yep, model aircraft, hydroplanes, boats... they were and are still used today, mainly found in buses and marine engines and some main battle tanks. The "diesel" part of the model engine refers to the compression ignition rather than the fuel type. The fuel as I recall was nitromethane (or something exotic that now comes with a Health and safety warning if you can still get it!)

The flat fronted buses after the routemasters - the name escapes me, used the same engine design as the Chieftan tank, but that was with two opposed pistons for every cylinder.