Dizel motorlarında yakıt sisteminin yanma olayına ve egzost gazları emisyonuna etkisinin araştırılması

Abstract

Dizel motorlarındaki, yanma olayı kullanılan yakıtın özellikleri/ püskürtme avansı açısı, sıkıştırma oranı, ha va fazlalık katsayısı motor devir sayısı, püskürtülen yakı tın atomizasyonu gibi çeşitli faktörler tarafından etkile nir. Bu faktörlerin bir çoğu ise günümüzde kullanılmakta olan dizel motorlarında yakıt sisteminin ayar ve bakım iş lemleriyle yakından ilgilidir. Yeni kullanıma başlayan bir dizel motorunda, motorun üretimi esnasında en iyi yan ma performansını verecek şekilde yukarıda belirtilen fak törler göz önüne alınarak yakıt sistemi seçilmiş ve dizayn edilmiştir. Bununla beraber kullanılmaya başlanan motorla rın yakıt sistemi zamanla çok çeşitli ayar ve bakım işlem lerine ihtiyaç gösterir. Günümüzde kullanılmakta olan dizel motorlu taşıtların egzostlarına baktığımızda motordaki kötü yanmanın bir sonu cu olarak çok koyu renkli ve zehirli egzost gazları çıkart tıkları görülmektedir. Bunun yanında yanmanın istenilen şartlarda gerçekleşmeyişi motor ömrünü de kısaltmaktadır. Bu çalışmada kara yollarında kullanılmakta olan dizel motorlarındaki yakıt sistemlerinin motordaki yanma olayına ve egzost emisyonlarına etkileri, deneysel olarak araştırıl mıştır. Yapılan deneysel çalışmaların yanında günümüz dizel motorlarında yaygın olarak kullanılmakta olan port ve helis kontrollü yakıt pompaları ve bunların dizayn yöntemler ide geniş bir şekilde incelenmiştir.

The fuel injection equipment in a diesel engine are intended to supply fuel to the cylinders, atomize and partly distribute it in the space of the combustion cham ber. i'ta functioning has a considerable influence on the power and economy charecteristics of an engine. * * The fuel injection equipment must ensure effective going on of the working process in a cylinder and the obtaining of charecteristics conforming with the condi tions of service of the engine on the vehicle. This equipment injects a precisely metered quantity of fuel at a specific time. The injection pump produces the pressure required for injecting the fuel and supplies the fuel to the cylin ders of the diesel engine. u The combustion process in the diesel engine depends decisively, on the amount of fuel supplied to the engine and how it supplied. The most important criteria are; a- The time at which the fuel is injected b- How long the fuel is injected c- The distribution of the fuel in the combustion chamber. d- The time of the start of combustion e- The service life of all the elenrsnts of the fuel feed system should not be less than that of the engine itself. f- The quantity of fuel supplied per degree crank angle4 g- The total amount of fuel supplied in accordance with the load on the engine. h- Identical supply of fuel in each cycle to all the cylinders of the engine. IX These criteria must be ideally met in order for the diesel engine to operate properly. In practice the fun- tions are performed by the fuel injection system or at least are greatly unfluenced by it. The classification of fuel feed systems is based on the design of its principal elements, the high pressure pump and the xnjector. Motor vehicle diesels mainly employ two types of fuel feed devices 1- Divided, with the high pressure pump and injector designed as two separate units. 2- Undivided, with the pump and injector combined in one unit. injectors may be of the open or closed type. There are various factors influencing combustion in diesel engines. They are properties of fuel, compression ratio, injection advance angle, atomization of fuel, dura tion of delivery, speed, and excess air coefficient value. The fuel feed devices of diesel engines which were being used today's motor vehicles, affects many of these factors and these factors affects combustion performance and exhaust emissions of diesel engines. A great injection advance increases the ignition lags, since the pressures and temperatures of the air are lower when injection begins. Accordingly the values of P and dp/dtf rise sharply not only because a greater amount of fuel accumulated in the combustion chamber by the moment of ignition, but also because most of the fuel burns near top dead center at V= Const. When the injection advance angles are small the ignition lags reduce, the engine operates smoother, but its power and completeness of com bustion decreases, since most of the fuel burns out during expansion. In this study the effect of early injection was found by experimenthali test of the test engine. The fuel pump of the test engine was separated from the engine and fixed again with 32 crank angle before top dead center injection x advance. Afterwards test engine was run and tested. When the motor was run, there were a big vibration and knock diesel engine due to greater ignition lags and fuel accumulated at this greater ignition lag periods. Improperly assembled fuel pumps cause this results. An another factor affecting combustion in a diesel engine is excess air coefficient. Excess air coefficient is a parameter which affects directly the out put and efficiency of the engine. In order to investigate the effect of "excess air coefficient on emissions of diesel engines, the experi ments were done at several excess air coefficient values. As a result, it was found that when the excess air coeffi cient was decreased, soot formation increased at the ex haust of the engine. Excess air coefficient decreases due to dirty air filters of diesel engines. As another experiment, the fuel pump of the engine was disassembled from the engine and it was tested in a fuel pump test bench. The fuel quantity which was deli vered by the pump to the glass tubes belonging each cylinder of the engine was mesured for 500 work cycles with the same rack stroke. The amount of oil in each glass tube was not the same. If the fuel differency in each glass tube becomes higher than % 3 percent for the same rack stroke, the pump must be adjusted. An another factor affecting the combustion on diesel engine is injection pressures. The fuel pressure generated by the injection pump acts in the pressure chamber on the exposed annular area of the nozzle needle. If the fuel pressure is higher than the opposed force of the pressure in the nozzle holder, the nozzle needle is lifted off its seat. The nozzle holes are open and fuel is injected into the combustion cham ber. The nozzle opening pressure is determined by (adjustable) inital tension of the pressure spring in the nozzle holder. The lift of the needle is limited by annular force undarneath the recessed valve stem. xi During injection the fuel takes the following path: fuel injection tubing-inlet bore-pressure chamber-spray orifices of injection nozzle-combustion chamber. The fuel leaking past the valve stem flows via the leak-off part on the nozzle holder and is returned to the fuel tank through return line. After injection of the fuel quantity, delivered by the injection pump, the pressure spring again forces the nozzle needle back onto its seat via the spindle and^the valve stem. The nozzle is thus closed again until the next lift. If the nozzle-opening pressure is different from its original pressure value (higher or lower) at this state, injection advance angle of the motor changes, atomization of fuel becomes bad, Power of the engine drops, soot formation starts, and specific consumption of the engine increases. Pump-injection systems are used for all types sizes of engines and all forms of the special forms of pump injec tion are always associated with certain forms of combus tion chamber and size of engine. Pump injection systems use both the open and closed types of spray nozzle. In general, the open nozzle is used where a relatively coarse spray is statisfactory, owing to the form of combustion chamber or the speed of the engine. Closed Pintle-type nozzles are used with turbulent combustion chambers. Multi-hole-type nozzles are used where dependence must be placed on the fuel jets for atomization and distribution of the fuel ih the combus tion chamber. In the case of variable-speed engines and especially those with a very wide speed range, some means must be provided for retarding the beginning of the fuel injection as the speed drops. As result of the test, the soot quantity values accor ding to excess air coefficient have been shown in the figures. xn Experiment X d(nım) F(N) n(rpm) R (mmss) Z(sec) Soot No (.g/cm3) In this study many factors affecting combustion in diesel engine has been investigated by being considered the effect of fuel injection equipment.