Editing Wankel engine (section)

==Wankel engine development==
[[Felix Wankel]] designed a rotary compressor in the 1920s, and received his first patent for a rotary type of engine in 1934.<ref name="Hege 2015 p. 31">{{cite book |last1=Hege |first1=J.B. |title=The Wankel Rotary Engine: A History |publisher=McFarland |series=EBL-Schweitzer |year=2015 |isbn=978-0-7864-8658-8 |page=31}}</ref> He realized that the triangular rotor of the rotary compressor could have intake and exhaust ports added producing an internal combustion engine. Eventually, in 1951, Wankel began working at German firm [[NSU Motorenwerke]] to design a rotary compressor as a [[supercharger]] for NSU's motorcycle engines. Wankel conceived the design of a triangular rotor in the compressor.<ref name="Hege 2015 p. 43">{{cite book |last1=Hege |first1=J.B. |title=The Wankel Rotary Engine: A History |publisher=McFarland |series=EBL-Schweitzer |year=2015 |isbn=978-0-7864-8658-8 |page=43}}</ref> With the assistance of Prof. {{ill|Othmar Baier|de}} from Stuttgart University of Applied Sciences, the concept was defined mathematically.<ref name="Hege 2015 p. 44">{{cite book |last1=Hege |first1=J.B. |title=The Wankel Rotary Engine: A History |publisher=McFarland |series=EBL-Schweitzer |year=2015 |isbn=978-0-7864-8658-8 |page=44}}</ref> The supercharger he designed was used for one of NSU's 50&nbsp;cm<sup>3</sup> one-cylinder two-stroke engines. The engine produced a power output of {{cvt|13.5|PS|kW|0}} at 12,000{{nbsp}}rpm.<ref name="Dieter Korp 1975 p. 62-63">{{cite book |last1=Korp |first1=Dieter |title=Protokoll einer Erfindung |publisher=Motorbuch Verlag Stuttgart| series= |year=1975 |isbn=3-87943-381-X |pages=62–63}}</ref>

In 1954, NSU agreed to develop a rotary internal combustion engine with Felix Wankel, based upon Wankel's supercharger design for their motorcycle engines. Since Wankel was known as a "difficult colleague", the development work for the DKM was carried out at Wankel's private Lindau design bureau. According to John B. Hege, Wankel received help from his friend Ernst Höppner, who was a "brilliant engineer".<ref name="Hege 2015 p. 48">{{cite book |last1=Hege |first1=J.B. |title=The Wankel Rotary Engine: A History |publisher=McFarland |series=EBL-Schweitzer |year=2015 |isbn=978-0-7864-8658-8 |page=48}}</ref> The first working prototype, DKM 54 (see figure 2.), first ran on 1 February 1957, at the NSU research and development department ''Versuchsabteilung TX''. It produced {{convert|21|PS|abbr=on}}.<ref name="JhYt4Sa">{{cite magazine |title=The Rotary Club |first1=Don |last1=Sherman |magazine=Automobile Magazine |date=February 2008 |pages=76–79}}</ref><ref name="NdA81">{{cite news| url= http://www.spiegel.de/auto/aktuell/0,1518,459789,00.html |title=Wankel-Jubiläum: Warten aufs Wunder |first1=Jürgen |last1=Pander |language=de |publisher=Der Spiegel Online |date=2007-01-21|access-date=2018-05-05|newspaper=Spiegel Online}}</ref> Soon after that, a second prototype of the DKM was built. It had a working chamber volume V<sub>k</sub> of 125&nbsp;cm<sup>3</sup> and also produced {{cvt|21|kW|PS|0}} at 17,000{{nbsp}}rpm.<ref name="Bensinger 1973 p. 56">{{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=56}}</ref> It could even reach speeds of up to 25,000{{nbsp}}rpm. However, these engine speeds distorted the outer rotor's shape, thus proving impractical.<ref name="Hege 2015 p. 52">{{cite book |last1=Hege |first1=J.B. |title=The Wankel Rotary Engine: A History |publisher=McFarland |series=EBL-Schweitzer |year=2015 |isbn=978-0-7864-8658-8 |page=52}}</ref> According to [[Mazda Motor]]s engineers and historians, four units of the DKM engine were built; the design is described to have a displacement V<sub>h</sub> of 250&nbsp;cm<sup>3</sup> (equivalent to a working chamber volume V<sub>k</sub> of 125&nbsp;cm<sup>3</sup>). The fourth unit built is said to have received several design changes, and eventually produced {{cvt|29|PS|kW|0}} at 17,000 rpm; it could reach speeds up to 22,000 rpm. One of the four engines built has been on static display at the [[Deutsches Museum Bonn]] (see figure. 2).<ref name="Basshuysen Schäfer Springer Fachmedien Wiesbaden 2017 p. 484">{{cite book |last1=Basshuysen |first1=Richard van |last2=Schäfer |first2=Fred |author3=Springer Fachmedien Wiesbaden |title=Handbuch Verbrennungsmotor Grundlagen, Komponenten, Systeme, Perspektiven |place=Wiesbaden |date=2017 |isbn=978-3-658-10901-1 |oclc=979563571 |language=de |page=484}}</ref>

