Algodoo

A 3 cyl engine making around 120 HP at peak RPMs. Weighs only 30kg, although you should ad dummy weight to get it to realistic values if you want to use it

The main idea of this engine is to optimize it noticeably, in terms of simulation performance - since it runs at only 120 Hz, complex scenes won't bog Algodoo down that much.

A diesel-style engine (3000 RPM max, very high torque) running at 60 Hz might even be possible, and is probably the next logical step

fix: now also compensates angle (i.e. runs upside down)