Thermal efficiency improvement with super

Thermal efficiency–related parameters in semi-premixed diesel combustion with a twin peak shaped heat release were experimentally investigated in a 0.55-L single-cylinder diesel engine. Here, the first heat release peak is realized with the premixed combustion at top dead center after the end of the first fuel injection with a sufficient ignition delay. The fuel injection quantity for the first combustion was maximized in a range to limit the rate of pressure rise below 0.6？MPa/°CA at 0.4？MPa IMEP, 0.8？MPa/°CA at 0.8？MPa IMEP, and 1.0？MPa/°CA at 1.3？MPa IMEP to ensure the large degree of constant volume heat release and to suppress smoke emissions. The second heat release peak is formed from the rate-controlled combustion with the second fuel injection immediately after the end of the first combustion. The influence of the intake oxygen concentration and the intake gas pressure on the thermal efficiency and the exhaust gas emissions was systematically examined at three load conditions (indicated mean effective pressure ≈0.4, 0.8, and 1.3？MPa). The results with two types of combustion chambers, a toroidal chamber expecting smaller cooling losses with weaker in-cylinder gas motion, and with a re-entrant chamber expecting better air utilization with stronger in-cylinder gas motion are compared. At the medium load, a significantly high indicated thermal efficiency exceeding 50% is established with a reduction in the intake oxygen concentration due to the smaller cooling loss. The indicated thermal efficiency improves with a decrease in the intake oxygen concentration as the reduction in the cooling loss is more significant than the increase in the exhaust loss. However, an excessive reduction in the intake oxygen concentration results in a deterioration in the indicated thermal efficiency due to a reduction in the combustion efficiency. At low load conditions, the indicated thermal efficiency is lower than at the medium load due to lower combustion efficiency and the improvement in the indicated thermal efficiency with reductions in the intake oxygen concentration is not significant as the combustion efficiency decreases with the decrease in the intake oxygen concentration. At the high load condition, the indicated thermal efficiency is lower due to a larger exhaust loss than at the low and medium load conditions and the indicated thermal efficiency decreases with the decrease in the intake oxygen concentration. With an increase in the intake gas pressure, the indicated thermal efficiency increases consistently mainly due to the reducing cooling loss. In