Lean Mixture Ignition Systems for CNG in Diesel Applications 2004-28-0017
A high compression ratio, single cylinder, open chamber diesel engine was converted to operate on homogenously charged compressed natural gas (CNG) with the aim of minimising pollutant emissions such as oxides of nitrogen, particulate matter and carbon dioxide. Three ignition systems were tested including spark ignition (SI), diesel pilot ignition (DPI) and hydrogen assisted jet ignition (HAJI). Irrespective of ignition system used, the efficiency of the engine operating on CNG was significantly reduced at part load compared to diesel. This was predominantly due to a greater amount of unburnt hydrocarbons, higher cycle-by-cycle variability, slow and partial burns and increased heat transfer to the walls. DPI and HAJI systems were able to extend the lean limit to lambda 2.7 and 3.3 respectively, however this did not result in efficiency gains. HAJI proved to be superior to DPI with higher peak efficiency, lower carbon dioxide, carbon monoxide and particulates, and significantly lower oxides of nitrogen in the absence of a locally rich ignition source.
Control of the air/fuel ratio through the combined use of HAJI and supercharging demonstrated that oxides of nitrogen can be controlled, even at higher loads, with little or no loss in efficiency. At 600kPa BMEP, NOx emissions were reduced to one-fortieth of their original value whilst maintaining an indicated efficiency of 40%.