of fuels, including diesel fuel. German engineer Dankwart Eiermann designed this engine at Wankel SuperTec (WST) in the early 2000s. It has a chamber volume of 500 cm³ and an indicated power ouput of 50 kW per rotor. Versions with one up to four rotors are possible. The WST engine has a common-rail direct injection system and operates 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 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 sparkplug is used to initiate combustion. The pressure at the end of the compression phase and during combustion is lower than in a conventional Diesel engine, and the fuel consumption is equivalent to that of a small IDI Diesel (i.e., >250 g/(kW·h)). Diesel-fuel-powered variants of the WST Wankel engine have been used as APUs in Deutsche Bahn Diesel locomotives. Hydrogen fuel 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. 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. 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. However, the exhaust gas behavior significantly worsened due to high NOx emissions caused by lean combustion, which meant that the vehicle failed to comply with Japan's SULEV emissions standard. To comply with emissions regulations, a supplementary stoichiometric mode had to be used, which lowered the thermal efficiency of the engine. Advantages Prime advantages of the Wankel engine are: A far higher power-to-weight ratio than a piston engine Easier to package in small engine spaces than an equivalent piston engine Able to reach higher engine speeds than a comparable piston