Design of . Variable Valve Actuation in I.C.Engine

These major issues are putting pressure on automakers to develop new technologies to increase the fuel economy and decrease the emissions while maintaining or improving the engine’s performance. Several new technologies have resulted. All of these technologies accomplish these goals by increasing the efficiency of an engine. As a whole these technologies are called variable valve actuation. These technologies achieve a higher efficiency by reducing the constants of the engine. However, the added variability increases the time to calibrate an engine. To address this, more testing is being performed using engine simulations instead of physical testing. This thesis focuses on how to create an engine model and how engine simulation can be used to optimize such an engine. In addition the benefits of a particular variable valve actuation technology, cam phasing, will be explored A conventional engine has static, mechanically-actuated valves and a compression ratio that is fixed once the components of the engine are chosen. A recently developed technology called variable valve actuation (VVA) enables added control of valve timing, lift and/or duration. With this additional freedom, the efficiency of an engine can be greatly increased. Not only can the compression ratio be increased with the addition of VVA, but also the necessity of throttling can be reduced[1,12] Although cam phasing has numerous benefits, it also has significant drawbacks. The largest drawback is a substantial increase in the amount of testing required to create an optimized engine map. By using engine modeling, the amount of testing required is reduced because most of the testing is done virtually through a simulation. The creating of an engine model requires a broad range of experimental data. To make an accurate model, the data must span the entire range of operating conditions. However, only a relatively small amount of data is needed. This thesis focuses on how to create an engine model and how to use the model to optimize engine development. In this study the abilities of GT-Power, an engine simulation program, will specifically be explored. Both the cycle resolved and cycle averaged data will be presented. The simulations will show the effect of intake and exhaust cam phasing on the trapped air mass, the trapped residual gases, intake air temperature, indicated mean effective pressure and combustion stability.[2,24] Variable Valve Actuation In standard IC engines, the compression ratio (set by the engine’s mechanical design) is also fixed for all engine conditions. The compression rate is thus limited by the engine condition with the lowest knock limit. Engine knock is caused by spontaneous combustion of fuel without a spark (auto-ignition).For spontaneous combustion to occur, the temperature and pressure must be sufficiently high. Therefore the limiting condition occurs at wide open throttle (WOT) and engine speeds close to redline. Likewise, lower engine speeds and throttled conditions (the most common operating conditions when driving a vehicle) have much less tendency to knock and can withstand higher compression ratios (hence the potential for higher efficiency). Figure 1: Efficiency Map of a Typical SI Engine (Guezennec, 2003)