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 membrane-based 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 oxygen-enrichment 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 °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 °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 NOx emissions evidently increased. [ABSTRACT FROM AUTHOR]