Taşıtlarda motor fren momenti karakteristiğinin deneysel olarak bulunması
Başlık çevirisi mevcut değil.
- Tez No: 75218
- Danışmanlar: PROF. DR. ALİ G. GÖKTAN
- Tez Türü: Yüksek Lisans
- Konular: Makine Mühendisliği, Mechanical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1998
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Makine Mühendisliği Ana Bilim Dalı
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 104
Özet
ÖZET TAŞITLARDA MOTOR FREN MOMENTİ KARAKTERİSTİĞİNİN DENEYSEL OLARAK BULUNMASI Bu çalışma kapsamında başlangıç olarak kara taşıtlarındaki güç iletimi konusu, hareket eden bir taşıta etkiyen seyir dirençleri, taşıtın bu dirençler altında istenilen hareketi yapması için tekerleklerinde ihtiyaç duyduğu moment ve bu taşıtı tahrik etmek üzere kendisine bağlanmış içten yanmalı motorun sağlayabildiği moment konularına değinilerek kısaca açıklanmıştır. Bu kapsamda içten yanmalı motorun arz karakteristiğinin taşıt gereksinim karakteristiğine uygun hale getirilmek üzere kademeli moment değiştirinin içten yanmalı motor ile birlikte kullanılmasına değinilmiştir. Çalışmaya gerekli teknik özellikleri bilinen bir taşıtın kaydedilmiş bulunan bir seyrine ait veriler kullanılarak yapılan temel güç iletimi hasapları ile devam edilmiştir. Konunun daha kapsamlı olarak incelenmesi için bu seyre ait hız, ivme ve vites konumları ile ilgili çeşitli istatistik hesaplar yapılarak elde edilen sonuçlar grafik halinde gösterilmişlerdir. Buradaki seyir kaydı seyir boyunca zamana bağlı olarak kaydedilmiş bulunan hız, motor devri, hava kelebeği açıklığı ve soğutucu akışkan sıcaklığı verilerini içermektedir. Bu kayıt taşıtın elektronik motor kontrol ünitesine bağlanan bir kayıt cihazı sayesinde gerçekleştirilmiştir. Son olarak hareket halindeki taşıtın sürücüsünün ayağını gaz pedalından ani olarak çekmesiyle motor kompresyonu yardımıyla oluşan ve motordan tekerleklere doğru giderken aktarma organlarının çevrim oranları ve verimleri ölçüsünde büyüyen motor fren momentinin, taşıta etkiyen seyir dirençlerinin yardımıyla, her seyir durumunda taşıt gereksininim momentinin motor arz momentine eşitliğinden faydalanılarak bulunması anlatılmıştır. 1. ve 2. vites kademelerinde gerçekleştirilen motor fren periyotları verilerinden teorik değerler ile uyumlu tek bir motor fren momenti karakteristiği çıkarılmış ve karakteristik dönüştürücü çevrim oranları kullanılarak her vites için tekerlek devir sayısına ve taşıt hızına bağlı tekerlek fren momenti eğrileri elde edilmişlerdir. xı
Özet (Çeviri)
SUMMARY THE EXPERIMENTAL DETERMINATION OF THE BREAK TORQUE CHARACTERISTICS OF THE INTERNAL COMBUSTION ENGINE OF A VEHICLE The necessary torque to be produced at the wheels of a vehicle is supplied by the internal combustion engine mounted in the vehicle body. The output torque of the engine is not suitable to drive a vehicle as long as it is not converted by the power train. The output torque of the engine is carried through the components of the power train to the driving wheels of the vehicle. The arrangements of these components differ from each other depending of the driven axis an its position according to the engine. In case of the front-wheel- drive vehicles with the engine mounted in the front, the engine output torque is carried by the clutch, transmission (gearbox), final drive gears and differential (all of which are mounted in the tranmission housing) and front drive shafts to the front wheels of the vehicle. One of the components of the power train is the friction disc clutch. It is used to couple and uncouple the engine and the gearbox. During operation dry friction between a disc and a flyweel is used to transmit engine power. To provide the necessary gear numbers and ratios and so that the necessary driving torque at the wheels the gearbox is made up of different and independent gears and shafts, since the internal combustion engine is not able to provide enough torque at lower rpms As the vehicle speed increases the torque demand decreases and rotating speed demand incerases. For this reason usually 5 gear speeds are used to provide a soft and stepwise conversion of the engine output torque. For a passenger car with a four speed gearbox a typical overall gear ratio of the first gear is about 12:1. The ratio of the first gear is chosen according to the maximum climbing capacity. The ratio of the last gear is chosen either for fuel economy or top speed. The gear ratios of the gears in between are chosen for the maximum acceleration ability. The tyre rolling resistance is directly proportional to the weight of the vehicle. The tyre rolling resistance is given as, xnFR = fRG where Îr is the so called rolling resistance coefficient and G is the weight of the vehicle. Gradient resistance is also proportional to the weight of the vehicle and is given as 'St Gp The air resistance of a vehicle is known to be proportional to the square of its driving speed v, to the cross sectional area A (projected frontal area normal to the direction of travel), to the drag coefficient Cw and to the density of the air p and to be calculated with (cross winds are neglected) FL = cwA(p/2)v2 There are three sources where the air resistance on vehicles originate from: One of them is the drag resistance, which is known as a function of aerodynamic shape. One another is the skin friction of the body and the last one is the air flow through the vehicle for engine cooling and interior ventilation purposes. Skin drag component of air resistance can considerably be reduced by smooting the outer surfaces of the vehicle. Typical values of A are 1.3-2.7 m2 and 0.3-0.5 for cw. The aerodynamic drag at higher speeds becomes more effective. The resistance of acceleration is the inertia force to overcome during acceleration of a vehicle and can be calculated with FB=/lmx where X is the factor of rotational masses, m is the mass of the vehicle and x is the acceleration in the direction of travel. The factor of rotational masses A, is, where Jt is the reduced moment of inertia of the whole power train, r is the static radius of driven wheels and R the dynamic radius of driven wheels (under slip). For a front-wheel-drive car, X is X=l + Xlllwhere Jr.a is the moment of inertia of the non-driven rear wheels, Jt,ö is the moment of inertia of the front driven wheels and drive shafts. Jd is the moment of inertia of the gearbox output shaft. Jm is the reduced total moment of inertia of the rotational masses of the engine (crankshaft, camshaft(s), flywheel etc.) and the gearbox input shaft. The fundamental equation of longitudinal vehicle dynamics is, » M. f ---^ Z = Yj- -= FR+R+FSt+FB= f.