| Low temperature combustion(LTC)has great potential to obtain high efficient and clean combustion in diesel engine,which is a hot research topic at present.Fuel properties are critical to LTC performances.And it's known that oxygenated fuels have the advantages in improving LTC combustion.Polyoxymethylene dimethyl ethers(PODE)is an emerging coal based oxygenated fuel,and has the properties of high oxygen content and high cetane number.In this paper,the effects of PODE/Diesel blends and combustion control parameters on LTC of diesel engine have been studied on a heavy-duty diesel engine to explore the approaches for achieving high efficiency and low emission combustion.The test results of PODE/Diesel blends showed that with the increasing of PODE blending ratio,soot,HC and CO emissions reduces and the brake thermal efficiency improves in medium and low load conditions,but it had small change in high load conditions.Therefore,increasing the blending ratio of PODEcan obviously improve LTC performance.In addition,proper early combustion phase and higher fuel injection pressure can achieve higher brake thermal efficiency and lower soot emission.As the load increases,the combustion phase should be appropriately retarded and the fuel injection pressure should be increased appropriately.Lower emission and higher thermal efficiency can be obtained by two stage turbocharged system than single-stage turbocharged system.As well,taking comprehensive thermal efficiency and emissions into account,the opening degree of variable geometry turbocharger(VGT)should be gradually increased with the increase of the load.Besides,Increasing the injector flow rate can alleviates the problems of prolonged injection duration and combustion performance caused by lowered LHV of PODE/Diesel blends,which can obviously improve the LTC performance at high load,especially at the condition of high PODE blending ratio,and thus resulting in higher brake thermal efficiency and lower soot emission.At low load conditions,it has comparative brake thermal efficiency between high and low flow rate injectors.So taking the fuel economy and emission at various operation conditions into account,it is beneficial to optimize the LTC of PODE/Diesel blends by appropriately increasing the injector flow rate.Particulate matter emission characteristics of PODE/diesel blended fuels were further studied.The test results show that the performance in improving accumulation mode particle number is more obviously as the PODE blending ratio increases,and the particle distribution shifts to smaller diameter region.Improving the fuel injection pressure can reduce the particulate number emissions,especially the accumulation particulate matter.When adopted the small pilot-main or main-post injection interval and less fuel mass of pilot and post injection,it can achieve lower particulate matter emission,but they play no obvious role in the peak value of particles number.With the increasing of exhaust gas recirculation(EGR),the number of particle emission and particle size increases.Meanwhile,the first peak range of particle number becomes narrow and the number of large size particles increases obviously.The thermal efficiency can be increased and the emission can be decreased by blending PODE coupled with the optimization of inlet and injection parameters,which is an effective approach to obtain high efficient and clean low temperature combustion for diesel engine. |
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