Investigations Of Combustion Processes In SI CNG Engine For Conforming The Chinese State Emission Standard â…£

The developments and applications of alternative fuels on automobile engines are accelerated by the pressure from environment pollution and energy shortage. In this dissertation, researches were carried in depth on the combustion process of single CNG (Compressed Natural Gas) fuel engines which are applied on urban vehicles and work in lean combustion modes. A combustion system that satisfies the Chinese State emission standardâ…£was developed through the investigations on the adaptable designs of combustion system to the combustion process of SI (Spark Ignition) CNG engine, the experiments on combustion system, the optimizations of combustion process and the control strategies of these engines.Key technologies to reach the Chinese State emission standardâ…£were studied profoundly according to numerical simulations, design calculations and steady flow experiments. Subsequently, the combustion chambers, inlet ducts and cam shafts of the CNG engines were redesigned. Numerical simulations were adopted on six combustion chambers of different configurations and dimensions by the software Fire. With the synthetically review that how the various parameters of the structure influenced the turbulent kinetic energy, the combustion heat release rate, the combustion pressure, the gas temperature in cylinders, the NOx emission and the accumulative quantity of heat, a conclusion was drawn that"Those combustion chambers with the same configuration but different size would affect the combustion process far less than those chambers with different configurations". Some combustion chambers'structure was preliminarily optimized to adapt the combustion processes of the CNG engines. By using the classical theories and modern computational methods on cam shapes, a whole suite of procedures to design the shapes of cams were established on the basis of MATLAB and some cam shapes were designed following the procedures. The experiments concerning the effects of spiral inlet-duct parameters on circulating characteristics were carried on a steady flow test bench, by which a suite of effective methods to adjust the circulating characteristics of spiral inlet ducts were explored and the conditions and the range of application of these methods were further researched and analyzed. Through the experiments, the author also went deep into the influence of the assemblies that compose the combustion systems of the CNG engines on the comprehensive performance of engines and a series of universal methods and procedures were developed.This dissertation also comprises the endeavor about some control strategies for the SI CNG engines to meet the Chinese State emission standardâ…£. From the total strategies of these engines, the viewpoint that open-loop and closed-loop control modes should be associative in overall working conditions was presented for the relative control objectives and the processes of open-loop and closed-loop strategies. To achieve the accurate control of A/F (Air/Fuel) ratio, strategies were schemed concerning the gas inlets, supply of fuel and spark timings for the CNG engines.The author first puts forward the specific concepts which are applicable to the range of running conditions,the lean combustion limits and the asymmetry of respective cylinders for CNG engines working at lean combustion mode with turbo chargers and intercoolers. Furthermore, experiments were taken to study the influence of different working parameters on the range of running conditions and the lean combustion limits. The author penetrated with the lean combustion limit that satisfies the power performance, economical efficiency and the emission guideline. A method was creatively summarized to solve the asymmetry of respective cylinders. Comprehensive experiments were implemented in order to apprehend the influences of the parameters (Excess-air Coefficient and Spark Advance Angle), inner Exhaust Gas Recycle(EGR)and compression ratio on the working processes of lean combustion CNG engines. In the end, optimizations were put into practice on the lean combustion CNG engines in multi-levels and multi-perspectives.