英文.docx

1、英文 Fuel economy in automobilesThe fuel economy of an automobile is the fuel efficiency relationship between the distance traveled and the amount of fuel consumed by the vehicle. Consumption can be expressed in terms of volume of fuel to travel a distance, or the distance travelled per unit volume of

2、 fuel consumed. Since fuel consumption of vehicles is a significant factor in air pollution, and since importation of motor fuel can be a large part of a nations foreign trade, many countries impose requirements for fuel economy. Different measurement cycles are used to approximate the actual perfor

3、mance of the vehicle. The energy in fuel is required to overcome various losses (wind resistance, tire drag, and others) in propelling the vehicle, and in providing power to vehicle systems such as ignition or air conditioning. Various measures can be taken to reduce losses at each of the conversion

4、s between chemical energy in fuel and kinetic energy of the vehicle. Driver behavior can affect fuel economy; sudden acceleration and heavy braking wastes energy.Units of measureFuel economy is the relationship between the distance traveled and fuel consumed.Fuel economy can be expressed in two ways

5、:Units of fuel per fixed distanceGenerally expressed as liters per 100 kilometers (L/100 km), used in some European countries, China, South Africa, Australia and New Zealand. Canadian law allows for use of either liters per 100 kilometres or miles per imperial gallon. Recently, the window sticker on

6、 new US cars has started displaying the vehicles fuel consumption in US gallons per 100 miles.Units of distance per fixed fuel unitMiles per gallon (mpg) is commonly used in the United States, the United Kingdom, and Canada (alongside L/100 km). Kilometres per litre (km/L) is more commonly used else

7、where in the Americas, Northern Europe, Asia, parts of Africa and Oceania. When the mpg unit is used, it is necessary to identify the type of gallon used, as the imperial gallon is 4.5 liters and the US gallon is 3.785 liters.Fuel economy statisticsWhile the thermal efficiency (mechanical output to

8、chemical energy in fuel) of petroleum engines has increased since the beginning of the automotive era, this is not the only factor in fuel economy. The design of automobile as a whole and usage pattern affects the fuel economy. Published fuel economy is subject to variation between jurisdiction due

9、to variations in testing protocols.The average fuel economy in 2008 for new cars, light trucks and SUVs in the United States was 26.4 mpg(US) (8.9 L/100 km).2008 model year cars classified as midsize by the US EPA ranged from 11 to 46 mpg(US)(21 to 5 L/100 km) However, due to environmental concerns

10、caused by CO2 emissions, new EU regulations are being introduced to reduce the average emissions of cars sold beginning in 2012, to 130 g/km of CO2, equivalent to 4.5 L/100 km for a diesel-fueled car, and 5.0 L/100 km for a gasoline (petrol)-fueled car.The average consumption across the fleet is not

11、 immediately affected by the new vehicle fuel economy, for example Australias car fleet average in 2004 was 11.5 L/100 km compared with the average new car consumption in the same year of 9.3 L/100 km Speed and fuel economy studiesFuel economy at steady speeds with selected vehicles was studied in 2

12、010. The most recent study indicates greater fuel efficiency at higher speeds than earlier studies; for example, some vehicles achieve better fuel economy at 100 km/hrather than at 70 km/h,although not their best economy, such as the 1994 Oldsmobile Cutlass, which has its best economy at 55 miles pe

13、r hour , and gets 2 mpg better economy at 65 mph than at 45 mph . The proportion of driving on high speed roadways varies from 4% in Ireland to 41% in Netherlands.When the US National Maximum Speed Laws 55 mph speed limit was mandated, there were complaints that fuel economy could decrease instead o

14、f increase. The 1997 Toyota Celica got 1 mpg better fuel-efficiency at 65 mph than it did at 45 mph , although almost 5 mpg better at 60 mph than at 65 mph, and its best economy at only 25 mph . Other vehicles tested had from 1.4 to 20.2% better fuel-efficiency at 55 mph vs 65 mph . Their best econo

15、my was reached at speeds of 25 to 55 mph .Officials hoped that the 55 mph limit, combined with a ban on ornamental lighting, no gasoline sales on Sunday, and a 15% cut in gasoline production, would reduce total gas consumption by 200,000 barrels a day, representing a 2.2% drop from annualized 1973 g

