Fuel economy in automobiles

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Image:2006 Honda Airwave fuel efficiency meter.jpg
Fuel economy monitor from a 2006 Honda Airwave

Fuel economy in automobiles is the amount of fuel required to move the automobile over a given distance. While the fuel efficiency of petroleum engines has improved markedly in recent decades, this does not necessarily translate into fuel economy of cars, as people in developed countries tend to buy bigger and heavier cars.

Contents

[edit] Units of measure

Image:Chart MPG to Litre-100km.svg
MPG to L/100km conversion chart (Imperial gallons)

The two most common ways to measure automobile fuel economy are:

  1. The amount of fuel used per unit distance; most commonly, litres per 100 kilometres (L/100km). Lower values mean better fuel economy: you use less fuel to travel the same distance.
  2. The distance travelled per unit of fuel used; most commonly, kilometres per litre (km/L) or miles per gallon (mi/gal, colloquially referred to as mpg). Higher values mean better fuel economy: you can travel farther for the same amount of fuel.

To convert between L/(100 km) and miles per U.S. gallon, divide 235 by the number in question. (For miles per imperial gallon, use 282 instead of 235.) For example, to convert from 30 mpg (U.S.) to L/(100 km), divide 235 by 30, giving 7.83 L/(100 km).

A related measure is the amount of carbon dioxide produced as a result of the combustion process, typically measured in grams of CO2 per kilometre (CO2 g/km). A petrol (gasoline) engine will produce around 2.3 kg of carbon dioxide for each litre of petrol consumed (19 lb/gal). [1] A typical diesel engine produces 2.8 kg/L (23 lb/gal)[2] though typically burns fewer litres per kilometre (and is thus typically more fuel efficient for an otherwise identical car). Since the CO2 emissions are relatively constant per litre, fuel efficiency is directly related to emissions of CO2 per kilometre.[3]

[edit] Fuel economy statistics

The choice of car and how it is driven drastically affects the fuel economy. A top fuel dragster can consume 6 U.S. gallons (23 L) of gasoline (petrol) for a quarter-mile (400 m) run in about 4.5 seconds, which comes out to 24 gallons per mile (5600 L per 100 km). The other extreme was set in 2005 by Pac-car II, a car driven by a hydrogen fuel cell. Its Swiss student team from ETH Zürich entered in the Shell Eco-Marathon and set a world record of 18.6 mL per 100 km (12,645.9 mpg) at an average speed of 30 kilometres per hour (18 miles/h).

Both such vehicles are extremes, and most people drive ordinary cars that typically average 15 to 40 miles per gallon (15 to 5.6 L per 100 km). However, due to environmental concerns 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 per 100 km (52 mpg) for a diesel-fueled car, and 5.0 L per 100 km (47 mpg) for a gasoline (petrol)-fueled car. [4] EU fuel economy testing is done on a rolling road with two segments, ECE15 and EUDC, which correspond to city and highway driving, respectively. The city driving cycle simulates a 4.052 km (2.5 mile) urban trip at an average speed of 18.7 km/h (11.6 mph) and at a maximum speed of 50 km/h (31 mph), while the highway cycle lasts 400 seconds (6 minutes 40 seconds) at an average speed 62.6 km/h (39 mph) and a top speed of 120 km/h (74.6 mph).[5]

The power to overcome air resistance increases roughly with the cube of the speed. Thus, above about 30 mph (48 km/h), wind resistance becomes a dominant limiting factor. By driving at 45 rather than 65 mph (72 rather than 105 km/h), the power to overcome wind resistance is about one-third, and much greater fuel economy can be achieved. Increasing speed to 90 mph (145 km/h) increases the power requirement by 2.6 times, and drastically decreases fuel economy. In practice, rather than doubling or halving the fuel economy, the difference is actually closer to 40-50%, since rolling resistance, which is broadly proportional to speed, is also a factor.

