Doğal gaz kullanım oranının bir taşıt çevriminde modellenmesi

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Tarih
1995
Yazarlar
Demirel, Kaan
Süreli Yayın başlığı
Süreli Yayın ISSN
Cilt Başlığı
Yayınevi
Fen Bilimleri Enstitüsü
Özet
Doğal gazın, egzoz ve gürültü emisyonları bakımından sahip olduğu avantajlar sebebi ile içten yanmalı motorlarda yakıt olarak kullanımı her geçen gün artmaktadır. Karayolu araçlarının motorlarından yayılan gürültü ve egzoz emisyonlarına oldukça dar limitler getiren veya zengin doğal gaz rezervlerine sahip olan ülkelerde doğal gazlı araçların artış hızı daha da fazladır. Doğal gazın taşınması, dağıtımı, depolanması gibi problemlerin çözümlenmesi ve doğal gazlı motorların verim, dayanıklılık ve fiyat açısından dizel motorlar ile rekabet edebilecek seviyelere gelmesi özellikle kamu taşımacılığında kullanılan doğal gaz motorlu araçların yaygınlaşmasına yardımcı olmuştur. Motorların dizel ve doğal gaz karışımı ile çalıştırılmaları ile de egzoz emisyonlarında azalma sağlanabilmektedir. Bu çalışmada, doğal gazın sahip olduğu özellikler verilmiş ve hem tamamen doğal gaz ile çalışan modern motorlar, hem de yakıt olarak doğal gaz ve dizel karışımını kullanacak şekilde dönüştürülmüş dizel orijinli motorlar hakkında bilgiler sunulmuştur. Avrupa, Japonya ve Amerika 'daki, motor üreticisi firmaların, doğal gazlı motorların tasarımında dikkate aldıkları ortak hususlardan söz edilmiştir. Ayrıca doğal gaz motorlarının, dizel motorlar ile egzoz emisyonu, gürültü ve ısıl verim bakımından karşılaştırmaları da yapılmıştır. Diğer bir bölümde ise egzoz emisyonlarının oluşum sebepleri ve emisyon azaltıcı önlemler konusundaki bilgilere yer verilmiştir. Doğal gaz kullanım oranının bir taşıt çevriminde modellenebilmesi için motora ait özelliklerin yanısıra, araca etkiyecek seyir dirençleri ve bu dirençlerin gerektirdiği motor gücünün hesabı da yapılmalıdır. Bu yüzden taşıt dinamiği ile ilgili konular ayrı bir bölüm başlığı altoda sunulmuştur. Çalışmanın diğer bir bölümü ise, EPA ( Environmental Protection Agency ) şehir çevriminin, ağır ticari araçlar için geçerli olan bölümünün doğal gaz ve dizel karışımı ile çalışan MAN marka motora sahip bir otobüse uygulanması ve doğal gaz kullanım oranının hesaplanması için hazırlanan bilgisayar programının anlatımım içermektedir. Bu programda, yalnızca sözkonusu motorun, İ.T.Ü. ' de yapılan testler sırasında elde edilen yakıt tüketim ve güç değerleri kullanılmaktadır. Ancak motorun uygulanacağı aracın özelliklerinin, programa dışarıdan girilmesi sağlanarak aynı motorun farklı araçlardaki yakıt tüketimi ve doğal gaz kullanım oram hesaplarının yapılabilmesine de olanak sağlanmıştır. Altıncı ve son bölümde de yapılan çalışmadan elde edilen sonuçlar ele alınarak bilgisayar programının verileri yorumlanmıştır.
