Wankel Diesel engine describes the idea of using the Diesel principle in a Wankel rotary engine. Several attempts to build such an engine have been made by different engineers and manufacturers in the 1960s and 1970s. Due to technical problems and the general disadvantages of the Wankel design, the Wankel Diesel engine never left the prototype stage, and designing a Wankel Diesel engine capable of running under its own power is thus considered unfeasible.
Design concept
A Wankel Diesel engine shares its basic design, a triangular-like rotor in an oval, epitrochoid-shaped housing, with a regular Wankel engine. The key difference is that it shares all of its operational characteristics with a Diesel engine. In addition to this, supercharging is required: A regular Diesel engine achieves compression ignition by a high compression ratio, which cannot be done in a Wankel engine due to the shape of the Wankel combustion chamber; supercharging ″artificially″ increases the compression ratio to ensure that compression ignition takes place. Several different approaches of designing a supercharger for a Wankel Diesel engine exist. Wankel designed an external supercharger, whereas Rolls-Royce added a secondary supercharging rotor of a bigger size, making the engine a two-stage rotary, which gave the housing an "8"-like appearance.
Reasons for designing a Wankel Diesel engine Compared with a Diesel engine, a regular petrol engine has a lower thermal efficiency. However, in the 1960s, the petrol engine advantages of lower mass, fewer vibrations and less noise emissions, all of which are important for passenger cars, were still prevailing. Low mass and smooth engine operation are also characteristics of the Wankel engine – combining Wankel design with Diesel characteristics would result in an efficient engine with low mass and few vibrations. Making a Wankel engine that can burn Diesel engine fuel, but which does not use the Diesel principle, can be useful if Diesel engine fuel is less expensive than petrol. In a 1974 study conducted for the EPA, it was attempted to determine whether or not a Wankel Diesel engine has a better exhaust behaviour than a regular Otto cycle petrol engine.
Causes of infeasibility The main cause of Wankel Diesel engine infeasibility is the shape of the combustion chamber, located in the rotary piston. It is elongated and convex, thus not allowing a high enough compression ratio (without too much heat loss), even in combination with a (solely crankshaft or exhaust gas driven) supercharger. This means that the engine is only functional if it is fed externally compressed air, as it requires more work to operate than it can produce. Also, designing a proper combustion chamber, correctly angled towards the injection nozzle to allow proper mixture formation, proved to be quite difficult; the EPA study proves that the Wankel Diesel engine's exhaust contains disproportionally high levels of carbon monoxide (CO) and hydrocarbons (HC), indicating incomplete combustion.
Prototype development
Diesel-Ring The Diesel-Ring was a joint venture of the German Diesel engine manufacturers Daimler-Benz, MAN, Krupp and KHD. Development was ceased in 1969, because the shape of the Wankel engine combustion chamber was considered unusable for Diesel combustion. The Diesel-Ring, attempts were made to modify the geometry of the epitrochoid and rotor in such a way that a higher compression ratio could be achieved. This was done without increasing the K-number and thus disproportionately increasing the overall volume of the engine. These attempts met with little success, as they resulted in vibrations of the apex seal due to excessive movement in the groove.
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