, many of the disadvantages of the Wankel engine.[74]

In 2015 a new system to reduce emissions and increase fuel efficiency with Wankel Engines was developed by UK-based engineers AIE (UK) Ltd, following a licensing agreement to utilise patents from Norton rotary engine creator, David Garside. The CREEV system (Compound Rotary Engine for Electric Vehicles) uses a secondary rotor to extract energy from the exhaust, consuming unburnt exhaust products while expansion occurs in the secondary rotor stage, thus reducing overall emissions and fuel costs by recouping exhaust energy that would otherwise be lost.[51] By expanding the exhaust gas to near atmospheric pressure, Garside also ensured the engine exhaust would remain cooler and quieter. AIE (UK) Ltd is now utilising this patent to develop hybrid power units for automobiles[53] and unmanned aerial vehicles.[75]

Traditional spark plugs need to be indented into the walls of the combustion chamber to enable the apex of the rotor to sweep past. As the rotor's apex seals pass over the spark plug hole, a small amount of compressed charge can be lost from the charge chamber to the exhaust chamber, entailing fuel in the exhaust, reducing efficiency, and resulting in higher emissions. These points have been overcome by using laser ignition, eliminating traditional spark plugs and removing the narrow slit in the motor housing so the rotor apex seals can fully sweep with no loss of compression from adjacent chambers. This concept has a precedent in the glow plug used by Toyota (SAE paper 790435), and the SAE paper 930680, by D. Hixon et al., on 'Catalytic Glow Plugs in the JDTI Stratified Charge Rotary Engine'. The laser plug can fire through the narrow slit. Laser plugs can also fire deep into the combustion chamber using multiple lasers. So, a higher compression ratio is permitted. Direct fuel injection, to which the Wankel engine is suited, combined with laser ignition in single or multiple laser plugs, has been shown to enhance the motor even further reducing the disadvantages.[73][76][77]

Homogeneous charge compression ignition (HCCI) involves the use of a pre-mixed lean air-fuel mixture being compressed to the point of auto-ignition, so electronic spark ignition is eliminated. Gasoline engines combine homogeneous charge (HC) with spark ignition (SI), abbreviated as HCSI. Diesel engines combine stratified charge (SC) with compression ignition (CI), abbreviated as SCCI. HCCI engines achieve gasoline engine-like emissions with compression ignition engine-like efficiency, and low levels of nitrogen oxide emissions (NO_x) without a catalytic converter. However, unburned hydrocarbon and carbon monoxide emissions still require treatment to conform with automotive emission regulations.

Mazda has undertaken research on HCCI ignition for its SkyActiv-R rotary engine project, using research from its SkyActiv Generation 2 program. A constraint of rotary engines is the need to locate the spark plug outside the combustion chamber to enable the rotor to sweep past. Mazda confirmed that the problem had been solved in the SkyActiv-R project. Rotaries generally have high compression ratios, making them particularly suitable for the use of HCCI.[78][79][80]

SPCCI incorporates spark and compression ignition. A spark is always used, to control exactly when combustion occurs. Depending on the load, it may be only spark ignition, other times SPCCI.

The spark ignites a small pulse of richer mixture injected into the combustion chamber. A fireball is created, acting like an air piston, and increasing the pressure and temperature. Compression-ignition of the very lean mixture occurs, with a rapid and even and complete burn leading to a more powerful cycle. The compression-ignition aspect makes the lean burn possible, improving engine efficiency up to 20–30%. It gives a rotary the ability to switch from the ideal, stoichiometric, 14.7:1 air-to-fuel mixture of a conventional gasoline burning engine to the lean-burn mixture of over 29.4:1. The engine is in lean-burn mode about 80% of running time. The combustion timing is controlled by the flame from the spark plug.

According to Mazda, SPCCI combines advantages of both petrol and diesel engines and gives high efficiency across a wide range of rpms and engine loads. Combined with a supercharger the compression ignition delivers an increase in torque of 20–30%.[81][82]

Rolls Royce R6 two stage rotary compression ignition engine

Rolls-Royce R1C compression ignition prototype.

