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Details of Fuel economy in automobiles

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, if people buy bigger and heavier cars.

Units of measure
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/100 km). 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, which may be U.S. gallon or imperial gallon, both of which are commonly abbreviated as mpg, although the imperial gallon is about 20% larger than the U.S. gallon. 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; or from 10 L/100 km to mpg U.S., divide 235 by 10 (23.5 mpg). To convert from L/100 km to km/L, divide between 100 and calculate the reciprocal of the result.

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 2.32 kg of carbon dioxide for each litre of petrol consumed (19.4 lb/gal). A typical diesel engine produces 2.66 kg/L (22.23 lb/gal)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.

Inverse scale
Because miles per gallon is the inverse of actual fuel consumption (gallons per mile), each additional mile per gallon represents less fuel saved to travel a given distance. For example, switching from a 15-mpg vehicle to a 17-mpg vehicle saves about as much gas as switching from a 50-mpg vehicle to an 82-mpg vehicle (about 117 gallons/year).

A vehicle getting 10 mpg would consume 0.1 gallons per mile. Reducing fuel consumption by increments of 0.01 gallons/mile results in the following measurements:

* 11.1 mpg-U.S. (0.09 U.S. gallons/mile or 21.2 L/100km)
* 12.5 mpg-U.S. (0.08 U.S. gallons/mile or 18.8 L/100km)
* 14.3 mpg-U.S. (0.07 U.S. gallons/mile or 16.4 L/100km)
* 16.7 mpg-U.S. (0.06 U.S. gallons/mile or 14.1 L/100km)
* 20 mpg-U.S. (0.05 U.S. gallons/mile or 11.8 L/100km)
* 25 mpg-U.S. (0.04 U.S. gallons/mile or 9.4 L/100km)
* 33.3 mpg-U.S. (0.03 U.S. gallons/mile or 7.0 L/100km)
* 50 mpg-U.S. (0.02 U.S. gallons/mile or 4.7 L/100km)
* 100 mpg-U.S. (0.01 U.S. gallons/mile or 2.4 L/100km)

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 nitromethane for a quarter-mile (400 m) run in about 4.5 seconds, which comes out to 24 U.S. gallons per mile (5600 L per 100 km). The other extreme was set by a French entrant in the Eco-Marathon in 2004, who managed 3401 km/l, or more than 8000 mpg-U.S.

Both such vehicles are extremes, and most people drive ordinary cars that typically average 15 to 40 miles per U.S. gallon (19 to 50 miles per imperial gallon) or (5.6 to 15 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 US, 63 MPG imperial) for a diesel-fueled car, and 5.0 L per 100 km (47 mpg US, 56 MPG imperial) for a gasoline (petrol)-fueled car.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).

It should be borne in mind that the average consumption across the fleet is not immediately directly affected by the new vehicle fuel economy, for example Australia's car fleet average in 2004 was 11.5 l/100 km (20.5 mpg-U.S.), compared with the average new car consumption in the same year of 25.3 mpg-U.S.

New Zealand fuel economy ratings
* 2008

United States EPA fuel economy ratings
* 2009

Physics background
The power to overcome air resistance increases roughly with the cube of the speed, and thus the energy required per unit distance is roughly proportional to the square of speed. Because air resistance increases so rapidly with speed, above about 30 mph (48 km/h), it becomes a dominant limiting factor. Driving at 45 rather than 65 mph (72 rather than 105 km/h), results in about one-third the power to overcome wind resistance, or about one half the energy per unit distance, and much greater fuel economy can be achieved. Increasing speed to 90 mph (145 km/h) from 65 mph (105 km/h) increases the power requirement by 2.6 times, the energy by 1.9 times, and drastically decreases fuel economy. In practice, rather than doubling or halving the fuel economy, the difference is actually closer to 40-50%, because engine efficiency varies greatly with the torque/speed operating point. Rolling resistance, which is broadly proportional to speed, is also a factor particularly at lower speeds.

There were complaints when the speed limit was lowered from 65 mph to 55 mph that it could lower, instead of increase fuel economy, and in fact the 1997 Toyota Celica gets 1 mpg better fuel-efficiency at 65 than it does at 55 (43.5 vs 42.5), although almost 5 mpg better at 60 than at 65 (48.4 vs 43.5). Other vehicles tested had from 1.4 to 20.2% better fuel-efficiency at 55 mph vs. 65 mph.
U.S. government regulations

U.S. Energy Tax Act
Main article: Energy Tax Act
The Energy Tax Act of 1978 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.

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.

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.
Because EPA figures are almost always higher than real-world fuel-efficiency, EPA has modified the method starting with 2008.

EPA testing procedure: 2008 and beyond
As a means of reflecting real world fuel economy more accurately, the EPA adds three new tests 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).

EPA tests for fuel economy do not include electrical load tests beyond climate control which may account for some of the discrepancy between EPA and real world fuel-efficiency. A 200-W electrical load can produce a 0.4 km/L reduction in efficiency on the FTP 75 cycle test.

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, 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.

State regulations
The states are pre-empted by federal law, and are not allowed to make fuel efficiency standards. However, California has a special dispensation from the Clean Air Act to make emissions standards (which other states may adopt instead of the federal standards). The California Air Resources Board is implementing some legislation which limits greenhouse gas emissions. A legal dispute has emerged over whether this is effectually a fuel efficiency standard.

European standards
In the European Union advertising has to show CO2-emission and fuel consumption data in a clear way as described in the UK Statutory Instrument 2004 No 1661.The industry usually places this information in footnotes along with price and other details.

