Measurements of ignition behavior, homogeneous reactor simulations employing detailed kinetics, and quantitative in-cylinder imaging of fuel-air distributions are used to delineate the impact of temperature, dilution, pilot injection mass, and injection pressure on the pilot ignition process. For dilute, low-temperature conditions characterized by a lengthy ignition delay, pilot ignition is impeded by the formation of excessively lean mixture. Under these conditions, smaller pilot mass or higher injection pressures further lengthen the pilot ignition delay. Similarly, excessively rich mixtures formed under relatively short ignition delay conditions typical of conventional diesel combustion will also prolong the ignition delay. In this latter case, smaller pilot mass or higher injection pressures will shorten the ignition delay. The minimum charge temperature required to effect a robust pilot ignition event is strongly dependent on charge O2 concentration. Measured fuel-air ratio distributions, coupled with the homogeneous reactor simulations, show this temperature dependency to be due to both kinetics and over-mixing caused by the long ignition delay associated with dilute mixtures.
The simulations also show that the impact of dilution on the ignition process is dominated by the added heat capacity of the diluent rather than by changes in the kinetic pathways. For practical dilution levels (14-20% charge O2 concentration) and moderate near-TDC temperatures (850-900 K), the simulations indicate that the optimal equivalence ratio to promote pilot ignition is between approximately 1.0 and 1.6, with the richer mixtures being appropriate to more dilute conditions. Lastly, pilot injection strategies are applied to a low-load, highly premixed operating condition. Robust pilot ignition events improve HC, CO and the COV of IMEP, but may deteriorate noise and soot emissions.
