The Valentine “Rasant” Diesel engine was designed and built by Ron Valentine in Germany circa 1993. It is among the smallest engines in the world. (Note the U.S. quarter dollar coin used as the support base for a size reference.) These 2-cycle Diesel engines burn a special Diesel fuel that is less viscous and produces combustion when head and pressure are applied. The piston and cylinder are lapped, a machining process that matches the piston to the cylinder so closely in size (within millionths of an inch) that piston rings are not needed.
A Diesel engine has no spark plug. As the piston compresses the fuel and air mixture into the top of the combustion chamber, the heat caused by compression eventually causes it to explode.