Effect of Increasing Compression Ratio in a Light-Duty Natural Gas-Fueled Engine on Efficiency and Emissions

As a result of CAFE (corporate average fuel economy) requirements, the trend in passenger car engine design is to smaller displacement engines of higher specific output which provide reductions in vehicle driving cycle fuel consumption without an accompanying decrease in maximum power output. Design features such as four valves per cylinder and compact combustion chambers give these engines significantly different combustion characteristics than traditional pushrod OHV (overhead valve) engines. In general, their combustion chambers are fast burning, enabling the use of higher compression ratios without knock on unleaded gasoline. Since fuel consumption decreases with increasing compression ratio, and since natural gas has a substantially higher octane rating than the best unleaded gasoline, it would appear to be desirable to operate with even higher compression ratios in a dedicated natural gas engine.

Tests at different speeds and loads were carried out with a natural gas fueled, 16 valve DOHC (dual overhead cam) two-litre four-cylinder engine representative of current passenger car design. Compression ratios of 10.0 and 11.5 were evaluated. Results show that increasing the compression ratio has little effect on NO_x but increases unburned hydrocarbons emissions. Modest part load fuel consumption improvements were observed. Torque output at WOT (wide open throttle) increased, but did not reach the value obtained with gasoline fueling at the lower compression ratio tested.