Effects of late intake valve closing (LIVC) and rebreathing valve strategies on diesel engine performance and emissions at low loads

Abstract

An experimental study has been conducted to explore the effects of five different valve strategies, including three intake valve closure (IVC) timing strategies and two rebreathing strategies (i.e., second opening of the intake/exhaust valves during the exhaust/intake processes, called 2IVO and 2EVO) on the combustion and emission characteristics at various low loads (1–5 bar gross indicated mean effective pressure, IMEPg) on a heavy-duty diesel engine. Then proper valve strategies to achieve clean combustion (Engine-out emission: NOx ~0.4 g/kW-hr, Smoke <0.1 FSN, CO and HC as low as possible) while maintaining relatively high thermal efficiency at various load conditions have been proposed. In order to control the NOx emissions within low levels, the externally cooled exhaust gas recirculation (Ex-EGR) was used and combined with three IVC strategies in this study, while internal EGR (In-EGR) was used with 2IVO and 2EVO strategies.The results show that low NOx emissions can be achieved for these valve strategies with high In-EGR or Ex-EGR. However, the differences among various valve strategies on other emissions (CO, HC and Smoke) and combustion characteristics are sensitive to engine loads. Improved combustion and emissions can be achieved with rebreathing strategies at low loads (1–2 bar IMEPg), however, higher Smoke emissions and lower thermal efficiency are observed at higher loads. The lowest Smoke emissions can be obtained with the late IVC strategy, but at the same time with high CO and HC emissions, especially at lower loads. The suggestion is to use the rebreathing valve strategies at lower engine load from 1 to 2 bar IMEPg and then change to the standard and late IVC strategies at higher loads.