New Zealand
Starting on 7 April 2008 all cars of up to 3.5 tonnes GVW sold other than private sale need to have a fuel economy sticker applied (if available) which shows the rating from one half star to six stars with the best economy cars having the most stars and the worst gas guzzlers the least, along with the fuel economy in L/100 km and the estimated annual fuel cost for driving 14,000 km. The stickers must also appear on vehicles to be leased for more than 4 months. All new cars currently rated range from 34 mpg-U.S. or 6.9 L/100km to 62 mpg-U.S. or 3.8 L/100km and received respectively from 4 1/2 to 5 1/2 stars.

Australia
Beginning in October, 2008, all new cars will need to be sold with a sticker on the windscreen showing the fuel economy and the CO2 emissions.Australia also uses a star rating system, from one to five stars, but it combines greenhouse gases with pollution, rating each from 0 to 10 with ten being best. To get 5 stars a combined score of 16 or better is needed, so a car with a 10 for economy (greenhouse) and a 6 for emission or 6 for economy and 10 for emission, or anything in between would get the highest 5 star rating.The lowest rated car is the Ssangyong Korrando Ssangyong with automatic transmission, with one star, while the highest rated was the Toyota Prius hybrid. The Fiat 500, Fiat Punto and Fiat Ritmo as well as the Citroen C3 also received 5 stars.The greenhouse rating depends on the fuel economy and the type of fuel used. A greenhouse rating of 10 requires 60 or less grams of CO2 per km, while a rating of zero is more than 440 g/km CO2. The highest greenhouse rating of any 2009 car listed is the Toyota Prius, with 106 g/km CO2 and 4.4 L/100 km (53 mpg–U.S. / 64 mpg–imp). Several other cars also received the same rating of 8.5 for greenhouse. The lowest rated was the Ferrari 575 at 499 g/km CO2 and 21.8 L/100 km (11 mpg–U.S. / 13 mpg–imp). The Bentley also received a zero rating, at 465 g/km CO2. The best fuel economy of any year is the 2004-2005 Honda Insight, at 3.4 L/100 km (69 mpg–U.S. / 83 mpg–imp).

Energy considerations
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. 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 drag power goes 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.Automatically controlled shifting of gearboxes that have the same internals as manual boxes will give the same efficiency as a pure manual gearbox plus the bonus of added intelligence selecting optimal shifting points
* Air conditioning. Parasitic losses due to the necessary power required for the engine to turn the compressor additionally decrease fuel-efficiency, though only when in use.
* Electrical systems. Headlights, battery charging, active suspension, circulating fans, defrosters, media systems, speakers, and other electronics can also significantly increase fuel consumption, as the energy to power these devices causes increased load on the alternator. Since alternators are commonly only 40-60% efficient, the added load from electronics on the engine can be as high as 3 horsepower at any speed including idle. In the FTP 75 cycle test, a 200 watt 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 and basic climate control.

Fuel-efficiency decreases from electrical loads are most pronounced at lower speeds because most electrical loads are constant while engine load increases with speed. So at a lower speed a higher proportion of engine horsepower is used by electrical loads. Hybrid cars see the greatest effect on fuel-efficiency from electrical loads because of this proportional effect.

Fuel economy-boosting technologies
Main article: Fuel saving devices
* Using lighter materials for moving parts such as pistons, crankshaft, gears and alloy wheels
* Designing the exterior of the vehicle to reduce aerodynamic drag
* Replacing tires with low rolling resistance (LRR) models.
* Using lower-friction lubricants (engine oil, transmission fluid, axle fluid)
* Incorporating Locking torque converters in automatic transmissions to reduce slip and power losses in the converter
* Augmenting a downsized engine with an electric drive system and battery (hybrid vehicles) hybrid electric vehicle
* Installing an alternator disconnect and supplying electrical system from deep cycle battery pack which is charged at home.
* Automatically shutting off engine when vehicle is stopped (mild hybrid)
* Recapturing wasted energy while braking (regenerative braking)
* Optimizing other engine combustion strategies:
o Stratified Charge combustion
o Lean burn combustion
o HCCI combustion
o Variable valve timing
o Supercharging or twincharging (when coupled with a downsized engine)
* Reducing vehicle weight by using materials such as aluminum, fiberglass, plastic, high-strength steel and carbon fiber instead of steel and iron

Future Technologies
Technologies that improve fuel efficiency, but are yet to be sold are:
* Six stroke engine - various methods to boost fuel efficiency
* BMW's Turbosteamer - using the heat from the engine to spin a mini turbine to generate power
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.

Fuel economy data reliability
The mandatory publication of the fuel consumption by the manufacturer led some to use dubious practices to reach better values in the past. 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. Test laboratories use a "golden car" that is tested in each one to check that each lab produces the same set of measurements for a given drive cycle.

Correctly aligning the vehicle wheels is something that should be normal practice for the vehicle users. Tire pressures and lubricants have to be as recommended by the manufacturer (Higher tire 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. The US Federal government retests 10-15% of models), to make sure that the manufacturer's tests are accurate.

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 drivers to maintain adequate air pressure in tires and careful acceleration/deceleration habits.

Fuel economy as part of quality management regimes
Environmental management systems EMAS as well as good fleet management do include record keeping of the fuel consumption of the fleet. Quality management on top of this uses those figures to steer the measures acting on the fleets. You may check whether procurement, driving, and maintenance in total have contributed to changes in the fleets overall consumption.
 

Type: Glossary & Dictionary