[edit] USA: Government regulations

[edit] U.S. Energy Tax Act

Main article: Energy Tax Act

The Energy Tax Act of 1978 [6] in the U.S. established a gas guzzler tax on the sale of new model year vehicles whose fuel economy fails to meet certain statutory levels. The tax applies only to cars (not trucks) and is collected by the IRS. Its purpose is to discourage the production and purchase of fuel-inefficient vehicles. The tax was phased in over ten years with rates increasing over time. It applies only to manufacturers and importers of vehicles, although presumably some or all of the tax is passed along to automobile consumers in the form of higher prices. Only new vehicles are subject to the tax, so no tax is imposed on used car sales. The tax is graduated to apply a higher tax rate for less-fuel-efficient vehicles. To determine the tax rate, manufacturers test all the vehicles at their laboratories for fuel economy. The U.S. Environmental Protection Agency confirms a portion of those tests at an EPA lab.

[edit] Current EPA testing procedure through 2007

Two separate fuel economy tests simulate city driving and highway driving: the city driving program consists of starting with a cold engine and making 23 stops over a period of 31 minutes for an average speed of 20 mph (32 km/h) and with a top speed of 56 mph (90 km/h); the highway program uses a warmed-up engine and makes no stops, averaging 48 mph (77 km/h) with a top speed of 60 mph (97 km/h) over a 10 mile (16 km) distance. The measurements are then adjusted downward by 10% (city) and 22% (highway) to more accurately reflect real-world results. A weight average of city (55%) and highway (45%) fuel economies is used to determine the tax.[7]

In some cases, this tax may only apply to certain variants of a given model - for example, the 2004–2006 Pontiac GTO did incur the tax when ordered with the four-speed automatic transmission, but did not incur the tax when ordered with the six-speed manual transmission.

[edit] New 2008- EPA testing procedure

As a means of reflecting real world fuel economy more accurately, the EPA adds three new tests [8] that will combine with the current city and highway cycles to determine fuel economy of new vehicles, beginning with the 2008 model year. A high speed/quick acceleration loops lasts 10 minutes, covers 8 miles (13 km), averages 48 mph (77 km/h) and reaches a top speed of 80 mph (130 km/h). Four stops are included, and brisk acceleration maximizes at a rate of 8.46 mph (13.62 km/h) per second. The engine begins warm and air conditioning is not used. Ambient temperature varies between 68 to 86 °F (30 °C).

The air conditioning test raises ambient temperatures to 95 °F (35 °C), and the vehicle's climate control system is put to use. Lasting 9.9 minutes, the 3.6-mile (5.8 km) loop averages 22 mph (35 km/h) and maximizes at a rate of 54.8 mph (88.2 km/h). Five stops are included, idling occurs 19 percent of the time and acceleration of 5.1 mph/sec is achieved. Engine temperatures begin warm. Lastly, a cold temperature cycle uses the same parameters as the current city loop, except that ambient temperature is set to 20 °F (−7 °C).

[edit] CAFE standards

Main article: Corporate Average Fuel Economy

The Corporate Average Fuel Economy (CAFE) regulations in the United States, first enacted by Congress in 1975,[citation needed] are federal regulations intended to improve the average fuel economy of cars and light trucks (trucks, vans and sport utility vehicles) sold in the US in the wake of the 1973 Arab Oil Embargo. Historically, it is the sales-weighted average fuel economy of a manufacturer's fleet of current model year passenger cars or light trucks, manufactured for sale in the United States. Under Truck CAFE standards 2008–2011 this changes to a "footprint" model where larger trucks are allowed to consume more fuel. The standards are limited to vehicles under a certain weight, but those weight classes will be expanding in 2011 if current law (as of April 2006) holds.

