Editing Wankel engine (section)

==Engineering==
{{Multiple image
| direction = vertical
| width = 250
| header = Seals, cooling
| image1 = ApexSeals.jpg
| caption1 = '''Figure 14.'''<br/>Apex seals, left [[NSU Ro 80]]; right Mazda 12A and 13B
| image2 = W-AR-Cooling.jpg
| caption2 = '''Figure 15.'''<br/>{{ubl
 |''Left'': Mazda L10A camber axial water cooling
 |''Middle'': Audi NSU EA871 axial water cooling, hot bow only
 |''Right'': Diamond Engines Wankel radial water cooling, hot bow only
}}}}

Felix Wankel managed to overcome most of the problems that made prior attempts to perfect the rotary engines fail, by developing a configuration with vane seals having a tip radius equal to the amount of "oversize" of the rotor housing form, relative to the theoretical epitrochoid, to minimize radial apex seal motion plus introducing a cylindrical gas-loaded apex pin which abutted all sealing elements to seal around the three planes at each rotor apex.<ref name="fAvRr">{{Citation|url= http://www.freedom-motors.com/history.html |work=Moller Freedom Motors, formerly Outboard Marine Corporation (Evinrude/Johnson) Rotary engines |title=Moller Skycar |url-status=dead |archive-url= https://web.archive.org/web/20150813225722/http://www.freedom-motors.com/history.html |archive-date=August 13, 2015}}</ref>

In the early days, unique, dedicated production machines had to be built for different housing dimensional arrangements. However, patented designs such as {{US patent|3824746}}, G.&nbsp;J. Watt, 1974, for a "Wankel Engine Cylinder Generating Machine", {{US patent|3916738}}, "Apparatus for machining and/or treatment of trochoidal surfaces" and {{US patent|3964367}}, "Device for machining trochoidal inner walls", and others, solved the problem.

Wankel engines have a problem not found in reciprocating piston four-stroke engines in that the block housing has intake, compression, combustion, and exhaust occurring at fixed locations around the housing. This causes a very uneven thermal load on the rotor housing.<ref name="Bensinger 1973 p. 110">{{cite book |last1=Bensinger |first1=Wolf-Dieter |title=Rotationskolben-Verbrennungsmotoren |place=Berlin, Heidelberg, New York |date=1973 |isbn=978-3-540-05886-1 |oclc=251737493 |language=de |page=110}}</ref> In contrast, four-stroke reciprocating engines perform these four strokes in one chamber, so that extremes of "freezing" intake and "flaming" exhaust are averaged and shielded by a boundary layer from overheating working parts. The University of Florida proposed the use of heat pipes in an air-cooled Wankel to overcome this uneven heating of the block housing.<ref name="SAE paper 2014-01-2160">SAE paper 2014-01-2160</ref> Pre-heating of certain housing sections with exhaust gas improved performance and fuel economy, also reducing wear and emissions.<ref name="fpsQZ">{{citation|title=Rotary Engine |first1=Kenichi |last1=Yamamoto |publisher=Toyo Kogyo |date= 1971 |pages=65–66 |url= https://www.academia.edu/45641685 |access-date=11 February 2024}}</ref>

The boundary layer shields and the oil film act as thermal insulation, leading to a low temperature of the lubricating film (approximate maximum {{convert|200|°C|°F|disp=or|-1}} on a water-cooled Wankel engine). This gives a more constant surface temperature. The temperature around the spark plug is about the same as in the combustion chamber of a reciprocating engine. With circumferential or axial flow cooling, the temperature difference remains tolerable.<ref name="8AMI9">{{citation |year=1971 |title=Rotary Engine |first1=Kenichi |last1=Yamamoto |publisher=Toyo Kogyo |at=p. 67 Fig 5.10, 11}}</ref><ref name="rTZvi">{{Citation |year=1981 |title=Rotary Engine |first1=Kenichi |last1=Yamamoto |publisher=Toyo Kogyo |at=pp. 32, 33 Fig. 3.39–41}}</ref><ref name="3mp3k">{{citation |first1=Richard F |last1=Ansdale |title=Der Wankelmotor |language=de |publisher=Motorbuch-Verlag |pages=141–50}}</ref>

