The most economical way to convert truck and bus DI-diesel engines to natural gas operation is to replace the injector with a spark plug and modify the combustion chamber in the piston crown for spark ignition operation. The modification of the piston crown should give a geometry well suited for spark ignition operation with the original swirling inlet port.Ten different geometries were tried on a converted VOLVO TD102 engine and a remarkably large difference in the rate of combustion was noted between the chambers. To find an explanation for this difference a cycle resolved measurement of the in-cylinder mean velocity and turbulence was performed with Laser Doppler Velocimetry (LDV).The results show a high correlation between in cylinder turbulence and rate of heat release in the main part of combustion. The very early part of combustion is more affected by other parameters but the intermediate part of combustion corresponding to 0.5-10% of the total heat released is influenced by both mean velocity and turbulence. There is a surprisingly good correlation between the average level of turbulence and the used squish area.
The objective of this paper is to investigate how the combustion chamber design will influence combustion parameters and emissions in a natural gas SI engine.Ten different geometries were tried on a converted Volvo TD102 engine. For the different combustion chambers emissions and the pressure in the cylinder have been measured. The pressure in the cylinder was then used in a one-zone heat-release model to get different combustion parameters. The engine was operated unthrottled at 1200 rpm with different values of air/fuel ratio and EGR. The air/fuel ratio was varied from stoichiometric to lean limit. EGR values from 0 to 30% at stoichiometric air/fuel ratio were used.The results show a remarkably large difference in the rate of combustion between the chambers. The cycle-to-cycle variations are fairly independent of combustion chamber design as long as there is some squish area and the air and the natural gas are well mixed.Geometries that give the fastest combustion give the highest NO x values at λ=1.2, but at λ>1.5, which is normally designated lean-burn, the differences are smaller. The lowest NO x values for lean burn were obtained with the geometries that gives fast combustion.The HC emissions display some correlation between high combustion rate and low levels of HC emissions, but combustion chambers with dead zones and large total combustion chamber areas give higher HC contents than the combustion rate alone would indicate.Indicated efficiency is reduced for combustion chambers with a large total combustion chamber surface area and thus large heat losses. High levels of turbulence also tend to reduce the efficiency for the same reason.
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