An Otto engine has an efficiency about 20-25%,
other 40% of fuel energy is in the exaust gas that has a temperature about 500°C, other 25% is in the radiator; this 65% is lost heat but it can be utilized from an external combustion engine to produce other 20-25% of fuel energy in mechanic energy.
Practically this system would have a global efficiency about 40-50% and there would be two engines with the same power!...
This is theory but there is a practical problem that is the dimension of an external combustion engine.
I have seen some of these engines, one of 35 kw
is great as an half car!
But we have to remember that if one is able to
make small this engine then he can double the efficiency of an Otto engine...

How about this:

A dual-function two-stroke diesel/steam engine?

The diesel injector draws fuel from the fuel tank and injects it into the cylinder just before TDC, then a few degrees after TDC, when the fuel has completely combusted, a water injector draws distilled water from the water jacket around the engine. This already hot water encounters white-hot temps in the combustion chamber and instantly flashes to steam; the steam increasing in volume by a factor of 1600, IIRC.

As the engine has internal cooling, you can get rid of the radiator, though you may need some kind of condenser if you wish to recover water vapor from the exhaust stream. You will of course need a water tank and feed line, as well as a pump to keep the water jacket pressurized.