Experimental Study On Ethanol Gasoline Homogeneous Charge Compression Ignition Combustion Characteristics

On the era background that energy crisis is gradually threatening human and alternative fuels are concerned universally, ethanol is considered as a renewable alternative fuel, which has advantages such as wide sources, short cycle period(the cycle period of crop is 1 year), and never captures food with human etc, so ethanol is the first choice of alternative fuels in all nations. The style of mix combustion of ethanol and gasoline is already a consensus. China has proposed the route of developing fuel E10 from the experiences of other nation's utility of ethanol. China's productivity of ethanol has reached 1.02 million tons per year at present, and the use of ethanol has spread in 9 provinces all over the country. Obviously, the product and consumption of ethanol has already certain scale in China.On the other side, a new mode of combustion-- homogeneous charge compression ignition(HCCI), has advantages of higher thermal efficiency, lower NOX and PM. With the engine's emission regulation being strict gradually, higher and higher request to the engine's performance, people working on the engine tech have attached importance to the technology of HCCI, and have achievement in this searching field.Colligating the two reasons mentioned above, this paper has made a research on the combustion characteristics of HCCI for ethanol gasoline, and analyzed the change law of combustion characteristics for ethanol gasoline in different boundary conditions. The combustion of HCCI is controlled by chemical kinetics, and the starting point of combustion is mainly affected by the fuel's characteristics and the boundary conditions. To make the control of boundary condition easy, the trials are conducted on the device RCM(rapid compression machine), which can simulate the process of engine's compressing combustion.RMC is mostly consisted of fuel/air mixture supplying system, driving system, combustion system, data-collecting and controlling system. The experimentapproximately are: Firstly, homogeneous fuel/air mixture with certain excess air ratio is formed in the supplying chamber, then it is delivered into the combustion chamber. The mixture is compressed to combustion by the driving system which is controled by controlling system.Finally, the displacement of piston in the process of compressing combustion and the mixture's pressure in cylinder are collected by data-collecting system.The PRF90(primary reference fuels, mixture of 90% iso-octane and 10% n-heptane in volume ratio ) is chosen instead of gasoline. The reasons are:①The main ingredient of gasoline is alkane and cyclic-paraffins of C5～C10. The average number of carbon atoms of PRF90's molecule belongs to the kind of alkane, and almost equal to the average number of carbon atoms of gasoline. And the octane number of PRF90 is close to that of gasoline. The physical and chemical character of both fuels can be considered as similar.②The structure of PRF90 is simple and clear, which is convenient to the accurate calculation of theoretic air-fuel ratio and low heat value, and the analysis of fuel's oxidation michanism. The mixture of 90% PRF90 and 10% ethanol is used instead of the fuel E10, ane then the mixture is called E10 in this paper.The combustion characteristics of E10 and PRF90 are analyzed in combination of chemical kinetics and mechanism of the fuel's oxidation. Referring to chemical kinetics, the affection of the parameters on chemical reaction rate are:①The reaction rate constant and reaction rate increase as the temperature ascends.②The pressure affects differently on the reaction rate in all levels. To the reaction in level 2, the reaction rate is proportional to the pressure as the temperature and reactant's concentration in certain level.③The reaction rate is proportional to the concentration in case that the change of oxygen's concentration is too limited.Referring to n-heptane , iso-octane and ethanol's oxidation mechanism ,the reaction activity of n-heptane is highest the next is iso-octane and the last is ethanol. The oxidation process of these fuels include low temperature reaction and high temperature reaction. At the low temperature , the reaction include H-atom abstraction reaction, O2 addition reaction and isomerization, and the decomposition ofits production. In this phase,the reaction releast heat, the temperature of air/fuel mixture was advanced, and many radicals were accumulated. They make the reaction into high temperature reaction. The fuel was completely oxidated through decomposition reaction and so on.The experiments are conducted on the rapid compression machine. The idiographic contents include: In the condition of changing the inlet temperature or the excess air ratio of air fuel mixture, the changing laws of pressure, pressure rise rate, heat release rate, combustion temperature, combustion starting point and combustion duration, which are in combustion of HCCI, are studied. The combustion characteristics of E10 is compared with that of PRF90 in the same boundary condition. The conclusions are obtained by experiments as following: 1. Through the combustion characteristics of HCCI for the fuel E10 whose excess air ratio is from 1 to 6, it can be achieved: As the excess air ratio is equal to 1, the process of combustion is abnormal, and the pressure fluctuates greatly, the pressure rise rate changes from positive to negative with huge fluctuation. As the excess air ratio is bigger than or equal to 4, the pressure and the temperature rise slowly, the combustion duration becomes longer, and the heat release rate gets lower.2. The maximum values of pressure rise rate and heat release rate decrease greatly as the excess air ratio rises. Asλ=2, the pressure rise rate and the heat release rate is approximately 3 times of that asλ=3.3. As the excess air ratio rises, the maximum pressure and temperature of combustion descend greatly, and the combustion duration gets longer obviously.4. When excess air ratios are equal to each other, as the intake temperature of air rises, the maximum pressure of combustion decreases, but not greatly. And it is considered that it's caused by the difference of pressure of combustion starting point.5. As the intake temperature of air rises, the maximum values of the rise rate of combustion pressure, the heat release rate and the temperature of combustion increases in certain level, but the margin is not great. And the combustionduration becomes longer.6. Through changing the temperature and excess air ratio of E10, it is found that the combustion starting point can get advanced dramatically by the enhancing the mixture's concentration and the temperature of intake air.7. In case of the same intake air's temperature and excess air ratio, the characteristics of E10 and PRF90 have no dramatic differences except the combustion starting point and combustion duration. The combustion duration of E10 is longer than that of PRF90, but the combustion staring point of E10 lags obviously compared with PRF90.