A heavy-duty 320 kW diesel engine has been converted to natural gas operation. Conversion technology was selected to minimize costs while reaching NOx emissions goals of less than 3.2 g/kW-hr. Two engines are being converted using quiescent and high swirl combustion systems. The first engine with low swirl cylinder heads of the base diesel engine, and a combustion system developed for it was tested on a steady state cycle that has been shown to simulate the US heavy duty transient test cycle. It shows NOx emissions of 2.9 g/kW-hr and total HC emissions of 5.4 g/kW-hr. It is suspected that the HC emission is high because of high valve overlap. Experience with other similar engines suggests that non-methane HC emission is about 0.4-0.8 g/kW-hr. It is also expected that modified valve events and/or an oxidation catalyst can reduce HC emissions to much lower levels. The efficiency of the low swirl natural gas engine at this NOx level is 36 percent at rated condition. The rated condition efficiency of the base diesel engine is 42 percent at an NOx level of 7.4 g/kW-hr.A unique wastegate control system has been designed and implemented to control load by wastegating plus throttling rather than throttling alone. It is shown that this technique improves part load efficiency to nearly 22 percent compared to 18 percent with throttling alone. Natural gas cools down considerably upon expansion from bottle pressure to metering pressure. A heat exchanger has been designed to use intake air to heat natural gas thus making air fuel ratio control independent of bottle pressure or ambient temperature.The development of the high swirl combustion system is not complete yet. Preliminary comparison shows that the high swirl system has higher NOx emissions and poorer thermal efficiency. In addition, hydrocarbon emissions at idle and light load conditions are very high. These effects are probably due to quenching of the flame in the squish area.In a companion project a heavy duty truck was modified to receive a CNG fuel system and engine. A field demonstration of CNG engine technology is under way.