Improvement of thermal efficiency of a high-boosted diesel engine with focus on peak cylinder pressure

The combination of high boosting and high exhaust gas recirculation (EGR) is a practical and effective strategy to achieve simultaneous reductions of fuel consumption and exhaust emissions in diesel engines. To obtain further improvements of fuel efficiency and emissions, the effects of engine parameters including compression ratio, peak cylinder pressure, and the timing of the start of combustion are investigated using a single-cylinder diesel engine under conditions of high boost pressure of 400 kPa and fuel injection pressure of 200 MPa, focusing particularly on the peak cylinder pressure. The experiments show that when the peak cylinder pressure is raised to 28 MPa by adjusting the timing of the start of combustion at −3° after top dead centre (ATDC) at a compression ratio of 18, the lowest specific fuel consumption rate of 185.6 g/kW h, which corresponds to the brake thermal efficiency of 45.2 per cent, is obtained. When the effective compression ratio is lowered by using a variable valve timing system while maintaining the peak cylinder pressure at 28 MPa, the minimum specific fuel consumption of 181.2 g/kW h, which corresponds to a brake thermal efficiency of 46.3 per cent, is obtained.