Natural gas has become the most popular alternative fuel for the internal combustion engines having it's specific properties as well as higher octane number, better emission levels etc. Natural gas due to it's high octane number ( research octane number is 120 ) provides stable and knock free combustion. It is flammable within large air / fuel ratio range so natural gas engine can operate either stoichiometric or lean mixture depending on design criteria. Natural gas has higher specific heat value then both diesel fuel and gasoline. Adding better exhaust emission potential to the properties above, natural gas have been accepted as an clean alternative fuel from almost all worldwide engine manufacturers. From engine manufacturers side, it is also important that the natural gas engines due to their working principal ( Otto cycle ) have less vibration and noise. Natural gas must be compressed up to 200 - 250 bar pressure in order to provide adequate fuel storage capacity for the vehicle which is equipped with natural gas engine. For this reason engines which run with compressed natural gas is so called CNG ( Compressed Natural Gas ) engines in general. If different CNG engine manufacturers overlooked, lots of design similarities can be found. For example, all state of the art CNG engines are the modified versions of the diesel ones. Power and torque rating targets during design concept is kept similar to diesel versions. Because replacement of CNG engine with diesel version must not cause a serious performance reduction on the same vehicle. As mentioned before CNG engine works as a gasoline engine and it means that thermal efficiency ( because low compression ratio and throttle loses ) is lower than diesel. To compensate this difference, addition of turbo charger and aftercooler units is necessary. In this case same torque and power figures can be achieved by the CNG engines. Another similarity for all CNG engines is the characteristic of the turbo unit with waste gate valve that operates certain ranges of boost pressure and provides flat torque curve at low engine speed. xiii With addition of by - pass valve and small diameter turbine, turbo unit have a quick response for engine air requirements. For the aftercooler unit both air to water or air to air aftercooling are possible and used. But air to air aftercooling is more general. Both water or air cooling is very useful for reducing the combustion chamber temperature levels resulting reduction of NOx formation during combustion. Because NOx formation is strongly depend on maximum combustion chamber temperature and the time period of it. Also aftercooler unit has an important influence to increase air amount which is inducted to the combustion chamber, since it can be clearly seen that the charge air temperature reduction, increases the air density. Spark plugs are used for starting the combustion on CNG engines. These plugs are located same place ( instead of injectors ) with injectors on diesel versions. This component interchange requires some design changes on cylinder head. And these applications ( if the CNG engine operates with lean mixture it may be effects ignition stability ) need more voltage then conventional gasoline engine spark plugs. Compression ratio of a natural gas engine must be less than diesel one. The reason for that is to prevent self ignition of the mixture. For reducing the compression ratio either piston profile or cylinder head design shuld be changed. For compression ratio 10-11 values are used. These new generation engines require different sealing as well as valve guide gaskets, modified piston rings etc. The aim is to minimize oil leakage to the combustion chamber. One of the most important difference between new CNG engines and their diesel versions is, CNG has completely electronically controlled. It means these engines equipped with many sensors as well as knock sensor ( the sensor that can detect self ignition of the mixture before ignition of the spark plug and causes partial pressure increase and unstable combustion ) manifold temperature sensor, engine speed sensor etc. An electronic control unit ( this control unit usually called ECU ) governs the fuel air mixer to maintain the quality of combustion resulting better performance and exhaust emissions. Natural gas passes through a first stage pressure regulator when the engine is started and the solenoid valve (which works as an on / off switch ) is turned on. At the first stage, pressure is reduced from 200 - 250 bar to 7 - 10 bar. First stage regulators must be heated by the engine cooling system to prevent icing due to large heat absorption of the expanded natural gas. On the second stage, gas pressure is again reduced close to the atmospheric pressure. Air gas mixing is provided by the mixer. It works as a carburetor that on gasoline engines. In general this same principal is used but there are also some different gas - air mixing systems as well as high pressure gas injectors should be considered. XIV If such a high pressure system is preferred there is no need to have a second stage reducer. Either low or high pressure mixing system, the main issue is to obtain precise air fuel mixture formation for all engine speed and load conditions. This is very important not only for the best thermal efficiency but also for exhaust emissions which are strongly related with air / fuel ratio of the mixture in each cylinder. :nous mixture To comply with stringent exhaust emission regulations homog© from cylinder to cylinder and cycle to cycle is a necessity. If the CNG engine have been designed to operate with stoichiometric air / fuel ratio ( X equal 1 ) with closed loop three way catalytic converter, mixture formation becomes more critical than the engine which runs with lean mixture. The reason is that any small