Problems arose during research in the 1950s and 1960s. For a while, engineers were faced with what they called "chatter marks" and "devil's scratch" in the inner epitrochoid surface, resulting in chipping of the chrome coating of the trochoidal surfaces. They discovered that the cause was the apex seals reaching a resonating vibration, and the problem was solved by reducing the thickness and weight of the apex seals as well as using more suitable materials. Scratches disappeared after introducing more compatible materials for seals and housing coatings. Yamamoto experimentally lightened apex seals with holes. Now, weight was identified as the main cause. Mazda then used aluminum-impregnated carbon apex seals in their early production engines. NSU used carbon antimony-impregnated apex seals against chrome. NSU developed ELNISIL coating to production maturity and returned to a metal sealing strip for the RO80. Mazda continued to use chrome, but provided the aluminum housing with a steel jacket, which was then coated with a thin dimensional galvanized chrome layer. This allowed Mazda to return to the 3mm and later even 2mm thick metal apex seals.<ref>Yamamoto, Kenichi (1971). Rotary Engine. Toyo Kogyo. Page 60-61</ref> Another early problem was the build-up of cracks in the stator surface near the plug hole, which was eliminated by installing the spark plugs in a separate metal insert/ copper sleeve in the housing instead of a plug being screwed directly into the block housing.<ref name="zZeTy">{{citation|title=The Wankel Engine: NSU and Citroën develops the Wankel |first1=Jan P. |last1=Norbye |pages=139 and 305 |publisher=Chilton |date=1971 |isbn=0-8019-5591-2}}</ref>

Toyota found that substituting a glow-plug for the leading site spark plug improved low rpm, part load, specific fuel consumption by 7%, and emissions and idle.<ref name="SAE paper 790435">SAE paper 790435</ref> A later alternative solution to spark plug boss cooling was provided with a variable coolant velocity scheme for water-cooled rotaries, which has had widespread use, being patented by Curtiss-Wright,<ref name="kM022">{{patent|US|3007460}}, M. Bentele, C. Jones, F. P. Sollinger, 11/7/61 and {{patent|US|3155085}}, C. Jones, R. E. Mount, 4/29/63 and {{patent|US|3196850}}, C. Jones, 7/27/65</ref> with the last-listed for better air-cooled engine spark plug boss cooling. These approaches did not require a high-conductivity copper insert, but did not preclude its use. Ford tested a Wankel engine with the plugs placed in the side plates, instead of the usual placement in the housing working surface ({{patent|CA|1036073}}, 1978).

===Torque delivery===
Wankel engines are capable of high-speed operation, meaning they do not necessarily need to produce high torque to produce high power. The positioning of the intake port and intake port closing greatly affect the engine's torque production. Early closing of the intake port increases low-end torque, but reduces high-end torque (and thus power). In contrast, late closing of the intake port reduces low-end torque while increasing torque at high engine speeds, thus resulting in more power at higher engine speeds.<ref name="Bensinger 1973 p. 75">{{cite book |last1=Bensinger |first1=Wolf-Dieter |title=Rotationskolben-Verbrennungsmotoren |place=Berlin, Heidelberg, New York |date=1973 |isbn=978-3-540-05886-1 |oclc=251737493 |language=de |page=75}}</ref>

A peripheral intake port gives the highest [[mean effective pressure]]; however, side intake porting produces a more steady idle,<ref name="J6imC">Yamamoto, Kenichi. ''Rotary engine'', fig 4.26 & 4.27, Mazda, 1981, p. 46.</ref> because it helps to prevent blow-back of burned gases into the intake ducts, which cause "misfirings" caused by alternating cycles where the mixture ignites and fails to ignite. Peripheral porting (PP) gives the best mean effective pressure throughout the rpm range, but PP was also linked to worse idle stability and part-load performance. Early work by Toyota<ref name="SAE790435">{{citation |first1=T |last1= Kohno |publisher= Toyota |title= SAE paper 790435|display-authors=etal}}</ref> led to the addition of a fresh air supply to the exhaust port. It also proved that a Reed-valve in the intake port or ducts<ref name="pKO20">SAE paper 720466, Ford 1979 patent {{patent|CA|1045553}}</ref> improved the low rpm and partial load performance of Wankel engines, by preventing blow-back of exhaust gas into the intake port and ducts, and reducing the misfire-inducing high EGR, at the cost of a slight loss of power at top rpm. Elasticity is improved with a greater rotor eccentricity, analogous to a longer stroke in a reciprocating engine.

