Cars - Fuel economy in automobiles

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	Cars - Fuel economy in automobiles

Fuel economy is usually expressed in one of two ways:

#As 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 efficient a vehicle is; #As 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 efficient a vehicle is. It is important to specify whether US gallons or Imperial gallons are referred to, as they are substantially different.

They are not equivalent, however. 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 = 282.481053 litres per 100 km and 1 litre per 100 km = 282.481053 miles per Imperial gallon

The Englishman obtains the following results. E1 has a fuel consumption of 94.2 mpg and E1 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 litres per 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.

The confusion arises because converting from miles per gallon to litres per 100 km, or vice versa, involves the use of the reciprocal function, which is not distributive.

Measurement cycles

Common, 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 subcompact car may manage motorway travel at 5 litres per 100 kilometres (47 mpg US) or 6.5 litres in city traffic (36 mpg US), with app. 140 grams of carbon dioxide emission per kilometre.

An average "car-shaped" US car produces circa 27 mpg (US) (9 L/100 km) highway, 21 mpg (US) (11 L/100 km) city; a large SUV usually gets 13 mpg (US) (18 L/100 km) city, 16 mpg (US) (15 L/100 km) highway. Pickup trucks vary considerably; whereas a light US pickup with a 4 cylinder engine produces circa 28 mpg (8 L/100 km), a full-size US pickup with extended cab with an 8 cylinder engine produces circa 13 mpg (US) (18 L/100 km) city, 15 mpg (US) (15 L/100 km) highway. An interesting example is the popular Smart ForTwo
car which can achieve up to 4.0 L/100 km (70.6 mpg) using a three-cylinder engine with turbocharger. The Smart is produced by DaimlerChrysler and is not sold in the US.

Diesel engines often produce greater fuel efficiency than petrol (gasoline) engines: 50% of all cars sold in the EU are now diesel vehicles.

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 cars are also being introduced in the 2006 model year (e.g., 2006 GM Chevrolet Impala).

The driving interval tests described here test emissions and fuel economy, but certainly not 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 dynamometer in a laboratory environment. As emissions standards have become more strict due to smog, some 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.

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