Experimental Study On Compound HCCI Combustion Based On Fuel Design And Management

Homogeneous charge compression ignition (HCCI) combustion is considered to be the most promising clean combustion with high efficiency to meet future environmental requirements. However, the ignition timing and heat release rate of HCCI combustion is difficult to control. Therefore, it is very difficult for HCCI combustion to realize the full load range of the engine, so far. The concept of compound HCCI combustion gradually attracts people's attention. This study focuses on a novel technology of compound HCCI combustion on basis of fuel design and management theory. This combustion technology can break the boundary between mode-transition compound HCCI combustion and stratified compound HCCI combustion, and take advantages of traditional DICI combustion, HCCI combustion and compound HCCI combustion. Through the control of parameters and modulation of heat release, this compound HCCI combustion mode can achieve the full load while keeping low emissions and high efficiency.In this paper, compound HCCI-DI combustion is realized through fuel port injection combined with in-cylinder direct injection. And for the combustion, two research pathways are carried out. One pathway mainly focuses on the depth of HCCI-DI combustion, which refers to investigating the effects of physical and chemical properties of direct injection fuels and intake air boost, EGR and other factors on HCCI-DI combustion. While the other study means the exploration of HCCI-DI combustion fueled with single and commercial fuel, which could promote HCCI-DI combustion from laboratory research to practical application. Another clue of this study is to eliminate the "trade-off" relationship between NO_x and soot emissions of HCCI-DI combustion.Firstly, dual-fuel HCCI-DI combustion with n-heptane port injection and PRFs direct injection is studied. Through the investigation of influencing factors, such as premixed ratio, overall equivalence ratio and physical and chemical properties of direct injection fuels, HCCI-DI combustion and emission characteristics are comprehended. The results indicate that the pressure rising rate of HCCI-DI combustion is dominated by port fuel equivalence ratio. CA10 and CO emission are also mainly determined by port fuel equivalence ratio. Among compound HCCI-DI combustion with different directly injected RON fuels, PRF25 has lower CO, HC and NO_x emissions overall, along with higher soot emission. Meanwhile, CO, HC emissions of PRF50 are relatively higher. The optimal premixed ratio of compound combustion is to maintain one at first, then gradually decrease, and finally increases slightly. That is, at low load of the engine, pure HCCI combustion mode is used, while at medium or higher loads, HCCI-DI combustion mode is adopted. After the optimization of premixed ratio, CO and HC emissions of HCCI-DI combustion are higher than the prototype engine, and the indicated thermal efficiency is similar to the prototype engine. NO_x emission is significantly lower than the prototype engine, within 120 ppm or less, but soot emission is higher than the prototype engine, which means that HCCI-DI combustion cannot reduce NO_x and soot emissions simultaneously.In order to eliminate the "trade-off" relationship between NO_x and soot emissions, the paper then chooses oxygen-containing alcohols as the direct injection fuels based on the fuel design concept. The study of this dual-fuel HCCI-DI combustion indicates that: the combustion processes of three different alcohols are identical, and the curves of heat release rate overlap. HC and soot emissions of ethanol HCCI-DI combustion are lower than other two types of alcohols. However, the level of CO and NO_x emissions are nearly the same. Soot emission of iso-butanol, iso-propanol and ethanol decreases gradually, in reverse order with their oxygen content. Iso-propanol and iso-butanol HCCI-DI combustion could reach the full load of the engine, while ethanol can reach 97% load. After the optimization of premixed ratio, NO_x emission of alcohols HCCI-DI combustion is significantly lower than the prototype engine, and maintains the same level of iso-octane HCCI-DI combustion, within 100 ppm or less; soot emission is less than 15% which is much lower than both the prototype engine and iso-octane HCCI-DI combustion. Moreover, ethanol HCCI-DI combustion is nearly smoke free. Alcohol HCCI-DI combustion can reduce soot emission, while keeping low NO_x emission. That means the problem of NO_x and soot trade-off is solved.The directions from dual fuels to a single fuel, from primary reference fuels to practical fuels are the inevitable trends for HCCI-DI combustion. Therefore, a study on a single PRF50 HCCI-DI combustion is then implemented. The results indicate that the "trade-off" relationship between NO_x and soot emissions appears again in single PRF50 HCCI-DI combustion. Neither changing intake charge temperature nor fuel supply advance angle can solve this problem. A high proportion of EGR is able to achieve Low Temperature Combustion and reduce NO_x and soot emissions of HCCI-DI combustion simultaneously. Further, this paper examines the influence of intake air boost on PRF50 HCCI-DI combustion. The study shows: with the increase of intake pressure, the peak pressure of PRF50 HCCI-DI combustion decreases firstly, and then increases; the initial timing of HTR gradually delays, while the peak value of heat release rate of HTR is significantly reduced; the peak value of heat release rate of LTR is slightly decreased, while the peak value of heat release rate of the third stage diffusion combustion increases slightly; CA10 and CA50 retard at first and then advance, while combustion duration shortens progressively. The results also show that: in the condition of low intensity boost, the inhibition effect of boost on HCCI combustion occupies the dominant position of the entire compound combustion, while in high intensity condition, the promotion effect of boost on the third stage of combustion plays a primary role. In the case of high premixed ratio,the effect of boost on compound combustion mainly performs as its influence on HCCI combution, while in low premixed ratio, the effect performs as its influence on DICI combustion. With intake boost, PRF50 HCCI-DI combustion could achieve the full load of the prototype engine. In contrast to natural aspiration, intake boost could reduce the concentration of NO_x and soot emissions both. Specifically, NO_x and soot emissions are almost 1/3 of prototype engine at the full load. In fact, intake boost could suppress the formation of NO_x and soot emissions of HCCI-DI combustion.Finally, an exploratory study on HCCI-DI combustion fueled with a single and practical fuel is conducted. The commercial gasoline and diesel are used to prepare the blended fuels. The study shows that: with the increase of premixed ratio, the peak values of heat release rate of LTR and HTR of HCCI-DI combustion increase gradually; on the contrary, the peak value of heat release rate of the third stage combustion reduces gradually. With the increase of total fuel consumption per cycle, the cylinder peak pressure increases, and both the initial timing of LTR and HTR advance gradually, while the peak values of heat release rate of LTR and HTR increase. This paper also examines the effect of intake boost on this HCCI-DI combustion. It indicates that: compared to aspiration condition, intake boost could retard the initial timing of LTR and HTR of HCCI-DI combustion, and reduce the peak value of heat release rate of HTR significantly, as well as maxium pressure rising rate. The optimization of premixed ratio and intake pressure of G30 HCCI-DI combustion shows that: G30 DICI combustion and HCCI-DI combustion with/without intake boost all could achieve the full load of the prototype engine. With the increase of the engine load, the optimal premixed ratio of G30 HCCI-DI combustion without intake boost decreases gradually, while the optimal intake pressure of G30 HCCI-DI combustion with intake boost increases. Compared to the prototype engine, G30 DICI combustion can greatly reduce soot emission, but the level of NO_x emission is quite the same. On the other hand, G30 HCCI-DI combustion can significantly reduce NO_x emission, but the level of soot emission is almost the same. That is, neither of them could achieve the purpose of reducing NO_x and soot emissions simultaneously. For G30 HCCI-DI combustion with intake boost, NO_x and soot emissions are far lower than the prototype engine. Specifically, NO_x emission could be maintained within 100 ppm, and soot emission is below 10% at the full load range.