Zdzisław Chłopek. 1. Introduction - PDF

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Article citation info: Chłopek Z. Research on energy consumption by an electrically driven automotive vehicle in simulated urban conditions. Eksploatacja i Niezawodnosc Maintenance and Reliability 2013;

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Article citation info: Chłopek Z. Research on energy consumption by an electrically driven automotive vehicle in simulated urban conditions. Eksploatacja i Niezawodnosc Maintenance and Reliability 2013; 15 (1): Zdzisław Chłopek Research on energy consumption by an electrically driven automotive vehicle in simulated urban conditions Badania zużycia energii przez samochód elektryczny w warunkach symulujących jazdę w mieście* In recent years, dynamic development of electric drives in automotive applications has been taking place. Electrically driven vehicles are considered to offer a possibility of solving the most important ecological problems posed by motorisation. The paper presents results of testing the energy consumption by an electric car in conditions corresponding to actual operation of such vehicles, i.e. at drive tests where urban, extra urban, and traffic jam conditions were simulated. The disitance energy consumption and total vehicle efficiency were determined at drive tests. An energy consumption characteristic was determined in pseudorandom conditions of urban operation of the car, with employing the Monte Carlo method for this purpose. Keywords: electric car, energy consumption, efficiency. W ostatnich latach następuje dynamiczny rozwój napędów elektrycznych w motoryzacji. W samochodach elektrycznych upatruje się możliwości rozwiązania najważniejszych problemów ekologicznych motoryzacji. W pracy przedstawiono wyniki badań zużycia energii przez samochód elektryczny w warunkach odpowiadających rzeczywistej eksploatacji takich pojazdów, mianowicie w testach jezdnych symulujących ruch w miastach, poza miastami, a także w zatorach drogowych. Wyznaczono drogowe zużycie energii i sprawność ogólną pojazdu w testach jezdnych. Wyznaczono charakterystykę zużycia energii w warunkach pseudoprzypadkowych użytkowania samochodu w mieście. Wykorzystano do tego celu metodę Monte Carlo. Słowa kluczowe: samochód elektryczny, zużycie energii, sprawność. 1. Introduction Motorisation poses significant hazards to the environment. A considerable part of these hazards is connected with using combustion engines to power automotive vehicles. The most conspicuous hazards include exhaust and noise emissions and using up of the natural resources necessary for the production of liquid and gaseous fuels. The use of electric motors for automotive applications makes it possible to eliminate exhaust emissions along transport routes and to reduce noise emission, because it is generally known that this has been made possible by the present day technologies of electric drives [10, 14, 16, 26]. At the same time, however, we must be aware of the fact that the platitudinous term zero emission vehicles is only a populist expression having merely a propaganda value and being of not very high standard at that. Firstly, there are a number of automotive pollutant sources other than the combustion engine, e.g. dust emission sources such as various tribological pairs in the vehicle, interaction between tyres and road surface, or stirring up of road dust [4]. Secondly, a vehicle powered by an electric motor does not emit combustion gases, but the generation of the electric energy used to power the vehicle results in environmental pollution, too. The electricity generation is still based to a considerable extent on the combustion of fossil fuels, predominantly hard coal. Moreover, the technologies of electricity generation with the use of hard coal are not, in many cases, adequately clean; therefore, not only the greenhouse gas emissions but also the emissions of pollutants harmful to human heath cannot be thus avoided. Obviously, a solution of the future is the use of renewable energy, chiefly the energy directly obtained from solar radiation (photoelectric cells), and nuclear energy; particularly high hopes are placed on the use of nuclear fusion [20]. To assess the pollutant emissions from automotive vehicles during the whole cycle of production and use of energy carriers, the Well-to-Wheel analysis, i.e. the analysis from the source (of an energy carrier) to the wheel (of a vehicle), may be employed [25, 27]. This cycle is divided into two stages, namely Well-to-Tank, i.e. from the source to the tank (of the energy carrier in the vehicle), and Tank-to-Wheel, i.e. from the tank to the wheel [25]. Another issue is the evaluation of the environmental benefits gained from the application of electric drives to automotive vehicles, carried out with employing the Life Cycle Impact Assessment (LCIA) method 1 [12]. In this method, not only the information obtained by inventorying the energy and pollutant emissions but also specific environmental hazards such as