Study On Premixed Combustion Mechanism Of Diesel Engine

The continuous development of automobile technology and the automobileexhaust legislations which become increasingly strict, had put forward a morerigorously challenge to the existing diesel engine technique, and the diesel enginewith traditional diffused combustion mode could not adapt the request of newexhaustion standard any longer. The diffused combustion is lord on the traditionaldiesel engine ,and the fuel and air mix asymmetrically, therefore in the district thatA/F ratio near to the chemistry equivalent ratio, the burning temperature is high,sothat more NOx is produced;whilein the district that A/F ratio is low , therewould be a great deal of carbon smoke because of lacking oxygen .To solve theantinomy that the diesel engine NOx and particulate exhaust radically, the onlymethod is to change the mix mode of the fuel gas mixture to make the mixturemix better and consenquently enhance the premix combustion scope and reducethe diffuse combustion quantity. The mixture has enough time to mix together so asto improve the uniformity and enhause the premix combustion scope if theinjection advance angle is large, but it brings disadvantages meantime, such asmaking the combustion burn in advance, increasing the compression negativepower, abnormal combustion, too fast combustion, and even knock. In order tominish negative influence of big fuel injection advance angle , EGRwas used.EGRcan lower the burn temperature and postpone combustion start point and ease theknock cylinder phenomenon that the big fuel injection advance angle brought .Therefore, in this paper premixed combustion mechanism of diesel engine wasstydied through changing the fuel injection advance angle and EGR rate under lightload.Generally speaking, EGR is an effective measure to lower NOx exhaust ofdiesel engine, currently, and also one of the basic ways for the control of catchingfire in combustion process. In this thesis its main action is to control fire timeinside cylinder. The principle is that one part of waste gases the namely not live gas(main for CO2) is led into inlet pipe and mix with the fresh air so that the the heatcapacity of the mixture increased so that respond speed before the burningdecelerates, time for mixture to catch fire is postponed. It is very easy for theengine tocatch fire in advance or knock using this method, hence, in order toavoid this, electronic control EGR system was adopted. The system is a highpressure loop EGR system,that is to say, circulate waste gas was derivates beforesupercharger , and then flowed through the EGR valve to restrict its flow, and wasled into the inlet pipe after the ventury pipe after being cooled by the EGRcooler,.The influence of EGR can also be experimental simulated through using theadditional gas or artificial composition gas;The imitating EGR gas physically is amixture of saving gas(such as the compression CO2) and the fresh air. This kind ofsimulation method can reappears EGR basic characteristic steadily under thecondition that the diesel engine waste gas couldnot be used. Having no combustiblecomponent is the main anti difference that imitates EGR to the actual EGR,however this contribute to stable running of diesel engine being using the very highCO2.Because this experiment design of high pressure loop of the EGR system couldget limited EGR rate, therefore, a set of imitate EGR system was designed to meetthe need of thesis study, big EGR rate could be available using this imitate EGRsystem.In this paper, the influence of fuel injection advance angle and the EGR rate onthe premixed combustion process of diesel engine is studied from the point ofengine performance and combustion characteristics. The result of the experimentindicatethat:1.The influence of fuel injection advance angle and the EGR rate on the dieselengine power, economy and exhausts characteristics.When engine speed, circlefuel amount and the EGR rate were fixed, along withfuel injection advance angle increasing, the NOx exhausts went up gradually, thesmoke degree descend, the HC and CO exhaust ascents slightly, the fuel consumerate(be)and the mean effective pressure didn't change greatly when the fuelinjection advance angle is under 20°BTDC;while the andvace angle was biggerthan 20 °BTDC of Advance angles, the NOx exhaust descend gradually,descending a degree very big, while the bigger fuel injection advance angle 47°BTDC, the NOx exhaust has already attained a few PPM, the smoke degreedescend, the HC and CO exhaust increase greatly, the fuel consume rate ascents,the mean effective pressure descends.When engine speed,circle fuel amount of and the fuel injection advance anglewere fixed. Along with the increase of the EGR rate, the NOX exhausts lower, thesmoke degree ascent, the HC and CO have ascent slightly, the fuel consume rateand mean effective pressure didn't change greatly much. Under the bigger fuelinjection advance angle and the bigger EGR rate, the fuel consume rate and HCexhaust ascents greatly.When circle fuel amount, fuel injection advance angle and the EGR rate werefixed, along with the revolving speed going up , the NOx exhaust goes up, thesmoke degree changes little, the HC and CO go lower, the fuel consume rate waslower and mean effective pressure changeed little when fuel injection advanceangle is above 20°BTDC,. Under the small fuel injection advance angle 20 °BTDC ,along with the increase of the revolving speed, the NOx exhaust reduces,the smoke degree goes up slightly, the HC and CO changes little, the fuel consumerate and mean effective pressure didn't change greatly much.2. The influence of fuel injection advance angle and the EGR rate on the dieselengine combustion characteristics.when engine speed, circile fuel amount and the EGR rate were fixed, alongwith fuel injection advance angle increasing, the combustion starts spot movesahead, maximum combustion pressure, the maximum pressure raising rate andmaximum releasing heat rates all enlargeed gradually, the corresponding crankshaftangle also continuously moved ahead. When fuel injection advance angle increaseto 20 °BTDC, combustion process changed obviously, low temperature powerreaction began to appear, maximum combustion pressure, the maximum pressureraising rate and maximum releasing heat rates all lower gradually, thecorresponding crankshaft angle postponed.When engine speed, circle fuel amount and injection advance angle werefixed, along with EGR rate increasing, under the small fuel injection advance angle,the combustion start spot postponed, and the maximum combustion pressurereduced, but the extent is not very big, maximum pressure raising rate andmaximum releasing heat rates changed little, the corresponding crankshaft anglepostponed;While fuel injection advance angle became bigger, the combustionstarts spot postponed, maximum combustion pressure, the maximum pressureraising rate and maximum releasing heat rates all declined, their peakcorresponding crankshaft angle postponed too.3,Through experimental study about influence fuel injection advance angle andEGR rate on premixed combustion mechanism of diesel engine, we can found that:under bigger fuel injection advance angle, leading into EGR can increase thepremixed combustion scope of diesel engine and reduce the diffusion combustionquantity and decrease NOx and particulate exhaust in the end and improve dieselengine exhaust performance.