Modelling Ignition and Combustion in GDI Engines

The previous chapter discusses various models describing the spray breakup and mixing. Once the fuel is injected into the combustion chamber, it mixes with the surrounding air. The mixing process is mainly influenced by the shape of the combustion chamber, in-cylinder air motion, and turbulence. After the desired type of mixture is formed in the combustion chamber (lean stratified, stoichiometric, or rich mixture), ignition and subsequent combustion of the mixture take place. In GDI engines, combustion of the fuel–air mixture is initiated with the help of a spark plug. The spark plug is mounted at a location where the equivalence ratio is stoichiometric or slightly rich at the time of ignition, under all the available modes of operation in order to avoid misfire. For lean stratified operation, the location of spark plug needs to be precise as it is difficult to initiate ignition in an ultra-lean mixture. The engine operation under a homogeneous mixture condition is relatively less sensitive to the location of the spark plug from the point of view of mixture richness around it. Ignition leads to combustion of the air–fuel mixture which has been already prepared inside the combustion chamber. The flame front travels across the combustion chamber consuming the fuel–air mixture through the process of combustion. The process of ignition involves the generation of spark across the electrodes of the spark plug, whereas the process of combustion is nothing but the evolution of chemical reactions as the generated flame front travels across the chamber. Thus, ignition and combustion are conventionally modelled using individual models. Therefore, this chapter first discusses some of the popular ignition models and then the focus is directed to the study of a few combustion models. A spark is generated by the ignition system according to the command received by the electronic control unit (ECU). The ignition phase includes electrical discharge, plasma breakdown, and shock wave propagation. Ignition takes place in a small volume ranging from the spark gap to the turbulence integral length scale within a period of less than 1 µsec. This chapter provides insights into the sub-processes that take place during ignition and the different models available to simulate them. Once the spark is generated, it travels across the combustion chamber consuming the charge inside it. The rate of propagation is a strong function of the local equivalence ratio and temperature. This propagation of flame is modelled based on several approaches. One of the popular approaches is to use the chemical reaction mechanism of the corresponding fuel surrogate. However, a reduced chemical mechanism is defined for a certain range of the equivalence ratio and temperature thus limiting it applicability to a wide range of operating conditions. On the other hand, using a more detailed chemical reaction mechanism would result in longer computational time. Another approach is to model the combustion process in the premixed charge using the popular three-zone model. According to this approach, the charge inside the combustion chamber is divided into three zones. The burned and the unburned zones are separated by a thin reaction zone where the chemical reactions are taking place. Based on the three-zone model, some of the popular combustion models are discussed in this chapter. Apart from these two popular approaches, this chapter also discusses some high-fidelity combustion models based on large eddy simulation (LES). As it is not possible to explain all the models developed for simulating combustion in GDI engines, the interested readers are encouraged to consult the open literature to gain further insight in the subject matter.