deviation from the stoichiometric ratio dramatically reduces the efficiency of the converter and thus increases the amount of pollutants emitted by the engine. It must be considered that the maximum conversion efficiency for CO, NOx and HC can only be achieved between 0.98 - 1 X values. Stoichiometric air / fuel ratio causes higher temperature level in the combustion chamber compared with lean mixture burning. Considering the CNG engines are the modified versions of the base diesel engines ( which have less burning temperature ) this temperature increase brings thermal loads and probably effects the durability of the engine. In spite of high burning temperature disadvantage, stoichiometric combustion gives higher power output and lower emission level if coupled with close loop three way catalytic converter. Compressed natural gas is stored in high pressure steel tanks over the roof of the vehicle and vehicle must be capable on operate in approximately 300 - 350 km distance without refueling. Providing such a fuel tank system ( with pipes, mountings, valves etc. ) results up to 1000 kg additional weight. In general 800 It total volume for the steel tanks is acceptable. It is also possible to run a diesel engine with dual fuel ( natural gas and diesel fuel ). In this case, natural gas enters the engine manifold with air to form a premixed mixture then certain amount of diesel fuel is injected at the end of compression stroke. Therefore such a system requires some extra equipment on the engine to form gas - air mixing and on the vehicle to store the natural gas. This is a quick and cost effective solution because the engine do not have to be redesigned. Also necessary equipment to perform this dual fuel conversion from diesel fuel to diesel fuel + natural gas are not so complicated. For dual fuel system, natural gas consumption of the engine is strongly depends on the working condition. For instance at idle speed engine runs with 100 % diesel fuel to ensure stability. XV As the speed and load increases the consumption of compressed natural gas also increases. As a result such engines operating mostly at high speed and load conditions, dual fuel system may have an advantage for emissions ( CNG does not produce soot during combustion ). If a converted engine will be operated in transient conditions ( like city bus route ) the benefits of such system is discussible. For this study a computer program was prepared to calculate the rate of natural gas and diesel fuel consumption of a converted engine. For the engine specifications, test results ( performed by Istanbul Technical University ) of a converted MAN engine were used and for working cycle simulation EPA ( Environmental Protection Agency ) transient cycle was chosen. On this program it is possible to change the vehicle properties as well as tyre rolling radius, weight, gearbox or final drive ratios, air drag coefficient etc. allowing the evaluate effects of these parameters on both fuel consumption and rate of each type of fuel is consumed by the engine. Quick Basic editor was chosen for preparing the program. This program named EPA.BAS is mouse controlled and allows configuration changes which can be performed by the user. EPA Transient test for heavy duty vehicles has been used to simulate both city traffic and highway conditions. This test cycle ( totally 1060 seconds ) captures all different working conditions such as acceleration, constant speed idling etc. and contains 93.34 km / h max speed, 30.35 km / h average speed and 8.937 km total distance. Program receives speed values of EPA transient cycle from another file named EPA. TXT. By using this and previous speed value, at the first step acceleration value of the vehicle ( it must be considered that the acceleration value can be positive negative or zero ) is determined. By using formulas that mentioned on section 3 of this study resistance of motion is calculated for that speed and acceleration value. Also engine speed and maximum power of the engine at this speed is found by using vehicle speed, gearbox / axle ratio and dynamic radius of tyres. If the engine power output matches the total resistance of motion, acceleration and speed values which are the requirements of EPA cycle are confirmed by the program. In opposite situation according to the engine output capacity at that point new values which are as close as possible to the requirements of EPA are determined. Than engine speed and mean effective pressure is found again. Computer program has a database containing fuel consumption values of the MAN engine which have been measured during the tests. For a specific engine speed and pressure condition ( by iteration method ) fuel consumption on that working condition is calculated. XVI As a result of this study, it has seen that the dual fueled engine consumes both natural gas and diesel fuel with very similar proportions ( close to 50 - 50 % ratio ) for a given test cycle which simulates typical city traffic conditions. Maximum consumption rate of the natural gas occurs at maximum vehicle speed ( 79 % ). On the other hand it must be considered that city busses can be used at maximum speed condition only very short period for during their lifetimes. Finally, even it is a practical and cost effective solution, the conversion of a diesel engine to dual fueled engine have limited capability reducing the exhaust emissions and diesel fuel consumption. In spite of their high initial costs, compressed natural gas engines which operates according to Otto cycle and consumes 100 % natural gas, have a good advantage to comply with current or forthcoming exhaust emission limits.
Açıklama
Tez (Yüksek Lisans) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 1995
Anahtar kelimeler
Doğal gaz, Otomotiv endüstrisi, Natural gas, Automotive industry
Alıntı