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Wankel engine
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===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 720197β720445 (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 (Ξt<sub>K</sub>>100 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 K under high load at engine speeds of 6000 rpm<sup>−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: 2004β2009|United States' low emissions vehicle (LEV-II) standard]].<ref name="Dobler 2000 pp. 440β442"/> 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 35β50% 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=2004β01β1790 |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>
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