Research has been undertaken into rotary compression ignition engines and the burning of diesel heavy fuel using spark ignition. The basic design parameters of the Wankel engine preclude obtaining a compression ratio higher than 15:1 or 17:1 in a practical engine, but attempts are continuously being made to produce a compression-ignition Wankel. The Rolls-Royce[83] and Yanmar compression-ignition[84] approach was to use a two-stage unit, with one rotor acting as compressor, while combustion takes place in the other.[85] Conversion of a standard 294-cc-chamber spark-ignition unit to use heavy fuel was described in SAE paper 930682, by L. Louthan. SAE paper 930683 (BSFC 330 g/KWhr), by D. Eiermann, resulted in the Wankel SuperTec line of spark ignition rotary engines (BSFC 270-310 g/KWhr),[86] not much less than motor 250/400 by Rudolf Diesel of the year 1897. But the Curtiss-Wright RC2-47 with stratified charge injection achieves consumption values around 226g/kWh which is on par with the common rail diesel engine MTU MB 873-Ka 501.[87][88]

Compression-ignition engine research is being undertaken by Pratt & Whitney Rocketdyne, which was commissioned by DARPA to develop a compression-ignition Wankel engine for use in a prototype VTOL flying car called the "Transformer".[89][90][91][92] The engine, based on an earlier concept involving an unmanned aerial vehicle called "Endurocore", powered by a Wankel diesel.[93] plans to utilize Wankel rotors of varying sizes on a shared eccentric shaft to increase efficiency.[94] The engine is claimed to be a 'full-compression, full-expansion, compression-ignition-cycle engine'. An October 28, 2010 patent by Pratt & Whitney Rocketdyne, describes a Wankel engine superficially similar to Rolls-Royce's earlier prototype, that required an external air compressor to achieve high enough compression for compression-ignition-cycle combustion.[95][96] The design differs from Rolls-Royce's compression-ignition rotary, mainly by proposing an injector both in the exhaust passage between the combustor rotor and expansion rotor stages, and an injector in the expansion rotor's expansion chamber, for 'afterburning'.

The British company Rotron, which specialises in unmanned aerial vehicle (UAV) applications of Wankel engines, has designed and built a unit to operate on heavy fuel for NATO purposes. The engines uses spark ignition. The prime innovation is flame propagation, ensuring the flame burns smoothly across the whole combustion chamber. The fuel is pre-heated to 98 degrees Celsius before it is injected into the combustion chamber. Four spark plugs are utilised, aligned in two pairs. Two spark plugs ignite the fuel charge at the front of the rotor as it moves into the combustion section of the housing. As the rotor moves the fuel charge, the second two fire a fraction of second behind the first pair of plugs, igniting near the rear of the rotor at the back of the fuel charge. The drive shaft is water cooled which also has a cooling effect on the internals of the rotor. Cooling water also flows around the external of the engine through a gap in the housing, thus reducing the heat of the engine from outside and inside eliminating hot spots.[97]

Mazda RX-8 Hydrogen RE hydrogen fuelled rotary engined car

Using hydrogen fuel in Wankel engines improved efficiency by 23% over gasoline fuel with near zero emissions.[60] Four-stroke reciprocating piston Otto cycle engines are not well suited for conversion to hydrogen fuel. The hydrogen/air fuel mix can misfire on hot parts of the engine like the exhaust valve and spark plugs, as all four stroke operations occur in the same chamber.[98]

As a hydrogen/air fuel mixture is quicker to ignite with a faster burning rate than gasoline, an important issue of hydrogen internal combustion engines is to prevent pre-ignition and backfire. In a rotary engine each pulse of the Otto cycle occurs in different chambers. The rotary has no exhaust valves that may remain hot and produce the backfire that occurs in reciprocating piston engines. Importantly, the intake chamber is separated from the combustion chamber, keeping the air/fuel mixture away from localized hot spots. These structural features of the rotary engine enable the use of hydrogen without pre-ignition and backfire.

