Ignition Delay in Low Temperature Combustion

Low temperature combustion (LTC) strategies present a means of reducing soot and oxides of nitrogen (NOx) emissions while simultaneously increasing efficiency relative to conventional combustion modes. By sufficiently premixing fuel and air before combustion, LTC strategies avoid high fuel-to-air equivalence ratios that lead to soot production. Dilution of the mixture lowers the combustion temperatures to reduce NOx production and offers thermodynamic advantages for improved efficiency. However, issues such as high heat release rates (HRRs), incomplete combustion, and difficulty in controlling the timing of combustion arise with low equivalence ratios and combustion temperatures. Ignition delay (the time until the start of combustion) is a way to quantify the time available for fuel and air to mix inside the cylinder before combustion. Previous studies have used ignition delay to explain trends seen in LTC such as combustion stability and HRRs. This study provides a novel method of integrating ignition delay into the investigation of LTC to determine what insights ignition delay calculations can provide for the different challenges associated with LTC strategies. To eliminate the need for large chemical kinetic models and interpolation to predict ignition delay, an ignition delay correlation is used. The flexibility gained using the correlation allows this method to be quickly adapted to target different LTC strategies and challenges. In this work, three cases of low temperature gasoline combustion (LTGC) with varying stratification levels were evaluated with an emphasis on the non-monotonic trend of the HRRs. The described method for ignition delay calculation is used to detail the stratification and ignition delay distributions for each case and provide insight into the observed trends.