Fuel efficiency sometimes means the same as thermal efficiency, that is, the efficiency of converting energy contained in a carrier fuel to kinetic energy or work.
Fuel efficiency can also mean the output one gets for a unit amount of fuel input such as “miles per gallon” or “liters per 100 kilometres” (l/100 km) for an automobile (sometimes called fuel economy). Here, vehicle-miles is the output, but for transportation, output can also be measured in terms of passenger-miles or ton-miles (of freight).
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. Nowadays, a hybrid vehicle is more fuel efficient: that is, consumes less fuel (and produces less carbon dioxide grams) than a conventional vehicle with the same engine.
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.
When comparing fuel consumption, it should be borne in mind that the use of different kinds of fuels has different consequences in terms of air pollution, greenhouse gas emission, and depletion of resources. When considering electric power produced from nuclear power, there are nuclear wastes produced as well. One cannot automatically say that a form of transportation having a lower fuel consumption than another is necessarily “better”.
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). This last term “litres per 100 km” is also a measure of “fuel economy” 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 liters 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. Which one do we use for converting fuel to energy?
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 that is obtained by burning a specific quantity of it (like a gallon, litre, kilogram, etc.). 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. 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. 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.
Fuel economy
Fuel economy in automobiles 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.
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.
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