If you want to have a vehicle that is very fuel efficient, diesel is what you want as your fuel. German engineer Rudolf Diesel designed and patented the diesel engine as the culmination of his quest to create truly thermodynamically and fuel efficient engines.
Every internal combustion engine works by mixing a fuel with air, and introducing it into a combustion/compression chamber. Work is performed when the mixtures explode (combusts), pushing a piston that is connected to a crankshaft, thus rotating the crankshaft. This happens many thousands of times per minute. At the rear end of the crankshaft is a flywheel which transfers the rotational energy of the engine to the transmission.
Diesel Engine Differences
There are two major differences between a diesel engine and a gasoline powered engine. The first and most significant of these differences is that in a diesel engine, combustion is caused by the heat generated by the compression of air and fuel. Whereas in a gasoline engine, the air and fuel mixture is ignited by a spark. The second main difference is that in a diesel engine, instead of the air and fuel being introduced into the cylinder during the intake stroke, the air is first introduced during the intake stroke, along with a small amount of fuel. Near the top of the compression cycle, more fuel is introduced through the injector into the cylinder. Because of the very high temperature and pressure present at this time in the cylinder, or combustion chamber, the mixture ignites, applying force to the crankshaft. Thus, rotating the transmission to provide motive force for the vehicle.
The preceding information was given so as to give you an understanding of how a fuel efficient diesel engine works and the basic differences between gasoline and diesel engines.