A High Efficiency, Dilute Gasoline Engine for the Heavy-Duty Market 2012-01-1979

A 13 L HD diesel engine was converted to run as a flame propagation engine using the HEDGE™ Dual-Fuel concept. This concept consists of pre-mixed gasoline ignited by a small amount of diesel fuel - i.e., a diesel micropilot. Due to the large bore size and relatively high compression ratio for a pre-mixed combustion engine, high levels of cooled EGR were used to suppress knock and reduce the engine-out emissions of the oxides of nitrogen and particulates. Previous work had indicated that the boosting of high dilution engines challenges most modern turbocharging systems, so phase I of the project consisted of extensive simulation efforts to identify an EGR configuration that would allow for high levels of EGR flow along the lug curve while minimizing pumping losses and combustion instabilities from excessive backpressure. A potential solution that provided adequate BTE potential was consisted of dual loop EGR systems to simultaneously flow high pressure and low pressure loop EGR. The use of this blended EGR concept allows the high EGR engine to fit on the compressor map, although with high levels of pumping work compared to the baseline diesel.

The combustion experiments were carried out at 17:1 and 14.5:1 CR with gasoline levels varying from 60% to 90% of the total fuel. The results at 17:1 CR showed that, even with 50% EGR, the load was limited by knock and combustion noise. At 14.5:1 CR, the load was again limited by combustion noise and knock, but was much closer to the baseline engine with acceptable combustion noise and low engine-out emissions. The fuel economy potential of the engine was shown to be high, with results of 195 g/kWh total (gasoline + diesel) at the A50 condition, while the high levels of EGR resulted in NOx and PM below the 2010 HD standard of 0.27 g/kWh NOx and 0.1 g/kWh PM respectively. A comparison between the two different gasoline-diesel dual fuel operations - i.e., diesel micro-pilot operation and PPC-style operation shows the tradeoffs between the two combustion modes and challenges of each.