Contact: Prof. Dr. rer. nat. Toralf Trautmann Phone:

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1 @ Contact: Prof. Dr. rer. nat. Toralf Trautmann Phone: Contents 1 Motivation 1 2 Comparison of an electric and a combustion engine car Theoretical background of energy storage Practical measurement with an electric car Measurement hardware and connection Calculation of energy consumption and total mileage Practical measurement on a combustion engine car Measurement hardware and connection Calculation of energy consumption and total mileage Conclusion Vehicle dynamics and driver assistance systems Vehicle dynamics sensors Electronic stability control Practical measurement of a stable and unstable turn Vehicle dynamics and behaviour Calculation of unstable situation detection Static circle-drive Surrounding sensors for vehicles 9

2 1 Motivation 5 You decided to visit the summer school of the HAWtech, which is the alliance of the most important universities of applied sciences in Germany. This means you will have practical sessions with applied science. At the end of this learning unit you will be able to understand how automotive engineering and mechatronics works. 2 Comparison of an electric and a combustion engine car 10 Two cars, one powered by a combustion engine, the other powered by an electric motor, will drive the same route in Dresden. Starting at the HTW going along the Bergstraße up the hill, turning at Südhöhe and back downhill to the HTW. The up and down will show you the advantage of an electric driven car, because of the opportunity to recuperate. 2.1 Theoretical background of energy storage Electrical mobility is an important aspect of the future. The challenge is, that batteries cannot store this much energy as gasoline. The unit for energy density per weight in Europe is MJ /kg (1MJ 0.28kWh 0.37HPh). Take a look at the figure below. Figure 2.1: Energy densities of common fuels (source: Wikipedia) 15 The energy density of a modern Lithium-Ion-battery is approximately 0.72 MJ /kg, the one of gasoline fuel is 47 MJ /kg, which is a ratio of 65. Sixtyfive times more energy per kilogram. This is the reason, why electrical driven cars do not have such a huge range as normal gasoline powered cars, although the efficiency of the engine is much better. 1

3 Figure 2.2: Sankey diagram of the coefficient of effectivity of electric and combustion engines 2.2 Practical measurement with an electric car 20 The laboratory of electrical mobility at the HTW Dresden has got a Renault Twingo, which is powered by electricity. Figure 2.3: HTWingo electric car Measurement hardware and connection 25 The HTWingo is equiped with a GPS system, which allows to log the driven route, speed and height. Current and voltage will be measured on the accumulator while driving. It is possible to view the power, which is nesseccary to move the vehicle Calculation of energy consumption and total mileage The power, which is required can be calculated with current (I) and voltage (U). The unit of measurement of power is Watt. P = U I 30 Figure 2.4: Energy consumption during route With numerical integration over time the work (or used energy) of the ride can be calculated. That allows to compare the exhausted electrical energy and the exhausted gasoline. 2

4 W = P t The energy density of gasoline can be assumed with Joule/L (fig. 2.1) Practical measurement on a combustion engine car The test vehicle is a Smart ForFour. Figure 2.5: Smart ForFour combustion engine car The CAN-Bus can be used, in order to record the fuel consumption over time and over driven distance. This enables the calculation of miles per gallon or, how it is calculated in Europe, the consumption in liter per 100km. Abbildung 2.6: Coherence between fuel consumption in mpg and L/100km (red=u.k. gallon, blue=u.s.a. gallon) Abbildung 2.7: Exemplary measurement of the fuel consumption of the Smart ForFour 3

5 Measurement hardware and connection Figure 2.8: Measurement setup 45 The Smart ForFour has got an interface to measure the signals on the CAN-Bus, which is necessary, in order to store the values time (in seconds s), speed (in kilometers per hour km /h) and fuel consumption (in liters per hour L /h). The data is stored in a text file, which can be used with Microsoft Excel for further calculations. Download is available in your tumblrs Calculation of energy consumption and total mileage 1. Calculate the average speed for your ride (v mean = vi i, i = number of relevant line in Excel) 2. Calculate the average consumption during the ride (c mean = ci i ) 3. Calculate the total time for your ride (t total = t i,end t i,beginning ) Calculate the driven distance (d = v mean t total ) 5. Calculate the fuel consumption in L/100km (c specific = 100 cmean t total d ) and take a look at fig Calculate how far you can drive, if your tank capacity is 40L (d max = 40 c specific 100 ) 2.4 Conclusion 55 Calculate the energy equivalent of 40L gasoline (take a look at fig. 2.1) and which weight an Lithium-Ion accumulator has to have for this energy to store. Furthermore answer these questions: Why does it make sence to drive electric driven cars in cities? 60 Which changes have to be made in order to help electric mobility to protect the environment? What is the main advantage of (renewable) electric energy? 4

6 3 Vehicle dynamics and driver assistance systems 3.1 Vehicle dynamics sensors 65 The required sensors to measure the vehicle dynamics are shown in figure 3.1. The sensors have to solve the following task: Steering-Angle In which direction does the driver want to drive? Yaw-rate The degrees per second, which the car turns on vertical axis Lateral-acceleration How much lateral acceleration is generated through the turn? Figure 3.1: RC-Car of the HTW Dresden with vehicle dynamics sensors 70 The installed sensors on the car measure the data with an offset. You can see the offset values in following table. Table 1: Sensor offsets on the car Value Offset Unit Steer Angle δ -0,0652 rad Yaw rate ψ 0,0242 rad/s Lateral acceleration a y -0,7264 m/s 2 5

