<div class="htmlview paragraph">An inherent characteristic of the rotary engine geometry is its high surface-to-volume ratio. This results in a high quenching effect at the trailing portion of the charge. Combination of rich mixtures in the end gases together with high quenching effects reduces the octane requirement. It also means, however, that the unburned hydrocarbon (HC) concentrations are relatively high, whereas the NO_x concentrations are quite low.</div> <div class="htmlview paragraph">Thermal reactors have been the Toyo Kogyo Co.'s principal approach to the control of CO and HC exhaust emissions.</div>