New incarnation for two-cycle engine

Science News, Oct 22, 1988

New incarnation for two-cycle engine

Automobile makers use a complex array of moving parts -- valves, rocker arms and rods -- to make the powerful "four-cycle" engine, which combusts fuel on every fourth stroke of each piston. the "two-cycle engine, on the other hand, does without those mechanisms. And although it is notorious for slurping fuel and exhaling a blue-smoke miasma of unburned fuel and oil, the two-cycle's simple design still makes it desirable for low-power use in lawnmowers, outboard motors and small motorcycles.

The lowly two-cycle engine may never replace its well-bred four-cycle cousin, but researchers at the Southwest Research Institute (SwRI) in San Antonio, Tex., have just sent the two-cycle engine to finishing school.

In traditional two-cycle engines, a gas, oil and air mixture that is pulled into the crankcase through a one-way valve lubricates the engine. As the piston descends after combustion it forces this mixture out of the crankcase and into the combustion chamber through an intake port, driving out the newly burned gases from the last stroke at the same time. But combustion in the two-cycle engine isn't very clean or complete because a lot of oil is incompletely burned with each stroke. In addition, some of the fresh fuel mixture rushes out the exhaust with the burned gases when both exhaust and intake ports are open.

The engine developed by SwRI researcher Susumi Ariga and colleagues (see diagram) uses a fuel injector rather than a carburetor to mix fuel with air for combustion. Because only air passes through the crankcase, no oil need be fixed in for lubrication, which is provided by a separate system. Because the exhaust port is closed before new fuel is injected, little unburned fuel escapes. Ariga also uses diesel fuel instead of gasoline in the engine.

A good burn depends on thorough mixing of air and fuel, though, and fuel injectors allow little time for mixing before combustion. Ariga solved this problem by adding an auxiliary chamber that catches a bit of diesel fuel from the injector. As combustion begins, a hot jet of gas from fuel burning in the auxiliary chamber increases mixing, and therefore combustion efficiency, in the main chamber.