7 Electronic stability control The job of the electronic stability control is to keep the vehicle on purposed trajectory. That means, if the side slip angle gets too big, the car has to stabilize itself. That occurs in specific breaking maneuvers of a single wheel in order to generate a yaw moment, which forces the car to minimize the side slip angle. +Z +yaw rate +velocity +Y -side slip angle +X Figure 3.2: Vehicle coordinate system (defined in German DIN70000) with side slip angle Practical measurement of a stable and unstable turn The model car drives twice through a left turn autonomously. The first time with activated electronic stability control, the second time without such a control. Figure 3.3: Left turn with activated electronic stability control - car passes Figure 3.4: Left turn with deactivated electronic stability control - car turns 80 The whole telemetry data will be transmitted to a PC and is accessible on a website: You will receive the login from your tutor. 1 only accessible inside HTW net 6

8 3.2.2 Vehicle dynamics and behaviour First of all, the behaviour of the vehicle has to be specified. An important parameter is the so called Eigenlenkgradient EG = m v c v m h c h 85 Variable m v = 5.4kg is the mass on the front axle, m h = 6.5kg the mass on the rear axle. The values of c v and c h can be assumed with c v = 550 N /rad and c h = 600 N /rad. If the EG (unit of measurement is s2 /m) has a negative value, the vehicle does oversteer and the driver has to reduce the steering angle while driving the same turn with higher velocity (drift). The EG is an important parameter in order to calculate the static yaw gain of the vehicle. ( ) ψ = δ stat v l + EG v 2 The static yaw gain increases with increasing speed. For understeering vehicles it increases until the speed reaches the characteristic speed v char = l 90 EG, then it decreases. For oversteering cars, as the one we are working with, the static yaw gain reaches an infinite value, which means the car does swing off. Figure 3.5: Static yaw gain of over- and understeering and neutral vehicle over speed Calculation of unstable situation detection 95 With the telemetry data, received from the practical measurement, it is possible to calculate the limits for stable and unstable situations. The yaw rate, which can be expected for a normal stable turn, depends on the track length of the car (l = 0.54m), the steering angle and the EG. ψ expect,1 = v l + EG v 2 δ 7

9 A second calculation for the maximum expected yaw rate is done with the lateral acceleration, which is assumed with a y,max = 3 m /s 2, and the measured velocity. ψ expect,2 = a y,max v 100 If the measured real yaw rate on the car is bigger than the most minimal of the expected yaw rates, the electronic stability control of the vehicle assumes that the car becomes unstable and that is why it starts to break one wheel. ψ measure! min( ψ expect,1, ψ expect,2 ) 105 When oversteering to the right (as showed on fig. 3.6), the front left wheel starts to break in order to generate a yaw moment in negative yaw rate direction (take a look at fig. 3.1 for definition of positive and negative direction of yaw rate). This action reduces the side slip angle and the car can pass the turn. Figure 3.6: Braking force generates a compensating moment (oversteering car) Keep in mind that everything happens in boundaries of physical laws, which means that passing a turn with too high speed is impossible. The electronic stability control only helps the driver to stabilize the car, because it is faster and more accurate than a surprised driver Static circle-drive The most important driving maneuver in order to specify the vehicle dynamics is the static circle-drive. This means the speed and circle radius is static (constant) and the vehicle drives around it. The angle, which has to be steered with, can calculated with δ = l R + EG a y 115 If the EG is a negative value, the steering angle will have to be reduced with increasing speed, because of the oversteering behaviour of the vehicle. You can calculate the EG of the vehicle by change over the equation. EG = δ R l a y R 8

10 Unfortunately, you do not know the radius of the driven circle. You can use the following formula to calculate a radius. R 1 = v ψ 120 In order to compare the radius and different sensors you can use another formula to calculate it twice. R 2 = v2 a y 4 Surrounding sensors for vehicles 125 The whole movement of the model car will be tracked by an IBEO Lux multi echo laserscanner. The laserscanner is a measuring instrument based on LIDAR technology (Light Detection and Ranging). It scans the surrounding area by means of a rotating infrared laser beam. The built-in laser transmits short rapid-fire pulses which are reflected by objects in the surroundings. The laserscanner can detect the reflections, which allows for a measurement of the pulses times of flight. From these times and the velocity of light, the distances to the objects can be determined. In parallel, the direction to each object is known from the angular position of the rotating mirror that deflects the laser beam. Figure 4.1: Measurement of the times of flight for laser beams; 1-object, 2-transmitted laser, 3-reflected impulse, 4-IBEO Lux sensor 130 The integrated ECU computer runs an object detection and tracking algorithm on the scan data. This algorithm splits the scan data into objects and tracks those objects through subsequent scans. The result is, instead of the scan data, a set of object data with information for each object like position, size and velocity. References [Trau09] Trautmann, T.: Grundlagen der Fahrzeugmechatronik: Eine praxisorientierte Einführung für Ingenieure, Physiker und Informatiker. Vieweg+Teubner, GWV Fachverlage GmbH, Wiesbaden,