Investigation of Performance and Emission Characteristics of a Heavy Duty Natural Gas Engine Operated with Pre-Chamber Spark Plug and Dilution with Excess Air and EGR

This article deals with application of turbulent jet ignition technique to heavy duty multi-cylinder natural gas engine for mobile application. Pre-chamber spark plugs are identified as a promising means of achieving turbulent jet ignition as they require minimal engine modification with respect to component packaging in cylinder head and the ignition system. Detailed experiments were performed with a 6 cylinder 9.4 liter turbo-charged engine equipped with multi-point gas injection system to compare performance and emissions characteristics of operation with pre-chamber and conventional spark plug. The results indicate that ignition capability is significantly enhanced as flame development angle and combustion duration are reduced by upto 30 % compared to those with conventional spark plugs at certain operating points. Maximum possible dilution (limited by combustion stability index, Coefficient of Variation (COV) of Gross Indicated Mean Effective Pressure (IMEPg)) with excess air and EGR were investigated experimentally at engine speed of 1500 rpm and 5, 12 and 18 bar IMEPg operating load and results indicate that the lean limit is extended by 0.8-1 Lambda unit and 5-8% EGR rate units. It was also observed that pre-chamber spark plugs cause charge pre-ignition at loads exceeding 10-12 bar IMEPg. To avoid this, the minimum amount of dilution, with excess air and then EGR, which is required to operate above 10 bar IMEPg was estimated experimentally. Finally, to compare performance and emission characteristics of operation with these two ignition techniques, the engine was tested on an ESC-like (European Stationary Cycle) 12 mode cycle. Results indicated marginal reduction in cycle averaged NOx emissions with maximum excess air dilution and about 50% reduction with maximum EGR dilution, whereas CO and HC emissions increased. Other operating characteristics like the flame development angle, combustion duration, brake efficiency etc. are also compared for the tested operating range of the test engine.