Improvement of startability of direct-injection diesel engines by oxygen-enriched intake air

The startability of diesel engines degrades as the ambient temperature falls, in particular, to the borderline or lower temperature. The essential cause is the deterioration of in-cylinder combustion resulting from worsening of combustion conditions. A membranebased oxygen-enriched intake air (OEA) method is proposed to improve in-cylinder combustion and then to reduce exhaust emissions during the start-up process. This paper investigates the effects of OEA on startability and exhaust emission behaviour during the engine start-up. The experiments were carried out on a direct-injection diesel engine. A membrane-based oxygenenrichment system was constructed to supply the test engine with intake air containing 21 vol %, 23 vol %, 25 vol %, and 27 vol % oxygen respectively. The in-cylinder pressure, instantaneous engine speed, and time-resolved engine-out emissions were recorded and compared for the OEA and the baseline methods (ambient air (AA), 21 vol % oxygen). The results show that, at the high intake air temperature (26'C), the engine could start smoothly. When the intake air temperature fell to the borderline temperature (5 degrees C), the startability significantly degraded, in-cylinder incomplete combustion was evident during the initial stage of start-up, and the start-up time increased. With OEA at 5 degrees C, the startability was markedly improved, and the engine experienced fewer cycles than those at the same intake air temperature with AA before it reached idle operation. With OEA, the engine-out hydrocarbon, CO, and smoke emissions throughout the whole start-up process were all reduced considerably, but NO, emissions evidently increased.