Influence of Engine Speed and Injection Phasing on Lean Combustion for Different Dilution Rates in an Optically Accessible Wall-Guided Spark Ignition Engine

Alternative combustion control in the form of lean operation offers significant advantages such as high efficiency and “clean” fuel oxidation. Maximum dilution rates are limited by increasing instability that can ultimately lead to partial burning or even misfires. A compromise needs to be reached between high tumble-turbulence levels that “speed-up” combustion and the inherent stochastic nature of this fluid motion. The present study is focused on gaining improved insight into combustion characteristics through thermodynamic analysis and flame imaging, in a wall-guided direct injection spark ignition engine with optical accessibility. Engine speed values were investigated in the range of 1000 to 2000 rpm, with commercial gasoline fueling, in wide open throttle conditions; mixture strength ranged from stoichiometric, down to the equivalence ratios that allowed acceptable cycle-by-cycle variations; and all cases featured spark timing close to the point of maximum brake torque. The effect of injection phasing was also scrutinized, with three different settings of start of injection during the intake stroke. For the “leanest” cases, fuel delivery timing exerted a significant influence on combustion stability; overall performance was comparable for all three settings. These results were further detailed with the evaluation of macro- and micro-characteristics of the flame front, as well as flame intensity that was correlated to the evolution of bulk oxidation and locally rich regions caused by fuel impingement. Natural flame emission spectroscopy revealed that indeed, the nature of the two flame types was related to “normally,” premixed combustion-related propagating fronts in the first case, while for the latter, “soot production” was the driving mechanism. The experimental results were also correlated to simulations of characteristic length within the reaction zone, using a quasi-dimensional approach. Both types of data confirmed a reduction of the characteristic length at high dilution rates, in line with results obtained on burner flames at high pressure.