Wankel engines operate better with a low-pressure exhaust system. Higher exhaust back pressure reduces mean effective pressure, more severely in peripheral intake port engines. The Mazda RX-8 Renesis engine improved performance by doubling the exhaust port area relative to earlier designs, and there have been studies of the effect of intake and exhaust piping configuration on the performance of Wankel engines.<ref name="Z8UtD">Ming-June Hsieh et al. SAE papers</ref> Side intake ports (as used in Mazda's Renesis engine) were first proposed by Hanns-Dieter Paschke in the late 1950s. Paschke predicted that precisely calculated intake ports and intake manifolds could make a side port engine as powerful as a PP engine.<ref name="van Basshuysen Schäfer 2017 p. 487">{{cite book |last1=van Basshuysen |first1=R. |last2=Schäfer |first2=F. |title=Handbuch Verbrennungsmotor: Grundlagen, Komponenten, Systeme, Perspektiven |publisher=Springer Fachmedien Wiesbaden |series=ATZ/MTZ-Fachbuch |year=2017 |isbn=978-3-658-10901-1 |language=de |page=487}}</ref>

===Materials===
As formerly described, the Wankel engine is affected by unequal [[thermal expansion]] due to the four cycles taking place in fixed places of the engine. While this puts great demands on the materials used, the simplicity of the Wankel makes it easier to use alternative materials, such as exotic alloys and [[ceramic]]s. A commonplace method is, for engine housings made of aluminum, to use a spurted [[molybdenum]] layer on the engine housing for the combustion chamber area, and a spurted steel layer elsewhere. Engine housings cast from iron can be induction-brazed to make the material suited for withstanding combustion heat stress.<ref name="Bensinger 1973 p. 137–138">{{cite book |last1=Bensinger |first1=Wolf-Dieter |title=Rotationskolben-Verbrennungsmotoren |place=Berlin, Heidelberg, New York |date=1973 |isbn=978-3-540-05886-1 |oclc=251737493 |language=de |pages=137–138}}</ref>

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, Daimler-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, [[Ferritic stainless steel|ferritic stainless]], Ferro-TiC, and other materials.<ref name="Bensinger 1973 p. 93">{{cite book |last1=Bensinger |first1=Wolf-Dieter |title=Rotationskolben-Verbrennungsmotoren |place=Berlin, Heidelberg, New York |date=1973 |isbn=978-3-540-05886-1 |oclc=251737493 |language=de |page=93}}</ref> The combination of housing plating and the apex and side seal 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 petrol fuel was the predominant type available in the first years of the Wankel engine's development. Lead is a solid lubricant, and leaded [[gasoline|petrol]] is designed to reduce the wearing of seals and housings. The first engines had the oil supply calculated with consideration of petrol's lubricating qualities. As leaded petrol was being phased out, Wankel engines needed an increased mix of oil in the petrol to provide lubrication to critical engine parts. An SAE paper by [[David Garside]] extensively described Norton's choices of materials and cooling fins.{{Citation needed|date=March 2023}}

===Sealing===
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 {{convert|50,000|mi|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.{{Citation needed|date=December 2010}} The problem was exacerbated when the engine was stressed before reaching [[operating temperature]]. However, Mazda Wankel engines solved these initial problems. Current engines have nearly 100 seal-related parts.<ref name="JhYt4Sa"/>

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 {{patent|US|3261542}}, C. Jones, 5/8/63, {{patent|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).<ref>Kenichi Yamamoto, Rotary Engine 1981, Page 50</ref>

===Fuel economy and emissions===
As is described in the [[#Thermodynamic disadvantages|thermodynamic disadvantages section]], the early Wankel engines had poor fuel economy. This is caused by the Wankel engine's design of combustion chamber shape and huge surface area. The Wankel engine's design is, on the other hand, much less prone to engine knocking,<ref name="Bensinger 1973 p. 86"/> which allows using low-[[octane rating|octane]] fuels without reducing compression. NSU tested low octane gasoline at the suggestion of Felix Wankel.

