Imagine how unstable a fusion combustion engine would be?
The premise would be that you'd have 3 pistons, each feeding a crank shaft connected to a generator.
You'd also have this underwater or some other coolant as to siphon the heat away, maybe add a turbine to the shaft to help move the liquid away from the pistons.
The first combustion either blows up everything, rips the shaft apart, or provides just enough force to force the next piston to combust.