A Wankel engine has stronger flows of air-fuel mixture and a longer operating cycle than a reciprocating piston engine, achieving a thorough mixing of hydrogen and air. The result is a homogeneous mixture with no hot spots in the engine, which is crucial for hydrogen combustion.[99] Hydrogen/air fuel mixtures are quicker to ignite than gasoline mixtures with a high burning rate, resulting in all the fuel being burnt with no unburnt fuel being ejected into the exhaust stream as is the case using gasoline fuel in rotary engines. Emissions are near zero, even with oil lubrication of apex seals.

Another problem concerns the hydrogenate attack on the lubricating film in reciprocating engines. In a Wankel engine the problem of a hydrogenate attack is circumvented by using ceramic apex seals.[100][101]

All these points lend the Wankel engine as ideal for hydrogen fuel burning. Mazda built and sold a vehicle that took advantage of the rotary's suitability to hydrogen fuel, a dual-fuel Mazda RX-8 Hydrogen RE that could switch on the fly from gasoline to hydrogen and back.[102][61]

Mazda 787B

In the racing world, Mazda has had substantial success with two-rotor, three-rotor, and four-rotor cars. Private racers have also had considerable success with stock and modified Mazda Wankel-engine cars.[123]

The Sigma MC74 powered by a Mazda 12A engine was the first engine and only team from outside Western Europe or the United States to finish the entire 24 hours of the 24 Hours of Le Mans race, in 1974. Yojiro Terada was the driver of the MC74. Mazda was the first team from outside Western Europe or the United States to win Le Mans outright. It was also the only non-piston engined car to win Le Mans, which the company accomplished in 1991 with their four-rotor 787B (2.622 L or 160 cu in—actual displacement, rated by FIA formula at 4.708 L or 287 cu in). In the C2 class, all participants had only the same amount of fuel at their disposal, besides the unregulated C1 Category 1 . In the category C1 was only naturally aspirated engines allowed. The Mazdas were allowed as naturally aspirated to start with 830 kg weight, 170 kg less than the supercharged competitors.[124] The cars under the Group C1 Category 1 regulations for 1991 were allowed to be another 80 kg lighter than the 787B.[125] In addition, Group C1 Category 1 had only permitted 3,5 liter naturally aspirated engines and had no fuel quantity limits.[126]

Formula Mazda Racing features open-wheel race cars with Mazda Wankel engines, adaptable to both oval tracks and road courses, on several levels of competition. Since 1991, the professionally organized Star Mazda Series has been the most popular format for sponsors, spectators, and upward bound drivers. The engines are all built by one engine builder, certified to produce the prescribed power, and sealed to discourage tampering. They are in a relatively mild state of racing tune, so that they are extremely reliable and can go years between motor rebuilds.[127]

The Malibu Grand Prix chain, similar in concept to commercial recreational kart racing tracks, operates several venues in the United States where a customer can purchase several laps around a track in a vehicle very similar to open wheel racing vehicles, but powered by a small Curtiss-Wright rotary engine.

In engines having more than two rotors, or two rotor race engines intended for high-rpm use, a multi-piece eccentric shaft may be used, allowing additional bearings between rotors. While this approach does increase the complexity of the eccentric shaft design, it has been used successfully in the Mazda's production three-rotor 20B-REW engine, as well as many low volume production race engines. The C-111-2 4 Rotor Mercedes-Benz eccentric shaft for the KE Serie 70, Type DB M950 KE409 is made in one piece. Mercedes-Benz used split bearings.

Norton Interpol2 prototype

The small size and attractive power to weight ratio of the Wankel engine appealed to motorcycle manufacturers. The first Wankel-engined motorcycle was the 1960 'IFA/MZ KKM 175W' built by German motorcycle manufacturer MZ, licensed by NSU.[128]

In 1972, Yamaha introduced the RZ201 at the Tokyo Motor Show, a prototype with a Wankel engine, weighing 220 kg and producing 60 hp (45 kW) from a twin-rotor 660-cc engine (US patent N3964448). In 1972, Kawasaki presented its two-rotor Kawasaki X99 rotary engine prototype (US patents N 3848574 &3991722). Both Yamaha and Kawasaki claimed to have solved the problems of poor fuel economy, high exhaust emissions, and poor engine longevity, in early Wankels, but neither prototype reached production.

