Analysis of diesel pilot-ignited natural gas low-temperature combustion with hot exhaust gas recirculation

Earlier efforts demonstrated the low NOx(< 0.2 g/kWh) and high efficiency (> 40%) benefits of the low temperature, advanced low pilot injection natural gas ( ALPING) combustion concept that utilized advanced injection (about 60 degrees BTDC) of small diesel pilots (2-3% on an energy basis) to compression-ignite a premixed natural gas-air mixture. At these injection timings, combustion was accompanied by increased unburned hydrocarbons (HC) ( mostly methane) and variations in torque fluctuations. In this article, hot exhaust gas recirculation (EGR) is proposed as a potential strategy to reduce HC emissions and torque fluctuations at low ( quarter and half) loads. It is shown that the addition of hot EGR leads to a combination of one or more of the following effects on the intake mixture entering the cylinder: oxygen depletion, increased temperatures due to mixing with exhaust gases, dilution due to introduction of high specific heat species, and active recycling of unburned hydrocarbons to effect reburn in subsequent cycles. In particular, hot EGR addition extends the ALPING operation regime from 50 degrees-60 degrees BTDC to 60 degrees-70 degrees BTDC, increases low-load efficiencies by more than 5 percentage points, substantially improves combustion stability, and drastically reduces HC emissions ( by more than 70%) with little associated penalty in NOx emissions.