the most efficient engine is probably the volkswagon tdi (turbo deisel) it gets 50 miles to the gallon, 300k plus miles per engine life expectancy and virtually have no maintenance. and an impressive amount of low end torque.
The Variable Specific Impulse Magnetoplasma Rocket (VASIMR)is very likely the most efficient rocket engine.
Diesel engine
In the United States the most fuel efficient pickup truck is the Ford Ranger with a 4 cylinder engine.
The most efficient to date is a 2-stroke design. it is a turbo desiel 2-stroke. but normally a 4-stroke is usually the more efficient of the two engine types.
Most of the energy is lost as heat.
Diesel engines are more efficient than gasoline engines.
A very small vehicle with a small engine is most likely to use less fuel.
The superlative of efficient is "most efficient."
The most efficient fuel engine is generally considered to be the hydrogen fuel cell engine, which converts hydrogen gas into electricity with water vapor as the only byproduct. Fuel cells can achieve efficiencies of up to 60%, and when used in hybrid systems, overall efficiency can be even higher. However, traditional internal combustion engines, particularly those using diesel fuel, can also be quite efficient, reaching around 30-40% efficiency under optimal conditions. The choice of the most efficient engine often depends on specific applications and energy sources available.
The diesel is more efficient but more expensive to build.
The smaller the engine the less fuel it will use. But then again an engine too small for the application can use more fuel than a larger engine that is appropriate for the application.
It will depend upon the conditions. Under normal operating conditions a diesel engine is more efficient than a petrol engine.