Precombustion chamber engines are among the lowest emission power sources available today. Due to excellent fuel-air mixing and complete combustion, HC and CO emissions are extremely low. NO2 levels are also relatively low because of the divided chamber “quench effect.”Because of the low HC and CO levels, effort has been concentrated on lowering NO2 levels. Retarded timing, inlet air cooling, through the use of aftercoolers, water induction, and exhaust recirculation have all shown beneficial results. Smoke levels are minimized by selective matching of engine and turbocharger to assure adequate air for combustion.