Thermodynamic modeling of HCCI combustion with recompression and direct injection.

摘要

Homogeneous Charge Compression Ignition (HCCI) engines have the potential to reduce pollutant emissions while achieving diesel-like thermal efficiencies. The absence of direct control over the start and rate of auto-ignition and a narrow load range makes implementation of HCCI engines into production vehicles a challenging affair. Effective HCCI combustion control can be achieved by manipulating the amount of residual gases trapped from the previous cycle by means of variable valve actuation. In turn, the temperature at intake valve closing and hence auto-ignition phasing can be controlled. Intake charge boosting can be used to increase HCCI fueling rates and loads, while other technologies such as direct injection provide means for achieving cycle to cycle phasing control.;Thermodynamic zero-dimensional (0D) models are a computationally inexpensive tool for defining systems and strategies suitable for the implementation of new HCCI engine technologies. These models need to account for the thermal and compositional stratification in HCCI that control combustion rates. However these models are confined to a narrow range of engine operation given that the fundamental factors governing the combustion process are currently not well understood. CFD has therefore been used to understand the effect of operating conditions and input variables on pre-ignition charge stratification and combustion, allowing the development and use of a more accurate ignition model, which is proposed and validated here.;A new empirical burn profile model is fit with mass fraction burned profiles from a large HCCI engine data set. The combined ignition model and burn correlation are then exercised and are shown capable of capturing the trends of a diverse range of transient HCCI experiments. However, the small cycle to cycle variations in combustion phasing are not captured by the model, possibly due to recompression heat release effects associated with variable valve actuation. Multi-cycle CFD simulations are therefore performed to gain physical insight into recompression heat release phenomena and the effect of these phenomena on the next cycle. Based on the understanding derived from this CFD work, a simple model of recompression heat release has been implemented in the 0D HCCI modeling framework.