Benzin ve doğalgaz motorunun çevrim analizinin matematiksel model ile karşılaştırılması

Abstract

ÖZET Yapılan çalışmada matematiksel model kurularak Otto çevrimi ile çalışan ve konstrüktif özellikleri bili nen bir benzin motorunun çevrim analizi teorik olarak incelenmiş, sonuçların gerçek motor değerlerine uygun olduğu gösterildikten sonra aynı matematiksel simülasyon modeli ile, elemansel analizi bilinen doğalgazın bu motor da kullanılması halinde çevrim analizi incelenerek, karşılaştırılmaları yapılmıştır. Bu çevrim analizinde, emme havası basıncı ve -sıcaklığı, egzoz açılma basıncı ve sıcaklığı, egzoz gazı karşı basıncı gibi parametreler seçilerek, taze dolgunun silindirdeki artık egzoz gazları ile karışmasıyla oluşan yeni karışımın termodinamik özellikleri belirlenmiştir. Sıkıştırma, yanma ve genişleme periyodunda basınç ve sıcaklığın KMA'na göre değişimi, enerji denklemleri ve ter modinamik bağıntılar yardımıyla Newton- Raphson iterasyon yöntemine göre istenen toleransta, adım adım hesaplanmış tır. Geliştirilen yanma modelinde, silindir içindeki karışımın homogen olduğu, bunun yanmamış, yanmış ve yanmak ta olan karışım olmak üzere üç bölgeye ayrılmış olduğu, kabul edilmiş, her adımda yanan yakıt miktarı verilen yanma kanununa göre bulunmuştur. Fakir karışımda ısıl dissossasyondan faydalanarak kısmi eksik yanma ürünleri, zengin karışımda ise su gazı denge reaksiyonlarından faydalanarak eksik yanma ürünle ri hesaplanmıştır. Çevrimin her bir sürecinde, yanma odasından çevreye geçen ısı miktarı, bilgisayar programı içinde ayrı bir altprogram ile hesaplanmıştır. Çeşitli hava fazlalık katsayısı ve dönme sayısı değerlerinde, basınç ve sıcaklığın çevrim boyunca değişi mi ile güç ve özgül yakıt tüketimi bulunmuş, benzin motoru için yapılan bu işlemler doğalgaz motoru için tekrarlanmıştır. Her iki motor için bulunan değerler biribir leriyle karşılaştırılarak doğalgaz motorunun üstünlükleri belirlenmiştir. - ix -THE COMPARISON OF THE CYCLE ANALYSIS OF GASOLÎNE AND NATURAL GAS ENGINES WÎTH THE MATHEMATICAL MODEL SUMMARY The rapid deplation of present fossil based fuels for internal combustion engines, together with the in crease in conventional fuel prices has led to a world wide increase in the research carried out on alternative fuels. Recent discovery of new natural gas reserves has introduced the idea of using this energy source for in ternal combustion engine purposes among other applica tions. Low exhaust emission of polluting materials as a result of natural gas combustion in automotive engines, is another advantage of the utilization of this alterna tive fuel. In this study, a theoretical model has been developed for a spark ignition engine operating with Otto cycle principle, to obtain the engine characteristics. The same model has been used, applying the same energy conservation equations to provide the engine char acteristics for a gas engine, for comparison. For an engine of known design parameters, opera tional conditions have been altered to obtain their ef fects on engine performance. Parameters such as induc tion air pressure and temperature, exhaust back pressure, exhaust discharge pressure and temperature have been kept constant while the engine rotational speed and equivalance ratio have been varied against temperature and pressure. Also the values of power output and specific fuel consump tion have been obtained for the engine. Spark ingition timing has also been optimized for the best performance of the theoretical cycle and the optimum value has been found to be 25° crank angle before TDC for a rotational speed of 40 (revulation/sec- ond). With the increase of engine speed, spark advence has also been increased. Combustion duration has been assumed to be 50° crank angle for the value of air-fuel ratio, X (reciprocal of equivalance ratio) of 0.95 and engine rotational speed of 40 (revulat ion/second). For leaner and richer mixtures, combustion dura tion increases due to the reduced flame speed. This has been included in the computations by starting the com bustion period at the same crank angle position, but terminating it later then the above referance value. On the other hand increasing the engine speed, increase the turbulent nature of the flow inside the combustion chamber, and results in a higher flame speed which in turn reduces the combustion duration in terms time (ie. seconds), but increase it in terms of crank angle -x-

