Experimental Investigation of a Natural Gas Lean-Burn Spark Ignition Engine with Bowl-in-Piston Combustion Chamber 2019-01-0559

On- and off-road heavy-duty diesel engines modified to spark-ignition natural gas operation can reduce U.S. dependence on imported oil and enhance national energy security. Engine conversion can be achieved through the addition of a gas injector in the intake manifold and of a spark plug in place of the diesel injector. This paper investigated combustion characteristics and engine performance at several lean-burn operating conditions that changed spark timing, mixture equivalence ratio, and engine speed, using methane as NG surrogate. The results show that the bowl-in-piston geometry separated the combustion process into two distinct events: an inside-the-bowl burning (due to the squish effect) that had a short duration and consumed a high fraction of fuel, and a slower inside-the-squish burning process, most probably due to the large surface/volume ratio (that increased the heat transfer to the boundaries) and to the lower in-cylinder pressure and temperature during the expansion stroke. While the operating conditions affected the overlapping of these two combustion stages, conditions that increased their phasing separation produced a second peak in the rate of heat release. Moreover, despite the variations in the magnitude and location of the 2^nd peak in the rate of heat release and the lean-burn operation, the standard deviation of peak cylinder pressure and combustion phasing were below 3.5 bar and 2 CAD, respectively, for all the conditions investigated here, suggesting acceptable engine operation. As the combustion phenomena inside the squish region is complex, the combustion strategy for such engine should optimize the fraction of the fuel that burns inside the squish region for better efficiency and reduced emissions.