By far, the most important part of the CVCC engine emission control system is the Compound Vortex Controlled Combustion (CVCC) cylinder head itself. Each cylinder has three valves: a conventional intake and conventional exhaust valve, and a smaller auxiliary intake valve. There are actually two combustion chambers per cylinder: a pre-combustion or auxiliary chamber, and the main chamber. During the intake stroke, an extremely lean mixture is drawn into the main combustion chamber. Simultaneously, a very rich mixture is drawn into the smaller precombustion chamber via the auxiliary intake valve. The spark plug, located in the precombustion chamber, easily ignites the rich pre-mixture, and this combustion spreads out into the main combustion chamber where the lean mixture is ignited. Due to the fact that the volume of the auxiliary chamber is much smaller than the main chamber, the overall mixture is very lean (about 18 parts air to one part fuel). The result is low hydrocarbon emissions due to the slow, stable combustion of the lean mixture in the main chamber; low carbon monoxide emissions due to the excess oxygen available; and low oxides of nitrogen emissions due to the lowered peak combustion temperatures. An added benefit of burning the lean mixture is the excellent gas mileage.