Abstract
A single cylinder, high compression ratio (16.5:1), open chamber diesel engine has been converted to operate on compressed natural gas (CNG). Diesel pilot ignition, spark ignition and Hydrogen Assisted Jet Ignition (HAJI) mechanisms were individually examined to see their effect on lean burn CNG performance. This performance was subsequently compared to diesel only fueling.
Results with CNG fueling have shown similar full load power output and thermal efficiency at the expense of higher oxides of nitrogen when compared to diesel-only operation. In the mid to light load region the efficiency was significantly reduced because of increased amounts of unburnt hydrocarbons, higher cycle-by-cycle variability, slow and partial burns and increased heat transfer to the walls. The mid to light load region was nevertheless accompanied by significant improvements in oxides of nitrogen and carbon dioxide emissions. Of the three ignition systems tested, the HAJI and diesel pilot systems offered a leaner misfire limit than spark ignition but this did not result in efficiency gains. The HAJI mode had better all round performance with higher peak efficiency, lower carbon dioxide and particulates, and significantly lower oxides of nitrogen in the absence of a locally rich ignition source.