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Fuel economy in automobiles

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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.

Contents

[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)
<ref>Automotive Handbook, 4th Edition, Robert Bosch GmbH, 1996. ISBN 0-8376-0333-1</ref>

[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 from miles per US gallon (3.785 L) to L/100 km is <math>\frac{235.2}{x}</math>, where <math>x</math> is the miles per gallon value. For miles per imperial gallon (4.546 L) the formula is <math>\frac{282.5}{x}</math>.

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 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 North American cars' approximate 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),[citation needed]

- 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.

The average fuel efficiency of European cars is over 40 mpg (5.9 L/100 km), Japanese cars 45 mpg (5.2 L/100 km), and North American cars 20.4 mpg (11.5 L/100 km).[citation needed]

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% [1]. 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.[2] 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 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[3]. 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 [4] 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

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 will prove rewarding to him or her, for reasons of personal, financial, and perhaps most importantly, global concern.

One method of improving fuel efficiency is by driving at constant, lawful 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 and 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.


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 usually make the ride harsher and the car handling better. Further increases degrade road holding also.

The generator 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 an 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. 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. 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.

In fuel economy competition and in gasoline/electric hybrids, the engine is shut off when not putting out power.

When running with closed throttle at above idle speed the engine efficiency becomes negative. This is the basis of the standard method of going down steep hills without overheating the brakes by using the lower gears. On the other hand, driving in the neutral gear when neither power nor braking is needed, which is illegal in some US states, can save small amounts of gasoline by removing engine drag. This has become less useful as transmissions have been made with more ratios.

Shutting off the engine at traffic lights costs some gasoline and wear when it is re-started but saves the idle consumption while it is off. Road and Track (See References) does not recommend it. On the other hand Car and Driver found that the automatic shutoff of a gasoline/electric hybrid did make a measurable difference. This technique may cause a net increase in pollution (except carbon dioxide) and can lead to blocking traffic, so it is probably illegal in some places. If the engine runs too little of the time for the generator to keep up with electrical equipment, such as lights and rear window defroster, the battery will gradually run down and eventually not re-start the engine.

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.

[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.

[edit] Fuel economy-boosting actions and technologies


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

[edit] References

<references/>

[edit] See also

[edit] External links

Consumer published articles

Sites and pages commissioned by various governments' institutions

Additional sites and pages with fuel economy tips

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