A single-cylinder, light-duty, diesel engine was used to investigate the effect of changes in intake pressure (boost) on engine performance and emissions in low-temperature combustion (LTC) regimes. Two different LTC strategies were examined: a dilution-controlled regime characterized by high rates of exhaust gas recirculation (EGR) with early-injection (roughly 30° BTDC), and a late-injection (near TDC) regime employing moderate EGR levels. For both strategies, moderate (8 bar IMEP) and low (3 bar IMEP) load conditions were tested at intake pressures of 1.0, 1.5, and 2.0 bar.
For both LTC strategies, increased intake pressure reduces emissions of unburned hydrocarbons (UHC) and CO, with corresponding improvements in combustion efficiency and indicated specific fuel consumption (ISFC), particularly at high load. Depending on the operating condition, UHC and CO emissions can stem from either over-lean or over-rich mixtures. UHC emissions can be further impacted by fuel from quench layers and liquid films. Increased intake pressure also reduces peak soot emissions at high load and shifts the peak soot emissions (the soot “bump”) towards lower oxygen concentrations. Due to this shift, the influence of intake pressure on soot emissions differs for different oxygen concentrations. Soot emissions are reduced with increased intake pressure at high oxygen concentrations, but increased at low oxygen concentrations. Already low NOx levels are reduced further at high intake pressures, though the influence of intake pressure is small compared to the influence of oxygen concentration.
Comparisons of the two LTC strategies at a fixed NOx emission index of 0.5 g/kg-fuel show that late-injection LTC offers improvements in engine noise and soot over dilution-controlled LTC. Conversely, dilution-controlled LTC yields lower emissions of UHC and CO and better combustion efficiency and ISFC than late-injection LTC.