gardenman
 

Most efficient RPM depends on the motor. I understand some of the old fishing boat single cylinder motors were super efficient and ran at 500 RPM. But they had a big flywheel. Seems that at over 1000 RPM the piston is travelling faster than the flame front for a considerable part of the power stroke. So these old fishing boat motors were good on fuel economy but lowsey on power to weight ratio, power band and just about every other means of comparing performance.Tractor motors cams are generlly are set up to run best at 1500 RPM which is a compromise between good power band , torque and fuel economy.
In a car One would have to dramatically increase the speed of the flame / combustion process to be able to benefit from slowing down the RPM below what the optimal design RPM for the engine would be. Converting to propane and or hydrogen injection might allow for better economy at slower RPM.

i don;t know.
i;ve never owned a diesel truck. when i;ve driven one, i never thought to compare the rpm gauge to gasoline trucks.

what i;ve been told is that diesels run at a lower rpm than gasoline trucks.

Taken to the extreme the Wurtzilla diesels that weigh something like 2800 tons run at 92 RPM directly coupled to the propeller, in those giant container ships.

Efficiency in the mid 50% range.

regards
Mech

The flame does not have to chase the piston. It starts when the piston is barely moving, and moves out through the charge, even as the charge changes shape. The flame, you might say, gets a tailwind from the expansion, so it's speed through the charge is less than it looks.

It's about the piston speed. IIRC Those big ol' engines that barely seem to be turning over have comparable piston speeds to the little fast stuff.