Thermal Efficiency Enhancement of a Turbocharged Diesel Engine
Dedicated for Hybrid Commercial Vehicle Application 2022-01-7053
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..