In 1974, Hercules produced W-2000 Wankel motorcycles, but low production numbers meant the project was unprofitable, and production ceased in 1977.[129]

From 1975 to 1976, Suzuki produced its RE5 single-rotor Wankel motorcycle. It was a complex design, with both liquid cooling and oil cooling, and multiple lubrication and carburetor systems. It worked well and was smooth, but being rather heavy, and having a modest power output of 62 hp (46 kW), it did not sell well.[130]

Dutch motorcycle importer and manufacturer Van Veen produced small quantities of a dual-rotor Wankel-engined OCR-1000 motorcycle between 1978 and 1980, using surplus Comotor engines. The engine of the OCR 1000, used a re-purposed engine originally intended for the Citroën GS Birotor car.[131]

In the early 1980s, using earlier work at BSA, Norton produced the air-cooled twin-rotor Classic, followed by the liquid-cooled Commander and the Interpol2 (a police version).[132] Subsequent Norton Wankel bikes included the Norton F1, F1 Sports, RC588, Norton RCW588, and NRS588. Norton proposed a new 588-cc twin-rotor model called the "NRV588" and a 700-cc version called the "NRV700".[133] A former mechanic at Norton, Brian Crighton, started developing his own rotary engined motorcycles line named "Roton", which won several Australian races.

Despite successes in racing,[134] no motorcycles powered by Wankel engines have been produced for sale to the general public for road use since 1992.

The two different design approaches, taken by Suzuki and BSA may usefully be compared. Even before Suzuki produced the RE5, in Birmingham BSA's research engineer David Garside, was developing a twin-rotor Wankel motorcycle. BSA's collapse put a halt to development, but Garside's machine eventually reached production as the Norton Classic.

Wankel engines run very hot on the ignition and exhaust side of the engine's trochoid chamber, whereas the intake and compression parts are cooler. Suzuki opted for a complicated oil-cooling and water cooling system, with Garside reasoning that provided the power did not exceed 80 hp (60 kW), air-cooling would suffice. Garside cooled the interior of the rotors with filtered ram-air. This very hot air was cooled in a plenum contained within the semi-monocoque frame and afterwards, once mixed with fuel, fed into the engine. This air was quite oily after running through the interior of the rotors, and thus was used to lubricate the rotor tips. The exhaust pipes become very hot, with Suzuki opting for a finned exhaust manifold, twin-skinned exhausted pipes with cooling grilles, heatproof pipe wrappings and silencers with heat shields. Garside simply tucked the pipes out of harm's way under the engine, where heat would dissipate in the breeze of the vehicle's forward motion. Suzuki opted for complicated multi-stage carburation, whilst Garside choose simple carburetors. Suzuki had three lube systems, whilst Garside had a single total-loss oil injection system which was fed to both the main bearings and the intake manifolds. Suzuki chose a single rotor that was fairly smooth, but with rough patches at 4,000 rpm; Garside opted for a turbine-smooth twin-rotor motor. Suzuki mounted the massive rotor high in the frame, but Garside put his rotors as low as possible to lower the center of gravity of the motorcycle.[135]

Although it was said to handle well, the result was that the Suzuki was heavy, overcomplicated, expensive to manufacture, and (at 62 bhp) a little short on power. Garside's design was simpler, smoother, lighter and, at 80 hp (60 kW), significantly more powerful.[136]

Wankel RC2-60 Aeronautical Rotary Engine

ARV Super2 with the British MidWest AE110 twin-rotor Wankel engine

Diamond DA20 with a Diamond Engines Wankel

Sikorsky Cypher Unmanned aerial vehicle (UAV) powered with a UEL AR801 Wankel engine

In principle, Wankel engines are ideal for light aircraft, being light, compact, almost vibrationless, and with a high power-to-weight ratio. Further aviation benefits of a Wankel engine include:

1. Rotors cannot seize, since rotor casings expand greater than rotors;
2. The engine is less prone to the serious condition known as "engine-knock", which can destroy a plane's piston engines in mid-flight.
3. The engine is not susceptible to "shock-cooling" during descent;
4. The engine does not require an enriched mixture for cooling at high power;
5. Having no reciprocating parts, there is less vulnerability to damage when the engine revolves at a higher rate than the designed maximum. The limit to the revolutions is the strength of the main bearings.

