Turbocharged SI-DI-engines in combination with a reduction of engine displacement (“Downsizing”) offer the possibility to remarkably reduce the overall fuel consumption. In charged mode it is possible to scavenge fresh unburnt air into the exhaust system if a positive slope during the overlap phase of the gas exchange occurs.
The matching of the turbo system in SI-engines always causes a trade-off between low-end torque and high power output. The higher mass flow at low engine speeds of an engine using scavenging allows a partial solution of this trade-off. Thus, higher downsizing grades and fuel consumption reduction potential can be obtained. Through scavenging the global fuel to air ratio deviates from the local in-cylinder fuel to air ratio. It is possible to use a rich in-cylinder fuel to air ratio, whereas the global fuel to air ratio remains stochiometrical. This could be very beneficial to reduce the effect of catalytic aging on the one hand and engine knock on the other hand. Since the global fuel to air ratio is stoichiometric a post-oxidation of the unburnt fuel of the rich combustion process in the exhaust port is possible, if the unburnt fuel gets in contact with the unburnt scavenged fresh charge. Because of the intermittent mass flow of internal combustion engines the mixing of burnt and unburnt gas is quite difficult. Therefore, the effect of post-oxidation is highly dependent on the engine geometry and the operating conditions as well. The post-oxidation leads to a temperature increase in the exhaust system. Thereby the enthalpy of the exhaust gas increases as well. This higher exhaust enthalpy could enable higher boost pressures at lower engine speeds and a better transient engine response.
In this paper an approach to model the effects of post-oxidation is presented. The simulations are done using 1-D flow calculation in combination with a phenomenological combustion and knock model. In the presented simulations a clear separation of the effect of scavenging with and without post-oxidation can be made.
In real driving conditions, scavenging is mainly used during acceleration phases, so additional simulations were made under transient accelerations, too. Here, the presented transient calculations are considering crank angle based gas dynamics at every cycle and are embedded in a virtual vehicle simulation model.
With these simulations the effects of post-oxidation in steady-state conditions as well as under transient acceleration conditions can be estimated and the benefit of a possible post-oxidation can be conceived.