Iíve always liked Stirling engines . They only require heat to run, so can use any fuel/heat source. This one is a ďKontax KS90Ē which will run from the heat of your hand, or of a cup of coffee.
I could not square buying the kit unless it was used for 3DS Max practice, so here it is. Itís rigged for animation, although I had problems with the Wagon Wheel effect, one day Iím going to rendering it running very slowly, just havenít done it yet.
The position of the pistons is not Simple Harmonic Motion, but is governed by the equations on this page. I used them in the Wired Parameters so that the rotation of the flywheel drives every other part of the animation.