Motorlu taşıtlarda bazı termik ve dinamik parametrelerin incelenmesi

Abstract

Bu çalışma, adından da anlaşılacağı gibi geniş bir araştırma ve incelemeyi gerektirmektedir. Motorlu taşıtlarda her parametre ayrı bir çalışma alanıdır. Tek tek incelenmesi lazım gelen ve daha sonra bir bütüne tamamlanması gereken bu çalışmalardan biri olan elinizdeki bu tez de sadece yakıt ekonomisine etki eden parametreler üzerinde durulmaktadır. Üç bölüm halinde sunulan bu çalışmada, motorlu bir aracın yakıt sarfiyatını hesaplayan bir bilgisayar programı hazırlanmıştır. İlk bölümde motor ve taşıtlara ait genel bilgiler anlatılmakta ve motorlu araçlarda simülasyonun önemine değinilmektedir. Taşıtların yol dirençleri alt bölümünde, yol dirençleri izah edilmekte ve bunların aracın yakıt sarfiyatı üzerindeki etkileri literatüre dayanılarak anlatılmaktadır. İkinci bölümde, hız - vites ilişkisinin belli olduğu sürüş tarzından hareketle yakıt sarfiyatı hesabında kullanılan matematik model kurulmaktadır. Bu amaçla önce performans eğrileri modellenmektedir. Bu esnada kullanılabilecek eğri uydurma yöntemlerinden bahsedilerek bunların incelenmesinden varılan sonuçlar anlatılmaktadır. Seyir halindeki bir taşıtın motorundan çekilen güç, seyir dirençlerinden hesaplanmakta, bu esnada motor gücü momenti ya da transmisyon veriminin diğer parametrelere bağlı ikinci derece, bir denklemle ifade edilebileceği gösterilmektedir. Ayrıca bilgisayar programında çevrimlerin matematik olarak ne şekilde modellendiği açıklanmaktadır. Üçüncü ve son bölümde bilgisayar programında, sayısal değerlerinden faydalanılan model taşıt tanıtılmıştır. Daha sonra Avrupa Seyir Çevrimi izah edilerek " bunun gerektirdiği sürüş tarzının otomobillere uyduğu, otobüs ve kamyon gibi taşıtlara uygun olmadığı vurgulanmaktadır. Bu sebeple bu çalışmada Değiştirilmiş Avrupa Seyir Çevrimi adı altında yeni bir çevrim sunulmaktadır. Buna ilaveten, bilgisayar programının kullanım alanlarına bir örnek teşkil edeceği düşüncesiyle şehiriçi yolcu otobüsleri için karşılaş­tırmalı bir çevrim de yer almaktadır. Ancak çevrimin şehiriçi otobüslerinin gerçek seyir çevrimlerine tam olarak uyduğu şeklinde bir iddia da bulunulmamaktadır. Çalışmaya ait bilgisayar programları ve bunlara ait örnek çıkışlar eklerde yer almaktadır.

