Fuel economy in automobiles

From Wikipedia, the free encyclopedia

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.

Contents 1 Energy content of fuel 2 Units 3 Measurement cycles 4 Fuel efficient driving 4.1 Acceleration methods 4.2 Drag 4.3 Engine efficiency 5 Gas Guzzler Tax 6 Fuel economy-boosting actions and technologies 7 References 8 See also 9 External links

[edit] Energy content of fuel

Fuel type	MJ/L	MJ/kg	BTU/imp gal	BTU/US gal	Research octane number (RON)
Regular Gasoline	31.60	42.70	151,600	126,200	Min 91
Premium Gasoline	32.84	43.50	157,500	131,200	Min 95
Autogas (LPG) (60% Propane + 40% Butane)	24.85	46.02	119,200	99,300	115
Ethanol	21.17	26.80	101,600	84,600	129
Methanol	15.56	19.70	74,600	62,200	123
Gasohol (10% ethanol + 90% gasoline)	30.63	41.11	146,900	122,300	93/94
Diesel	35.50	42.50	170,200	141,700	N/A (see cetane)

Note: The above energy values are higher heating values. For calculating actual vehicle fuel economy the lower heating value is used. The lower heating values are around 90% of the energy defined above.^[1]

[edit] 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).

The formula for converting between miles per US gallon (3.785 L) and L/100 km is , where x is the miles per gallon value and y is the liters per 100 kilometers value, or vice versa. For miles per imperial gallon (4.546 L) the formula is .

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 (as arithmetic mean) 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 mpg + 23.5 mpg) /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.

[edit] Measurement cycles

Government-mandated fuel efficiency measurements generally have two regimens or driving cycle patterns: a city or urban cycle, and a highway or extra-urban cycle. In Europe, the two standard measuring cycles for "L/100 km" value are motorway travel cycle up to max. 120 km/h and rush hour city traffic. A reasonably modern European car like an VW Golf or an Renault Megane turbo diesel direct injection may manage motorway travel at 4.3 L/100 km (54.7 mpg US) or 6.7 L/100 km in city traffic (35.1 mpg US), with carbon dioxide emissions of around 140 g/km (overall 5.2 L/100 km or 45.2 mpg).

Here are some comparisons about cars' approximate consumptions:

	mpg (US)	l/100 km	km/l
Japanese cars
Overall average ²	45	5.2	19
European cars
Overall average ²	40+	5.9−	17+
Smart ForTwo[citation needed]	69.2	3.4	29
Lupo 3L / Audi A2[citation needed]	78	3	33
North American cars
Overall average ²	20.4	11.5	8.70
North American, highway
Average mid-size car[citation needed]	27	9	11
full-size SUV[citation needed]	16	15	6.7
Pickup trucks vary considerably:
4-cylinder-engined light pickup ¹	28	8	13
V8 full-size pickup with extended cabin[citation needed]	13	18	5.6
North American, city
Average mid-size car[citation needed]	21	11	9.1
full-size SUV[citation needed]	13	18	5.6
Pickup trucks vary considerably:
4-cylinder-engined light pickup ¹	28	8	13
V8 full-size pickup with extended cabin[citation needed]	15	15	6.7

¹ No specific highway vs. city data available, figure listed in both sections for comparison purposes.^{[citation needed]}

² [1]

An interesting example of fuel economy is the popular microcar Smart ForTwo, which can achieve up to 3.4 L/100 km (69.2 mpg) using a turbocharged three-cylinder Diesel-engine. The Smart is produced by DaimlerChrysler and is currently only sold by one company in the United States (see external link ZAP). The current record in fuel economy of production cars is held by Volkswagen, with a special production model of the Volkswagen Lupo (the Lupo 3L) and the Audi A2 that can consume as little as 3 litres per 100 kilometres (78 miles per US gallon or 94 miles per Imperial gallon). The production lines for the Audi A2 und Volkwagen Lupo closed in June 2005.

Diesel engines often achieve greater fuel efficiency than petrol (gasoline) engines. Diesel engines have maximum energy efficiency of 45% and Petrol engines of 30% [2]. That's one of the reasons why Diesels have better fuel economy that equivalent petrol cars. A common margin is 40% more miles per gallon for an efficient turbodiesel. For example, the current model Skoda Octavia, using Volkswagen engines, has a combined Euro mpg of 38.2 mpg for the 102 bhp petrol engine and 53.3 mpg for the 105 bhp — and heavier — diesel engine. The higher compression ratio is helpful in raising efficiency, but diesel fuel also contains approximately 11% more energy per unit volume than gasoline.[3] Diesels consume less fuel also when operating at low power output or at idle, because they can run with leaner mixtures than can spark ignition engines. On the other hand the weight difference is significant for powerful cars.

Most of 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 annual Mobil Economy Run (1936–1968, except during World War II) provided fuel efficiency numbers during a coast to coast test on real roads with regular traffic and weather conditions. Today, 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[4]. The EPA tests used through 2007 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.

