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Fuel economy is the amount of fuel required to move a vehicle 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.

Energy content of fuel

Fuel type     MJ/L     MJ/kg     BTU/imp gal     BTU/US gal     Research octane
number (RON)
Gasoline 29.0   45      150,000 125,000 91–98
LPG 22.16 34.39 114,660 95,475 115
Ethanol 19.59 30.40 101,360 84,400 129
Methanol 14.57 22.61 75,420 62,800 123
Gasohol (10% ethanol + 90% gasoline) 28.06 43.54 145,200 120,900 93/94
Diesel 40.9   63.47 176,000 147,000 N/A (see cetane)

Units

Fuel economy is usually expressed in one of two ways:

  • The amount of fuel used per unit distance; for example, litres per 100 kilometres (L/100 km). In this case, the lower the value, the more economic a vehicle is (the less fuel it needs to travel a certain distance);
  • The distance travelled per unit volume of fuel used; for example, kilometres per litre (km/L) or miles per gallon (mpg). In this case, the higher the value, the more economic a vehicle is (the more distance it can travel with a certain volume of fuel).

Converting from mpg or km/L to L/100 km (or vice versa) involves the use of the reciprocal function, which is not distributive. Therefore, the average of two fuel economy numbers gives different values if those units are used. If two people calculate the fuel economy average of two groups of cars with different units, the group with better fuel economy may be one or the other.

Consider the following example: a Frenchman and an Englishman argue about whether English cars are more fuel efficient than French cars. To resolve the issue they obtain 2 French cars, (F1 and F2) and two English cars (E1 and E2). Each man tests the fuel economy of all four cars. The Englishman works in miles per Imperial gallon, while the Frenchman works in litres per 100 km. Note that 1 mile per Imperial gallon = 282.48 litres per 100 km.

The Englishman obtains the following results. E1 has a fuel consumption of 94.2 mpg and E2 has a fuel consumption of 23.5 mpg. The average fuel consumption of the English cars is therefore (94.2 + 23.5)/2 = 58.9 mpg. For the French cars he obtains a fuel consumption of 47.1 mpg for each car. Thus he concludes that the British cars have better fuel consumption on average.

The Frenchman obtains the same results, but expresses them in L/100 km. He measures the English cars E1 and E2 to consume 3 L/100 km and 12 L/100 km respectively. The average is therefore 7.5 L/(100 km). For the French cars he obtains 6 L/100 km for both. Thus he concludes that the French cars have lower fuel consumption.

Measurement cycles

Government-mandated fuel efficiency measurements generally have two regimens or driving cycle patterns: a city or urban cycle, and an highway or extra-urban cycle. In Europe, the two standard measuring cycles for "L/100 km" value are motorway travel at 90 km/h and rush hour city traffic. A reasonably modern European supermini may manage motorway travel at 5 L/100 km (47 mpg US) or 6.5 L/100 km in city traffic (36 mpg US), with carbon dioxide emissions of around 140 g/km.

Here are some comparisons about North American car's approximately consumptions:

Average mid-size car 27 mpg (9 L/100 km) highway, 21 mpg (US) (11 L/100 km) city;

- full-size SUV 16 mpg (15 L/100 km) highway and 13 mpg (US) (18 L/100 km) city;

- Pickup trucks vary considerably:

- 4 cylinder-engined light pickup produces circa 28 mpg (8 L/100 km),

- V8 full-size pickup with extended cabin produces circa 13 mpg (US) (18 L/100 km) city and 15 mpg (US) (15 L/100 km) highway.

An interesting example of fuel economy is the popular microcar Smart ForTwo, which can achieve up to 4.0 L/100 km (70.6 mpg) using a turbocharged three-cylinder engine. The Smart is produced by DaimlerChrysler and is currently only sold by one company in the United States (see external link ZAP).

Diesel engines often produce greater fuel efficiency than petrol (gasoline) engines: above 50% of all cars sold in the European Union are now diesel vehicles, because of tax benefits, due to the truck companies lobby.

