Effect of Fuel and Thermal Stratifications on the Operational Range of an HCCI Gasoline Engine Using the Blow-Down Super Charge System

In order to extend the HCCI high load operational limit, the effects of the distributions of temperature and fuel concentration on pressure rise rate (dP/dθ) were investigated through theoretical and experimental methods. The Blow-Down Super Charge (BDSC) and the EGR guide parts are employed simultaneously to enhance thermal stratification inside the cylinder. And also, to control the distribution of fuel concentration, direct fuel injection system was used.

As a first step, the effect of spatial temperature distribution on maximum pressure rise rate (dP/dθmax) was investigated. The influence of the EGR guide parts on the temperature distribution was investigated using 3-D numerical simulation. Simulation results showed that the temperature difference between high temperature zone and low temperature zone increased by using EGR guide parts together with the BDSC system. Experiments were conducted by using a four-cylinder gasoline engine equipped with the BDSC with EGR guide system to investigate the effect of the EGR guide on the heat release rate and the in-cylinder pressure rise rate. Experimental results showed that 50% and 90% mass fraction burned timing (CA50 and CA90) were delayed and combustion duration became longer when the EGR guide was used to enhance thermal stratification. As a result, the maximum pressure rise rate could be decreased and the HCCI high load limit successfully extended. Meanwhile 10% mass fraction burned timing (CA10) was not affected by the thermal stratification generated by the EGR guide. This is probably because the fuel is also spatially stratified such that the fuel concentration becomes lean in the high temperature zone.

Next, the effect of the fuel distribution on high load HCCI operation was investigated. Numerical analysis using a multi-zone combustion model considering detailed chemical reactions was carried out. The simulation results showed that the maximum pressure rise rate was decreased by 27% when the fuel distribution was uniform with temperature distribution generated by the BDSC with EGR guide system. Then, to obtain a uniform fuel distribution while keeping the temperature distribution generated by the BDSC with EGR guide system, a direct injection system was employed and the effect of fuel direct injection on maximum pressure rise rate was experimentally investigated. As a result, if 20% of the total fuel was injected directly into the cylinder during the exhaust stroke, the spatial distribution of the fuel concentration (G/F; fuel mass ratio to the total mass of in-cylinder mixture) became more homogeneous and maximum pressure rise rate was decreased by 10%. And also, 20% of the total fuel directly injected during the compression stroke, maximum pressure rise rate was decreased by 20%.

Finally, a simple method to predict the ignition timing using Livengood-Wu integral and 1-D numerical simulation code was examined. It was found that the proposed method can predict the ignition timing with small deviation around ±2 degrees.