Two-stroke engines are associated with smoky exhaust for a number of reasons. Primarily, the carbureted,
crank-case-scavenged engines that are commonly used for motorcycles pass approximately one third of the fuel
in an unburnt state out the exhaust pipe due to a process known as “fuel short circuiting”. Injecting the fuel
directly (DI) into the cylinder eliminates this problem, reducing emissions by about 88% and improving fuel
consumption by approximately one third. For direct injection to work, the fuel must be completely atomized very
quickly. Several different techniques are available for DI systems. One such system pioneered by Orbital Engine,
Corp., uses a blast of compressed air to “blow” the fuel into the cylinder. The air is supplied by an air
compressor running directly off the crankshaft.
We decided to reduce the complexity and cost of the Orbital-style injection system by extracting pressure from the combustion chamber (immediately prior to ignition), storing this pressure in a “mixing cavity” (where the fuel is added) and injecting the mixture back into the combustion chamber. This system eliminates the need for an air compressor but uses most of the rest of the proven “air blast” DI system.
We were able to demonstrate operation of the “compression pressurized” DI system on a test vehicle and compare
it to the carbureted and the air-blast DI systems. The results showed similar fuel savings and emissions
reductions to the air-blast DI system.