6. RESULTS AND ANALYSIS

1. Experimental Determination of Rolling Resistance and Aerodynamic Drag Coefficients by Using Coastdown Method MAK 461E Experimental Methods in Mechanical Engineering – Automotive Section 2009-2010 SpringTerm Group members: Burak Yanalak, Eyüp Çelebi, Neslihan Albayrak, Sinan Kaya Department of Mechanical Engineering, Istanbul Technical University, Gumussuyu, Istanbul, Turkey Objectives:With the coastdown method, the rolling resistance and aerodynamic drag coefficients might be extracted simultaneously with the effects of real life loading conditions on road. Simultaneous measurement of speed and time is examined under the equation of motion by using the least square methods. 1. INTRODUCTION & BACKGROUND 3. EXPERIMENTAL METHODS AND TESTING EQUIPMENT 6. RESULTS AND ANALYSIS Least square approach results in the tabulated data. Three different real vehicles and one hypothetical vehicle are examined according to the supplied data from the literature. Two vehicles show unacceptable behaviour with their rolling resistance coefficients dependence on vehicle speed. The spatial filter type speed detector is a very special sensor that extracts only specific unevenness in the reflection (uneven color, uneven surface,etc.), from the irregular patterns formed by particles of various sizes such as small stones, sand and asphalt on the road surface and railroad tracks, and patterns formed by tire marks. The fluctuations in the amount of reflected light generated by these components are converted to electrical signals and send to the speedometer where the signals are passed through a bandpass filter, shaped into a waveform, converted to a pulse train, and calculated. The surface pattern of the road surface is passed through the objective lens and the slit, and an image is formed on the top of the special light-receiving elements of the comb-shaped structure which acts as a spatial filter. Furthermore, as the vehicle that is being measured moves, variations in the photoelectric current occur. • The determination of aerodynamic and rolling resistance coefficients has a great importance on the analysis of vehicle performance characteristics. • As rolling resistance and aerodynamic resistance coefficients increase, the vehicle speed drops with a higher deceleration when the other factors are considered to be constant. The common values of these coefficients are given as 0.20-0.40 for Cd and 0.010-0.015 for fr. • The main causes of the rolling resistance may be the hysteresis in the tire materials, friction between tire road caused by sliding, the air circulating inside the tire and the fan effect of the rotating tire. The aerodynamic resistance is generated by flow over the exterior of the vehicle body (90%) and flow through the engine radiator and the interior of the vehicle for purposes of cooling, heating and ventilation (10%). • The rolling resistance of tires are measured in the laboratory on a smooth rolling drum or on a moving flat belt. The equipments used in the drum tests are needed to be mounted with a high caution not to cause inaccurate measurement. The aerodynamic drag coefficient are measured in the laboratory by placing the vehicle in a wind tunnel. The wind tunnels are expensive to be constructed. • In the laboratory tests for obtaining the rolling resistance of the tires, just one tire is located on the experimental set-up. The inflation pressure, tire type, vertical load and speed are changed to extract their effect on the rolling resistance coefficient. • In the coast down test method, the vehicle is in the real life condition that is no artificial loading is applied to the vehicle. The vehicle behaves with its natural tendency. Coast down test method makes it possible to derive an expression for the rolling resistance coefficient with the consideration of all of the tires mounted on the vehicle that is the combined effect of the tyres are possible to be examined. The rolling resistance coefficient and the aerodynamic drag coefficient are obtained at the same time with the coast down method without distinct experiments to find each coefficient seperately. Speed and time measurement are enough to extract the respected coefficients thus the experimental set-up is not full of confused measurement procedures. The vehicle may be examined as a whole and the tire effect may be analysed by changing all of the tires. The weight effect may be examined by loading the vehicle. Different vehicles may be compared with the same tyres but with a different weight and shape. 