Until the early seventies it was not of such a great care that energy demand would increase exponentially. Petroleum crisis that took place in 1973 has updated the discussions about energy economy. The fuel consumption of a vehicle with a Internal Combustion Engine depends on an efficiency of the engine, the behaviour of the driver and the resistance forces acting on it. in this study that consists of three parts; a computer program to çalçulate the fuel consumption of a vehicle with an I.C. Engine has been prepared. General parameters related with the I.C. Engines and vehicles are explained in the first part. The sum of the slope, aerodynamic, rolling and acceleration forces give the total resistance force acting on the vehicle. Reducing these resistance forces is of a vital importance to decrease the fuel consumption of a vehicle. For instance if the aerodynamic resistance is reduced by two thirds, about 10 % percent fuel economy can be expected. Reducing the slope to the half can lessen the fuel consumption by 10 % percent. Rolling resistance reduced by two thirds may supply about 5 % percent fuel economy. Measuring the fuel consumption of a vehicle requires quite high expenses. Instrumentation of laboratory in order to make fuel consumption measurements costs about 150,000 $. Further, just öne measurement in such a laboratory costs about 500 $. The term system simulation means observing a synthetic system that imitates the performance of a real system and simulation is used when it is not possible or not economical to observe the real system. It is much more economical to caluclate the fuel consumption of a vehicle by employing system simulation due to the high expences to measure it. A hartware that costs about 1,200 $ is enough to compute the fuel comsumption. Another situation wher system simulation is employed is where the system is stili in the design state and no real system yet exist. The performance ör control of the system at off-design may be of interest, so the planned system is "run" in advice of its construction. This gives the advantage to test an imaginary vehicle. Also, the effect of changing some elements of the vehicle such as gears ör resistance forces may be observed. Mathematical model that is used to calculate the fuel consumption, in case velocity and gear relations are specified, is formed in the second chapter of this study. So that, first the performance curves are modeled. Although a line of constant brake specific fuel consumption is similar to an ellipse, equation of ellipse can not be used to model these lines. The development of numerical computers gives the advantage to fit some curves that are difficult to be expressed analytically. Polynomial Representation, Lagrange Interpolation and Newton-Gregory Forward methods that may be used to fit polinomals in two dimensions are studied in chapter 2.1. In order to calculate brake mean specific fuel consumption, performance curves are modeled such that brake % mean effective pressure (pme) and effective rate of the engine (ne) are the variables. Newton-Gregory Forward Polynornîal interpolation method is used in main computer program. Since modeling performance cursev requires a lot of data, using the genaral form of these methods causes the polynomials to be of extremely high degrees. The higher the degree of the polynomial, the larger the errors and the longer the time needed to çalçul ate the brake mean specific fuel consumption. Using less data will reduce the errors and shorten the time to interpolate. in this study it has been decided to use an area of data consisting of three rows and four columns. The other data is not considered at this moment. This is achieved by a logic to select the area of data to be used. Using lower step size will alsa reduce the errors. Errors increase to the power of the degree of the step size. The logic mentioned above has been applied only in Newton-Gregory Forward polynomial interpolation certainly it could be applied to the other methods. Driving cycles are experiments which give the advantage to the vehicles to be compared with their fuel consumption ör exaust emission. Simulating the driving cycles allows them to be imitated by computers. in the third capter Europian Driving Cycle has been explained and it is ernphasized that this driving cycle is favorable för automobiles but not för vehicles such as buses and trucks. So that Modified Europian Driving Cycle is submitted in this study. in addition, thinhing that it would be an eMample to the erase that this computer program can be used, A Comparative Driving Cycle för City Passenger Buses is also presented. However, it should be pointed out that there is no claim that this cycles fits the real driving cycles of the city buses. An accelerating vehicle consumes some additional fuel. in such situations, because the engine works closer to full throttle conditions the fuel-air miMtüre enjected into the cylinder has to be rich. That means a bad fuel economy. Calculating the acceleration resistance force requires knowledge about the moment of inertia of the engine, the transmission system and the tires. it is difffcult to calculate these moments of inertia. It has been tried to calculate soma of these values approxİmately. Power taken from the engine of a travelling vehicle is calculated by means of resistance forces and it is porved that the effective power, the effective torque or the transmission efficiency of the vehicle can be stated by a second degree equation depending on the other parametres. If the effective power or effective torque or transmission efficiency is known then the brake mean effective pressure may be calculated. Also driving cycles allow the effective rate of the engine to be specified. These two parameters are used to interpolate the brake mean effective fuel consumption. Model vehicle which the numerical values given on Addition Bi are taken from,is the OTOMARSAN Mercedes-Benz 0302 S. Ali other values related with the vehicle are calculated approximitely. The flom chart of the computer program is given in chapter 3.3. Also a numerical applicaton of the program is given in chapter 3.4. Studying the curves demonstrating the change of the cuınulative fuel consumtion through time shows that the fuel consumption increases when the vehicle accelerates. This increase is because of the acceleration forces acting on the vehicle as resistance forces. Also it may be seen from the m Tyt-t curves that the vehicle should not travel with a large acceleration at high gears relative to the lower gears in order to reduce the fuel consumption. The driving cycle named "A comparative Driving Cycle for City Passenger Buses" considers a distance of B00 meters between two bus stations. The fuel consumption of the bus for 10 bus stations is calculated. Then the distance betueen the bus stations is extended to 1000 meters and the fuel consumption is calculated for S bus station. The total travelling distance is constant for both situations. Thus the fuel consumption difference gives the fuel economy for the constant distance that is 8000 meters. If the model vehicle does not carry any extra load, there is no slope and the vehicle travels on a paved road (asphalt), the result is of 45 % percent fuel economy. Since no power is transmitted to the tires of the vehicle, the effective engine power and thus the brake mean effective pressure can not be determined. So that the specific fuel consumption of the vehicle, when the clutch pedal is pressed ör when no power is transmitted to the tires of the vehicle from its engine can not be determined by using the performance curves. in such cases the fuel consumption of the vehicle is calculated by means of idling fuel cosumption of the engine. If consumption is directly equal to the idling fuel consumtion. However, vhen the engine rate is different to the idling engine rate, the fuel consumption is different from the idling fuel consumption too. The fuel consumption in these situations is calculated by the equation my =myR*neR /ner where "myR" is the idling fuel consumption, "neR" is the idling engine rate and "ne " is the engine rate just before the clutc pedal is pressed. The computer program includes 13 subroutines. When the program is run, a selection menu is offered and according to the selection some of the subroutines are used. An important point to be emphasized is that the performance curves given in this study are drawn aproximately by means of the full throttle curves. The real performance curves of the model engine are supposed to be different. The computer program submitted in this study may be used to optimize the behaviour of the driver by adding some subroutines or making some reorganizations. For instance if the distance and the time are certain, the behaviour of the driver to minimize the fuel consumption can be determined. However, this topic does not exist in this study.