Effect of variable valve timing on SI engine performance

Abstract

In the modern world, one of the biggest concerns from the energy point of view is the ever depleting supply of fossil fuels. The automotive industry is especially impacted. In recent years, the price of fuel oils has fluctuated substantially and the price of crude oil has reached record highs. The high price of gasoline coupled with the uncertainty of its availability and future price has put a high priority on efficiency and fuel economy of an engine. In internal combustion engines, particularly for spark ignition (SI) engines, valve events and their timings have a major influence on the engine’s overall efficiency and its exhaust emissions. Because the conventional SI engine has fixed valve timing and synchronization between the camshaft and crankshaft, a compromise results between engine efficiency and its maximum power. By using variable valve timing (VVT) technology, it is possible to control the instantaneous valve lift, phase, and valve timing at any point on the engine map, with the result of enhancing the overall engine performance. The VVT technology ensures a higher efficiency by adding flexibility to the valve actuation of the engine. However, the added variability needs more experimental development which is expensive. In this regard, more investigation can be done by using engine simulations instead of experimentation. In this thesis, a first order mathematical model of a spark ignition engine has been developed in SIMULINK. The model includes instantaneous mass of gasses inside cylinder, temperature, exhaust gas residual and pressure dynamics, including combustion and heat transfer effects. The model has been checked against experimental data obtained from a specific SI engine and validated against the overall performances of the engine. Different valve timing strategies for maximizing engine power and minimizing Bsfc in terms of the opening and closing timings of the intake and exhaust valves of a commercial SI engine were studied. The engine simulation was used to investigate the effects of valve timing on brake power, volumetric efficiency and fuel consumption of the engine and thereby optimize the engine valve timings for improving its performance. In addition, an unconventional valve lift pattern has been tried out and its effects were studied under some simulated engine conditions.