Abstract
This paper reviews the current technologies on the Heavy Duty (HD) natural gas engines for automotive use. NOx from premixed engine is well bellow the 1994 US EPA STD or Japanese long-term (year 2000) emission target. Natural Aspirated (NA) lean burn (LB) engine which is operated with stoichiometrically at higher load shows comparable NOx as the engine with TWC. High BMEP LB engine with catalyst shows not only low NOx but also low THC satisfying CARB ULEV STD. Further the thermal efficiency of the well designed High BMEP LB engine is competitive as the predicted future low NOx, particurate diesel engine. Good performance of natural gas direct-injection engine has been clarified, and the high level HC and low thermal efficiency associated with a partial load operation of a dual-fuel engine has been improved by hot EGR.