Unlike cars and motorcycles, a Wankel aero-engine will be sufficiently warm before full power is asked of it because of the time taken for pre-flight checks. Also, the journey to the runway has minimum cooling, which further permits the engine to reach operating temperature for full power on take-off.[137] A Wankel aero-engine spends most of its operational time at high power outputs, with little idling. This makes ideal the use of peripheral ports. An advantage is that modular engines with more than two rotors are feasible, without increasing the frontal area. Should icing of any intake tracts be an issue, there is plenty of waste engine heat available to prevent icing.

The first Wankel rotary-engine aircraft was in the late 1960s being the experimental Lockheed Q-Star civilian version of the United States Army's reconnaissance QT-2, essentially a powered Schweizer sailplane. The plane was powered by a 185 hp (138 kW) Curtiss-Wright RC2-60 Wankel rotary engine. The same engine model was also used in a Cessna Cardinal and a helicopter, as well as other airplanes.[20][138][139] In Germany in the mid-1970s, a pusher ducted fan airplane powered by a modified NSU multi-rotor Wankel engine was developed in both civilian and military versions, Fanliner and Fantrainer.

At roughly the same time as the first experiments with full-scale aircraft powered with Wankel engines, model aircraft-sized versions were pioneered by a combine of the well-known Japanese O.S. Engines firm and the then-extant German Graupner aeromodeling products firm, under license from NSU/Auto-Union. By 1968, the first prototype air-cooled, single-rotor glow plug-ignition, methanol-fueled 4.9 cm³ displacement OS/Graupner model Wankel engine was running, and was produced in at least two differing versions from 1970 to the present day,[140] solely by the O.S. firm after Graupner's demise in 2012.[141]

Aircraft Wankel engines are increasingly being found in roles where the compact size, high power-to-weight ratio and quiet operation are important, notably in drones and unmanned aerial vehicles. Many companies and hobbyists adapt Mazda rotary engines, taken from cars, to aircraft use. Others, including Wankel GmbH itself, manufacture Wankel rotary engines dedicated for that purpose.[142][143] One such use is the "Rotapower" engines in the Moller Skycar M400. Another example of purpose-built aircraft rotaries are Austro Engine's 55 hp (41 kW) AE50R (certified) and 75 hp (56 kW) AE75R (under development) both appr. 2 hp/kg.[144]

Wankel engines have been fitted in homebuilt experimental aircraft, such as the ARV Super2, a couple of which were powered by the British MidWest aero-engine. Most are Mazda 12A and 13B automobile engines, converted to aviation use. This is a very cost-effective alternative to certified aircraft engines, providing engines ranging from 100 to 300 horsepower (220 kW) at a fraction of the cost of traditional piston engines. These conversions were initially in the early 1970s. With a number of these engines mounted on aircraft, as of 10 December 2006 the National Transportation Safety Board has only seven reports of incidents involving aircraft with Mazda engines, and none of these were a failure due to design or manufacturing flaws.

