The use of direct fuel injection in Homogeneous Charge Compression Ignition (HCCI) engines has shown potential to improve combustion and expand the engine operating range. The current study analyzes experimental and CFD simulation results to obtain enhanced insight into the nature of the stratified-charge HCCI combustion system in two different research engines: a CFR engine and a Yamaha engine.
In the direct-injection CFR-engine study, current simulations showed the mixture formation was strongly influenced by the momentum of the spray. Based on the modeling results, varying the injection timing from 300? to 180? bfTDC produced a small change in the charge stratification in the fuel-air mixture. Retarding the injection timing from 180? to 45? bfTDC showed a significant change in the distributions of equivalence ratio and gas temperature. The degree of charge stratification and the residence time that the mixture is exposed to high temperatures are of importance to avoid high NOx generation. Results from the kinetic rate calculations suggest that in a stratified air-fuel mixture, high NOx are likely to be generated from the regions where the equivalence ratio is greater than 0.5.
In the Yamaha engine study, the use of direct fuel injection during a negative valve overlap (NVO) period produces fuel reformation and NVO heat release that increases the gas temperature in the cylinder prior to intake valve closure. The amount of NVO energy release was found to be affected by the percentage and the timing of NVO fuel injection, the oxygen concentration in the exhaust, and the residual gas temperature. The findings in this study indicate that the use of NVO fuel injection has the ability to control the combustion phasing, expand the operating range, and improve the combustion efficiency for an HCCI engine.