Due to its complicated design with a stationary center shaft, the DKM engine was impractical.<ref name="Hege 2015 p. 53"/> [[Wolf-Dieter Bensinger]] explicitly mentions that proper engine cooling cannot be achieved in a DKM engine, and argues that this is the reason why the DKM design had to be abandoned.<ref name="Bensinger 1973 p. 54">{{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=54}}</ref> NSU development chief engineer Walter Froede solved this problem by using Hanns-Dieter Paschke's design and converting the DKM into what would later be known as the KKM (see figure 5.).<ref name="Hege 2015 p. 53"/> The KKM proved to be a much more practical engine, as it has easily accessible spark plugs, a simpler cooling design, and a conventional power take-off shaft.<ref name="Hege 2015 p. 54"/> Wankel disliked Froede's KKM engine because of its inner rotor's eccentric motion, which was not a pure circular motion, as Wankel had intended. He remarked that his "race horse" was turned into a "plough horse". Wankel also complained that more stresses would be placed on the KKM's apex seals due to the eccentric hula-hoop motion of the rotor. NSU could not afford to finance developing both the DKM and the KKM, and eventually decided to drop the DKM in favor of the KKM, because the latter seemed to be the more practical design.<ref name="Hege 2015 p. 57">{{cite book |last1=Hege |first1=J.B. |title=The Wankel Rotary Engine: A History |publisher=McFarland |series=EBL-Schweitzer |year=2015 |isbn=978-0-7864-8658-8 |page=57}}</ref>

Wankel obtained the US patent 2,988,065 on the KKM engine on 13 June 1961.<ref>{{cite book |last1=Hege |first1=J.B. |title=The Wankel Rotary Engine: A History |publisher=McFarland |series=EBL-Schweitzer |year=2015 |isbn=978-0-7864-8658-8 |page=65}}</ref> Throughout the design phase of the KKM, Froede's engineering team had to solve problems such as repeated bearing seizures, the oil flow inside the engine, and the engine cooling.<ref>{{cite book |last1=Hege |first1=J.B. |title=The Wankel Rotary Engine: A History |publisher=McFarland |series=EBL-Schweitzer |year=2015 |isbn=978-0-7864-8658-8 |page=66}}</ref> The first fully functioning KKM engine, the KKM 125, weighing in at only {{cvt|17|kg|lb|1}} displaced 125&nbsp;cm<sup>3</sup> and produced {{cvt|26|PS|kW|0}} at 11,000{{nbsp}}rpm.<ref>{{cite book |last1=Hege |first1=J.B. |title=The Wankel Rotary Engine: A History |publisher=McFarland |series=EBL-Schweitzer |year=2015 |isbn=978-0-7864-8658-8 |page=67}}</ref> Its first run was on 1 July 1958.<ref name="Faith 1975 p. 45">{{cite book |last1=Faith |first1=N. |title=Wankel: The Curious Story Behind the Revolutionary Rotary Engine |publisher=Stein and Day |year=1975 |isbn=978-0-8128-1719-5 |page=45}}</ref>

In 1963, NSU produced the first series-production Wankel engine for a car, the KKM 502 (see Figure 6.). It was used in the [[NSU Spider]] sports car, of which about 2,000 were made. Despite its "teething troubles", the KKM&nbsp;502 was a powerful engine with decent potential, smooth operation, and low noise emissions at high engine speeds. It was a single-rotor PP engine with a displacement of {{cvt|996|cm3|in3|0}}, a rated power of {{cvt|40|kW|hp|0}} at 6,000{{nbsp}}rpm and a [[Mean effective pressure|BMEP]] of {{cvt|1|MPa|lb/in2|0}}.<ref name="Bensinger 1973 p. 127-131">{{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=127–131}}</ref>