eutrophication, acidification, noise, vibrations, smog, electromagnetic radiation, dust, land-use change, damage 1 In the English to Polish translation, it has become a common but incorrect and reprehensible practice to use the term życie (literally life ), borrowed from English, in relation to objects other than organisms, and not only in official documents but also in scientific publications at that! According to the Dictionary of the Polish Language, życie is an organism s state consisting in an uninterrupted train of processes making it possible for the organism to react to stimuli and, usually, to move. According to the Encyclopaedia issued by Państwowe Wydawnictwo Naukowe (Polish Scientific Publishers PWN), życie is a biological phenomenon, complex and multidimensional, which cannot be described with the use of one simple definition. This phenomenon is exclusively known from the Earth; in this context, it is defined as having two basic meanings: it describes the state of a substance (referred to as an organism) that lasts from the coming up (birth) of the organism till the end of its individual existence, i.e. its death in most cases, or it describes the dynamic process that began on the Earth about 3.8 billion years ago and covered all the organisms that existed in the past and live now and derive from one initial form, including any mutual interrelationships and dependencies and their environmental impact. The fact that the term life is used in English in the meaning of existence does not entitle the Poles to disregard the culture of the Polish language and to translate this term as życie (incorrect, literally life ) instead of istnienie (correct, literally existence ). (*) Tekst artykułu w polskiej wersji językowej dostępny w elektronicznym wydaniu kwartalnika na stronie Eks p l o a t a c j a i Ni e z a w o d n o s c Ma i n t e n a n c e a n d Reliability Vo l.15, No. 1, to resources, e.g. water and raw materials used to produce fossil fuels, climate changes, ozone layer depletion, etc., are taken into account [12]. With respect to automotive vehicles, the LCIA method makes it possible to assess their environmental impact at all the vehicle existence stages, from production through operation right to the management of the vehicles when worn-out. Such an analysis may also cover the vehicle operation infrastructure. When the life cycle impact assessment method is applied to electrically driven automotive vehicles, many factors that are very harmful to the environment, especially the production and use of batteries and the management of the batteries that have been withdrawn from service, should be taken into consideration [1, 11, 13, 16, 18, 21, 27]. This cautious approach to the problem of electric vehicles is by no means inconsistent with the dynamic work on development of electric drives not only in small passenger cars but also in commercial vehicles [1, 8 11, 13 16, 18, 21 23, 26, 27] and in single track vehicles [24]. An important problem is the evaluation of energy consumption by electric vehicles in conditions corresponding to the typical conditions in which such vehicles are actually used. At the current state of technical development, electric vehicles are thought of as being chiefly intended for urban traffic [1, 3, 8 11, 14 16, 18, 21 23, 26, 27]. In this connection, the conditions corresponding to the actual operation of electric vehicles represent urban traffic inclusive of special cases, i.e. the traffic in central urban areas and in suburbs. In this study, the traffic models to represent the actual electric vehicle operation conditions were adopted in accordance with the European UDC (Urban Driving Cycle) and American FTP-75 (Federal Transient Procedure) type approval tests, see Figs. 1 and 2, respectively [28]. The UDC test is a typical model of the driving of passenger cars and light duty goods vehicles in towns, while the FTP-75 test covers both the urban and suburban traffic conditions. The test program described herein was Fig. 3. The Stop and Go test additionally extended by adding to it the Stop-and-Go test procedure, which represents the street jam traffic type, Fig. 3 [2, 6]. The traffic models adopted cover most of the passenger car traffic conditions occurring in urban areas. 2. The system of quantities adopted to describe the energy consumption by an electric vehicle The system of quantities adopted to describe the energy consumption by an electric vehicle has been presented below. For an electric vehicle without braking energy recuperation, the efficiency system is defined as follows: efficiency of the vehicle drive: η D R = N N T (1) efficiency of the battery charging: η CH T = N N CH (2) total efficiency: ηg = ηch ηd (3) Fig. 1. The UDC test where: N T electric vehicle drive power; N R resistance to motion 2 power; N CH battery charging power. For an electric vehicle with braking energy recuperation, the efficiency system is defined as follows. efficiency of the vehicle drive: N η R D = NT NU (4) efficietncy of the braking energy recuperation: η U U = N N B (5) Fig. 2. The FTP 75 test 2 The term resistance to motion should be understood in this context as the phenomenon rather than the forces. 