Could we build a four-stroke engine that runs on the exhaust heat of another engine? My idea is to have a four stroke engine **** in the exhaust from another running engine, instead of fresh air. Compress that hot exhaust on it’s compression stroke, then at TDC inject a burst of steam, through some kind of injector. The boiler water could be preheated by the cooling system of the first running engine. Does this sound at all feasible?