I searched for the engine briefly, and found no mention of anyone thinking of this.
This post comes from this comment of mine.
In my original comment, I note that I immediately think the strain would blow the entire thing a part.
What are your reasons of why you think would not work, or why no one has tried, or maybe you know someone who did try?
Note, Not the size of a car. More like the size of a cargo ships diesel engine. ("Wärtsilä-Sulzer RTA96-C" as an example)
Imagined design. Original Post
I think you are missing the point of fusion as we know it.
Pistons are pushed by small controlled explosions a thousand or so times a minute. Each explosion pushes the piston, which makes the crankshaft turn.
Fusion, however is a continuous source of heat to produce steam that turns a turbine to make electricity. It's meant to be an alternative to nuclear power just with much less danger involved. Fusion can't melt down like nuclear. Fusion also would require very little fuel to keep going.
Unless I am missing the point of your update, I just don't think it's possible to make a fusion piston. I believe the only way is to use fusion to make steam to power a turbine.