There are two types of Diesel engines. The one mentioned by Nick fires by the heat of compression and in many cases has a blower of some type acting to pre-compress the charge although some compression Diesels do not employ a blower. As Nick indicated this type of engine is quite heavy for its’ physical size. This type of engine is normally of the two-stroke type and runs at a lower RPM than the engine type described below
However there is another type of Diesel engine (referred to as Hesselman Cycle) and this type of engine uses glow plugs to ignite the fuel and as such does not require the robust structure employed on a compression Diesel engine. As a result they are much lighter in weight. There are several German companies developing radial Diesel engines for use on aircraft.