Ignition delay prediction of multiple injections in diesel engines

New correlations have been developed to predict the ignition delay of the main and multiple pilot injections as a function of the operating conditions in diesel engines. The ignition delay was first modeled through a global-mechanism approach, which accounts for the physical and chemical contributions separately. Semi-empirical correlations were then developed to predict the ignition delay of the pilot and main pulses for model-based control applications. Interest in this kind of application has in fact increased among car manufacturers over the last few years. An experimental investigation has been set up and carried out on a Euro 5 diesel engine at ICEAL-PT (Internal Combustion Engine Advanced Laboratory at the Politecnico di Torino), in order to assess the dependence of the ignition delay of each injection pulse on several parameters. The physical delay has been evaluated, with reference to the global-mechanism model, starting from a scaling law for the evaluation of the liquid length of the spray that was developed by Sandia National Laboratories. It has been verified that the physical delay depends on the charge and fuel thermodynamic conditions, as well as on the injector nozzle characteristics and injection pressure. The chemical delay, for the pilot injections, has been modeled by means of an Arrhenius-like expression that takes into account the effects of the charge density, temperature and oxygen concentration evaluated at the end of the physical delay. The chemical delay of the main injection has been modeled using a similar expression that includes an additional parameter, i.e., the total injected fuel quantity of the pilot injection shots. The thermodynamic and chemical conditions of the charge at the start of the main injection are in fact influenced to a great extent by the burning process of the pilot injection shots. The control-oriented approach, which is based on semi-empirical correlations to predict the ignition delay of the pilot and main pulses, was then developed. These correlations are robust and easy to apply, and are therefore suitable for integration with low-throughput combustion control algorithms that can be implemented in the engine control unit. Finally, the global-mechanism model and the control-oriented approach have been assessed and applied for ignition delay prediction in both steady-state and transient conditions.