Simulation of The Effects of Valve Timing Misalignment on Performance in Spark Ignition Engines

Engine performance can be improved by changing the valve opening (or closing) timing without any cam profile changes. In this study, a simple simulation model was created for valve timing misalignment, which is an assembly defect in the engine. Due to misalign, the opening angles of the valves have been changed between +20 degrees and −20 degrees compared to the normal opening angles. The engine performance resulting from this advanced and retarded valve timing was examined for a four-stroke, spark ignition, single-cylinder engine with an engine volume of 393 cc. In this study conducted for the 1000-7000 rpm engine speed range, first the in-cylinder pressure data were examined in detail and then the general engine performance parameters were examined. Accordingly, opening the valves earlier than necessary at low and medium engine speeds increases the maximum in-cylinder pressure, and at high engine speeds, it reduces the maximum in-cylinder pressure due to excessive decrease in volumetric efficiency. It was observed that the volumetric efficiency, which was 0.89 at medium speeds, decreased to 0.70 due to misalignment. Regardless of whether the misalignment is positive or negative, pressure fluctuations increase during the valve lapping process. Maximum braking torque occurs at medium engine speeds. Positive misalignment reduces braking torque, especially for low and high engine speeds. However, especially at high speeds, negative misalignment reduces the pumping torque. While the pumping torque for high engine speeds was −3.78 Nm, it increased up to −5.04 Nm due to positive misalignment. Whether it is positive misalignment or negative misalignment, brake specific fuel consumption tends to increase in both cases. At low and medium engine speeds, negative misalignment or positive misalignment always increases residual gas fraction. As a result of the study, it was seen that misalignment negatively affected engine performance. However, it is seen that the value accepted as reference is the optimum value for the operating speed range of the engine. With this study, it has been understood that valve timing, as well as valve system design, is vital for engine performance.