The Effects Of The Fuels Properties On Homogeneous Charge Compression Ignition (HCCI) Combustion

With the fast developing of global economy, the problems of energy crisis and environment pollution become more and more serious. Automotive engines as the main source of petroleum consumption and emissions to atmosphere, have received increased attentions. Homogenous Charge Compression Ignition (HCCI) combustion technology has got increased attention from academic and industry area, for its ultra-low NOX and PM emissions and high thermal efficiency. HCCI combustion process is hard to control because it is dominated by chemical kinetics of fuel air mixture. In addition, the HCCI operating range is narrow.The n-heptane is chosen as a testing fuel, mainly because its cetane number is 56, almost the same with the diesel oil, although its physical property is much different from diesel oil, their burnable characteristic is similar, so through studying the n-heptane, we can get some combustion characteristics of the diesel oil which is applied to burn in the HCCI engine. This research contains important theoretical meanings for investigating the burnable reaction mechanism of the high cetane number fuel (diesel oil) .In addition, because the process of catching fire and combustion is controlled completely by chemical reaction kinetics of fuel, the octane number of the fuel has important influence on the HCCI combustion characteristic. This paper takes the n-heptane and the iso-octane as basic fuels and gets the different octane number fuels by mixing two fuels according to different ratios. The experimental study on HCCI combustion fueled with n-heptane and fuels whose octane number are 25, 50, 75 is carried by a new device---RapidCompression Machine, research octane number's influence to combustion process, this research contains important theoretical meanings for looking for the control strategies of the time of ignition and velocity of combustion and widening the HCCI operating range.HCCI combustion's mixture is diluted and homogenous ,which is ignited by multi-kindling points at the same time ,whose combustion process is controlled mainly by chemical reaction kinetics of fuel .The process depending on self-ignition ,without flame diffusing ,is controlled by indirect methods ,such as different engine's boundary conditions which include the inlet temperature and excess air number etc ,to control the time of ignition and the velocity of combustion . The paper's experiments on HCCI combustion are carried out by a new device---Rapid Compression Machine which has better controls to Boundary conditions, following works are carried through:1. Boundary conditions (inlet temperature and Excess Air Number) are confirmed, the accurate temperature and the formula of Excess Air Number is got.2. Through the analysis and calculating of blow-by area of Combustion cylinder and proofread of the procedure, effective blow-by area is got. To make sure the quality of mixture of air and fuel, the blow-by quantity before the combustion ignition timing is calculated.3. The analysis releasing energy rate method is confirmed through analysis of releasing energy law with the characteristics of Rapid Compression Machine. At the same time, the pressure increasing rate, the ignition timing, the period of combustion and the combustion temperature and so on can be got by calculating.4. Through coherent controlling the combustion boundary conditions and the research octane number to HCCI combustion, the experimental study of n-heptane, and fuels whose octane number are 25, 50, 75 are carried out and the process of HCCI combustion is analyzed. The effects of inlet temperature and Excess Air Number on HCCI combustion and the effects of researchoctane number on HCCI combustion are analyzed. The research results are as following:(1) Both the activity of fuel molecule and the probability of impacts among the molecules increase as the inlet temperature rises, the time of the low temperature reaction's release heat becomes early, the produce velocity of active particles during the low temperature increases and the fuel's velocity of chemical reaction accelerates, which makes the mixture produce active particles and release energy at low-temperature chemical reaction. Therefore, the mixture with higher inlet temperature is much faster to engage the high-temperature chemical reaction, which makes the ignition timing in advance. When the process of high-temperature chemical reaction starts, the mixture with higher inlet temperature has faster oxidation, quicker combustion, higher temperature and fast changed pressure. Therefore, with the inlet temperature of mixture rising, the period of combustion decreases, the maximum temperature of combustion rises, the releasing energy rate increases, and the rate of maximum pressure increase becomes higher. At the same time, the time point of the highest pressure increasing rate appears in advance, and the time of the maximum pressure advanced.(2) As Excess Air Number increasing, the mixture concentration of n-heptane decreases, which makes the fuel molecule decomposition reaction and dehydrogenization difficult to occur affect oxidation rate directly. At the same time, the fuel produced less activity particles as the Excess Air Number of mixture increasing, the released heat reduced at the low temperature, as a result the timing of high temperature reaction delays, the ignition timing of high temperature reaction phrase delays and the temperature of ignition increases. Therefore, as the Excess Air Number increases, the energy from combustion, the highest combustion temperature of n-heptane, the maximum of releasing energy rates, the pressure increasing rates and combustion reaction rates all decreases, but the combustion period increases.(3) Different octane number indicates that the activity and rate of thereaction is different at the same boundary conditions. The higher the fuel octane number is, the higher the ignition self-ignite temperature is, and the fastest reactions which consume fuel fastest are those reactions with hydroxyl. Owing to the velocity of the hydroxyl produced reduces as the octane number increases .Almost all reactions which can produce the hydroxyl can not occur below the temperature of 1000K. However, the fuel with lower octane number has produced much active particles, the activity and rate of the reaction is higher than the fuel with higher octane number at the low temperature. Therefore, as the octane number increases, the ignition timing delays, the combustion period prolongs, the maximums of releasing energy rate and combustion temperature decreases at the same time, the maximum of pressure increasing rate decreases, the pressure increasing rate delays and the burning temperature rises.