Variable Valve Actuation Systems

Internal combustion (IC) engines today represent a class of heat engines marked by their high power-to-weight ratio, making them the suitable choice for portable power solutions. Being reliable and robust, their widespread use in commercial vehicles is, therefore, implicitly justified. Being a heat engine, the efficiency and performance of an internal combustion engine are limited by the temperature of heat addition and rejection. Moreover, with the inherent irreversible and non-ideal nature of the various processes of the power cycle, a fraction of the ideal thermodynamic efficiency is realised accounting for the low overall thermal efficiency. The text that follows is centred around the gas exchange process in an IC engine. The working of conventional camshaft-driven valve train systems, which have been in use for quite a long time, has been discussed followed by its limitations and their repercussions on the performance and efficiency of an IC engine. The origins of the unavoidable pumping losses accompanying load control using a throttle valve have been explained. An overview of the various strategies and methods used in commercial vehicles to mitigate such losses (variable valve timing and variable valve lift) has been given while providing some insight into the working of some experimental variable valve actuation systems. The discussion then shifts to fully flexible camless valve actuation systems explaining the working of some popular actuation systems, highlighting their advantages and limitations. The basic control logic of such systems is then discussed followed by a list of some unique attributes and advantages of the same. Few experimental results from the literature have also been cited to substantiate the utility of variable valve actuation systems.