High-speed flow and fuel imaging study of available spark energy in a spray-guided direct-injection engine and implications on misfires

The spark energy transferred under the highly stratified conditions during late injection in a spray-guided spark-ignition direct-injection (SG-SIDI) engine is not well characterized. The impact of high pressures, temperatures, velocities, and variations in local fuel concentration along with temporal and/or spatial variations on spark performance must be better characterized. Previous spark ignition studies have not addressed the full range of conditions that are present in SG-SIDI engines. Therefore, high-speed particle image velocimetry (PIV) experiments are conducted to characterize the spark energy dependence for a wide range of well-defined homogeneous fuel–air equivalence ratios (W50–2.9) and average air velocities (0–8 m/s) in an optical SG-SIDI engine. A moderate dependence of spark energy on equivalence ratio is shown to exist with average values of spark energy increasing by 21 per cent for the equivalence ratio range of W50– 2.3. Air injection into the motored engine is used to prepare well-defined flow conditions without the complications of fuel concentration gradients that are present during fuel injection. This allows the study of the effects of velocity, shear strain rate, and vorticity on spark energy. The spark energy increases with velocity at the spark plug. This observation is consistent with findings reported in the literature for low-pressure conditions. A linear increase is shown between shear strain rate and spark energy, while vorticity and spark energy are only weakly correlated. Simultaneous high-speed PIV, planar laser-induced fluorescence, and spark-discharge electrical measurements are also performed in the optical SG-SIDI engine to measure flow properties and fuel concentrations under late injection. Operating parameters are chosen to be near peak indicated mean effective pressure performance, but they occasionally provide a random misfired or partial burned cycle. Misfired cycles occur under stoichiometric-to-lean mixtures and low velocities near the spark plug. The lower spark energies observed under these conditions are in agreement with the observations made under well-controlled mixture and flow conditions reported in this study. All mixture conditions found in misfiring and partially burning cycles are within the ignitability range and fall within the general population of all, predominantly well-burning, cycles. There is no predominant impact of shear strain rate and vorticity under late injection operation on misfires and partial burns.