On a trial basis 40-octane gasoline was produced by BV Aral, which was used in the Wankel DKM54 test engine with a compression ratio of 8:1; it ran without complaint. This upset the petrochemical industry in Europe, which had invested considerable sums of money in new plants for the production of higher quality gasoline.<ref>Dieter Korp, Protokoll einer Erfindung - Der Wankelmotor, Motorbuch Verlag Stuttgart 1975 {{ISBN|3-87943-381-X}} p. 77-78</ref><ref name="Ansdale Keller 1971 p. 161">{{cite book |last1=Ansdale |first1=R.F. |last2=Keller |first2=H. |title=Der Wankelmotor: Konstruktion und Wirkungsweise |place=Stuttgart| publisher=Motorbuch-Verlag |year=1971 |language=de |page=161}}</ref><ref name="Yamamoto_1971_104">'Rotary Engine', Kenichi Yamamoto; Toyo Kogyo, 1971, p. 104</ref><ref name="K. Yamamoto, T. Muroki 1972">K. Yamamoto, T. Muroki, T. KobayakawaSAE Transactions, Vol. 81, SECTION 2: Papers 720197–720445 (1972), pp. 1296-1302 (7 pages) page 1297 test run down to 56 Oktan</ref><ref>Rotary Engine and Fuel Kenichi Yamamoto 8th World Petroleum Congress Moskow 1971, Paper Number: WPC-14403</ref>

Direct injection stratified charge engines can be operated with fuels with particularly low octane numbers. Such as diesel fuel, which only has an octane number of ~25.<ref>SAE Paper 2001-01-1844/4263 Direct injection stratified charge wankel engines</ref><ref>Direct Injection Stratified Charge Rotary Engine Zachary Steven Votaw .A., Wright State University, 2011 p. 6</ref> As a result of the poor efficiency, a Wankel engine with peripheral exhaust porting has a larger amount of unburnt [[hydrocarbons]] (HC) released into the exhaust.<ref name="Bensinger 1973 p. 87">{{cite book |last1=Bensinger |first1=Wolf-Dieter |title=Rotationskolben-Verbrennungsmotoren |place=Berlin, Heidelberg, New York |date=1973 |isbn=978-3-540-05886-1 |oclc=251737493 |language=de |page=87}}</ref><ref name="36xrO">{{Citation |first1=Ritsuharu |last1=Shimizu |first2=Haruo |last2=Okimoto |first3=Seijo |last3=Tashima |first4=Suguru |last4=Fuse |title=SAE Technical Paper Series |publisher=SAE |chapter-url=http://papers.sae.org/950454 |chapter=The Characteristics of Fuel Consumption and Exhaust Emissions of the Side Exhaust Port Rotary Engine |year=1995| volume=1 |doi=10.4271/950454}}</ref> The exhaust is, however, relatively low in [[NOx|nitrogen oxide]] (NOx) emissions, because the combustion is slow, and temperatures are lower than in other engines, and also because of the Wankel engine's good [[exhaust gas recirculation]] (EGR) behavior. [[Carbon monoxide]] (CO) emissions of Wankel and Otto engines are about the same.<ref name="Bensinger 1973 p. 86"/>

The Wankel engine has a significantly higher (Δt<sub>K</sub>>100&nbsp;K) exhaust gas temperature than an Otto engine, especially under low and medium load conditions. This is because of the higher combustion frequency and slower combustion. Exhaust gas temperatures can exceed 1300&nbsp;K under high load at engine speeds of 6000&nbsp;rpm<sup>&minus;1</sup>. To improve the exhaust gas behavior of the Wankel engine, a thermal reactor or [[catalyst|catalyst converter]] may be used to [[redox|reduce]] hydrocarbon and carbon monoxide from the exhaust.<ref name="Bensinger 1973 p. 87"/>

Mazda uses a dual ignition system with two spark plugs per chamber. This increases the power output and at the same time reduces HC emissions. At the same time, HC emissions can be lowered by reducing the pre-ignition of the T leading plug relative to the L trailing plug. This leads to internal afterburning and reduces HC emissions. On the other hand, the same ignition timing of L and T leads to a higher energy conversion. Hydrocarbons adhering to the combustion chamber wall are expelled into the exhaust at the peripheral outlet.<ref>Rotary Engine', Kenichi Yamamoto; Toyo Kogyo, 1971 lower HC emisions with dual ignition with leading and trailing spark plug, p. 104</ref><ref>Rotary Engine', Kenichi Yamamoto; Toyo Kogyo, 1971 lower HC emisions with dual ignition with leading and trailing spark plug, Fig.13.9 p. 141-</ref>