All these previously-cited fuel economy values are for operation on petrol, gasoline. New US light vehicles designated as flexible fuel vehicles (FFVs) running on E85 (85% ethanol, 15% gasoline) will typically achieve from 5% to 15% less fuel economy in mpg on pure E85 than when operated on pure gasoline. Older non-turbo-charged fuel-injected FFVs running on E85 will typically achieve about 25% to 30% less fuel economy on E85. Over 4 million FFVs are currently operated on US roadways as of 2005; most tend to be light trucks or van vehicles, although newer "car-shaped" high performance autos are also being introduced in the 2006 model year (e.g., 2006 GM Chevrolet Impala).

The driving interval tests described here test laboratory derived emissions and calculated fuel economy, but certainly not on-the-road fuel efficiency. In the United States, the Environmental Protection Agency (EPA) is the government body that makes the calculations that auto manufacturers use when advertising their vehicles. Separate numbers are given for city and highway driving. The EPA tests do not directly measure fuel consumption, but rather calculate the amount of fuel used by measuring emissions from the tailpipe based on a formula created in 1972. The cars are not actually driven around a course, but are cycled through specific profiles of starts, stops, and runs on a chassis dynamometer in a laboratory environment. As emissions standards have become more strict due to smog, most of the resulting numbers do not directly correspond to what people actually experience when driving. Most often, the EPA estimate of mileage is several percent higher than what the average driver manages to achieve in practice, although there are some cases where the difference is nearly 200% higher than what the average driver achieves. Correcting this discrepancy by means of an updated, more conservative, testing procedure which would understate rather than overstate MPGs, would help force automakers to improve fuel economy without changing the Corporate Average Fuel Economy (CAFE) standard. This tends to be fought vehemently by automakers and is politically unattractive. This is because the vehicles they produce would need to achieve better fuel economy just to meet the current standard.

In the United Kingdom, the Vehicle Certification Agency has initiated a similar fuel economy rating system in accordance with European Community Directive [http://europa.eu.int/eur-lex/lex/LexUriServ/LexUriServ.do?uri=CELEX:31993L0116:EN:HTML 93/116/EC. The ratings are based on an urban and extra-urban driving cycle. The urban cycle is a cold start followed by "a series of accelerations, steady speeds, decelerations and idling. Maximum speed is 31 mph (50 km/h), average speed 12 mph (19 km/h) and the distance covered is 2.5 miles (4 km)." The extra-urban cycle is conducted immediately following the urban cycle and consists of roughly half steady-speed driving and the remainder accelerations, decelerations, and some idling. Maximum speed is 75 mph (120 km/h), average speed is 39 mph (63 km/h) and the distance covered is 4.3 miles (7 km).

The raw averages for all 2005 vehicles rated in the United Kingdom are: Urban cycle, 11.3, extra-urban 6.4 (L/100 km). This converts to 20.9 and 36.5 mpg, respectively, in United States measurements.

Consumer ability to increase fuel efficiency

See also: Fuel efficient driving

Consumers are also able to adopt certain methods to increase fuel efficiency for their benefit, and for that of humanity.

Reducing the vehicle's weight, especially in city driving, and frontal area, especially in highway driving are the two largest contributing factors. This is because in city driving, the vehicle stops more often, and inertia plays a larger role. Weight increases the initial energy needed to get the vehicle moving. In contrast, in highway driving, the vehicle is generally constantly moving at a relatively faster speed so air resistance plays a role.

The type of engine a consumer chooses is a major contributing factor in fuel efficiency. The most efficient mass-production engines are petrol engines under a litre displacement, turbo-diesels under 1.4 litres, standard diesels and electric hybrids. Out of these, turbo-diesels tend to be the most efficient in terms of power per unit displacement. However, some consumers may not be interested in acceleration rates, so a standard diesel engine is the most efficient per litre displacement.

Fuel efficiency can be hampered if the air pressure in the tires of vehicles is too low. Abrupt acceleration and deceleration can also decrease fuel efficiency. In order to avoid this, drivers generally accelerate as gradually as possible, especially uphill, and try to keep a stable speed. They also time their movement to minimize slowing or complete stops, and coast whenever possible.

One way Hybrids and other more efficent cars achieve good efficency is their use of Low-rolling resistance tires which are up to 10% more efficent than standard tires.