Peter Garrison, contributing editor for Flying magazine, has said that "in my opinion ... the most promising engine for aviation use is the Mazda rotary."[145] Mazda rotaries have worked well when converted for use in homebuilt aircraft. However, the real challenge in aviation is to produce FAA-certified alternatives to the standard reciprocating engines that power most small general aviation aircraft. Mistral Engines, based in Switzerland, developed purpose-built rotaries for factory and retrofit installations on certified production aircraft. The G-190 and G-230-TS rotary engines were already flying in the experimental market, and Mistral Engines hoped for FAA and JAA certification by 2011. As of June 2010, G-300 rotary engine development ceased, with the company citing cash flow problems.[146]

Mistral claims to have overcome the challenges of fuel consumption inherent in the rotary, at least to the extent that the engines are demonstrating specific fuel consumption within a few points of reciprocating engines of similar displacement. While fuel burn is still marginally higher than traditional engines, it is outweighed by other beneficial factors.[147][148]

At the price of increased complication for a high pressure diesel type injection system, fuel consumption in the same range as small pre-chamber automotive and industrial diesels has been demonstrated with Curtiss-Wright's stratified charge multi-fuel engines, while preserving Wankel rotary advantages[149] Unlike a piston and overhead valve engine, there are no valves which can float at higher rpm causing loss of performance. The Wankel is a more effective design at high revolutions with no reciprocating parts, far fewer moving parts and no cylinder head.[150]

The French company Citroën had developed Wankel powered RE-2 helicopter in the 1970s.[151]

Since Wankel engines operate at a relatively high rotational speed, at 6,000 rpm of output shaft, the Rotor makes only 2,000 turns. With relatively low torque, propeller driven aircraft must use a propeller speed reduction unit to maintain propellers within the designed speed range. Experimental aircraft with Wankel engines use propeller speed reduction units, for instance the MidWest twin-rotor engine has a 2.95:1 reduction gearbox. The rotational shaft speed of a Wankel engine is high compared to reciprocating piston designs. Only the eccentric shaft spins fast, while the rotors turn at exactly one-third of the shaft speed. If the shaft is spinning at 7,500 rpm, the rotors are turning at a much slower 2,500 rpm.

Pratt & Whitney Rocketdyne has been commissioned by DARPA to develop a diesel Wankel engine for use in a prototype VTOL flying car called the "Transformer".[89][90][91][92] The engine, based on an earlier unmanned aerial vehicle Wankel diesel concept called "Endurocore".[93]

The sailplane manufacturer Schleicher uses an Austro Engines AE50R Wankel[152][153] in its self-launching models ASK-21 Mi, ASH-26E,[154] ASH-25 M/Mi, ASH-30 Mi, ASH-31 Mi, ASW-22 BLE, and ASG-32 Mi.

In 2013, e-Go aeroplanes, based in Cambridge, United Kingdom, announced that its new single-seater canard aircraft, the winner of a design competition to meet the new UK single-seat deregulated category, will be powered by a Wankel engine from Rotron Power, a specialist manufacturer of advanced rotary engines for unmanned aeronautical vehicle (UAV) applications. The first sale was 2016. The aircraft is expected to deliver 100 knots (190 km/h; 120 mph) cruise speed from a 30 hp (22 kW) Wankel engine, with a fuel economy of 75 mpg_‑imp (3.8 L/100 km; 62 mpg_‑US) using standard motor gasoline (MOGAS), developing 22 kW (30 hp).[155]

The DA36 E-Star, an aircraft designed by Siemens, Diamond Aircraft and EADS, employs a series hybrid powertrain with the propeller being turned by a Siemens 70 kW (94 hp) electric motor. The aim is to reduce fuel consumption and emissions by up to 25%. An onboard 40 hp (30 kW) Austro Engines Wankel rotary engine and generator provides the electricity. A propeller speed reduction unit is eliminated. The electric motor uses electricity stored in batteries, with the generator engine off, to take off and climb reducing sound emissions. The series-hybrid powertrain using the Wankel engine reduces the weight of the plane by 100 kg compared with its predecessor. The DA36 E-Star first flew in June 2013, making this the first ever flight of a series-hybrid powertrain. Diamond Aircraft state that the technology using Wankel engines is scalable to a 100-seat aircraft.[156][157]

Structure of a series-hybrid vehicle. The grey square represents a differential gear. An alternative arrangement (not shown) is to have electric motors at two or four wheels.