Mazda used 3 spark plugs in their R26B engine per chamber. The third spark plug ignites the mixture in the trailing side before the squish is generated, causing the mixture to burn completely and, also speeding up flame propagation, which improves fuel consumption.<ref>Mazda Motor Corp.: Ritsuharu Shimizu, Tomoo Tadokoro, Toru Nakanishi, and Junichi Funamoto Mazda 4-Rotor Rotary Engine for the Le Mans 24-Hour Endurance Race SAE Paper 920309 Page 7</ref> According to Curtiss-Wright research, the factor that controls the amount of unburnt hydrocarbons in the exhaust is the rotor surface temperature, with higher temperatures resulting in fewer hydrocarbons in the exhaust.<ref name="LyHft">{{Citation |first1=C |last1=Jones |publisher=SAE |format=PDF |url=http://papers.sae.org/790621 |title=790621 |year=1979| doi=10.4271/790621|url-access=subscription }}</ref> Curtiss-Wright widened the rotor, 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.<ref name="NIGaB">SAE paper 710582</ref> Quenching is the dominant source of hydrocarbon at high speeds and leakage at low speeds.<ref name="Wum7B">{{Citation |first1=GA |last1=Danieli |publisher=SAE |format=PDF |url=http://papers.sae.org/740186 |title=740186 |year=1974| doi=10.4271/740186|url-access=subscription }}</ref> Using side-porting which enables closing the exhaust port around the top-dead center and reducing intake and exhaust overlap helps improving fuel consumption.<ref name="36xrO"/>

Mazda's [[RX-8]] car with the [[Renesis (Engine)|Renesis]] engine (that was first presented in 1999), met in 2004 the [[United States vehicle emission standards#Phase 2: 2004–2009|United States' low emissions vehicle (LEV-II) standard]].<ref name="Dobler 2000 pp. 440–442"/> This was mainly achieved by using side porting: The exhaust ports, which in earlier Mazda rotary engines were located in the rotor housings, were moved to the side of the combustion chamber. This approach allowed Mazda to eliminate overlap between intake and exhaust port openings, while simultaneously increasing the exhaust port area. This design improved the combustion stability in the low-speed and light load range. The HC emissions from the side exhaust port rotary engine are 35–50% less than those from the peripheral exhaust port Wankel engine. Peripheral ported rotary engines have a better [[mean effective pressure]], especially at high rpm and with a rectangular-shaped intake port.<ref name="y5HHx">{{Citation |publisher=SAE |series=Technical Paper |number=2004–01–1790 |title= Developed Technologies of the New Rotary Engine (RENESIS)}}</ref><ref>SAE Paper 950454 Page 7</ref> However, the RX-8 was not improved to meet [[Emission standard#European Union|Euro 5 emission regulations]], and it was discontinued in 2012.<ref name="EjVRK">{{cite web|url= http://www.autocar.co.uk/car-news/motoring/mazda-kills-rx-8-sports-coupe |title=Mazda kills off RX-8 sports coupe |work=Autocar |access-date=2014-02-01}}</ref> The new Mazda 8C of the Mazda MX-30 R-EV meets the Euro 6d-ISC-FCM emissions standard.<ref>{{Cite web|url=https://www.auto-motor-und-sport.de/tech-zukunft/alternative-antriebe/mazda-mx-30-wankelmotor-range-extender-technik/|title=Mazda MX-30 R-EV Wankelmotor als Range Extender: Der neue Wankelmotor von Mazda im Detail|first1=Torsten|last1=Seibt|date=January 30, 2023|website=auto motor und sport}}</ref>