76 Eks p l o a t a c j a i Ni e z a w o d n o s c Ma i n t e n a n c e a n d Reliability Vo l.15, No. 1, 2013 where: N B electric machine braking power; N U braking energy recuperation power. The distance energy consumption is defined as derivative of the energy consumed relative to the distance travelled. In particular: for an electric vehicle without braking energy recuperation, it is: A schematic diagram of the power flow in the powertrain 3 of an electric vehicle with electricity recuperation has been shown in Fig. 4. dl s c = ds T ( ) (6) where: s distance travelled by the vehicle; L T (s) work of the electric vehicle drive as a function of the distance travelled. for an electric vehicle with braking energy recuperation, the distance energy consumption is: ( ( ) ( )) d LT s LU s c = (7) ds where: L U (s) braking energy recuperated as a function of the distance travelled. The average value of the distance energy consumption for the test is defined by the following formulas. for an electric vehicle without braking energy recuperation, it is: () () L AV N t c T T AV = = (8) s AV vt where: t time; L T work done by the electric drive of the vehicle during the test with test duration time t f, see below: tf LT = NT() t dt 0 s distance travelled by the vehicle during the test, see below: s tf v t dt 0 = () (9) (10) AV averaging operator. for an electric vehicle with braking energy recuperation, the distance energy consumption is: L L AV N t AV N t c T U T U AV = = () () (11) s AV vt where: L U braking energy recuperated during the test, see below: tf LU = NU() t dt 0 () (12) Fig. 4. Schematic diagram of the power flow in the powertrain of an electric vehicle with electricity recuperation. Legend: CH battery charging system; A battery; D vehicle driving system; U braking energy recuperation system; NCH battery charging power; NT electric vehicle drive power; NR resistance to motion power; NB electric machine braking power; NU braking energy recuperation power Noteworthy is the fact that the energy balance in the driving system of a vehicle determines whether the vehicle state is static, i.e. the vehicle moves with a constant speed, or dynamic, i.e. the vehicle accelerates or decelerates. The electric machine braking power differs from the total vehicle braking power because the latter additionally includes the power dissipated in the braking system by friction brakes. A part of the electric machine braking power may be recuperated: the electric energy recovered during the braking process may be stored in the battery. 3. Results of empirical tests of an electric vehicle carried out on a chassis dynamometer Empirical tests of the energy consumption by an electric vehicle were carried out on a chassis dynamometer at the Environmental Protection Centre of ITS (Motor Transport Institute) [3]. The test specimen was an electric passenger car Zilent Courant, made in the People s Republic of China. In the Zilent Courant car, the electric motors were exclusively powered from a battery of electric storage cells. The vehicle powertrain had no braking energy recuperation system. The vehicle running mass was kg. The power rating of the vehicle s electric motor was 8.5 kw. The car was provided with 10 maintenance free lead acid batteries, each of 12 V and 100 Ah rated voltage and capacity, respectively. The maximum and economical speed of the car was 85 km/h and 40 km/h, respectively. The vehicle range when driven at the economical speed was claimed as not less than 150 km. The car tested was classified in the category of vehicles of simple construction. It was provided with neither battery systems of new generation nor braking energy recuperation system. The utility indicators did not qualify the car tested to the category of modern electric vehicles, either. The vehicle range was short, the maximum speed was low, and the dynamic characteristics of the vehicle were all the more unsatisfactory. These features had an effect on the sceptical opinion about the passive safety and comfort of use of the vehicle. The tests were carried out on a single-roller chassis dynamometer with controlled load characteristics, manufactured by AVL-Zöllner [3]. The parameters measured during the tests carried out on the chassis dynamometers included: vehicle speed measured on the chassis dynamometer roller; voltage of the battery set; 3 The term driving system is to be understood, consistently with the traditional meaning adopted in automotive sciences, as the system to transmit mechanical energy from the motor to the road wheels of a vehicle ( power transmission system ). The driving system taken together with the motor and the energy storage reservoir is referred to in this paper as powertrain. Eks p l o a t a c j a i Ni e z a w o d n o s c Ma i n t e n a n c e a n d Reliability Vo l.15, No. 1, current in the electric drive wiring of the vehicle. The characteristic curve of the power absorbed by the chassis dynamometer