If you want pistons the absolute most efficient is a free piston engine be it opposed or double ended with a linear alternator as the power take off point. Livermore labs has one that hits a record high 65% BSFC to electrons and can burn anything flammable. That’s fuel cell good but no platinum catalysts. There is no more efficient way to convert chemical energy into electrons. A couple of companies have commercialized linear piston engines one runs on natural gas and is used for on-site cogen the other is looking to use theirs to power a class 8 truck as the range extender like a diesel locomotive only sending DC current to the electric motors. They tested it and it doubles the class 8 tonnes per mile of fuel burnt. Still not as good as a locomotive which is one tonne moved over a distance of 400+ miles per gallon of diesel. Ship’s are 1200 btw trucks are in the double digits all long distance freight should be moved by rail or ship’s period full stop. Look at it this way a truck tops out at 80,000lbs gross , 15,000 of that is the trailer and 20,000 is the tractor for a net of 55,000lbs, at full load your class 8 is getting 7 mpg of diesel....Do the math it’s pitiful compared to rail or ship’s which includes inland barges.
Grrr typo the lightest class 8 tractors no sleeper cab are 10,000lbs most are in the 15-20 range so not 55,000 lbs net 45,000lbs net.