Large Eddy Simulation of Turbulent Combustion in a Spark-Assisted Homogenous Charge Compression Ignition Engine

Abstract

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. IntroductionIn light of today's public concern on green house gas (CO 2 ) 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][2][3][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 combus...

Modelling Study of Cycle-To-Cycle Variations (CCV) in Spark Ignition (SI)-Controlled Auto-Ignition (CAI) Hybrid Combustion Engine by Using Reynolds-Averaged Navier–Stokes (RANS) and Large Eddy Simulation (LES)

“…Computational fluid dynamics (CFD) simulations have been applied to understand the SI-CAI hybrid combustion process and the causes of its CCV. Joelsson et al [22] applied large eddy simulations (LES) to investigate the SACI combustion. They found that the first stage flame propagation is mainly affected by turbulence, whereas the second stage HCCI combustion is governed by the initial temperature.…”

Computational study of the influence of in-cylinder flow on spark ignition–controlled auto-ignition hybrid combustion in a gasoline engine

“…Persson et al 21 experimentally found that increasing the turbulence levels could increase the flame expansion speed at both high and low residual rates, while the auto-ignition is delayed by increased turbulence at high residual rates. Joelsson et al 23 analyzed the effect of initial temperature and turbulence fields on the SACI using large eddy simulation (LES) and found that turbulence plays a significant role in the first stage SI flame propagation. Yoo et al 24 performed two-dimensional direct numerical simulations (DNS) on the effect of turbulence on SACI and found that the high-flow turbulence significantly enhances the overall combustion by inducing many deflagration waves.…”