Mazda2 EV prototype

Due to the compact size and the high power to weight ratio of a Wankel engine, it has been proposed for electric vehicles as range extenders to provide supplementary power when electric battery levels are low. There have been a number of concept cars incorporating a series hybrid powertrain arrangement. A Wankel engine used only as a generator has packaging, noise, vibration and weight distribution advantages when used in a vehicle, maximizing interior passenger and luggage space. The engine/generator may be at one end of the vehicle with the electric driving motors at the other, connected only by thin cables. Mitsueo Hitomi the global powertrain head of Mazda stated, "a rotary engine is ideal as a range extender because it is compact and powerful, while generating low-vibration".[158]

In 2010, Audi unveiled a prototype series-hybrid electric car, the A1 e-tron, that incorporated a small 250-cc Wankel engine, running at 5,000 rpm, which recharged the car's batteries as needed, and provided electricity directly to the electric driving motor.[159][160] In 2010, FEV Inc said that in their prototype electric version of the Fiat 500, a Wankel engine would be used as a range extender.[161] In 2013, Valmet Automotive of Finland revealed a prototype car named the EVA, incorporating a Wankel powered series-hybrid powertrain car, utilizing an engine manufactured by the German company Wankel SuperTec.[162] The UK company, Aixro Radial Engines, offers a range extender based on the 294cc-chamber go-kart engine.[163]

Mazda of Japan ceased production of direct drive Wankel engines within their model range in 2012, leaving the motor industry worldwide with no production cars using the engine. The company is continuing development of the next generation of their Wankel engines, the SkyActiv-R. Mazda states that the SkyActiv-R solves the three key issues with previous rotary engines: fuel economy, emissions and reliability.[24][164][165] Takashi Yamanouchi, the global CEO of Mazda said: "The rotary engine has very good dynamic performance, but it's not so good on economy when you accelerate and decelerate. However, with a range extender you can use a rotary engine at a constant 2,000rpm, at its most efficient. It's compact, too."[74] No Wankel engine in this arrangement has yet been used in production vehicles or planes. However, in November 2013 Mazda announced to the motoring press a series-hybrid prototype car, the Mazda2 EV, using a Wankel engine as a range extender. The generator engine, located under the rear luggage floor, is a tiny, almost inaudible, single-rotor 330-cc unit, generating 30 hp (22 kW) at 4,500 rpm, and maintaining a continuous electric output of 20 kW.[166][167][168] In October 2017, Mazda announced that the rotary engine would be utilised in a hybrid car with 2019/20 the targeted introduction dates.[169][158][170]

Mazda has undertaken research on Spark Controlled Compression Ignition (SPCCI) ignition on rotary engines stating that any new rotary engines will incorporate SPCCI. SPCCi incorporates spark and compression ignition combining the advantages of gasoline and diesel engines to achieve environmental, power and fuel consumption goals.[82]

Mazda confirmed that a rotary equipped range extended car would be launched a year late in 2020. There may be a choice of a larger battery bank to give full EV running with battery charging from the grid, with the engine performing the dual functions of a range-extender and battery charger when the battery charge is too low. When running on the engine, the electric motor is used to assist in acceleration and take off from stationary.[171][26]

UEL UAV-741 Wankel engine for a UAV

Small Wankel engines are being found increasingly in other applications, such as go-karts,[172][173] personal water craft, and auxiliary power units for aircraft.[174][175] Kawasaki patented mixture-cooled rotary engine (US patent 3991722). Japanese diesel engine manufacturer Yanmar and Dolmar-Sachs of Germany had a rotary-engined chain saw (SAE paper 760642) and outboard boat engines, and the French Outils Wolf, made a lawnmower (Rotondor) powered by a Wankel rotary engine. To save on production costs, the rotor was in a horizontal position and there were no seals in the down side. The Graupner/O.S. 49-PI is a 1.27 hp (950 W) 5-cc Wankel engine for model airplane use, which has been in production essentially unchanged since 1970. Even with a large muffler, the entire package weighs only 380 grams (13 oz).[176][177]

