Modeling the Effect of Variable Cam Phasing on Volumetric Efficiency, Scavenging and Torque Generation

In a mean value engine model, the mass flow of air through the cylinders is, up to a constant factor, described as the product of the air density in the intake manifold, the engine speed, and the volumetric efficiency. The volumetric efficiency is traditionally modeled as a function of the engine speed and the pressure in the intake and exhaust manifolds, but for modern engines the model must also account for the effect of variable valve timings. The engine that is modeled here is equipped with variable cam phasing on both the intake and the exhaust valves. In order to reduce the complexity, we will only model the effect of the valve overlap, which is the number of crank angle degrees that both valves are open simultaneously. When the valve overlap is significant, there may be fresh air that flows directly through the exhaust valve, known as scavenging. The scavenging will cause the air/fuel ratio of the gas mixture in the cylinder to be different than the global air/fuel ratio, and it has effects both on the torque generation from the combustion and on the temperature and pressure of the exhaust gases that drive the turbine as well as on the emission levels from the engine. The degree of scavenging cannot be measured directly on the engine and for that part of the modeling we use GT POWER simulations. The simulations show that the scavenging can be modeled as a function of the volumetric efficiency. A torque model is derived as a tool to validate the scavenging model. The new models are shown to improve the predictions of the engine torque significantly.