Numerical and Experimental Study of Flame Propagation and Knock in a Compressed Natural Gas Engine

Abstract

One of the major objectives during the development process of new products is to reduce costs and time to market. Increasing computational power and continuous improvements of models for internal combustion engine applications show promise with respect to replacement of some optimisation steps by computer simulations. A prerequisite for such a substitution is that trends can be reasonably predicted and that calculations adequately incorporate the physics. The flame propagation and the knock behaviour of compressed natural gas engines have been studied in the present work. The aim is to improve the physical understanding on one hand and to develop physically based models for cycle simulation tools on the other hand. These models have been used to optimise a new engine concept which combines ultra-low emissions, high efficiency and driveability. An empirical combustion model based on experimentally determined burn rate curves has been developed to predict the engine behaviour for a wide range of operating conditions. It was found that global qualitative trends can be predicted quite well. Some relevant parameters characterising the combustion process the crank angle at 5% burned, the crank angle at 50% burned and the burn duration defined as 5% to 90% burned have been computed and compared with experimental data. The limitations of such a model have been shown by evaluating this model for a different combustion chamber geometry and various operating conditions. Therefore, a new model based on physical formulations has been developed. The phenomenological combustion model dedicated to compressed natural gas engines developed in this work can be used for projections and additionally to support the understanding of experimental results. A characteristic mean flame front area has been defined by applying some submodels describing the laminar flame speed, the turbulent flame speed and the turbulence intensity. Furthermore, the expansion factor describing the flame propagation due to the ratio of the densities of burned and unburned mixture has been considered. Good agreements between experimental data and computed results have been observed by applying this model to a different combustion chamber geometry. The characteristic mean flame front area was redefined for these new geometrical properties. It was shown, that the new flame front area can be approximated based on considerations concerning flame propagation and based on the known mean flame front area. Research and development activities often focus on increasing the efficiency of spark ignited engines, but many modifications leading to higher engine efficiency in part load operation lead to higher risk of knock occurrence at full load operation. These contradictory requirements clearly indicate the necessity of accurate physical formulations of the knock phenomena. The model developed in this work is based on a one step chemistry approach leading to the so called knock integral method. Due to the varying gas composition of compressed natural gas five well-defined compositions of synthetic gases have been tested to investigate the influence of the individual components. Furthermore, the model considers different operating conditions of an engine meaning that intake pressure, intake temperature, engine speed and spark timing have been varied. The differentiation between non-knocking and knocking combustion has been found to be a key factor for the quality of the model and has been thoroughly investigated. The widely used analysis of the maximum amplitudes of the pressure oscillations has been replaced by the analysis of the burn rate, where a new knock detection method has been developed. A clearly defined initiation of knocking combustion was observed. The parameters of the knock model determined finally can adequately describe the dependencies on the gas composition.