Influence of cooling loss on the energy and exergy distribution of heavy-duty diesel engines based on two-stage variable supercharging, VVT, and EGR

Abstract

The application of mechanisms such as exhaust gas recirculation (EGR) coupled with variable valve timing (VVT) and a variable geometry turbocharger (VGT) can improve engine efficiency; however, the energy laws and loss distribution after EGR, VVT, and VGT changes are unclear, restricting the optimization of engine structures and corresponding strategies. Herein, a six-cylinder engine is studied, revealing that the cooling loss of the high-pressure (HP) EGR loop is an important factor affecting the engine energy distribution. The cooling loss accounts for 10.00%–20.00% of the total energy, with an average increase of 1.73%, surpassing other energy losses growth rates. The low-pressure (LP) EGR loop considerably reduces cooling losses. The cooling loss of the LP EGR loop is only 64.05% of the HP EGR loop at a 20% EGR rate. When the EGR rate is >10%, the resulting lower cooling losses effectively improve the engine efficiency and the indicated thermal efficiency (ITE) of the LP EGR loop is 0.20%–0.33% higher than that of the HP EGR loop; when the EGR rate is 21%, the ITE of the LP EGR loop reaches 49.52%. By studying the variation in exergy with operating parameters, it is found that while increasing the EGR rate from 15% to 20%, the proportion of available exergy increases by adjusting the VVT to −85° crank angle after top dead center (CA ATDC) or adjusting the VGT to 47.5% under the original operating scheme of the LP EGR loop (−146° CA ATDC; VGT = 42.5%). The available exergy increases from 71.22%–71.42% (−146° CA ATDC; VGT = 42.5%; original device) to 71.88%–71.58% (−146° CA ATDC; VGT = 47.5%) and 72.02%–72.21% (−85° CA ATDC; VGT = 42.5%). This study explores the energy distribution under different operating schemes, providing theoretical guidance for further improving the thermal efficiency of the entire device.