+p + yl-- G+--/>-v2-cwA Z is the tractive force at the wheels of the vehicle acting at the contact surface of the tyres and the ground. Mt is the required torque at wheels of the vehicle to overcome the sum of the tractive resistances. The output torque of the engine (MM), is multiplied by the gear ratios of the power train while its being transmitted to the driving wheels, the gear ratios can be selected by the driver. (iş,i, io). In case of positive engine torque, the torque available at the output shaft of the engine is multiplied with the total gearbox efficiency t]k ('HK='nş,ir|D) which causes a loss in the value of the torque at the wheels.. This can be expressed as follows, MT=r|biTlDibiiDMM where iş,j is the gear ratio and rjŞ;i is the efficiency of the selected gear. In case of negative engine torque the torque at the wheels is increeased by the efficiencies and can be calculated by, MT=- - ibiiDMM + Mfren where Mfren is the moment at the wheels produced by the service brake of the vehicle. In the third section statistical analysis of a cruise data is performed. The necessary tractive and brake force is determined for a recorded driving data collected by the help of a data acquisition equipment which is connected to the electronic control unit of the engine of a vehicle. The sampling rate of the equipments about 150 miliseconds. The acquised data include, -the time in seconds, -the translational speed v of the vehicle in km/h, -the rotating speed n of the engine in rpms, -the displacement of the intake air throttle valve (accel. pedal) in Volts, -The temperature T of the coolant of the engine in °C. xivThe values of the instantaneous acceleration are derived from the recorded vehicle speed. The gear numbers are determined by means of calculated v/n ratios. The characteristics listed below are obtained for the given driving cycle: - the frequency distributions of vehicle speed and acceleration values, - some statistics about gear shifting, - the distribution of vehicle speed on vehicle acceleration, - the distribution of gear position values on vehicle speed. To define a city-cycle to measure the harmful emissions of the exhaust gases on a test stand, a simulation of the traffic conditions of a city is necessary. To perform this plenty of recordings taken in different routes in the city traffic are needed in order to be used as test pattern. To obtain the tractive force Z to be produced at the wheels of a vehicle, resistant forces acting on the driven vehicle are to be determined. It is known that the collected road data recorded on a single route in the city traffic contains certain information. This information consists of the vehicle and engine speed, time, position of air throttle valve. By means of this information the tyre rolling resistance, acceleration resistance, air resistance and gear number can be calculated. Since it is known that the inclination of the track is so small that it can be neglected without a significant error. By the help of this information the tractive force is calculated by summing up the determined resistances. At the end of this chapter, the calculation of the engine torque from the tractive force values is discussed. xvBrake Force at the Front Wheels (1. Gear.) -500 -: -1000 -1500 -2000 -2500 Rpmof the Engine [rev/mln] Figure 1. The Brake Force Characteristics at the Wheels of the Vehicle When the Engine Brake Torque is Utilized as the Source of the Brake Moment The final step is to calculate the brake torque of the engine when the gas pedal is released without using the service brake and keeping the gear lever position as it is. The measurements of this are performed on a selected track whose inclination is determined by collecting the test data with driving the vehicle on this track on the neutral gear so that the resistances acting are the tyre rolling resistance, air resistance and acceleration resistance. Since the engine torque is zero the only unknown resistance which is the inclination resistance is calculated. The collected test data from engine brake periods of 1. and 2. gear are used to determine the brake torque of the engine. To perform this every engine brake period is analysed and the irrelevant periods are omitted according to the theoretical brake torque value at the engine speed at maximum power. Then for each gear all of the data obtained from suitable engine brake periods are used to find a single engine brake torque characteristics. The static radius of the wheel, the total reduction ratio of the transmission and the transmission efficiency are used to calculate the engine brake torque when the engine works under compression and acting as a brake to decelerate the vehicle body. By the utilization of the reduction ratio of the transmission, the brake torque at the wheels of the vehicle with respect to the vehicle speed is determined. The graph of that is shown on figure 2. xviThe Brake Torque at the Wheels Originating From The Engine Working Under Compression For Each Gear 0 50 100 150 200 Vehicle Speed [kmAi] 250 Figure 2. Brake Torque At The Wheels Of The Vehicle With Respect To The Vehicle Speed The results found are in correlation with the theoretical values. The brake torque of the engine at the engine speed at maximum power deviates only about 10% from the theoretical value which cannot be more than 30 % of the tractive engine torque at this engine speed. When it is not possible to dissasembly the engine of a vehicle in order to measure the brake torque under compression or in the absence of a wheel dynamometer on which the brake torque at the wheels of a vehicle can easily be measured, the experiment explained in the study can be utilized as long as enough engine brake periods undertaken on a flat test track covering a wide range of engine speed in the second gear. The second gear is recomended since the tyre brake torque is big enough to overcome the disturbances from the environment and a considerably smooth brake torque charactesitics can be obtained without much harmful vehichle and engine vibrations. xvu
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