Due to concerns over the accuracy of this method, additional tests were devised and used for years but were not counted as part of the final EPA MPG number. They were approved in 2006 for use beginning with model year 2008 vehicles[5]. The new tests are additional to the original tests and more closely reflect the average American's driving habits of rapid acceleration and fast highway speeds. They also make use of the AC and test the car in cold conditions. The fuel economy ratings for vehicles tested under the new system are expected to drop an average of 12 percent on 'city' mileage, and 8 percent on 'highway' mileage. However, fuel usage is still measured by carbon in the car's exhaust.

The old test method was particularly favorable to hybrid cars, as the driving style used low speeds (highway speeds of only 45mph) and mild acceleration where a hybrid can turn the gas engine off. AC usage negatively impacts a hybrid's mileage, especially during idle, and cold weather can prevent a hybrid from turning the gas engine off because of the catalytic converter cooling to below optimal filtering temperature. As such, these vehicles may see the largest decrease in fuel economy ratings – city economy is expected to drop by 20 to 30 percent, and highway economy by 10 to 20 percent.

However, this new rating will not in any way affect the Corporate Average Fuel Economy (CAFE) rating for a company. This standard is mandated and controlled by the Department of Transportation, and not the EPA; the fuel economy ratings used to calculate a manufacturer's CAFE score is not the test utilized by the EPA to produce the ratings displayed on the window sticker.

In the United Kingdom, the Vehicle Certification Agency [6] has initiated a similar fuel economy rating system in accordance with European Community Directive 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, overall 8.2 (L/100 km). This converts to 20.9 and 36.5 mpg, overall 28.7 mpg, respectively, in United States measurements.

[edit] Fuel efficient driving

This article or section may contain original research or unattributed claims. Please help Wikipedia by adding references. See the talk page for details.

Fuel-efficient driving is a manner of driving intended to reduce the fuel consumption of an automobile. Aside from driving technique, a number of other factors contribute, including technical aspects of the car, road conditions, and fuel quality. Though in some cases these may be outside the driver's control, any attempt to conserve fuel may prove rewarding to him or her, for reasons of personal, financial, or global concern.

One method of improving fuel efficiency is by driving at constant, conservative speeds. This allows the engine to operate powerfully without being hindered by the aerodynamic drag associated with high speed.

[edit] Acceleration methods

This section deals with fuel efficient acceleration techniques. When constant speed is reached the minimum amount of depression of the accelerator to maintain the desired speed is the optimum.
Stop and go driving: The best method is to try to anticipate the future speed (a few seconds ahead in time) of the stop and go traffic and to accelerate or coast to this speed. This can be accomplished by leaving a growing gap in front when the traffic in front is accelerating and shortening this gap when traffic in front is braking. This may cause an accordion effect as less-knowledgeable motorists accelerate and brake rapidly. These same motorists also unknowingly slow down the overall traffic flow and reduce roadway throughput.

Other ways to improve efficiency include: In some gear systems, dropping down a gear while drifting removes fuel consumption completely. This can be seen in some digital displays of European cars "L/100km".

It is also more efficient to perform any required acceleration during level or downhill regions before approaching an opposing inclination, as more energy is required to increase velocity uphill.

Traffic and signal light anticipation reduces the need for stopping, starting and acceleration. Often slowing down before an intersection allows the driver to choose an empty lane. Much of the energy cost is derived from fighting against the mass of the initially stationary vehicle.

After the vehicle has reached cruising speed, there are still ways improve fuel efficiency. While following large transportation vehicles such as trucks or buses can be tedious, on motorways or highways they assist in reducing drag, pushing through the air and leaving your vehicle a large pocket of turbulence to drive behind.

Reviewing the basic physics, the engine converts the chemical energy in the fuel and air to the kinetic energy of the car's speed, with an efficiency that depends mostly on the engine and on its speed and power output (throttle opening). Additional energy is required to maintain speed, balancing air drag, tire drag, and mechanical drag. When the brakes are used, they provide a controllable drag to slow the car. Each type of drag converts some of the kinetic energy to heat.

[edit] Drag

The formula for air drag is usually written as a "constant" times the frontal area times the square of the car's speed. The constant depends mostly on streamlining and cooling air flow but is also weakly dependent on the area and speed, through the effect of the Reynolds number. Therefore, at high speeds, where air drag dominates the total drag and the engine efficiency changes slowly, the fuel consumption is roughly proportional to the square of the car's speed. Air drag is greater with the windows open or the top down.

The brakes intentionally convert energy from the useful form of kinetic energy to useless or undesirable heat. So driving in ways that reduce use of the brakes will generally reduce fuel consumption. Looking at it in terms of when the energy is lost to heat, driving slowly or reducing speed, before applying the brakes, reduces the amount of kinetic energy (from gasoline) that is dumped in the brakes at a stop. The equation for this is E = m v² / 2. That is, coasting down to half the peak speed by letting off the gas saves three quarters of the kinetic energy for use in overcoming other types of drag. Looking at the same thing from the point of view of when the gasoline is actually burned, the distance coasted is essentially free.

