Study On Spray And Combustion Characteristics Of Automotive IBE/diesel Blends

The usage of alternative fuels in vehicles attracts much attention in recent years because of its potential to reduce exhaust emissions and alleviate energy shortage.Acetone-butanol-ethanol(ABE)is the intermediate fermentation product of butanol,which can be directly used as an automotive alternative fuel to avoid the high separation and purification cost during butanol fermentation.However,the acetone in ABE cannot be used as a fuel because of its high flash point and corrosivity.ABE can be converted to an isopropanol-butanol-ethanol(IBE)mixture by a simple bio-fermentation step.In comparison with ABE,IBE is more suitable as automotive alternative fuel.In general,the alternative fuels can reduce the soot emissions but will increase NOx emissions.Low temperature combustion(LTC)is a new technology to effectively reduce exhaust emissions of engines.It has the potential to reduce NOx and soot emissions simultaneously.Currently,most research on the LTC are based on engine bench tests with few of them based on the spray and flame evolution.Moreover,the LTC research on the spray combustion of IBE/diesel blends are much less.To investigate the spray combustion process of IBE/diesel blends,a constant volume chamber(CVC)has been utilized.All the effects of the differences between IBE/diesel blends and diesel,component ratio of IBE,IBE to diesel ratio,difference among butanol,IBE and ABE have been studied.To simulate LTC,normal combustion and EGR conditions,the ambient temperature was selected from 800 K to 1200 K at the step of 200 K and the ambient oxygen concentration was selected from 13% to 21% at a step of 4%.Additionally,the diesel engine test was conducted to verify the results collected from the CVC.With a reduction in ambient temperature and oxygen concentration,the spray,combustion and flame evolution process of all tested fuels present some general trends such as an increase in liquid penetration length and spray area,an increase or decrease in maximum combustion pressure and heat release rate,a prolonged ignition delay,a shortened or prolonged combustion duration,a retarded combustion phasing,and a increase in flame lift-off length and air-fuel mixing area.The spatially integrating natural flame luminosity(SINFL)of all tested fuels decreases as the ambient temperature decreases.When the temperature fixed at 800 K and 1000 K,a similar trend of SINL for all tested fuels can be found with a decrease in the oxygen concentration.When the temperature is fixed at 1200 K,SINFL of diesel and small-ratio IBE/diesel blends decreases as the temperature decreases;but for pure butanol,IBE,ABE and IBE80,SINFL increases as the temperature decreases.Fuel properties have an obvious impact on the spray,combustion and flame evolution process under LTC conditions,but lesser impacts under normal temperature conditions.The results show that the spray characteristics improve,liquid penetration shortens and spray area reduces with the addition of IBE into diesel.The higher the IBE to diesel ratio is,the better the spray performance was.Unfortunately,the combustion pressure and heat release rate reduced with blending ratio increases,which is not good for maintaining the thermal efficiency of engine.The lift-off length and air-fuel mixing area of IBE/diesel blends is larger than those of pure diesel.Also,compared with pure diesel,the SINFL of IBE/diesel blends is smaller.It can be found that with an increase of the volume fraction of isopropanol in IBE,the liquid penetration length of tested fuels shortens,the spray area decreases,the lift-off length slightly prolongs and the SINFL reduces drastically.Those results indicated that the increase of the volume fraction for isopropanol in IBE is favorable in improving spray characteristics and reducing soot emissions.The spray characteristics of pure butanol,pure IBE and pure ABE are very close to each other,and only present some slight differences under the condition of 800 K.The liquid penetration is minimal for ABE,moderate in IBE,and maximal for butanol.Compared with IBE and ABE,the SINFL of butanol is larger.At the temperature of 1000 K,the SINFL of ABE is slightly larger than that of butanol,but drastically larger when temperature is increased to 1200 K.The engine results show that when blending IBE into diesel,the soot emissions could be drastically reduced but NOx emissions increased.With the decreased oxygen concentration,the NOx emissions for all tested fuels remarkably decreases,and the soot emissions for diesel and IBE15 increase.However,the increase of soot emissions for IBE30 is very little.That is to say,IBE30 coupled with a proper EGR ratio has the potential to reduce NOx and soot emissions simultaneously.