===Laser ignition===
Laser ignition was first proposed in 2011,<ref name="autocar.co.uk">{{cite web|url= http://www.autocar.co.uk/car-news/motoring/mazdas-radical-new-rotary-tech |title=Mazda's radical new rotary tech |publisher=Autocar |date=2011-06-27 |access-date=2014-02-01}}</ref><ref>{{Cite web|url=https://patents.google.com/patent/DE102011083450A1/de|title=Rotationskolbenbrennkraftmaschine und Betriebsverfahren hierfür}}</ref> but first studies of laser ignition were only conducted in 2021. It is assumed that laser ignition of lean fuel mixtures in Wankel engines could improve fuel consumption and exhaust gas behavior. In a 2021 study, a Wankel model engine was tested with laser ignition and various gaseous and liquid fuels. Laser ignition leads to a faster center of combustion development, thus improving combustion speed, leading to a reduction in NO<sub>x</sub> emissions. The laser pulse energy required for proper ignition is "reasonable", in the low single-digit mJ-range. A significant modification of the Wankel engine is not required for laser ignition.<ref name="Loktionov Pasechnikov 2021 p=012031">{{cite journal |last1=Loktionov |first1=E Yu |last2=Pasechnikov |first2=N A |title=First tests of laser ignition in Wankel engine |journal=Journal of Physics: Conference Series |publisher=IOP Publishing |volume=1787 |issue=1 |date=2021-02-01 |issn=1742-6588 |doi=10.1088/1742-6596/1787/1/012031 |page=012031| bibcode=2021JPhCS1787a2031L |doi-access=free }}</ref>

===Compression-ignition Wankel===
{{Main article|Wankel Diesel engine}}

[[File:Two Stage Rotary RR R1C.jpg|thumb|'''Figure 16.'''<br/>Rolls-Royce R1C compression ignition prototype]]

Research has occurred into rotary compression ignition engines. The basic design parameters of the Wankel engine preclude obtaining a compression ratio sufficient for Diesel operation in a practical engine.<ref name="Eichlseder Klüting Piock 2008 p. 222">{{cite book |last1=Eichlseder |first1=Helmut |last2=Klüting |first2=Manfred |last3=Piock |first3=Walter F. |title=Grundlagen und Technologien des Ottomotors |place=Wien |date=2008 |isbn=978-3-211-25774-6 |oclc=255415808 |language=de |page=222}}</ref> The Rolls-Royce<ref name="zqxX7">''Autocar'' magazine, week ending Dec 17, 1970</ref> and Yanmar compression-ignition<ref name="tqVfR">SAE paper 870449</ref> approach was to use a two-stage unit (see figure 16.), with one rotor acting as compressor, while combustion takes place in the other.<ref name="LfWIG">Wolf-Dieter Bensinger: Rotationskolben-Verbrennungsmotoren, Springer, Berlin/Heidelberg/New York 1973, {{ISBN|978-3-642-52174-4}}. p. 141</ref> Both engines were not functional.<ref name="Eichlseder Klüting Piock 2008 p. 222"/>

===Multifuel Wankel engine===
A different approach from a compression ignition (Diesel) Wankel engine is a non-CI, multifuel Wankel engine that is capable of operating on a huge variety of fuels: diesel, petrol, kerosene, methanol, natural gas, and hydrogen.<ref name="auto">{{Cite web |url=https://www.energy-saxony.net/fileadmin/Inhalte/Downloads/Veranstaltungen/2020/Lausitzer_Energiefachtagung/Pitches/03_Wankel_SuperTec_Dr._Holger_Hanisch.pdf |title=Hydrogen & Multi-fuel Engines for Sustainable Power & Mobility |archive-url=https://web.archive.org/web/20210225160308/https://www.energy-saxony.net/fileadmin/Inhalte/Downloads/Veranstaltungen/2020/Lausitzer_Energiefachtagung/Pitches/03_Wankel_SuperTec_Dr._Holger_Hanisch.pdf |archive-date=2021-02-25}}</ref><ref>Wankel Journal, No. 74, January 2015, p. 23</ref> German engineer Dankwart Eiermann designed this engine at Wankel SuperTec (WST) in the early 2000s. It has a chamber volume of 500&nbsp;cm<sup>3</sup> (cc) and an indicated power output of {{convert|50|kW|hp|sigfig=3|abbr=on}} per rotor. Versions with one up to four rotors are possible.<ref name="Wankel Journal p. 22">{{Cite journal |date=January 2015 |journal=Wankel Journal |number=74 |page=22}}</ref>