16、asoline consumption levels.This was partly based on a belief that cars achieve maximum efficiency between 40 and 50 mph and that trucks and buses were most efficient at 55 mph .However, the United States Department of Transportations Office of Driver Research found total fuel savings of the 55 mph l

17、imit to be 1%, and independent studies found a 0.5% savings.Energy considerationsSince the total force opposing the vehicles motion (at constant speed) multiplied by the distance through which the vehicle travels represents the work that the vehicles engine must perform, the study of fuel economy (t

18、he amount of energy consumed per unit of distance travelled) requires a detailed analysis of the forces that oppose a vehicles motion. In terms of physics: For a vehicle whose source of power is a heat engine (an engine that uses heat to perform useful work), the amount of fuel energy that a vehicle

19、 consumes per unit of distance (level road) depends upon:1 The thermodynamic efficiency of the heat engine;2 The forces of friction within the mechanical system that delivers engine output to the wheels;3 The forces of friction in the wheels and between the road and the wheels (rolling friction);4 O

20、ther internal forces that the engine works against (electrical generator, air conditioner etc., water pump, engine fan etc.);5 External forces that resist motion (e.g., wind, rain);6 Non-regenerative braking force (brakes that turn motion energy into heat rather than storing it in a useful form elec

21、trical energy in hybrid vehicles).Ideally, a car traveling at a constant velocity on level ground in a vacuum with frictionless wheels could travel at any speed without consuming any energy beyond what is needed to get the car up to speed. Less ideally, any vehicle must expend energy on overcoming r

22、oad load forces, which consist of aerodynamic drag, tire rolling resistance, and inertial energy that is lost when the vehicle is decelerated by friction brakes. With ideal regenerative braking, the inertial energy could be completely recovered, but there are few options for reducing aerodynamic dra

23、g or rolling resistance other than optimizing the vehicles shape and the tire design. Road load energy, or the energy demanded at the wheels, can be calculated by evaluating the vehicle equation of motion over a specific driving cycle. The vehicle powertrain must then provide this minimum energy in

24、order to move the vehicle, and will lose a large amount of additional energy in the process of converting fuel energy into work and transmitting it to the wheels. Overall, the sources of energy loss in moving a vehicle may be summarized as follows: Engine efficiency, which varies with engine type, t

25、he mass of the automobile and its load, and engine speed . Aerodynamic drag force, which increases roughly by the square of the cars speed, but note that drag power goes by the cube of the cars speed. Rolling friction. Braking, although regenerative braking captures some of the energy that would oth

26、erwise be lost. Losses in the transmission. Manual transmissions can be up to 94% efficient whereas older automatic transmissions may be as low as 70% efficient .Automatically controlled shifting of gearboxes that have the same internals as manual boxes will give the same efficiency as a pure manual

27、 gearbox plus the bonus of added intelligence selecting optimal shifting points Air conditioning. The power required for the engine to turn the compressor decreases the fuel-efficiency, though only when in use. This may be offset by the reduced drag of the vehicle compared with driving with the wind

28、ows down.The efficiency of AC systems gradually decrease due to dirty filters etc; regular maintenance prevents this. The extra mass of the air conditioning system will cause a slight increase in fuel consumption. Power steering. Older hydraulic power steering systems are powered by a hydraulic pump

29、 constantly engaged to the engine. Power assistance required for steering is inversely proportional to the vehicle speed so the constant load on the engine from a hydraulic pump reduces fuel efficiency. More modern designs improve fuel efficiency by only activating the power assistance when needed;

30、this is done by using either direct electrical power steering assistance or an electrically powered hydraulic pump. Cooling. Older cooling systems used a constantly engaged mechanical fan to draw air through the radiator at a rate directly related to the engine speed. This constant load reduces effi

31、ciency. More modern systems use electrical fans to draw additional air through the radiator when extra cooling is required. Electrical systems. Headlights, battery charging, active suspension, circulating fans, defrosters, media systems, speakers, and other electronics can also significantly increas

32、e fuel consumption, as the energy to power these devices causes increased load on the alternator. Since alternators are commonly only 4060% efficient, the added load from electronics on the engine can be as high as 3 horsepower (2.2 kW) at any speed including idle. In the FTP 75 cycle test, a 200 wa

33、tt load on the alternator reduces fuel efficiency by 1.7 MPG. Headlights, for example, consume 110 watts on low and up to 240 watts on high. These electrical loads can cause much of the discrepancy between real world and EPA tests, which only include the electrical loads required to run the engine an