Alternatif motor yakıtları ve LPG'nin motor yakıtı olarak benzinle deneysel olarak karşılaştırılması

Abstract

Bu çalışma, çağımızda dünyamızın şüphesiz en büyük sorunlarından olan enerji dar boğazı ile ilgilidir. İlk olarak motor yakıtı olarak kullanılabilen alternatif yakıtlardan olan doğal gaz, hidrojen ve LPG'nin elde edilişi, fiziksel ve kimyasal özellikleri ile depolanması, taşınması ve motorlarda kullanımı sonucu oluşacak olan egzoz emisyonlarından bahsedilmiştir. Bu yakıtların araçlarda kullanılmasının avantajlarına ve dezavantajlarına değinilmiştir. Ayrıca günümüzde kullanılmakta olan gaz karbüratörlü LPG sistemlerinden örnekler verilerek incelenmiştir. Çalışmanın esas amacı ise yeni bir yakıt - hava ön karışım modeli geliştirmektir. Bu nedenle Motorlar Laboratuvarında bulunan Renault 12 tipi deney motorunun LPG ile çalışabilmesi için motor kafasında bazı değişiklikler yapılmıştır. Bunun için geliştirilen dahili karışım hazırlama yönteminde, emme supabı hareketiyle kontrol edilen bir mekanizma yardımı ile, gaz halindeki LPG doğrudan yanma odasına gönderilmektedir. Karışım oluşumu emme supabı oturma yüzeyinde gerçekleşmektedir. Emme supabı oturma yüzeyinde çevresel olarak 1 mm çapında 12 adet delik bulunmakta olup bu delikler çevresel bir kanal ile birbirlerine ve motor gövdesinde bulunan LPG giriş hattına bağlıdır. Yakıt gönderme işlemi kam milinin hareketine bağlı olarak supabın yuvasından kalkması ile açılan delikler üzerinden yapılmaktadır. Deney motorunun performansı ve egzoz emisyonları benzin ile çalıştırılarak bir seri deney ile belirlenmiştir. Aynı deneyler LPG kullanılarak tekrarlanmış ve elde edilen sonuçlar birbirleriyle mukayeseli olarak grafikler halinde verilmiştir.

The rapid depletion of conventional energy sources together with the introduction of strict limitations on emissions of polluting combustions products, has lead to an intensive search for alternative fuels. Natural gas and Hydrogen with clear advantages over fossil based conventional fuels is one of the strong candidates for future applications. For heavy duty engines pure natural gas or nature! gas /diesel dual fuel is the best short term solution for meeting the present emission targets. Adaptation of diesel engines to nature! gas operation can be achieved by two different approaches. Dedicated nature! gas engines operating with premixed homogenous charge under Otto cycle, can either use lean or stochiometric mixture. Stochiometric mixture operations are preferred for additional 3- way catalytic converter aplications at the exhaust system to reduce emissions, where as lean operation is more economical. Another method for the utilization of nature! gas in present heavy duty diese! engines is the use of pilot injection of diese! fuel to ignite premixed naturel gas- air mixture. This applications is widely used for the conversion of diese! engines that are already in use. With all these applications reduction of PM emissions is achieved white keeping the CO, HC and NOx emissions at low levels. Hydrogen being the cleanest fuel for internal combustion engines, will be the long term solution for automative emissions. Use of gaseous hydrogen reduces power output of the engine to a certain degree, but produces excellent results in terms of exhaust of emissions as the fuel used does not possess any carbon molecular. This elimanates the production of CO, HC, C02 and PM. In the second part of this study LPG is told as an engine fuel. Liquid Petroleum Gases (LPG) is a hydrocarbon that is mainly made up of a propane and butane mixture. These hydrocarbons are present in nature only in petrol and obtained during the refinement of crude oil or its derivatives. With today' s refinement technology only 3 to 4 % of cruide oil can be obtained as LPG. Therefore, LPG has been never considered as an overall alternative to gasoline. Thus, LPG conversion of passenger cars is limited to 3 to 4 percent of passenger car park in time world. Table 1 lists the physical and chemical characteristics of butane and propane. In the fifties LPG was introduced in Italy, Netherlands and some other European countries as an automative fuel. The growth of the industry was based on beneficial running cost since for a customer real benefit comes after a certain break even mileage per year. The break - even mileage is determined mainly by the following :. initial investment c fuel prices (LPG - gasoline). exces road tax Today Italy and Netherlands are two leading countries in Europe in LPG automative applications and the conversion market. Because of the governmental policies LPG was attractive to use in automobiles xix Table 1. Physical and chemical properties. Burning velocity (cmfe)inair 32 32 In Turkey, because there was no governmental policy in favor of LPG as an automative fuel and the lack of LPG automotive know how until today, there has not been a serious enterprise on this subject. Besides, for time being LPG or CNG conversion of any any type of car and truck etc. is still illegal. The related Turkish law has been rearranged recently and will be applicable by the end of this year.