The Effect of Homogeneous Lean Combustion on Efficiency and Emissions Trends in Natural Gas-Fueled Small Engines 2021-01-0652

Alternative combustion modes for spark ignition engines, such as homogeneous lean combustion, have been extensively researched in transportation and large stationary power applications due to their inherent emissions and fuel efficiency benefits. However, these types of approaches have not been explored for small engines (≤ 30 kW), as the various applications for these engines have historically had significantly different market demands and less stringent emissions requirements. However, going forward, small engines will need to incorporate new technologies to meet increasingly stringent regulatory guidelines. One such technology is jet ignition, enables lean combustion via air dilution through the use of a pre-chamber combustor.

This work constitutes a continuation of the development of a small-displacement natural gas-fueled jet ignition engine as part of the Department of Energy’s Advanced Research Projects Agency-Energy (DOE ARPA-E) GENerators for Small Electrical and Thermal Systems (GENSETS) program. In this study, a jet igniter concept previously optimized for use in a prototype single cylinder, 390cc engine (Gen1) was incorporated into a modified Kohler Aegis 2-cylinder, 747cc engine (Gen2). The Gen2 engine was operated in two configurations with ignition initiated either via (1) the jet igniter or (2) a conventional centrally-located spark plug. Analysis of a parametric sweep of normalized air-fuel ratio, λ, and engine power elucidates the mechanisms for improving combustion stability with jet ignition over conventional spark ignition under lean conditions (λ>1.4). An efficiency loss breakdown based on the First Law of Thermodynamics highlights the thermal efficiency benefits of lean operation. Furthermore, it is shown how lean operation in conjunction with downspeeding is used to achieve a peak brake thermal efficiency of 37.6% at a 5kW power level in the Gen2 engine. These results are compared and contrasted with those of the Gen1 engine in order to evaluate relative changes in efficiency loss mechanisms with moderate increases in power in the sub-10kW range. Finally, the implications of these results on the viability of robust lean combustion for the small engine segment are discussed.