Tire and mechanical drag are most reduced by getting a lighter car, but type and pressure of tires and types of lubricants can also make some difference. There are low-drag tires, and increasing the tire pressure reduces drag, but there are trade-offs with ride, handling and tire life. Increasing the pressure beyond the limit listed on the side wall increases the chance of a blowout. Small increases in pressure can be felt in the form of a slightly harsher ride; such changes will also affect the handling characteristics of the car, though depending on the car and the original pressures, the changes may be beneficial or detrimental.

The alternator and air conditioner obtain their power by putting drag on the engine. So the use of the air conditioner or electrical equipment increases fuel consumption slightly.

[edit] Engine efficiency

Main article: Fuel efficiency

The efficiency of a gasoline (petrol) internal combustion engine is typically greatest at between three quarters and full load (throttle opening) and at around the same speed as the maximum torque figure^{[citation needed]}. So accelerating as slowly as is common in the US does not save significant fuel, but running the engine above its torque peak does rapidly increase fuel consumption. It is generally most efficient to run the engine at about the lowest speed at which it can supply the desired power^{[citation needed]}. This is done by automatic transmissions, when left in "drive". For manual transmissions, it should be done by the driver. It is carrying around a powerful engine and keeping it running that is expensive, not letting it work for brief periods.

Engine shut-off becomes efficient during stops exceeding 3 seconds since fuel consumed during start is less than that consumed during 3 seconds of idle. Some manufacturers have begun developing cars that shut off the engine when not in use and automatically restart it when the brake pedal is released. Engine wear during restart is considered to be negligible.

Diesel engines on the other hand do not become vastly less efficient at lower throttle settings, with normally aspirated diesel engines fuel consumption in a specific gear is directly related to the position of the accelerator pedal (with turbo engines the boost pressure also affects the volume of fuel injected per cycle). Examples would be the 1990 Rover Montego 2.0 TD which was advertised as doing 100mpg (imperial gallon, not US) at a steady 30mph, a gasoline (petrol) engine does not increase its mpg nearly as much by going slower.

The reason gasoline (petrol) engines lose efficiency more at lower throttle settings (thus partially negating the positive effect of driving slower) is the decrease in compression due to the partially closed throttle reducing the inlet manifold pressure. A diesel engine does not have a throttle restricting the airflow into the engine, the accelerator pedal simply increases or decreases the amount of fuel injected during each firing cycle of the engine, whilst the compression remains constant (turbocharger notwithstanding, the effects of which are positive anyway), thus keeping fuel efficiency high over a much wider range of speeds.

[edit] Gas Guzzler Tax

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 gas guzzler tax applies only to cars (not trucks) and is collected by the IRS. The purpose of the Gas Guzzler tax is to discourage the production and purchase of fuel-inefficient vehicles. The gas guzzler tax was phased in over ten years with rates increasing over time, The tax 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. Two separate fuel economy tests simulate city driving and highway driving. A weight average of city (55%) end 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.

[edit] Fuel economy-boosting actions and technologies

• Reducing vehicle weight by using materials such as aluminum, fiberglass, plastic, high-strength steel and carbon fiber instead of steel and iron
• Designing the exterior of the vehicle to reduce aerodynamic drag
• Using lower-viscosity lubricants (engine oil, transmission fluid, axle fluid)
• Replacing incandescent light bulbs with Light Emitting Diodes to reduce power consumption
• 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)
• Automatically shutting off engine when vehicle is stopped (mild hybrid)
• Recapturing waster energy while breaking (regenerative braking)
• Optimizing other engine combustion strategies:
  • Stratified Charge combustion
  • Lean burn combustion
  • HCCI combustion
  • Variable valve timing
  • Supercharging or twincharging (when coupled with a downsized engine)

Aftermarket consumer products exist which are purported to increase fuel economy; many of these claims have been discredited.

[edit] References

1. ^ Automotive Handbook, 4th Edition, Robert Bosch GmbH, 1996. ISBN 978-0-8376-0333-9

[edit] See also

• ACEA agreement
• Corporate Average Fuel Economy
• Emission standard
• Energy conservation
• Hypermiling

[edit] External links

Consumer published articles

• How to increase auto fuel efficiency
• In-depth advice to help increase fuel efficiency
• Fuel efficient driving and modifying

Sites and pages commissioned by various governments' institutions

• US EPA Green Vehicle Guide
• US government's FuelEconomy.gov
• Canadian Energuide: Vehicles
• Green Vehicle Guide Australia
• European Community Directive 93/116/EC — European Commission Directive 93/116/EC of 17.12.1993 adapting to technical progress Council Directive 80/1268/EEC relating to the fuel consumption of motor vehicles

Additional sites and pages with fuel economy tips

• Miles Per Gallon Calculator
• FuelEconomyTips.com
• mpgresearch.com
• www.omninerd.com/2006/07/16/articles/57 - A study conducted via the engine's OBDII interface on how driving habits influence fuel economy. (For whatever reason, the site is blacklisted but the empirical results are worth linking.)
• Online Fuel Economy Database Search
• Fatter U.S. drivers guzzle more gas, spend extra $2.8 billion annually