The WST engine has a common-rail direct injection system operating on a stratified charge principle. Similar to a Diesel engine and unlike a conventional Wankel engine, the WST engine compresses air rather than an air–fuel mixture as in the four-cycle engine compression phase. Fuel is only injected into the compressed air shortly before top-dead centre, which results in stratified charge (i.e., no homogeneous mixture). A spark plug is used to initiate combustion.<ref>Wankel Journal, No. 74, January 2015, p. 24</ref> The pressure at the end of the compression phase and during combustion is lower than in a conventional Diesel engine,<ref name="Wankel Journal p. 22"/> and the fuel consumption is equivalent to that of a small [[Indirect injection#Precombustion chamber|indirect injection compression ignition]] engine (i.e., >250&nbsp;g/(kW·h)).<ref>Wankel Journal, No. 74, January 2015, p. 27</ref>

Diesel-fuel-powered variants of the WST Wankel engine are being used as [[Auxiliary power unit|APUs]] in 60 Deutsche Bahn diesel locomotives. The WST diesel fuel engines can produce up to {{convert|400|kW|hp|sigfig=3|abbr=on}}.<ref name="lr-online.de 2020">{{cite web |title=Wirtschaft: Wankel Supertec forscht in Cottbus mit Uni-Nachwuchs |website=lr-online.de |date=2020-09-05 |url=https://www.lr-online.de/lausitz/cottbus/wirtschaft-wankel-supertec-forscht-in-cottbus-mit-uni-nachwuchs-51085230.html |language=de |access-date=2023-01-29}}</ref><ref name="auto"/>

===Hydrogen fuel===
[[File:Mazda RX8 hydrogen rotary car 1.jpg|thumb|right|'''Figure 15.'''<br/>[[Mazda RX-8 Hydrogen RE]] hydrogen-fuelled rotary-engined car]]

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 cycle of the Otto cycle occurs in different chambers. Importantly, the intake chamber is separated from the combustion chamber, keeping the air/fuel mixture away from localized hot spots. Wankel engines also do not have hot exhaust valves, which eases adapting them to hydrogen operation.<ref>{{cite journal |url= https://www.researchgate.net/publication/323548015 |doi=10.1016/j.ijhydene.2018.01.202|title=Recent studies on hydrogen usage in Wankel SI engine |year=2018 |last1=Ozcanli |first1=Mustafa |last2=Bas |first2=Oguz |last3=Akar |first3=Mustafa Atakan |last4=Yildizhan |first4=Safak |last5=Serin |first5=Hasan |journal=International Journal of Hydrogen Energy |volume=43 |issue=38 |pages=18037–18045 |bibcode=2018IJHE...4318037O |s2cid=103553203 }}</ref> 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.<ref name="WQkIw">1980 BMF report hydrogen Audi EA871 comparison to a hydrogen reciprocating piston engine page 11. Page 8 higher lubricating oil consumption caused by hydrogen</ref><ref name="V4ltk">{{cite web |url= http://www.wankel-spider.de/journal/downloads/journal_12_2003/Seite%2028%2B29%20-%20Wasserstoffwankel.pdf |archive-url= https://web.archive.org/web/20040611070850/http://www.wankel-spider.de/journal/downloads/journal_12_2003/Seite%2028%2B29%20-%20Wasserstoffwankel.pdf |url-status=dead |archive-date=June 11, 2004 |title=The rotary engine is ideally suited to burn hydrogen without backfiring that can occur when hydrogen is burned in a reciprocating piston engine |access-date=2011-01-05}}</ref>

In a prototype Wankel engine fitted to a [[Mazda RX-8]] to research hydrogen operation, Wakayama et al. found that hydrogen operation improved thermal efficiency by 23% over petrol fuel operation. Although the lean operation emits little NOx, total amount of engine-out NOx
exceeds Japanese SULEV standard. The supplementary stoichiometic operation combined with a catalyst
provides additional NOx reduction. Accordingly, the vehicle satisfies the SULEV standard.<ref name="cder.dz">{{cite conference |last1=Wakayama |first1=Norihira |last2=Morimoto |first2=Kenji |last3=Kashiwagi |first3=Akihiro |last4=Saito |first4=Tomoaki |title=Development of Hydrogen Rotary Engine Vehicle |conference=16th World Hydrogen Energy Conference |date=13–16 June 2006 |url=https://www.cder.dz/A2H2/Medias/Download/Proc%20PDF/PARALLEL%20SESSIONS/%5BS22%5D%20Internal%20Combustion%20Engines/13-06-06/169.pdf |access-date=19 January 2023}}</ref>