Experimental Study On HCCI Combustion Of A Multi-cylinder Gasoline Engine Based On Fuel Design Concept

In the face of energy security and environmental pollution problems becoming increasingly serious, HCCI(Homogeneous Charge Compression Ignition) engine, with high efficiency and low emissions, has attracted more and more attention. HCCI combustion has received more and more attention for the internal combustion engine research. However, ignition & combustion control and narrow load range have prevented HCCI engine from being widely operated in practical applications. Therefore, by the means of fuel design concept, an investigation of optimizing HCCI combustion and expanding load region of HCCI engine was carried out with real-time changes of fuel characteristics.A four-cylinder gasoline SI and HCCI dual- mode engine based on OKP(Optimized K inetic Process) was developed. This OKP engine could be operated in HCCI mode stably with the high temperature intake air which was heated by the exhaust heat exchanger and the coolant heat exchanger. Based on the comparison with spark ignition operation mode, a detailed research on the gasoline HCCI combustion was carried out. Results showed that the lower air input and the higher CTCV(Cylinder-to-Cylinder Variation), both attributed to higher temperature, are the two main reasons limiting the load region of the HCCI engine. Although the load region of the HCCI engine was expanded with supercharging and higher compression ratio, it was still difficult to meet the requirements of engineering applications.With real-time control of gasoline and n- heptane injection, a strategy to further expand the load region of the HCCI engine was raised by the means of fuel design concept. The effect of dual fuels on HCCI combustion was investigated. The requirement of the intake air temperature and the coolant temperature for stable HCCI combustion could be reduced by dual fuels. With lower intake air temperature, the air input was increased. C TCV of HCCI engine was decreased with the lower temperature gradient among four cylinders, attributed to the low intake air temperature and the proportional control of fuels in each cylinder. Critical values of the intake air temperature and the EGR rate existed for the HCCI combustion, at which the thermal efficiency was highest and COV(Coefficient of Variation), PRRmax(the Maximum of Pressure Rise Rate) and emissions were low. With the increase of RG(ratio of gasoline in the fuel), the critical value of intake air temperature decreased, while the critical of EGR rate increased. Based on fuel design concept, combined with the control of the intake air and coolant temperature, EGR, supercharging etc., the load region of HCCI engine was expanded significantly. With supercharging, the torque of dual- fuel HCCI engine could reach 86N?m, which was about 70% of original SI engine at the same speed. Compared with original SI engine, BSFC(Brake Specific Fuel Consumption) of dual- fuel HCCI engine was reduced by 20.8% and 16.7% in the conditions of 28.5 N?m/1600 rpm and 60 N?m/1600 rpm respectively, and NOx emissions were reduced by 99.9% and 99.4%.An investigation of the transition between SI mode and HCCI mode of the engine based on fuel design was carried out. With dual fuels, the transition could be executed at very low load and speed, even at idling condition. Compared with the previous mode-switch strategy, based on the intake air temperature control and the fuel design, the duration of transition was shortened to only 10 engine cycles. The transition of SI/HCCI with dual fuels could be a solution to the problem of the cold-start of the gasoline HCCI engine.The effect of spark ignition on HCCI engine was investigated. Results showed that the effect of spark ignition on HCCI combustion was related to HCCI combustion phase. Spark ignition at low temperature reaction period, would lead to low COV and PRRmax and high CO emission; spark ignition at high low temperature reaction period would lead to high thermal efficiency and high COV and PRRmax.