Particulates from a CNG DI SI Engine during Warm-Up

To assist efforts reducing harmful emissions from internal combustion engines, particulate formation was investigated in a compressed natural gas (CNG) Direct Injection single-cylinder SI engine in warm-up conditions. This involved tests at low engine speed and load, with selected engine coolant temperatures ranging from 15 to 90 °C, and use of a gasoline direct injection (GDI) system as a standard reference system. Total particulate number (PN), their size distribution, standard emissions, fuel consumption and rate of heat release were analyzed, and an endoscope with high-speed video imaging was used to observe combustion luminescence and soot formation-related phenomena.

The results show that PN was strongly influenced by changes in coolant water temperature in both the CNG DI and GDI systems. However, the CNG DI engine generated 1 to 2 orders of magnitude lower PN than the GDI system at all tested temperatures. The PN decreased in both systems when the coolant temperature increased. The results also show that PN was sensitive to a broader engine coolant temperature range in the GDI system. However, PN was around two orders of magnitude higher at the lowest coolant temperature (15 °C) than at the highest temperature (90 °C) in the CNG DI system. In homogeneous CNG combustion (unlike gasoline combustion) high-speed video images revealed no diffusion or yellow flame anywhere in the cylinder, even at the lowest coolant temperature. Thus, no soot formation location could be determined from the images in CNG cases. Overall, engine measurements showed that the CNG DI engine emitted lower standard emissions (CO₂, CO, HC, NO_x) and PN than the GDI system across the experimental range of engine coolant temperatures.