(1995). Most of the present LPG cars and light trucks on the road are based on production gasoline engines. Various retrofit systems for LPG use are offered from different producers. The necessity for a large range of adaptibilitiy to various engine types leaves limited spaces for indivual optimization. Lack of equipment in some service stations is another reason for limited performance of many LPG vehicles on the road. Only the careful selection of a suitable LPG equipment and its best adaptation to the engine makes the full utilization of all advantages of this excellent engine fuel possible. Still better results can be obtained if the engine vehicle cocept is optimized to the LPG fuel as well. A lean bum engine promises low emissions at good performance, where the economy of the vehicle i.e. the operation cost depend on the local fuel prices in the different countries. There are some essential differences between normal gasoline and LPG, which consist of a propan-butane mixture fed into the engine as a gas, thus facilatating optimal mixture preparation and an even mix with the intake air. In contrast to normal liquid fuels, LPG is the totally vaporized during the intake cycle and thus takes up a grater volume ; moreover, the latent heat of vaporazition does not affect the tiermodynamic processes in the engine because its equalized in the cooling water heated vaporizer before entering the gas mixer. Since the composition of the LPG varies greatly from one region to another it can be expected that the characteristics will vary accordingly. In Europe, for example, the propane- butane ratio in different countries fluctuates between 96 % and 20 % propane content, which accounts for the differences in the characteristics. Similiar differences can be observed in other countries depending on the availability of the components and climatic conditions ; propane gives much higher pressures at given temperatures. The usual variation of the propane-butane ratio on the market means otherwise a difference of approximately 9 % in calorific value and stoichiometric air-fuel ratio and up to 17 octane numbers.(Tab!e 2). XX Table 2. Variîion of engine relevant LPG properties by different composition. PROPANE N-BUTANE 96 % Density 15% KG/L Vol. Calorific Value MJfL Stoichiometric Air Fuel Ratio (Volumetric) Octane Numbers RON yoN Advantages and disadvantages in the application of LPG are classified as the below. Advantages: High octane numbers Potential for better fuel distribution Effective combustion Lean burning possible Low emissions Low strain on lubricants Less deposits in combustion chamber High durability of exhaust system. Disadvantages : High weight of fuel tank Space requinment of fuel tank Reduced power output No additives possible Deposits in intake systems and evaparator/ pressure regulator. If we look at emission levels we find that the air - fuel ratio related CO concentration is independent of the fuel. However, NO* and HC emissions show significant diferences compared with the traditional gasoline engine. The NOx values are significantly higher for LPG than for gasoline, whereas the HC values are lower. Furthermore, under normal driving conditions, the gas driven engine allows leaner mixtures at acceptable driveability. Hence the limit for the lean mixture operation can be extended and consequently lower NOx emissions can be achieved. During the trials for the engines under consedaretion here the acceptable limit for lean operation with gasoline was at 10 to 15 % lean mixture, whereas with gas the values can be stretched to over 35 % lean, without the driving characteristics suffering in any way. Nowadays, if gas is required as a fuel, it is usual to modify production vehicles with gasoline engines after they have left the factory, in which case the vehicle can be run on either gasoline or gas, as desired. Production vehicles, however, already have certain features for example the principal design and installation situation, the air filter, mixture formation, the intake pipe and the spark timing. The gas system must be adapted so as to conform to this given situation which sometimes means restrictions. Various manufacturers of LPG retrofit equipments have range of systems on the market to meet the different requirements. In every system there is abasic similarity in the operation of the xxi vaporizer - pressure regulator, which is heated by the cooing water; also the quantity of gas flowing through is usually determined with respect to the pressure in front of the throttle valve of the intake system. There are various principles governing the mixing of the