THE COMPARISON OF THE CYCLE ANALYSIS OF GASOLÎNE AND NATURAL GAS ENGINES WÎTH THE MATHEMATICAL MODEL SUMMARY The rapid deplation of present fossil based fuels for internal combustion engines, together with the in crease in conventional fuel prices has led to a world wide increase in the research carried out on alternative fuels. Recent discovery of new natural gas reserves has introduced the idea of using this energy source for in ternal combustion engine purposes among other applica tions. Low exhaust emission of polluting materials as a result of natural gas combustion in automotive engines, is another advantage of the utilization of this alterna tive fuel. In this study, a theoretical model has been developed for a spark ignition engine operating with Otto cycle principle, to obtain the engine characteristics. The same model has been used, applying the same energy conservation equations to provide the engine char acteristics for a gas engine, for comparison. For an engine of known design parameters, opera tional conditions have been altered to obtain their ef fects on engine performance. Parameters such as induc tion air pressure and temperature, exhaust back pressure, exhaust discharge pressure and temperature have been kept constant while the engine rotational speed and equivalance ratio have been varied against temperature and pressure. Also the values of power output and specific fuel consump tion have been obtained for the engine. Spark ingition timing has also been optimized for the best performance of the theoretical cycle and the optimum value has been found to be 25° crank angle before TDC for a rotational speed of 40 (revulation/sec- ond). With the increase of engine speed, spark advence has also been increased. Combustion duration has been assumed to be 50° crank angle for the value of air-fuel ratio, X (reciprocal of equivalance ratio) of 0.95 and engine rotational speed of 40 (revulat ion/second). For leaner and richer mixtures, combustion dura tion increases due to the reduced flame speed. This has been included in the computations by starting the com bustion period at the same crank angle position, but terminating it later then the above referance value. On the other hand increasing the engine speed, increase the turbulent nature of the flow inside the combustion chamber, and results in a higher flame speed which in turn reduces the combustion duration in terms time (ie. seconds), but increase it in terms of crank angle -x-rotation (ie. degrees crank angle). This phenomena has also been considered in the calculations of combustion duration. In spark ignition engines, in the case of in complete combustion, the amount of heat released is lower than the heat capacity of the fuel-air mixture introduced into the cylinder. So combustion efficiency has also been considered in the present model calcula tions, for values of X less then one. The amount of CO and H2 in the exhaust gases has been calculated in order to obtaine the combustion efficiency for rich operating conditions. The amount of fuel burnt during an engine cycle can be given by an empiricial combustion function. In this study the function given by VÎBE, 8-6. m =£-EXP{-6.908 ( AA ) * } y A9 y has been used. The amount of burnt fuel in each step has been calculated by a different subroutine in the computer program acconding to this function and it is assumed that the whole of the fuel is burned at the end of the burning period. The transfer of heat between the combustion chamber of variable temperature to cooling water at constant temperature taking piece the the cycle has been taken in consideration in the calculations. The transfer area has been determined for the position of the piston for each step and the heat transfer coefficient function was assumed to be 9 UIG - 1000 - 5000. Cos(10) 2 depending are engine speed and crank angle. Air-fuel mixture introduced into the combustion chamber of the engine at atmospheric pressure and tem perature conditions, mixes with the gases left over in the cylinder from the previous cycle, thus changes the pressure and temperature of the final mixture. At the first stage of this study, thermodynamic properties of this new mixture has been obtained. For the combustion model, combustion chamber has been assumed to be consisting of three diffirint - regions such as `burnt zone`, `burning zone` and `unburnt - xi -zone`. Compression, combustion and expansion processes are calculated using basing thermodynamic relation. Gas pressure and temperature and the composition of the combustion products are obtained by step-by-step comput ation of the following equations. First law of thermodynamics states that, dQ - dW = dE - Qvs» dny where dW is the work done, dE is the change in internal energy, dQ is the amount of heat transfer to the surround ings and Q is the heat of reaction. Here, * vs dW = p,2. dV = (p-0.5dp) «dV dE = EtT-dT,!