The Effects of Intake Valve Closing Timing on Engine Performance and Emissions in a DME Compression Ignition Engine at Low Load Cold Start Condition

Abstract

AbstractThe effective compression ratio reduction by means of late intake valve closing (LIVC) strategy was applied in a di-methyl ether (DME) compression ignition engine to investigate its potential effects on the engine performance and emission at cold start condition. The single injection timing of the DME was varied from the beginning to the end of compression stroke. The DME was injected directly into the cylinder with an injection pressure of 60 MPa. The indicated mean effective pressure (IMEP), heat release rate and combustion duration was investigated at two different intake valve closing (IVC) conditions—base IVC of 28 degree after bottom dead center (ABDC) and LIVC of 43.9 degree ABDC. The other experimental conditions such as injection duration and the environmental temperature remained fixed. The IMEP characteristics with respect to injection timings of two different IVC timing showed similar trend at conventional combustion regime. The IMEP distribution was shifted towards advanced injection timing direction for LIVC condition. In other words, the injection timing of LIVC condition had to be advanced compared to that of base IVC timing in order to produce equal power output. The reduction in the compression ratio had resulted in lower compression pressure and the temperature, so the ignition delay was increased and the overall heat release rate was retarded to retarded crank angle. However, the combustion characteristics in terms of combustion duration and the heat release rate curve did not show great differences at early injection timings (earlier than −30 crank angle degrees ATDC (after top dead center)). The NOx emission was reduced by around 10 % due to the reduced effective compression ratio. The prolonged ignition delay which enhanced the mixture homogeneity was also considered to have contributed on the reduction of NOx emission. The HC and CO emissions of LIVC condition were relatively higher than those of base IVC condition due to the lowered in-cylinder temperature. The smoke formation was low due to the intrinsic properties of DME. The exhaust gas temperature was higher for the LIVC timing condition. The expelled portion of the charge during the compression resulted in lower heat capacity of the working gas. The in-cylinder gas temperature increased more when the same amount of fuel energy input was delivered to the gas with lower heat capacity. It was found that, malfunction of the piezo injector occurred when applying DME fuel with inappropriate setup. It was assumed that the vaporization of the DME might occur inside the injector when the engine coolant temperature increased. The movement of the piezo stack was not able to be translated into injection events due to the gas phase inside the injector. The fuel injector was restored and able to maneuver with the injection events again when the fuel return line was pressured above the vapor pressure. In future, further experiments need to be carried out at fully warmed-up condition in order to reveal its potential of reducing effective compression ratio on improving engine performance.KeywordsDME (di-methyl ether)Compression ignitionLate intake valve timingEffective compression ratio