The simplicity of the Wankel engine makes it well-suited for mini, micro, and micro-mini engine designs. The Microelectromechanical systems (MEMS) Rotary Engine Lab at the University of California, Berkeley, has previously undertaken research towards the development of Wankel engines of down to 1 mm in diameter, with displacements less than 0.1 cc. Materials include silicon and motive power includes compressed air. The goal of such research was to eventually develop an internal combustion engine with the ability to deliver 100 milliwatts of electrical power; with the engine itself serving as the rotor of the generator, with magnets built into the engine rotor itself.[178][179] Development of the miniature Wankel engine stopped at UC Berkeley at the end of the DARPA contract. Miniature Wankel engines struggled to maintain compression due to sealing problems, similar to problems observed in the large scale versions. In addition, miniature engines suffer from an adverse surface to volume ratio causing excess heat losses; the relatively large surface area of the combustion chamber walls transfers away what little heat is generated in the small combustion volume resulting in quenching and low efficiency.

Ingersoll-Rand built the largest-ever Wankel engine, with two rotors, which was available between 1975 and 1985, producing 1,100 hp (820 kW). A one rotor version was available producing 550 hp (410 kW). The displacement per rotor was 41 liters, with each rotor being approximately one meter in diameter. The engine was derived from a previous, unsuccessful Curtiss-Wright design, which failed because of a well-known problem with all internal combustion engines: the fixed speed at which the flame front travels limits the distance combustion can travel from the point of ignition in a given time, thereby limiting the maximum size of the cylinder or rotor chamber which can be used. This problem was solved by limiting the engine speed to only 1200 rpm (400 kW per rotor)[180] and the use of natural gas as fuel. That was particularly well chosen, since one of the major uses of the engine was to drive compressors on natural gas pipelines.[181] The engines used "homogeneous charge", and some ran for over 30,000 hours.[180]

John Deere acquired the Curtiss-Wright rotary division in February 1984, also making large multi-fuel prototypes, some with an 11-liter rotor for large vehicles.[180][150][182] The developers attempted to solve the flame front issue (which hampered Ingersoll-Rand's homogeneous combustion) by stratifying the fuel in the combustion chamber.[180] The technology was transferred to RPI in 1991.[183][184]

Yanmar of Japan produced some small, charge-cooled rotary engines for chainsaws and outboard engines.[185] One of its products is the LDR (rotor recess in the leading edge of combustion chamber) engine, which has better exhaust emissions profiles, and reed-valve controlled intake ports, which improve part-load and low rpm performance.[186]

In 1971 and 1972, Arctic Cat produced snowmobiles powered by Sachs KM 914 303-cc and KC-24 294-cc Wankel engines made in Germany.

In the early 1970s, Outboard Marine Corporation sold snowmobiles under the Johnson and other brands, which were powered by 35 or 45 hp (26 or 34 kW) OMC engines.

Aixro of Germany produces and sells a go-kart engine, with a 294-cc-chamber charge-cooled rotor and liquid-cooled housings. Other makers are: Wankel AG, Cubewano, Rotron and Precision Technology USA.

The American M1A3 Abrams tank may use an rotary Diesel APU,[187] developed by the TARDEC US Army lab. It has a high-power-density 330-cc rotary engine, modified to operate with various fuels such as standard military JP-8 jet fuel.[188]

Ogura Wankel Air conditioning system compressor

In addition for use as an internal combustion engine, the basic Wankel design has also been used for gas compressors, and superchargers for internal combustion engines, but in these cases, although the design still offers advantages in reliability, the basic advantages of the Wankel in size and weight over the four-stroke internal combustion engine are irrelevant. In a design using a Wankel supercharger on a Wankel engine, the supercharger is twice the size of the engine.

The Wankel design is used in the seat belt pre-tensioner system[189] in some Mercedes-Benz[190] and Volkswagen[191] cars. When the deceleration sensors detect a potential crash, small explosive cartridges are triggered electrically, and the resulting pressurized gas feeds into tiny Wankel engines which rotate to take up the slack in the seat belt systems, anchoring the driver and passengers firmly in the seat before a collision.[192]