Study of Unconventional Cycles (Atkinson and Miller) with Mixture Heating as a Means for the Fuel Economy Improvement of a Throttled SI Engine at Part Load

This contribution is focused on an investigation of two well-known techniques, i.e. the modified Atkinson working cycle with a late intake valve closing (LIVC) and the Miller working cycle with an extreme early intake valve closing (EIVC) in order to increase the fuel economy of a throttled SI engine at a part load (high throttled mode). However, the application of the Atkinson and Miller cycle causes a decrease in the in-cylinder charge temperature before the compression stroke. In the case of a constant value of the geometric compression ratio, the in-cylinder charge temperature at the beginning of the combustion is also decreased and the combustion is then slower (compared to a standard Otto cycle). This could negatively influence the indicated efficiency of the unconventional cycle. In order to avoid this, increase in the in-cylinder charge temperature was provided due to mixture heating in the intake manifold of the engine. A commercial 1-D code was utilized in order to calculate the in-cylinder pressure and the fuel economy improvement for both unconventional cycles with and without mixture heating. Numerical simulations were carried out in two various engine speeds. A predictive turbulent flame model for SI engine, as a part of applied 1-D code, was used for combustion calculation. Data from experiments carried out on the SI engine with the Otto cycle were used for calibration of the engine model as well as of the predictive model of combustion. Based on results from simulations, a significant increase in fuel economy improvement has been observed compared to the Otto cycle, especially due to the application of the Miller cycle. Further fuel economy improvement has been achieved due to a combination of both unconventional cycles and the mixture heating. During simulations certain limits have been observed at both Atkinson and Miller cycles.