Editing Wankel engine (section)

===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>