İçten yanmalı motorlarda doğal gaz kullanımı

Abstract

Doğal gazın motorlarda kullanımı, çevre kirlenmesinde büyük payı olan egzoz emisyonlarını ortadan kaldırmak ve tükenmekte olan petrole alternatif bulmak amacıyla başlamıştır. Bu çalışmada doğal gazın içten yanmalı motorlarda kullanımı incelenmiştir. Bu inceleme altı ana bölümde yapılmıştır. Birinci bölümde, gaz yakıtların motorlarda kullanılma nedenleri ve Türkiye' nin doğal gaz potansiyeli anlatılmıştır. İkinci bölümde, doğal gazın fiziksel ve kimyasal özellikleri ele alınmış ve bu özelliklerin motorlara etkisi tartışılmıştır. Üçüncü bölümde ise motorlarda gaz yakıt kullanımı ve doğal gazın içten yanmalı motorlarda çeşitli kullanım şekilleri ve özellikleri karşılaştırılmıştır. Dördüncü bölümde, doğal gaza dönüştürülen bir motorun en fazla değişiklik gören kısmıolan yakıt sistemi anlatılmıştır. Bu bölümde taşıtlar hafif ve ağır taşıtlar olarak iki aşamada ele alınmış, doğal gazlı motorlarda kullanılan elektronik enjeksiyon sistemi tarif edilmiştir. Beşinci bölümde MAN firması tarafından üretilen E2866 DUH / DF doğal gaz motoru çeşitli açılardan incelenmiştir. Altıncı ve son bölüm, tüm bu anlatılanlar gözönünde tutularak doğal gazın taşıtlarda kullanımının sonuçları tertışılmış ve öneriler sunulmuştur.

Two new factors are now coming into play which are expected to give a strong boost to the use of gaseous fuels in internal combustion engines, especially for road transport, these are: * Increased pressure on the conservation of the environment * Dwindling oil reserves These are expected to further intensify the research and development into these excellent fuels and correspondingly excellent results are expected. For various reasons natural gas will gain increasing importance as a fuel for commercial vehicles. On the one hand, countries with great natural gas fields wish to use gas as a fuel for motor vehicles for economic reasons. On the other hand, there is a substantial potential for reducing the emissions of commercial vehicles by using natural gas in combination with advanced engine concepts. Natural gas is a compound of several gases, the main portion being methane with a share of 80% to 98% in the total volume, depending on the area where the gas comes. Methane weights only about half as much as air. Therefore, escaping methane will rise very quickly into the atmosphere where it will rapidly dilute. Methane has a high ignition temperature of about 650 °C. So it has advantages with respect to self-ignition in the event of the combination of a gas system leakage and hot components. The octane number of methane is very high. This permits a relatively high compression ratio which has a positive effect on energy consumption. In the 1950's the application of LPG as automotive fuel began. Later the work was extended to include natural gas and other alternative fuels. In Netherlands by now about 700.000 cars are running on LPG. In the region of 50 cars and vans some 17 buses are running on natural gas. TNO has always contributed in the development of components required by these vehicles. At a later stage TNO has contributed by developing dedicated engines for the motor industry. TNO's involvement is quite international. It has for example made a substantial contribution to the LPG buses in Vienna, Austria. In the past a lot of work has been done on mixed fuel systems in diesel engines, in which the gaseous fuel is mixed with the combustion air and the amount of diesel fuel is accordingly restricted. With regard to sparkignited engines parallel developments of stochiometric and lean burn engines were started. Originally the developed systems were of a relatively simple nature. That means all controls were run by simple electronics and components such as diaphragms, springs and ventures. XI Due to the increasing pressure to obtain optimal exhaust emission results, four years ago TNO started the development of micro-electronic electronically controlled motor management systems. Gaseous fuels can be used both in diesel engines (dual fuel) and in spark ignited engines (mono fuel). An overview of the characteristics of spark ignited, dual fuel and diesel engines is made with the following possibilities: - Spark ignited lambda = 1 - Spark ignited lean burn - Pilot injection diesel / gas ( carburetted as well as double injection ) - Mixed operation diesel / gas - Glow plug assisted engine - Diesel engine All of these engine types can be either naturally aspirated or turbo charged. TNO has much experience with the engine types except for pilot injection engines with double injection and glow plug assisted engines. Emission legislation in the Netherlands for light duty vehicles is such that CNG converted vehicles have to meet the same emission standards as their petrol equivalent. To meet stringent emission legislation more and more vehicles are introduce to the market which are equipped with micro-processor controlled petrol injection and ignition systems. Vehicles running on gaseous fuels equipped with electronically controlled gas-carburetted systems have an explosive mixture in the inlet XII manifold. When the ignition system of the vehicles does not function correctly, the mixture can ignite spontaneously in the inlet manifold. The electronic system as well as the mechanical fuel injection system can be damaged. Without micro-processor controlled gaseous fuel systems it is not possible to convert vehicles to a gaseous fuel without disturbing the petrol injection and engine management system. These problems were main reason for CNG / LPG equipment manufactures to start the development of micro-processor controlled fuel injection systems for the use of alternative fuels like CNG. New emission legislation has been introduced in United States for heavy duty engines for the period 1991-1998. There is a real chance that clean burning fuels could be introduced in view of the increasing stringency of emission levels. To meet the stringent emission legislation for heavy duty engines one of the equipment manufacturers has developed a micro processor controlled single point gas injection system. The result obtained so far endorse our view that the use of CNG be considered as a potential emission reducing strategy. A new electronically controlled fuel system is proposed for direct injection of natural gas in diesel engine. It consist of the solenoid controlled injectors interfaced with a metering valve under stepper motor control. This system proved to operate well in different configurations with metering valve being located at various distances from the injector; two injectors interfaced by one metering valve were also tested. In all cases, the required maximum and minimum gas doses were obtained, either by the change of the metering XIll valve flow area or the injector opening time. The gas dose was measured using a special closed chamber. The conclusion drawn from these test were used for the proposal of improvement of the injector design in order to reduce its size and weight and obtain more reliable operational characteristics. MAN has developed two types of natural gas engines for vehicles: the E2866 DUH and E2866 DF. Both engines are naturally aspirated. The E2866 DUH is an engine for buses and is installed horizontally. The E2866 DF is an engine for trucks and is installed vertically. Both types of gas engines base on their Diesel counterparts. The gas engines operate on the Otto-cycle, have a throttle valve, a controlled mixture formation device and a three-way catalyst.