Optimization of variable valve timing for maximizing performance of an unthrottled SI engine-a theoretical study

Previous investigations have demonstrated that improvements in gasoline engine performance can be accomplished if the valve timing is variable. In this work valve timing strategies for maximizing engine torque and minimizing bsfc in terms of the exhaust opening (EO), intake opening (IO) and intake closing (IC) timings of a commercial SI engine are studied. The MICE (Modeling Internal Combustion Engines) computer program, which simulates an actual SI cycle, has been used. Overall performance characteristics such as the cycle efficiency, engine power, and exhaust gas composition are calculated. The model has been calibrated with data obtained from a measured indicator diagram, and validated against the overall performances of the engine. It is concluded that when both valves and spark timings are optimized, the optimal timing of each valve, depends apparently linearly on the engine load, linearly (in a good approximation) on the engine speed, while the slope depends in a weak manner on the engine load. When VVT is employed, the maximum engine power has been increased by 6%, and the engine bsfc has been decreased by 13%. The maximum torque has been shifted towards a lower engine speed. The present results are summarized as working maps for the engine designer. These show the influence of the intake and exhaust valve timing on the engine performance at the entire range of operation conditions (engine load and speed).