gas and air. In most cases a gas mixer is inserted in the air filter or in front of the carburettor. Such a mixer must fullfill certain requirements :. It must produce an equlized mixture so that there is uniform distribution of the air - fuel ratio in all cylinders.. The vacuum signal necesary for the regulation of the gas flow must be faithfully passed on to the vaporizer - pressure regulator by the mixer.. The mixer should not interfere with any future operation using normal gasoline. This sometimes would seem to be a problem, particularly during high load operation, float chambers and also the vacuum - controlled enrichment at full load are both more or less affected by the mixer. Other constructions place the gas supply behind the throttle valve and thus avoid some of the problems metioned above : however, they require a more complex regulating system to control the gas flow, like, for instance, an additional linkage controlled throttle valve for the gas. Since the assembly kits commercially available must cater for awide variety of vehicles, they must also be adjustable in the necesarry range. The performance of the vehicle after adaptation depends on the exact match between the gas system and the engine ; this in turn will depend onnthe quality of the service station doing the fitting and its facalities for carrying out the measurements involved in the job. Factory - modified LPG vehicles are offered on the automotive market only in some countries. For instance in Japan some companies produce special LPG modified vehicles with their own gas mixture and metering systems. In many Japanese cities nearly all taxis use LPG. To achieve the best possible result an LPG system should be favorably monovalent, that is, designed and optimized to use only LPG as a fuel. This obviates any problems associated with having to match the system to various fuels. As far as the combustion chamber, the compression ratio, the spark timing and the mixture preparation are concerned, the system is designed such that the advantages of LPG can be utilized to the full. This also the case for the gas mixing system which can be constructed without regard to possible restrictions imposed by dual gasoline - LPG operation. The traffic increase influences more and more negatively the environment and the energetic consumption. The transport field is particularly responsible for emissions of carbon and nitrogen monoxide, unburnt hydrocarbons, lead, carbon bioxide, sulphurous anhydride, coal particles (diesel) etc... in the environment. Combustion is inevitably polluting. Emissions coming from it are linked to the chemical and physical properties of fuel, to the composition of the fuel - comburent mixture but also to the combustion process and tothe environment characteristics. When comparing petrol, unleaded fuel, diesel and LPG emissions, we can note that all these fuels produce carbon and nitrogen monoxide as well as unburnt hydrocarbons while lead is only produced by petrol. Moreover, LPG does not produce sulfurous anhydride and aromatics. All products coming from combustion negatively influence upon the air quality. However, it is more important to know the toxicity grade of each element and define the real injuriousness (table 3) than considering quantity in absolute values. xxii Table 3. Compounds and toxicity parameters after combustion. From the above table, we can see that the more toxic products of combustion, such as sulfurous anhydride and lead, are not present in LPG. Moreover, LPG unburnt hydrocarbons are less toxic than the other fuelsones because LPG has neither additives nor aromatics. It has to be underlined that LPG pollutes little because :. combustion is done during the gases phase. As LPG is a gas, it suits better this process and assures a more homogeneous mixture without any coal particles.. High thermodynamics characteristics ease and better consumption. there are no additives such as lead, sulphur and aromatics. In the last part of this study I compared LPG with gasoline. LPG is tested in a four cylinder Renault 12 engine. LPG has been introduced directly into the combustion chamber through the passages connected to the gallery placed under the inlet valves seats. Direct and intermittent induction of LPG into cylinders prevented the pre - ignition and back fire problems.The main purpose was to investigate the performance characteristics of a Renault 12 engine operating with LPG / gasoline, in terms of power output and exhaust emissions. The results showed that PM, CO, NOx and HC emissions can be reduced with a little loss in power output of the engine. Moreover the results showed that LPG is more economical than gasoline and also the use of LPG resulted in a loss maximum power output of the engine but it is proved that LPG can give approximately same power output values with lean mixtures as gasoline.