A Study of Air to Fuel Transient Response and Compensation with Different Fuels 941931
In order to fulfill the future LEV exhaust emission standards, with different fuel specifications it is necessary to achieve a good control of the air to fuel ratio under transient conditions especially during the warm-up period. The objective in this report was to clarify the controlling factors of the exhaust auto fuel ratio response dependency of different fuels in order to provide a correct compensation strategy.
A mathematical model based on a manifold air flow model and a fuel film model, together with a time delay between air and fuel supply to the engine during the transient, was used for data matching of the measurements. The fuel film model includes a deposit factor (X), an evaporation time constant (τ) and a time delay (Δt).
Tests were carried out with three different fuels, typical European unleaded fuel. US California phase 2 fuel and US low volatility fuel.
The results from the models of delay and wall wetting effects were used for compensation strategy development.
Since wall wetting varies with e.g. fuel properties and engine ageing, observer models are discussed to adapt X and τ from the λ error feedback. This adaptation is made complicated due to the load and engine speed dependency of the wall film mass.
The results show that:
The deposit factor X and the time constant τ are affected by the engine speed, load, temperature and fuel properties.
When the engine is hot only T70 to T100 of the distillation curve is relevant for the determination of the X parameter. However, when the engine is cold, a much larger proportion of the distillation curve must be considered.
The time constant τ is affected most of the temperature in the interval between 60-90°C.
The injection timing strongly affects the wall wetting mass. It can be reduced by up to 50 % by an open valve injection compared to a closed valve injection.
The mass in the fuel film is also strongly affected by engine load, temperature and fuel properties.
The wall wetting model with load, engine speed and temperature dependent X and τ is ‘reversed’ to yield a non-linear compensator, which will compensate for both load as well as engine speed variations.
The time constant of the wall wetting compensation is smaller than the wall wetting evaporation time constant.