was identified based on empirical vehicle coast down tests [3]. The signals representing the quantities measured were recorded with 1 s sampling time. Each signal value recorded constituted an averaged result of a series of 10 measurements carried out with 0.1 s time intervals. The signals recorded were preliminarily processed to eliminate gross errors and to reduce the share of high frequency noise. The gross errors were searched by analysing the current signal variance. To reduce the share of high-frequency noise in the signals recorded, the signals were subjected to low pass filtration, with a Golay-Savitzky filter being used, where both-side approximation from 2 data points on each side to a polynomial of degree 2 was applied. Results of the empirical vehicle tests carried out to the Stop-and- Go test procedure have been presented in Figs. 5 and 6. The former shows time histories of the current drawn from the battery set and of the voltage measured on the battery set terminals while time histories of the electric vehicle drive power and of the resistance to motion power can be seen in the latter. Fig. 8. Electric vehicle drive power and resistance to motion power vs. time, at the UDC test Fig. 9. Current drawn from the battery set and voltage measured on the battery set terminals vs. time, at the FTP 75 test Fig. 5. Current drawn from the battery set and voltage measured on the battery set terminals vs. time, at the Stop and Go test adopting of the battery charging efficiency. The vehicle drive and battery charging processes do not take place at the same time; therefore, the notion of total vehicle efficiency is rather symbolic and, in principle, the total efficiency may only be evaluated in the conditions of energy balance. The battery charging efficiency values that can be found the literature significantly differ from each other, depending on battery type. As an example, the battery charging efficiency has been specified in publication [10] as 0.86, while significantly lower values, even of the order of 0.6, were recorded for lead acid batteries at the tests described in report [3]. Finally, the battery charging efficiency value was assumed as 0.65 for the purposes of the analyses presented herein. The total vehicle efficiency values as recorded at specific tests have been shown together in Fig. 11. Fig. 6. Electric vehicle drive power and resistance to motion power vs. time, at the Stop and Go test Results of the empirical vehicle tests carried out to the UDC test procedure have been presented in Figs. 7 and 8. Results of the empirical vehicle tests carried out to the FTP-75 test procedure have been presented in Figs. 9 and 10. Based on the experimental tests, the total vehicle efficiency was determined. The total efficiency was calculated as the product of efficiency of the vehicle drive and efficiency of the battery charging. The efficiency of the vehicle drive was determined from empirical tests carried out. However, a problem was encountered with correct Fig. 10. Electric vehicle drive power and resistance to motion power vs. time, at the FTP 75 test 78 Eks p l o a t a c j a i Ni e z a w o d n o s c Ma i n t e n a n c e a n d Reliability Vo l.15, No. 1, 2013 Fig. 11. Total vehicle efficiency as determined from the Stop and Go, UDC, and FTP 75 tests Fig. 12. Average distance energy consumption as determined at the Stop and Go, UDC, and FTP 75 tests Fig. 13. Average distance energy consumption as determined in the dynamic states of positive and negative acceleration at the Stop and Go test Fig. 15. Average distance energy consumption as determined in the dynamic states of positive and negative acceleration at the FTP 75 test The total vehicle efficiency values recorded at the tests carried out for type approval purposes are similar to each other, although the tests significantly differed from each other in their dynamic parameters. However, these tests show much closer similarity to each other than to the Stop-and-Go test in respect of the average speed values. A considerably lower value of the total vehicle efficiency was recorded at the Stop-and-Go test, characterised by frequent acceleration and deceleration, with the average vehicle speed being about 5.8 km/h. The specific conditions of this test are likely to cause much higher energy losses. A similar situation can also be observed in the case of automotive vehicles powered with combustion engines [2, 6]. The average distance energy consumption has been presented in Fig. 12. Fig. 14. Average distance energy consumption as determined in the dynamic states of positive and negative acceleration at the UDC test The average distance energy consumption values were similar to each other at the Stop-and-Go and FTP-75 tests, while this value recorded at the UDC test was visibly lower. This is probably related to the dynamic characteristics of the speed programs followed during the tests carried out. The FTP-75 and Stop-and-Go test programs are more dynamic in comparison with that
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