[edit] Energy considerations

Please help improve this article or section by expanding it.
Further information might be found on the talk page or at requests for expansion. (June 2007)

Ideally, a car traveling at a constant velocity and constant grade 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. With ideal regenerative braking, this energy could be completely recovered. In real-world conditions, energy is lost in a number of ways:

  • Engine efficiency, which varies with engine type, the mass of the automobile and its load, and engine speed (usually measured in RPM).
  • Aerodynamic drag force, which increases roughly by the square of the car's speed, but note that Aerodynamic drag power, increases roughly by the cube of the car's speed.
  • Rolling friction.
  • Braking, although regenerative braking captures some of the energy that would otherwise 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.[9])
  • Air conditioning. Parasitic losses due to the necessary power required for the engine to turn the compressor additionally decrease fuel mileage, though only when in use. Headlights, media systems, and other electronics can also increase fuel consumption, as the energy to power these devices comes from the charging system, increasing the load on the alternator.

[edit] Fuel economy-boosting technologies

Many aftermarket consumer products exist which are purported to increase fuel economy; many of these claims have been discredited. In the United States, the Environmental Protection Agency maintains a list of devices that have been tested by independent laboratories and makes the test results available to the public. "Gas Saving and Emission Reduction Devices Evaluation". Retrieved on 2008-01-06.[citation needed]

[edit] Fuel economy data reliability

The mandatory publication of the fuel consumption by the manufacturer has led to a doubtful practice to reach better values. Those consist of measures the users may imitate but others which are dangerous if used in road traffic or may increase maintenance costs. The measures in detail:

  • selection of high quality vehicles from the production
  • use of special expensive lubricants
  • increase tire pressure above feasible thresholds
  • disable energy consumers and start with a loaded battery
  • turn the outer mirror(s) in-line with the wind
  • correct the alignment of the wheels

If the test is on a test stand, the vehicle may detect open doors and adapt the engine control. Also when driven according to the test regime, the parameters may adapt automatically. This is summarized under the term "golden car" which is used to produce better mileage figures.

NOTE: Exact regulations vary between countries. The below is true of current European and US regulations, upon which most other countries' regulations are based. Although the "cheats" above may have been true historically, most of them are now generally illegal. Correctly aligning the vehicle wheels however is something that should be normal practice for the vehicle users. Tyre pressures and lubricants have to be as recommended by the manufacturer (Higher tyre pressures are required on a particular dyno type, but this is to compensate for the different rolling resistance of the dyno, not to produce an unrealistic load on the vehicle). Normally the quoted figures a manufacturer publishes have to be proved by the relevant authority witnessing vehicle/engine tests. A lot of Governments independently test emissions from customer vehicles, and as a final measure can force a recall of all of a particular type of vehicle if the customer vehicles do not fulfil manufacturers' claims within reasonable limits. The expense and bad publicity from such a recall means manufacturers should be very cautious not to publish unrealistic figures.

[edit] Fuel economy maximizing behaviors

Main article: Fuel economy-maximizing behaviors

Governments, various environmentalist organizations, and companies like Toyota and Shell Oil Company have historically urged customers and citizens to maintain adequate air pressure in tires and careful acceleration/deceleration habits. These concepts are marketed through the use of terms like hypermiler, ecodriving, and Nempimania.

[edit] References

This article needs additional citations for verification.
Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (May 2007)
  1. ^ http://www.dpiw.tas.gov.au/inter.nsf/WebPages/MCLE-5WV8R7?open
  2. ^ http://timeforchange.org/what-is-a-carbon-footprint-definition
  3. ^ sugre.info conversion tool
  4. ^ European strategy targets car emissions
  5. ^ Vehicle test cycles
  6. ^ http://www.fueleconomy.gov/feg/info.shtml#guzzler
  7. ^ How the EPA Tests and Rates Fuel Economy
  8. ^ Earthcars: EPA fuel economy ratings - what's coming in 2008
  9. ^ http://www.sae.org/servlets/productDetail?PROD_TYP=PAPER&PROD_CD=1999-01-1259

[edit] See also

[edit] External links

nl:Brandstofverbruik ja:燃費 sv:Bränsleförbrukning

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