Thermodynamic Equilibrium Analysis On Diesel Methanol Dual Fuel Combustion

The results given by theoretical analysis and experimental research show that,the diesel methanol dual-fuel(DMDF)combustion is the most suitable way to apply methanol on compression ignition(CI)engine.To be brief,the DMDF combustion is realized by injecting low-pressure methanol into the intake manifold.The formed methanol-air mixture will be ignited by diesel bundle around TDC.When adopting the DMDF combustion,the NOx and PM emission in diesel engine can be reduced together,and the emission property is improved combining with simple aftertreatment devices.Except that,the engine thermal efficiency also has a remarkable increasement,leading to reduction in CTU.So far,the mechanism on how the DMDF reduce engine emission has been studied widely and its potential to improve emission property is explored sostenuto.However,studies on high efficiency combustion mechanism of DMDF engine is rare to be reported.The results from engine bench tests show that,the DMDF can achieve much lower replacement ratio than theoretical value.Meanwhile,some unique characteristics may appear when converting to DMDF combustion.For example,some condensate water may appear at the surface of the intake manifold after large amount of methanol injection,and the exhaust temperature and cooling water temperature both have a certain extent of drop.These phenomena mean that the thermodynamic balance of the engine is changed,leading to less in energy loss and more in effective work.Therefore,it is an efficient path to analyze the DMDF combustion from the thermodynamic equilibrium point of view.By calculating the quantity of all loss items and their variation,the high efficiency mechanism of DMDF combustion can be revealed,and the diesel methanol dual fuel combustion theory is more substantial after absorbing the thermodynamic equilibrium analysis.In this study,an EUP diesel engine equipped with a methanol injection system at the intake manifold was selected as the main experimental subject.Basing on the first and second law of thermodynamics,the calculation models for energy and exergy balance of DMDF engine were built by selecting the control volumes as engine test system and cylinder separately.Then the DMDF combustion was analyzed from the thermodynamic equilibrium point of view.The effects of operating parameters,such as engine load,cooling water temperature,intake air temperature and methanol temperature,on the energy and exergy balances of DMDF engine were investigated.In the end,the incylinder combustion property,which is the decisive factor of the DMDF thermal behavior,was fully discussed.The main conclusions are listed below:The energy and exergy balance of DMDF engine has significant variation with engine load.The replacement ratio is higher than its theoretical value 2.16 at low-medium load,but it becomes much lower than 2.16 at medium-high load condtions.The explanations from the energy balance point of view are as follows: The cooling loss in DMDF combustion is always lower the the correspondent D mode.At low-medium load,the DMDF has much higher incomplete combustion loss,but this reversed at medium-high load condition with a much lower exhaust loss.For the exergy balance point of view,the exergy loss due to heat transfer is lower for DMDF at the whole load range,but its combustion irreversibility is also higher.The difference of combustion irreversibility between the two modes becomes narrow at medium-high load.The emission physical exergy of the two modes all increase with rising engine load and their difference appears only in medium-high load.The emission chemical exergy for DMDF mode drops to be lower than D mode at medium-high load,while their relationship is reversed at low-medium load.The intake air temperature has the most important effect on energy and exergy balance of DMDF engine.From the energy balance point of view,when increasing the intake air temperature,the cooling loss increases incidentally,but the exhaust loss almost unchanged.By increasing the intake air temperature from 35? to 65?,the thermal efficiency of DMDF combustion improved significant.This is the result of reduction in incomplete combustion loss and intercooler loss.At 35? to 65? range,the increasing intake air temperature lowers the replacement ratio,but the overall thermal efficiency has little improvement.This can be explained that,the incomplete combustion loss does not decrease at higher intake air temperature.Meanwhile,the intercooler does not absorb heat from intake air but offer some through its heating module,causing increasement in total input energy,and the increased energy cannot be reflected in replacement ratio.From the exergy balance point,the improving intake air temperature leads to increase in heat transfers exergy loss and exhaust physical exergy loss.The exhaust physical exergy loss decreases with improving replacement rate.While the exhaust chemical exergy loss decrease with rising intake air temperature and increases with rising replacement rate.The exergy efficiency improvement at 65? to 100? is much lower than the 35? to 65? range,which is the result of little reduction in combustion irreversibility at higher intake air temperature range.Controllable methanol quick combustion(CMQC)is the core characteristic of DMDF combustion.The addition of methanol has two main effects on engine combustion,i.e.putting off the ignition timing and accelerating the combustion process.The accelerating effect appears in the whole engine operating range and enhanced at heavy load condition.Therefore,the sharply increased combustion rate at heavy load will lead to great improvement in isochoric degree.Then the thermodynamic balance of the engine is changed,resulting in reduction in the overall energy loss and improvement in thermal efficiency.At constant operating parameter and boundary conditions,both the putting off and accelerating effects caused by methanol are enhanced with increasing methanol concentration.The putting off effect weakened as the air temperature increases,but the accelerating effect becomes more effective.Therefore,the isochoric degree for DMDF deteriorates with increasing air temperature at low methanol concentration.As the concentration rises,the isochoric degree will improve with increasing methanol concentration.By analyzing the function law of load,concentration and temperature,high efficiency combustion of CMQC can be realized by adopting these items as controls parameters.