Dilute operation of a SI engine offers attractive performance incentives. Lowered combustion temperatures and changes in the mixture composition inhibit NOx formation and increase the effective value of the ratio of burned gas specific heats, increasing gross indicated efficiency. Additionally, reduced intake manifold throttling minimizes pumping losses, leading to higher net indicated efficiency. These benefits are offset by the reduced combustion speed of dilute fuel-air mixtures, which can lead to high cycle-to-cycle variation and unacceptable engine behavior characteristics.
Hydrogen enhancement can suppress the undesirable consequences of dilute operation by accelerating the combustion process, thereby extending the dilution limit. Hydrogen would be produced on-board the vehicle with a gasoline reforming device such as the plasmatron. High dilution at higher loads would necessitate boosting to meet the appropriate engine specific power requirements.
Experiments were performed on a 12:1 compression ratio, single cylinder research engine to study the effects of hydrogen enhancement with both lean and EGR-diluted mixtures. Various parameters were monitored including overall efficiency, NOx emissions, and combustion speed. Results under partial load conditions show that lean dilution can improve engine efficiency by as much as 12 percent while EGR dilution delivers 8 percent improvement.
Either form of dilution can reduce NOx emissions by 98 percent or more when the engine is operated close to the dilution limit. While the efficiency improvement trend is similar for both forms of dilution, NOx emissions behavior differs. Lean dilution causes NOx levels to peak near a relative air-fuel ratio, lambda (λ), of 1.1 and then decline out to the dilution limit whereas increased levels of exhaust dilution always lead to decreased NOx emissions. Converting these data for lean and EGR operation to a thermal dilution parameter (based on the thermal capacity of the diluent) shows that the effect of EGR - at equal dilution - on NOx is substantially greater than the effect of excess air.