3D-Simulation On The Working Process Of Diesel Engine And Research On Valve Timing Optimization

With advantages in engine power and fuel economy, diesel engine finds its wide applications in many industries. However, people have to take measures to further improve its performances in many aspects due to global energy and environment issues. Valve-timing optimization is an important way of improving engine performance, increasing thermal efficiency and reducing emissions. When engine is running at different speed or load, the airflow velocity and cylinder pressure varies, therefore, the best valve-timing is different for each case. However, valve-timing is fixed in traditional valve train system; engine could only perform very well in one condition. Variable Valve Timing System is introduced for this reason. Traditional way, either rule of thumb or experiment, in which engine performance is studied and optimized cannot meet the requirements of modern engine Variable Valve Timing system, for the accuracy and applicability of rule-of-thumb calculation is always questioned; the high cost, long research period and limited experiment data also make the experiment method less applicable. Therefore it has become the aim for practitioners to find an economic, effective and fast way to solve the above situation.In such case, this thesis simulates the complete engine working process of a turbocharged diesel engine by employing the CFD method. The simulation is then verified by experiment data. Based on that, valve timing optimization is made under different working conditions in order to determining valve-timing for Variable Valve Timing System.In this thesis, different inlet valve-timing schemes for four running conditions are evaluated; trapped air and scavenged air are quantified for each case, analysis is made on how the amount of inlet backflow and trapped air in cylinder will change when Inlet Advance Angle varies. Additionally, the best inlet valve-timing of each working condition is concluded based on the principle that cylinder will have maximum charge air in itself and minimum inlet air through inlet valve with appropriate scavenging, when the inlet valve-timing is the best; on the other hand, different outlet valve-timing schemes for four running conditions are evaluated as well, the expansion loss and forced exhaust loss of each case are calculated, the interplay of engine speed and Outlet Advance Angle is analyzed. Additionally, the best outlet valve-timing of each working condition is concluded based on the principle that the sum of expansion loss and forced exhaust loss is the least when the outlet valve-timing is the best.The calculation results suggest that the optimized valve timing is able to improve indicated engine power, weaken inlet backflow and reduce exhaust loss. The analysis shows that the CFD method could be employed as an effective way of determining valve timing for Variable Valve Timing System, Variable Camshaft System and even Non-cam Driven Valve System.