7. DISCUSSIONS The main purpose of the analysis of experimental results is the testing of the mathematical model used to extract the respected drag coefficients. No experiment is done with the related experimental set-up or the vehicles. The speed-time data is taken from the literature. Nissan 84 300ZX gives the best results with its rolling resistance coefficient and aerodynamic drag coefficient. A unique result is obtained by 93 Chevrolet Cavalier and MGA since their rolling resistance coefficients decrease with increasing speed. Aerodynamic drag coefficients of these vehicles are respectively high that are not consistent with the common values. 4. PROCEDURE • The vehicle is accelerated up to a speed 10 km/h greater than the speed of the first measurement point. • The vehicle is kept at that speed and warmed up until the tires are in the steady state condition . • The gear is shifted to neutral and the gas pedal is released to let the vehicle decelerate with the effect of rolling resistance and aerodynamic resistance. • During the slow down of the vehicle, the speedometer records the speed and time data. The mounting of the speedometer must be of a type that it must have no effect on aerodynamic resistance and the measurement quality of the system must not be affected by the natural operating condition of the vehicle. • For the sake of high accuracy of the measurements, a set of experiments must be done for the same vehicle with the same weight and tires. The moving direction might even be changed without the labour of turning to the previous start point thus saving time. • Different vehicles might be compared by their drag coefficients with the same procedure in addition to extracting the coefficients of the specified vehicles. • Serious caution must be given to satisfy the similarity of individual tests by maintaining the vehicle in the same operating conditions i.e. closed window throughout the test. 8. CONCLUSIONS & FUTURE DIRECTIONS Consequently, coast down method is an effective method to extract the rolling esistance coefficient and the aerodynamic drag coefficient simultaneously and in a short time respectively. Measurements of speed and time are enough to result the experiment. Higher the number of data points and higher the number of experiment sets contribute to higher accurate results. One specific vehicle may be examined by changing their tire type, tire size, tire inflation pressure, weight and controlling the change in the resistance coefficients. Another comparison might be done between different vehicles with their original equipment. The results obtained from the coast down test must be compared with the results obtained from the other testing methods such ad drum test or wind tunnel test to consolidate the reliability of the coast down method. 2. THEORY AND BASIC EQUATIONS: 9. REFERENCES • 1. M.J.Nunney, 2007, “Light and Heavy Vehicle Technology”, Butterworth-Heinemann, 4th Edition, p.443. • 2. L.Vlacic, M.Parent, F.Harashima, 2001, “Intelligent Vehicle Technologies”, Butterworth-Heinemann, pp.92-94. • 3. P. Hong, B.Marcu, F.Browand, A. Tucker, 1998, “Drag Forces Experienced by Two Full-Scale Vehicles at Close Spacing”, University of California. • 4. U.A.Toyran, E.Tönük, Y.S.Ünlüsoy, 2003, “Experimental Determination of Rolling Resistance of Automobile Tires According to SAE J1269 and SAE J1270”, Middle East Technical University. • http://www.onosokki.net • http://209.85.229.132/search?q=cache:g8Iz629WDYsJ:www.geocities.com/plymouthsd1950/testing/vehicletest.xls+coastdown+filetype:xls&hl=tr&ct=clnk&cd=16&gl=tr&client=firefox-a 5. (EXPECTED) RESULTS The rolling resistance coefficient increases with increasing vehicle speed. The governing model for the dependency is given as fr=b0+b1V2 where b0 and b1 are constants. The common value for fr is given as 0.008-0.015 for passenger cars. In the conducted experiment, the dependency is slightly changed to the model fr=b0+b1V. In this case the constant part of the model is expected to be higher in order to compensate the model difference. The aerodynamic drag coefficient is a function of vehicle design and operational factors. Fore body, after body, underbody, drip-rails, window recesses, external mirrors, mud flaps, ground clearance, ground clearance, loading conditions, radiator open/blanked, window open/closed, angle of attack are some of the contributing factors to the drag coefficient. The common values for passenger cars are given as 0.3-0.52 and the test results are expected to be consistent with that since the vehicles to be examined are passenger cars.