Modeling Alternative Prechamber Fuels in Jet Assisted Ignition of Gasoline and LPG 2009-01-0721
Gas assisted jet ignition is a prechamber combustion initiation system for conventional spark ignition engines. With the system, a chemically active turbulent jet is used to initiate combustion in lean fuel mixtures enabling reliable combustion over a much broader range of air-fuel ratios. The extended range is due to the distributed ignition source provided by the jet, which can overcome the problems of poorly mixed main chamber charges and slower burning fuels. In addition, the ability to reliably ignite lean mixtures improves the thermal efficiency and enables ultra low emission levels.
Experiments together with flame propagation modeling completed using STAR-CD with CHEMKIN Kinetics were done in order to examine the effects of numerous prechamber fuels on the ignition of the main fuel, which consisted of either liquefied petroleum gas (LPG) or gasoline. The modeling and experimental results showed similar trends, with the modeling results giving insight into the physicochemical process by which main fuel combustion is initiated in the jet ignition process.
Both the modeling and experimental results indicate that the level of ignition enhancement provided by jet ignition is highly dependent on the generation of chemical species and not solely on the energy content of the prechamber fuel. In addition, the effects of the rich jet ignition prechamber are examined and reveal that the chemical properties of the rich prechamber jet are essential in promoting lean main chamber combustion in the jet ignition process and that the thermal and turbulence effects of the jet play only a secondary role.
Experimental and modeling results both showed that although hydrogen was the most effective prechamber fuel with this jet ignition process, especially when operating in the ultra lean region, other alternative fuels were effective at operating between λ=2-2.5. These lean limits are about twice the value possible with spark ignition (λ=1.25-1.4) in this engine at similar load conditions. In addition, the LPG results are very encouraging as they offer the possibility of a system similar to HAJI but using a commonly available fuel in both the main chamber and prechamber.