Abstract
Recently Continuously Variable Valve Timing (CVVT) system has been widely used for the fuel economy and better torque at low engine speed. Optimization of valve timings according to various engine operating conditions is essential to improve these merits. However it is not easy to estimate the residual gas fraction in cylinder and intake manifold. Thus in this study engine simulation was executed to verify the distribution of residual gas in cylinder and intake manifold at part load condition by different valve timings and the experiments were performed with same conditions used on simulation to find the effects of residual gas. By these variable intake and exhaust valve conditions, the effects of not only valve overlap but negative valve overlap (valve underlap) were examined and the influences of carbon dioxide in residual gas were analyzed as internal EGR (Exhaust Gas Recirculation). In addition, unburned THC in residual gas can affect the cyclic combustion. Therefore, the characteristics of cyclic THC emission were acquired using Fast Response Flame Ionization Detector (FRFID) in intake port and exhaust port. The engine stability was discussed when the valve timing was changed rapidly and it was clarified that improvements of fuel economy, torque and low emission could be available with optimized valve timing condition consequently.
Keywords: Variable Valve Timing, Fast Response FID, Valve Overlap, Negative Valve Overlap, Residual Gas Fraction