Abstract
Mixture quality in a CAI engine is one of the key parameters that affect the higher autoignition reliability and thermal efficiency. In this paper, four intake-valve strategies were applied to control the flow characteristics of the intake air. The variation of the intake valve lift induces different swirling and tumbling intensities of the air flow. Also, the direct fuel injection into the cylinder can be used as one of the most effective means of controlling the combustion phasing because it can form various mixtures. In this study, 3D full cycle simulations were carried out to investigate the effects of intake valve strategies and fuel injection timings on the auto-ignition and to obtain subsequent combustion and emission characteristics of the CAI engine. Computational results showed that a valve lift of 6 mm of one of the intake valves with another intake valve being closed yielded the largest swirling (helical motion toward the axial direction) and tumbling, which in turn rendered the optimal fuel-gas mixing. We also found that the fuel distribution affected the auto-ignition sites (or spot) and fuel injection timing had a large effect on the combustion characteristics. The combustion phasing was advanced, as the injection was retarded further into the compression stroke. This is more likely due to the mixture stratification. Also, the presence of over-lean and over-rich mixture increased the peak pressure and reduced the combustion duration. Regarding the emission characteristics, NO emission is known to be heavily dependent on the local temperature of the cylinder. In most cases, a higher peak cylinder temperature leads to higher NO emission rate.
Keywords: CAI (Controlled Auto-Ignition), Uniformity, Swirl ratio, SOI (Start Of Injection)