Implementation of conditional moment closure in a zero-dimensional model for HCCI engines and comparison with experiment

Implementation of conditional moment closure in a zero-dimensional model for HCCI engines and comparison with experiment

Homogeneous charge compression ignition (HCCI) engines have been an active research area recently due to their advantages in reducing emissions levels. Regulatory bodies, such as those in Europe, the United States and Japan, are imposing stringent vehicle emissions quality standards. Most automotive manufacturers are moving towards fuel efficient vehicles by developing hybrid (combination of two or more power sources) vehicles or improving on any conventional engine technology that can reduce emissions levels. Hybrid vehicles are receiving increasing attention from most manufacturers because they offer advantages including reduced emissions and providing good mileage per fuel tank. The HCCI engine has the potential to replace the current conventional engine used in hybrid vehicles, which can reduce the emissions levels further.

To improve the development work, simulations are undertaken to reduce research costs while maintaining good productivity because of their cost efficiency compared to experiments. For engine research, a zero-dimensional model is known for its advantage in reducing computational time compared with a multi-dimensional Computational Fluid Dynamics (CFD) approach. CFD yields more accurate results but requires greater computational resources and time, while a zero-dimensional model offers versatility in a reduced simulation time. However, the zero-dimensional model has the limitation of a shorter combustion duration and rapid pressure rise compared to the experiment. Also, the zero-dimensional model is incapable of using the actual intake air temperature and needs to be set higher than the actual.

Conditional Moment Closure (CMC) is a model for the mixing in the combustion chamber at modest computational cost, which considers the turbulent-chemistry interactions. An implementation of CMC into a zero-dimensional model for an HCCI engine application is new in the literature; most of the CMC studies use the CFD approach. The combined model is expected to improve the zero-dimensional model limitation while still keeping the advantages of the latter of not using relatively modest computational resources and time.

The goals of the thesis are to write a new computer program that implements the combined model and develop a new experimental test rig for HCCI. The main focus of the thesis is to obtain an improved result for the combined model against the zero-dimensional model, while the experimental results will be used for the purpose of validation. The thesis consists of nine chapters. The first two chapters after the introduction cover the HCCI engine background and performance irrespective of the fuel being used and a background to turbulence, which introduces some turbulent theories and conservation equations. Chapter 4 discusses the details of the numerical part, which consists of all the formulations being used in the combined model. Then Chapter 5 will validate the combined models against two experimental works from others, which use diesel and gasoline fuelled HCCI engines. Chapters 6 and 7 introduce the experimental work setup and engine performance comparison between SI and HCCI modes. Chapter 8 continues the validation of the combined model based on the HCCI engine developed in the experimental work, followed by the Conclusion.

The results show that the combined model has improved the zero-dimensional model limitation by using the actual intake air temperature instead of artificially increasing it. To some extent, the combined model has shown the ability to reduce the short burn duration in the zero-dimensional model, where the maximum in-cylinder pressure trace is slightly lower than with the zero-dimensional model, with a smooth profile in the vicinity of the peak. However, the combined model has a limitation in predicting the ignition point accurately when the air-to-fuel ratio varies. Besides these observed limitations, the combined model shows good agreement with the experiments. The experimental results that were obtained are consistent with the literature, including the limitation that the HCCI engine only operates with a low load operating condition. The emissions levels also agree with the literature, where the HCCI engine produces high unburned hydrocarbon and carbon monoxide in the exhaust. Therefore, future work is recommended to improve the combined model and also further develop the HCCI engine.