Large eddy simulation of turbulent combustion in a spark assisted homogenous charge compression ignition engine

A large eddy simulation (LES) model was developed to simulate the combustion process in a spark-assisted homogeneous charge compression ignition (SACI) engine. First, an ignition and flame propagation model based on a reaction progress variable is presented. The reaction progress variable is defined based on the normalized cumulative heat release. Transport equation for the progress variable is derived where the source terms due to flame propagation and auto-ignition are modelled. The model is then applied to simulate the SACI combustion process with special focus on the interaction between the flame propagation introduced by the spark and the auto-ignition of the homogeneous charge. The engine simulated is a 0.5 litre experimental HCCI engine, with operation conditions ranging from spark-ignition controlled flame propagation to auto-ignition controlled HCCI combustion. In the first stage of SACI combustion, between the spark-ignition and the onset of HCCI auto-ignition, turbulence field governs the heat release rate and pressure-rise-rate in the cylinder. Increasing turbulence promotes the contribution of SI flame to the overall heat release. The second stage combustion, which is in the HCCI auto-ignition mode, is rather sensitive to the temperature field. The numerical results showed that with low initial temperature the SI flame mode prevails; with high initial temperature the HCCI mode prevails. With moderate initial temperature SI flame and HCCI ignition interact more closely, which results in higher sensitivity to the initial temperature and turbulence conditions. This may be the reason of having high cyclic variation found in the previous experiments. Introduction In light of today’s public concern on green house gas (CO2) emission and air pollution from combustion of fossil fuels, modern internal combustion engines are developed to have high efficient with low emissions. The benefits and shortfalls of the two major combustion concepts, spark ignition (SI) and compression ignition (CI), have over the past decades led to the development of homogenous charge compression ignition (HCCI) engines that can achieve high efficiency and low emissions of NOx and soot by using high compression ratio and excessive air or exhaust gas recirculation (EGR) in the fuel/air mixtures [1-4]. In HCCI engines the combustion phasing is controlled by the auto-ignition of the lean charge. As the ignition delay time is sensitive to temperature of the charge the combustion phasing becomes rather sensitive to the initial flow and intake flow conditions. Furthermore, in HCCI engines the reaction fronts propagate at a velocity typically of an order of magnitude higher than the turbulent flame speed, the combustion duration in HCCI engine can be short if care is not taken to generate a suitable flow and temperature field in the cylinder. This is especially a serious problem when the engine runs at high load, where the pressure-rise-rate can be rather high, resulting in high noise level [5]. A recent review on HCCI combustion can be found in Yao et al. [6]. One way to control the ignition timing (combustion phasing) in HCCI engines is to ignite the fuel/air mixture with a spark before the onset of auto-ignition [7-10]. This strategy is known as spark-assisted HCCI (SACI), which may also be viewed as a natural extension of the gasoline SI engine operation in a HCCI mode at low load by trapping hot residual gas (internal EGR) or external EGR. In SACI a flame kernel is first initiated, followed by autoignition of the remaining charge. A successful SACI operation would depend on the success in manipulation of the SI flame/auto-ignition interaction. Several experimental studies have been conducted to investigate how the engine operation conditions, e.g. spark timing, load and amount of EGR in the mixture [11], swirl and thereby level of turbulence [12], fuel stratification by using secondary direct injection [13], on SACI combustion. Important information has been obtained in these investigations; for example, by using inlet valve deactivation to increase swirl and thus the level of turbulence, the SI flame contribution to the overall heat release is increased and the HCCI auto-ignition process is delayed at high-level EGR conditions [12]. It is fairly well recognized that turbulence can directly interact with the premixed flame propagation initiated by the spark, e.g. by wrinkling the flame fronts; however, the effect of turbulence on HCCI combustion can be rather problem dependent. It is generally accepted that temperature stratification plays an important role in HCCI combustion [14-16]. With large temperature stratification the combustion duration can be longer and the pressure-rise-rate can be lower for a given combustion phasing. One important role of turbulence played in HCCI engines is by modulating the temperature stratification in the engine cylinder; turbulence can affect the mixing of the intake gas with the residual gas and similarly it can affect the heat transfer between the intake gas and the hot residual gas, and between the cylinder/piston walls and the charge [17,18]. Under certain conditions, e.g. low intensity turbulence and high temperature stratification [19], or when the integral scale of turbulence eddies are comparable with the length scales of the hot/cold spots [16], turbulence can directly interact with the ignition front. It is evident that the interaction between the SI flame and the HCCI combustion in a SACI engine can be highly nonlinear and under certain EGR level the EGR nonlinear feedback mechanism can lead to oscillatory combustion and cyclic variation [20]. It is yet unclear how the two processes interact each other under different initial mixture and engine operation conditions. The goal of this work is to gain more insights to interaction between the SI flames and the HCCI auto-ignition fronts. A SACI model based on large eddy simulation (LES) approach is presented in this paper; the model is used to simulate an experimental SACI engine [13] where incylinder pressure measurement was reported. The transition region between SI flame and HCCI combustion is simulated. Description of the SACI LES model A LES model is developed for SACI combustion. The model is based on a reaction progress variable that is defined based on the normalized cumulative heat release [21]. The SI premixed flame propagation model is based on the flame surface density concept [22-24]. Spatially filtered Navier-Stokes equations and energy transport equations are coupled with the progress variable equation. Inside the SI premixed flame kernels the combustion products are computed using tabulated flamelet database [25,26]; outside the SI flame kernels the species and temperature are computed using an ignition tabulation database based on enthalpy, pressure and the ignition progress variable [19]. The SI flame and the HCCI ignition process interact through the incylinder pressure, temperature, as well as the heat and mass transfer by turbulence between the SI flame kernels and the unburned charge. The reaction progress variable for SACI combustion First, we introduce a reaction progress variable defined as the following [21,19],