Modelling of the auto-ignition angle in diesel HCCI engines through D-optimal design

The HCCI (Homogeneous Charge Compression Ignition) process is one of the most promising combustion processes developed to reduce pollutant emissions from automotive vehicles. However, there are practical difficulties concerning the control of the onset of ignition, and thus the availability of simple models which allows to simulate the auto-ignition phenomena may be very interesting for the development of new HCCI engines. In this work, the onset of ignition in a HCCI engine and the auto-ignition angle were modelled (OAM and AAM respectively) through experimental plans based on the D-optimal criterion. The experimental values were obtained by using the chemical kinetic code CHEMKIN together with an appropriate diesel fuel surrogate. The models developed have an acceptable goodness-of-fit and predictive capability (differences lower than 3 CAD were obtained between modelled and real auto-ignition angles for all the cases). The relative fuel/oxidant ratio and the intake temperature were the most significant engine parameters affecting the onset of auto-ignition, while the intake temperature and pressure appear as the most important parameters determining the auto-ignition angle. These models could be used by the Engine Control Unit (ECU) as an on-board diagnostic technique to control the HCCI combustion in real time. The optimal engine parameters for five specific operating conditions (chosen to cover the most common light duty diesel vehicles operating modes) were also calculated by using the above mentioned models (OAM and AAM) and by solving two non-linear optimization problems. To achieve optimization, a desirability function was defined. The optimization methodology proposed can be used to obtain the optimum engine parameters, which are used by the ECU, matching different vehicle requirements.