Fuel efficiency , is a form of thermal efficiency, meaning the efficiency of a process that converts chemical potential energy contained in a carrier fuel into kinetic energy or work. Overall fuel efficiency may vary per device, which in turn may vary per application, and this spectrum of variance is often illustrated as a continuous energy profile. Non-transportation applications, such as industry, benefit from increased fuel efficiency, especially fossil fuel power plants or industries dealing with combustion, such as ammonia production during the Haber process.
In the context of transport, "fuel efficiency" more commonly refers to the energy efficiency of a particular vehicle model , where its total output (range, or mileage ) is given as a ratio of range units per a unit amount of input fuel (gasoline, diesel, etc.). This ratio is given in common measures such as liters per 100 kilometers (L/100 km) (common in Europe, Canada and Australia) or litres per mil (Norway/Sweden) or miles per gallon (mpg) (prevalent in the USA, UK, and often in Canada, using their respective gallon measurements) or kilometres per litre (km/L) (prevalent in Asian countries such as India and Japan). Though the typical output measure is vehicle range , for certain applications output can also be measured in terms of weight per range units (freight) or individual passenger-range (vehicle range / passenger capacity).
This ratio is based on a car's total properties, including its engine properties, its body drag, weight, and rolling resistance, and as such may vary substantially from the profile of the engine alone. While the thermal efficiency of petroleum engines has improved 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 (i.e. SUVs will get less range per unit fuel than an economy car).
Hybrid vehicle designs use smaller combustion engines as electric generators to produce greater range per unit fuel than directly powering the wheels with an engine would, and (proportionally) less fuel emissions (CO 2 grams) than a conventional (combustion engine) vehicle of similar size and capacity. Energy otherwise wasted in stopping is converted to electricity and stored in batteries which are then used to drive the small electric motors. Torque from these motors is very quickly supplied complementing power from the combustion engine. Fixed cylinder sizes can thus be designed more efficiently.
Energy-efficiency terminology
Energy efficiency is similar to fuel efficiency but the input is usually in units of energy such as British thermal units (BTU), megajoules (MJ), gigajoules (GJ), kilocalories (kcal), or kilowatt-hours (kW·h). The inverse of "energy efficiency" is "energy intensity", or the amount of input energy required for a unit of output such as MJ/passenger-km (of passenger transport), BTU/ton-mile (of freight transport, for long/short/metric tons), GJ/t (for steel production), BTU/(kW·h) (for electricity generation), or litres/100 km (of vehicle travel). Litres per 100 km is also a measure of "energy intensity" where the input is measured by the amount of fuel and the output is measured by the distance travelled. For example: Fuel economy in automobiles.
Given a heat value of a fuel, it would be trivial to convert from fuel units (such as litres of gasoline) to energy units (such as MJ) and conversely. But there are two problems with comparisons made using energy units:
- There are two different heat values for any hydrogen-containing fuel which can differ by several percent (see below).
- When comparing transportation energy costs, it must be remembered that a kilowatt hour of electric energy may require an amount of fuel with heating value of 2 or 3 kilowatt hours to produce it.
Energy content of fuel
The specific energy content of a fuel is the heat energy obtained when a certain quantity is burned (such as a gallon, litre, kilogram). It is sometimes called the heat of combustion. There exists two different values of specific heat energy for the same batch of fuel. One is the high (or gross) heat of combustion and the other is the low (or net) heat of combustion. The high value is obtained when, after the combustion, the water in the exhaust is in liquid form. For the low value, the exhaust has all the water in vapor form (steam). Since water vapor gives up heat energy when it changes from vapor to liquid, the high value is larger since it includes the latent heat of vaporization of water. The difference between the high and low values is significant, about 8 or 9%. This accounts for most of the apparent discrepancy in the heat value of gasoline. In the U.S. (and the table below) the high heat values have traditionally been used, but in many other countries, the low heat values are commonly used.
Neither the gross heat of combustion nor the net heat of combustion gives the theoretical amount of mechanical energy (work) that can be obtained from the reaction. (This is given by the change in Gibbs free energy, and is around 45.7 MJ/kg for gasoline.) The actual amount of mechanical work obtained from fuel (the inverse of the specific fuel consumption) depends on the engine. A figure of 17.6 MJ/kg is possible with a gasoline engine, and 19.1 MJ/kg for a diesel engine. See specific fuel consumption for more information.
Fuel efficiency of vehicles
Main article: Fuel economy in automobilesThe fuel efficiency of vehicles is usually expressed in one of two ways:
- Fuel consumption is 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); this is the measure generally used across Europe.
- Fuel economy is the distance travelled per unit volume of fuel used; for example, kilometres per litre (km/L) or miles per gallon (MPG) , where 1 MPG (imperial) = 0.354013 km/l. 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). This measure is popular in the USA and the UK (mpg), India and Latin America (km/L).
Converting from mpg or 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.
The formula for converting to miles per US gallon (3.785 L) from L/100 km is
, where x is value of L/100 km. For miles per Imperial gallon (4.546 L) the formula is
.
In Europe, the two standard measuring cycles for "litre/100 km" value are "urban" traffic with speeds up to 50 km/h from a cold start, and then "extra urban" travel at various speeds up to 120 km/h which follows the urban test. A combined figure is also quoted showing the total fuel consumed in divided by the total distance traveled in both tests. A reasonably modern European supermini and many mid-size cars, including station wagons, may manage motorway travel at 5 L/100 km (47 mpg US/56 mpg imp) or 6.5 L/100 km in city traffic (36 mpg US/43 mpg imp), with carbon dioxide emissions of around 140 g/km.
An average North American mid-size car travels 27 mpg (US) (9 L/100 km) highway, 21 mpg (US) (11 L/100 km) city; a full-size SUV usually travels 13 mpg (US) (18 L/100 km) city and 16 mpg (US) (15 L/100 km) highway. Pickup trucks vary considerably; whereas a 4 cylinder-engined light pickup can achieve 28 mpg (8 L/100 km), a V8 full-size pickup with extended cabin only travels 13 mpg (US) (18 L/100 km) city and 15 mpg (US) (15 L/100 km) highway.
European-built cars are generally more fuel-efficient than American vehicles. While Europe has lots of higher efficiency diesel cars, gasoline vehicles are also more efficient. Most European vehicles cited in the CSI study run on diesel engines, which tend to achieve greater fuel efficiency than gas engines. Selling those cars in the United States is difficult because of emission standards, notes Walter McManus, a fuel economy expert at the University of Michigan Transportation Research Institute. “For the most part, European diesels don’t meet U.S. emission standards,” McManus said. Another reason why many European models are not marketed in the United States is that labor unions object.
An interesting example of European cars' capabilities of fuel economy is the microcar Smart Fortwo cdi, which can achieve up to 3.4 l/100 km (69.2 mpg US) using a turbocharged three-cylinder 41 bhp (30 kW) Diesel engine. The Fortwo is produced by Daimler AG and is currently only sold by one company in the United States. Furthermore, the current (and to date already 10 year old) world record in fuel economy of production cars is held by the Volkswagen Group, with special production models (labeled "3L") of the Volkswagen Lupo and the Audi A2, consuming (NEDC ratified) as little as 2.99 litres of diesel fuel per 100 kilometr
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