Experimental investigation of partially premixed combustion in a DISI natural gas engine: Emissions behaviour

Experimental results of partially premixed combustion using direct injection of natural gas during the compression stroke in a spark ignited single cylinder research engine are presented. A constant overall 0.70 equivalence ratio was used, with 34 and 69 kPa gauge intake pressures. Two injector nozzle designs were employed, one with a flat injection plane and one with a narrower 100 degree injection cone. Injection timing was varied to change the degree of premixing, from fully premixed with injection near BDC, to stratified combustion with injection near TDC. The effects on engine-out emissions of NO, CO, unburnt hydrocarbons, and particulate matter were assessed, along with the combustion efficiency determined from exhaust gas composition. Later injection timings resulted in reduced time for mixing before ignition, concentrating the fuel mass and increasing the local equivalence ratios of combustion. Unburnt hydrocarbon levels decreased with later injection timings as the fuel concentrated and injection was directed into the piston bowl. NO increased as local equivalence ratios approached stoichiometric, before reducing at richer local equivalence ratios. Late injection timings resulted in order of magnitude increases in CO and particulate matter as the low mixing time resulted in combustion at high local equivalence ratios. Combustion efficiency reached a maximum at moderate degrees of premixing. The residence time of the fuel between the end of injection and ignition was found to be a critical metric in defining the resulting emissions. The cone nozzle confined the fuel to the piston bowl at earlier injection timings, resulting in lower unburnt hydrocarbon emissions. Operation at 66 CAD BTDC injection timing and 34 kPa gauge intake pressure (giving 34.2 CAD of residence time) with the cone nozzle was found to give the optimal emissions profile. Unburnt hydrocarbon emissions decreased by 73.8%, but CO levels increased by a factor of 7.5 from 1338 to 11443 ppm compared to fully premixed operation with injection near BDC. NO levels were moderate at 612 ppm, and particulate matter emissions increased moderately to 5.6E+06 #/cm3, compared to the motored concentration of 3.1E+05 #/cm3. The combustion efficiency of 90.3% was moderate in comparison to other operating points. •Later injections during compression stroke gave higher spatial fuel concentration.•NO emissions increased with combustion locally near stoichiometric.•Hydrocarbon emissions fell with fuel