The Wankel design engine,is a subject,all of its own.If one thinks about it,the design is relatively simple.It being merely a rotary compressor,modified to run on combustion.The problems arose,much due to high temperatures,with the combustion process,such as rotor seals,lube break down,etc.Being of a basic"turbine"design,for want of a better word,they were never very efficient in terms of fuel usage,etc.The usage,was attempted on many application,such as snowmobiles,autos,motorcycles[experimental],but never gained acceptance.Perhaps,as technology evolves,they may make a come back.