Thermal Efficiency Enhancement of a Turbocharged Diesel Engine Dedicated for Hybrid Commercial Vehicle Application

Hybrid powertrain has been proven to be an effective fuel-saving technology in commercial vehicles, but many hybrid commercial vehicles still use conventional diesel engines, resulting in limited fuel savings. The main purpose of this study is to enhance the thermal efficiency of a dedicated hybrid diesel engine focusing on the characteristic operating conditions. Via fundamental thermodynamics process analysis of internal combustion engine, steel piston with high compression ratio, air system involving two-stage turbocharger(2TC) with an intercooler, and late intake valve closing(IVC) timing are proposed to improve the thermal efficiency of the engine. Experimental results show that high compression ratio and lower thermal conductivity of the combustion chamber surface lead to lower heat release rates, requiring optimization of piston profile to accelerate the mixing rate. Besides, high compression ratio also leads to higher mechanical losses. As a result, high compression ratio only improves BSFC at medium load. Steel piston improves BSFC due to low thermal conductivity and higher combustion peak pressure. 2TC system with an intercooler evidently improves the brake specific fuel consumption (BSFC) by reducing pumping loss and raising air fuel ratio. Pumping loss is obviously improved by late IVC timing. Although the air flow rate is reduced by late IVC timing, CA50-CA90 becomes shorter. The BSFC is slightly improved by late IVC timing. The IVC timing of -92 °CA after top death center(ATDC) is acceptable for the hybrid dedicated diesel engine. These strategies have led to a reduction in minimum BSFC from 187 g/kWh to 178.5 g/kWh and a wide range of the high-efficiency region. Further, NOx emission increases significantly at the low speed medium load and high load conditions, increasing the demand for higher conversion efficiency after-treatment system..