Development of a Combustion Delay Model in the Control of Innovative Combustions

In modern internal combustion engines the research for innovative solutions aimed at the simultaneous reduction of engine-out pollutants and fuel consumption requires synergies from different application areas: the thermo-fluid dynamic design of the combustion chamber, the study and production of specific components for air and fuel supply, the development of sensors and related methods of analyzing their signals to control the combustion process. The most promising innovative combustion methodologies suitable to achieve high efficiency and low emissions, commonly named Low Temperature Combustions (LTC), usually require sophisticated techniques for the management of the combustion phase. With respect to the combustion angular position control, directly performed in traditional spark ignition engines through the ignition from the spark plug and in compression ignition engines by the timing of fuel injection, the ignition mechanisms of LTC combustions are characterized by a high sensitivity to the thermal conditions of the combustion chamber which greatly modifies the angular position of the combustion, mainly due to the combination of high ignition delays and lean homogeneous mixture. Once the hardware of the air and fuel supply systems has been defined, it is therefore essential to ensure the correct management of the combustion phase. In this paper a model for the estimation of the delay between the start of injection and the start of combustion is presented. The model has been developed analyzing the experimental data from a modified cylinder of a diesel engine, fueled with gasoline, while the other three cylinders were still running with Diesel fuel. This solution represents a first step that allows analyzing the behavior of the combustion of gasoline in a Diesel engine, with the final goal to inject gasoline in all the engine cylinders. In particular, the approach used is similar to the one already applied in a traditional turbocharged gasoline engine, where the goal was to estimate the time delay between the spark firing and the start of combustion, mainly to detect the presence of undesired pre-ignition due to the presence of hot spots related to slightly knocking conditions. As it is well known, the role of the pilot injection is to reduce the ignition delay of the main injection. However, to significantly accelerate the ignition of the fuel injected with the main injection, it is necessary to burn a sufficient quantity of the fuel injected by the pilot before the Top Dead Center position (TDC). The application of this model has to allow the implementation of a feed-forward control to stabilize the whole combustion process and achieve the best conversion efficiency from energy to work, taking into account the operational constraints that must be satisfied to guarantee the integrity of the engine and the compliance with the homologation rules.