Let’s put things in perspective. YankeeDiesel is an invention. In particular, it is an internal combustion engine. An internal combustion engine is a machine that controllably converts combustion into mechanical power. So getting down to basics, YankeeDiesel is an engine, not a motor. The first figure clears up some confusion between motors and engines. There is a difference and the words are frequently used interchangeably. So YankeeDiesel is an engine that produces heat and converts that heat into mechanical power.
The second figure is useful and it shows that diesels are more efficient than gasoline. Diesels have higher combustion temperatures than gasoline and hence greater expansion. The bigger expansion is converted directly into power and hence has greater efficiency. Also diesel fuel has more energy than gasoline – pound for pound. But gasoline engines have one advantage over diesels – they have a throttle. Throttles allow engines to maintain a perfect air/fuel (stoichiometric) mixture over all power ranges. Diesels do not have throttles. If they did, the compression ratio would not be sufficient to heat the air enough to start combustion. The YankeeDiesel does have a throttle but maintains the same compression ratio by reducing the size of the combustion chamber.
The next two figures are background information – two-stroke and four-stroke engines. An obvious difference between them is that a two-stroke engine produces power for every revolution of the crankshaft. A four-stroke produces power every other revolution. A power stroke on every revolution requires the exhaust and intake process to occur very quickly. The rapid intake requires the incoming air charge to be forced into the combustion chamber.
So the first four figures help clarify the YankeeDiesel as an engine, burning diesel fuel and of the two-stroke type. The last figure is of the generic type that shows the combustion chamber pressure during one revolution of a two-stroke engine. During compression, the air in the combustion chamber gets hotter. The power to compress the air comes from a flywheel or another cylinder. Then after ignition, the pressure in the combustion chamber increases and the piston reverses direction. Now all the power to compress the air is returned plus extra power because it is at higher pressure. Then near the bottom of the stroke, the exhaust valve opens and the hot gases escape. Next, the inlet ports are exposed and fresh air is forced into the combustion chamber and the remaining combustion gases are pushed out. If the exhaust valve closes early, it is possible to pressurize the combustion chamber with fresh air. This is sometimes referred to as “boost”.