Analysis of the Multi-Zone SI-Engine Model for Fuel-Flexible Engine Design Applications

Development of fuel-flexible spark-ignition engines, working on CNG, LPG, hydrogen-enriched fuels or with mixtures of gaseous fuel/gasoline requires models for prediction of heat release rate, which can capture the effect of fuel composition and combustion chamber geometry on engine performance and emissions. Multi-zone models with explicit tracking of turbulent flame surface can be used for this purpose. Coupled with detailed chemical kinetic mechanisms, these models can also predict self-ignition of unburned charge ahead of the flame front. When optimizing engine performance and emissions in a fuel-flexible mode, the key question is sensitivity of the multi-zone model parameters to the properties of the fuel. In the present work, the multi-zone model of the CFR engine is developed based on Blizard-Keck eddy burn-up flame propagation approach for prediction of flame propagation and heat release rate. Model parameters are time constant for flame development, eddy entrainment rate constant for unburned mixture, time constant of burn-up in turbulent eddies. The simulations are performed for number of fuels: PRF and LPG components. The set of mentioned parameters is determined for every fuel and corresponding compression ratios used in experiments. It is shown that these parameters demonstrate only a slight sensitivity to fuel selected and can be used for prediction of fuel effects on engine performance. It is identified that for the model parameters to be true constants, the correct selection of turbulence integral length scale is required. Also, it is shown that the accurate correlation for laminar flame speed as function of unburned mixture state is required for the model to provide reasonable accuracy and predictive capability. New laminar flame speed correlations for n-butane, iso-butane and propylene are proposed.