Fuel efficiency can also be hampered by unnecessary friction in the engine due to drivers not taking advantages of situations where engine speed can be reduced. This tends to be especially pertinent to those driving with a manual transmission. Generally, conscious drivers use the highest reasonable gear, and shift up early and shift down late. Downshifting in a manual transmission car on a downhill will provide adequate speed control (without using the brakes to reduce wear) and will save fuel as long as no throttle is applied. It is a commonly misconceived notion that shifting into neutral on a downhill will save fuel. This is not true because the engine uses more fuel to idle (while in neutral and coasting) than it does in gear and coast (due to the wheels spinning the engine. It makes slightly more difference for carburetor cars, while cars with fuel injection - or carburetor cars with a fuel cut-off solenoid - benefit from the fuel cutoff when the car is left in gear. (The second half of this paragraph may not be factually correct and is being challenged on the Discussion page)

Fuel efficiency can also be increased by reducing the time when the engine operates but does not need to. For example, drivers may shut it down whenever it is unused for more than 30 seconds when not in traffic or stopped at a lengthy light or intersection. (Turning off the key while the car is moving disables power steering and brakes, if fitted, and the steering wheel will lock if the driver follows habit by taking the key out.) Using air conditioning sparingly and relying on neutral settings where the outflow is neither hot nor cold increases fuel efficiency . It is estimated that the air conditioning running at maximum setting can lower fuel efficiency by 5-25%.

However, during traveling at high speeds, if the windows are open, this creates a lot of aerodynamic drag. Thus this may consume more fuel than operating the air conditioner sensibly. High speed driving can drastically reduce fuel efficiency. Gasoline powered cars operate with maximum efficiency in the highest gear at the lowest speed in that highest gear without engine lugging. This effect is largely due to aerodynamic drag. In highway driving over 80-90 km/h (50-55 mph) the aerodynamic drag will rise sharply, thus increasing fuel consumption.

Medium-sized motorcycles can get from 37-60 miles per US gallon (6.4–3.9 L/100 km) depending on how they are ridden. For example, the same 600 cm³ race bike which is capable of getting 50 mpg (4.7 L/100 km) will only get 30 mpg (7.8 L/100 km) if raced. 50 cm³ scooters, which do not require a motorcycle license to operate, can get upwards of 100 mpg (< 2.4 L/100 km).

As fuel efficiency is related to the complete combustion per fuel molecule in order to get the most energy out of each unit of fuel, the air intake plays a significant role. Keeping a vehicle's air intake clean and free from obstruction and replacing it's air filter regularly will benefit efficiency. Installing an aftermarket intake that allows a greater and freer airflow can increase fuel efficiency depending on the vehicle. A lifetime air filter (such as K&N, Barry Grant, AEM Dryflow, etc.) will eventually save it's own cost and more since it doesn't require replacement. Certain vehicles respond better to higher flowing filters than others, depending on the quality level of factory engineering.

A larger exhaust system can increase fuel efficiency by reducing back pressure allowing the engine to turn more freely. However, this is normally true with larger exhausts that are only slightly larger than stock. Ideally, you want the diamter to be as large as possible while achieving the highest power, but when it is over this point, going any larger will release too much back pressure and there will be power loss. The only scientific method to determine the optimal diameter exhaust for a certain rpm is on a dyno.

The main benefit for split or multi electrode spark plugs is to prevent misfire and not to give a more powerful spark even though they can. When residual ionization builds up, the spark can't jump the gap and there is a misfire. If there is another equidistant electrode, the spark will just jump there instead of misfiring.

The quality and grade of fuel used can also significantly impact fuel economey. Using premium when your engine doesn't require it can actually REDUCE fuel economey, due to the 92/93 octane fuel actually having less BTU's than regular unleaded. The only benefit to using the higher grade fuels when not required is for their generally higher amount of additives.

The quality of virtually any carbon based fuel can be cheaply improved with additives that reduce smog by enhancing efficiency which gives a horsepower gain and fuel mileage gain. Only use additives that have been tested and validated by the D.O.T.

Fuel economy-boosting technologies

See also

External links

Consumer published articles Sites and pages commissioned by various governments' institutions

Automobiles

 

This article is licensed under the GNU Free Documentation License. It uses material from the "Fuel economy in automobiles".

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