Improvement of DME HCCI Engine Performance by Fuel Injection Strategies and EGR 2008-01-1659
The combustion and exhaust emission characteristics of a DME fueled HCCI engine were investigated. Different fuel injection strategies were tested under various injection quantities and timings with exhaust gas recirculation (EGR). The combustion phase in HCCI was changed by an in-cylinder direct injection and EGR, due to changes in the in-cylinder temperature and mixture homogeneity. The gross indicated mean effective pressure (IMEPgross) increased and the hydrocarbon (HC) and carbon monoxide (CO) emissions decreased as the equivalence ratio was augmented. The IMEPgross with direct injection was greater than with the port injection due to retarded ignition timing resulting from latent heat of direct injected DME fuel. It was because that most of burn duration was completed before top dead center owing to higher ignitability for DME with high cetane number. However, HC and CO emissions were similar for both injection locations. In the case of direct injection, the IMEPgross increased until direct injection timing was 260 crank angle degrees (CAD) and decreased afterwards. In the case of port injection, the injection timing was not an influential parameter for IMEPgross change as the combustion characteristic changed from a relatively lean to locally-rich combustion due to a change in the mixture homogeneity with the delay of direct injection timing. Locally-rich combustion is supposed to lead to a higher combustion temperature, resulting in nitrogen oxide (NOx) formation at the retarded injection timing. In this study, optimal direct injection timing for HCCI combustion was approximately 260 CAD. NOx increased after direct injection timing was retarded more than 260 CAD. EGR could increase IMEPgross because combustion phase was shifted from before to after top dead center due to tardy combustion, while HC and CO emissions increased due to the lower combustion temperature. The combustion efficiency was improved by direct injection and a higher equivalence ratio, resulting in a higher combustion temperature. The thermal efficiency was increased due to late combustion, as the EGR rate was augmented and the direct injection timing was retarded. Additionally, there was a drop of thermal efficiency due to an increased equivalence ratio resulting in early auto-ignition.