^) - E(T,n±) Q = I n.. AÜ- (T ) -En.» AU -. (T ) vs pr. i f s rea.i f,ı s For the combustion and expansion processes these equations are solved in an iterative manner until the temperature values at the end of the computational step increament`convErges within the required tolerances, then the pressure is calculated, knowing the temperature values. For the compression stroke, as and dn = 0 y E0(T ) - E, (T ) Z 3 IS the energy equation will take the following form, dQ - dW = Z h±(c ±«dT) This energy equation can be written as, f(T). = Mln.'fc. (T)»dT>- Q »An - AW - AQ j. i vi vs y for the j. th iteration. Empiricial functions have for the calculation of internal energy and specific heat values of the mix ture of reactans and products. For the calculation of products of rich mixture combustion conditions, water gas dissociation reaction has been used, where as for lean mixtures thermal disso ciation reaction has been applied. Water gas dissociation - xn -reaction constant K^, is given as a function of tempera ture in a polinomial of 5th degree. The constant K^G has been utulised in the calculation of products due to incomplete combustion. In the lean operation calcula tions thermal dissociation reactions are applied to ob tain the dissociation constants KCQ2 and K`2o for the computation of incomplete combustion products. The mathematical model developed is first applied to petrol fueled spark ignition engines to obtain results for variation of pressure and temperature at various X values and at a constant engine speed, of 40 (revulation /sekond). Specific fuel consumtion and power output have also been computed. As a result of these computations the pressure, temperature and power output values have appeard to be high for rich mixtures, while specific fuel consumption is found to be dropping by leaning the mixture. Specific fuel consumption is also calculated against various engine 'speeds while keeping X at a con stant value at 0.95. Specific fuel consumption calcul ations showed that it increasing engine rotational speeds. Specific fuel consumption is also calculated for two different heat transfer coefficients for low rotational speeds. The predictions of the present model is found to be in good agreement with the real experimental data found in literature. After the confirmation of the model with applica tions to petrol fueled engines, natural gas has been used as a fuel instead of petrol while keeping all engine spec ifications and environmental conditions constant. In this case, for value of X at 0.95, 1.0 and 1.05 with constant engine speed, pressure and temperature profiles, power output and specific fuel consumption of the engine are again calculated. Calculated power output, pressure and temperature values have appenred to be higher for rich mixtures, while for lean mixtures specific fuel consumption is found to be low. When the result of petrol engine calculations are compared with the calculations for natural gas, the power output of the engine reduces by 6.29 % from 16.99 kW to 15.92 kw where as the specific fuel consump tion also reduces by 8.81 % from 346.8 g/kWh to 316.24 g/kWh. Maximum pressure and temperature values of the petrol fueled engine, which are 58.81 bar and 2822 °C also reduce to 54.97 bar and 2698 °C for the natural gas fueled engine with 6.52 % and 4.39 % reductions respectively. - xiii -With stoichiometric and lean mixture, engine maximum power output, specific fuel consumption and maximum pressure and temperature values also decrease when using natural gas fuel instead of convertional gasoline fueled operation. In the engine working natural gas for each value of air-fuel ratio due low heat value of the fuel the power obtained is lower than the petrol engine and the specific fuel consumption is reduced. The resistance to detonation of the natural gas been higher this would facilitate the augmentation of the compression-ratio of the natural gas engine thus increasing the thermal efficiency.. On the other hand the pressure and temperature conditions in the natural gas engine being more favor able the elements of this engine will be subjet to lower thermal and mechanical stresses. - xiv -