Study of Valve Timing Effects on Combustion Behavior at High Load Boost Conditions

This paper analyzes the potential of a Fully Variable Camshaft Timing (FVCT) system for optimizing the performance of a highly boosted downsized ethanol SIDI engine running at high loads and reducing the emissions of pollutant gases. The FVCT influences the gas exchange process, which in turn modifies the charge composition and the cylinder filling. In order to achieve higher fuel conversion efficiency in conjunction with better emission levels, a FVCT strategy was defined seeking to characterize the best scavenging behavior. For all engine test conditions, the results showed that the FVCT adjusted to 5° before top dead center (BTDC) provided the lowest specific fuel consumption and, consequently, the greatest fuel conversion efficiency, despite the highest cyclic variability achieved. At this FVCT position, the pollutant emissions of HC and CO were reduced due to a more complete combustion process provided by higher gas temperatures into the combustion chamber, which, on the other hand, leads to the increase of NOx emissions. The CO₂ emission levels reflect the fuel consumption behavior, so that the lowest level of CO₂ was also achieved for FVCT adjusted to 5° BTDC. At higher loads, the FVCT adjustment provides greater fuel conversion efficiency by improving the scavenging process. Therefore, FVCT system proves to be an effective tool in reducing the engine specific fuel consumption.