An Experimental And Kinetic Modeling Study Of N-Butano/N-Heptane Ignition Characteristic In Rapid Compression Machine

With the serious energy crisis and enhanced awareness of environmentalprotection, a new engine combustion technology called HCCI (HomogenousCharge Compression Ignition) and the automobile alternative fuels attractedthe attention of researchers. Although there are several renewable energieshave the potential to replace the gasoline and diesel, they are not widely usedbecause some defects of themselves. Compared with the common usedalternative fuels methanol and ethanol, n-butanol is a new promising bio-fuelwith higher energy content, lower vapor pressure and lower solubility inwater. In addition, it can be produced by advanced fermentation techniquesusing various feed stocks. So it has become a potential renewable alternativefuel. To learn about the compression ignition features of n-heptane andn-butanol/n-heptane mixture, the study discussed the effects of temperature, pressure and equivalence ratio on the fuels’ ignition delay in rapidcompression machine (RCM). A computational model was established toverify the experimental results. Then the model was used to study thesensitivity analysis and the reaction path of n-heptane, as well as n-butanol.The conclusions can be summarized as follows:(1) Experimental and modeling results indicate that ignition delay ofn-heptane in RCM can be divided into three regions, and that is lowtemperature, medium temperature, and high temperature region. In low andhigh temperature region, the ignition delay shortened obviously with thetemperature increased. While in medium temperature region, the ignitiondelay changed slowly with the temperature increased, which is callednegative temperature coefficient (NTC) phenomenon. When then-butanol/n-heptane mixture contained n-heptane, the ignition delay of themixture showed the same trend with pure n-heptane. With the increasedproportion of n-butanol, ignition delay of the mixture prolonged. Modelingresults indicate that ignition delay of pure n-butanol has no NTC.(2) For the fuels researched in this study, increasing of equivalence ratioand compression pressure will shorten the ignition delay. Variations inequivalence ratio didn’t change the NTC range, while increasing pressurewill shift the NTC region to higher temperatures. (3) Sensitivity analysis of ignition delay was conducted with the model,and the results indicate that the ignition delay of fuels were different whenthe temperature changed. When combined the sensitivity analysis with fuelreaction path, the study find that: at low temperature, fuels mainly consumedvia low temperature chain branching, so the ignition delay was more sensitiveto these kind of reactions; at high temperature, the ignition delay was moresensitive to small radicals because the fuel and large radical are mainlyconsumed to form small radicals by dissociation; at medium temperature, theoxidation path of n-heptane changed, which reduced its low temperaturechain branching, and generated aldehyde, olefin substances which are morestable under medium or low temperature, and that’s why the fuel mixturescontained n-heptane showed a NTC phenomenon.