Fuel efficiency, in its basic sense, is the same as 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 transportation, "fuel efficiency" more commonly refers to the energy efficiency of a particular vehicle model, where its total output (range, or "mileage" [U.S.]) 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 and Canada or "miles per gallon" (mpg) (prevalent in USA, UK, and often in Canada, using their respective gallon measurements) or "kilometers per liter"(kmpl) (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 (CO2 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 effeciently.
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:
|Fuel type||MJ/l||MJ/kg||BTU/Imp gal||BTU/US gal|| Research octane|
|Regular Gasoline / Petrol||34.83||~47||150,100||125,000||Min 91|
|Premium Gasoline / Petrol||~46||Min 95|
|Autogas (LPG) (60% Propane + 40% Butane)||25.5 - 28.7||~51||108 - 110|
|Gasohol (10% ethanol + 90% gasoline)||33.7||~45||145,200||120,900||93/94|
|Diesel||38.60||~48||166,600||138,700||N/A (see cetane)|
|Vegetable oil (using 9.00 kcal/g)||34.32||37.66||147,894||123,143|
|Jet fuel, naphtha||35.5||46.6||153,100||127,500|
|Jet fuel, kerosene||37.60||162,100||135,000|
|Liquefied natural gas||25.3||~55||109,000||90,800|
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 economy is usually expressed in one of two ways:
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 is value of L/100km. For miles per Imperial gallon (4.546 L) the formula is .
In Europe, the two standard measuring cycles for "L/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 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.
In general, cars are far more fuel-efficient in Europe. While Europe has lots of higher efficiency diesel vehicles, gasoline vehicles are also more efficient. The problem in the US is that American car buyers have downplayed the importancy of energy usage and pollution, relative to flexibility and cost of ownership.
An interesting example 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 hp (30 kW) Diesel engine. The Fortwo is produced by Daimler_AG 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) that can consume as little as 3 litres per 100 kilometres (78 miles per US gallon or 94 miles per Imperial gallon). The last VW Lupo was built in July 2005: it was replaced by the VW Fox.
Diesel engines often achieve greater fuel efficiency than petrol (gasoline) engines. Diesel engines have energy efficiency of 45% and petrol engines of 30%. That is one of the reasons why diesels have better fuel efficiency 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 European fuel efficiency 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 the energy efficiency, but diesel fuel also contains approximately 10-20% more energy per unit volume than gasoline which contributes to the reduced fuel consumption for a given power output.
The common distribution of a flame under normal gravity conditions depends on convection, because soot tends to rise to the top of a flame, such as in a candle, making the flame yellow. In microgravity or zero gravity, such as an environment in outer space, convection no longer occurs, and the flame becomes spherical, with a tendency to become more blue and more efficient. There are several possible explanations for this difference, of which the most likely one given is that the cause is the hypothesis that the temperature is evenly distributed enough that soot is not formed and complete combustion occurs. Experiments by NASA in microgravity reveal that diffusion flames in microgravity allow more soot to be completely oxidised after they are produced than diffusion flames on Earth, because of a series of mechanisms that behaved differently in microgravity when compared to normal gravity conditions. Premixed flames in microgravity burn at a much slower rate and more efficiently than even a candle on Earth, and last much longer.
Fuel efficiency directly affects emissions causing pollution and potentially leading to climate change by affecting the amount of fuel used. However, it also depends on the fuel source used to drive the vehicle concerned. Cars can, for example, run on a number of fuel types other than gasoline, such as natural gas, LPG or biofuel or electricity which creates various quantities of atmospheric pollution.
A kilogram of petrol, diesel, kerosene and the like in a vehicle leads to approximately 3.15 kg of CO2 emissions, or 2.3 kg/L (19 lb/gal). This figure is only the CO2 emissions of the final fuel product and does not include additional CO2 emissions created during the drilling, pumping, transportation and refining steps required to produce the fuel. Additional measures to reduce overall emission includes improvements to the efficiency of air conditioners, lights and tires.
There is also a growing movement of drivers who practice ways to increase their MPG and save fuel through driving techniques. They are often referred to as hypermilers. Hypermilers have broken records of fuel efficiency, averaging 109 miles per gallon driving a Prius. In non-hybrid vehicles these techniques are also beneficial. Hypermiler Wayne Gerdes can get 59 MPG in a Honda Accord and 30 MPG in an Acura MDX.
Hybrid vehicles can conserve petroleum fuel and therefore be more efficient than conventional vehicles.
The most efficient machines for converting energy to rotary motion are electric motors, as used in electric vehicles. However, electricity is not a primary energy source so the efficiency of the electricity production has also to be taken into account. Currently railway trains can be powered using electricity, delivered through an additional running rail, overhead catenary system or by onboard generators used in diesel-electric locomotives as common on the UK rail network. Pollution produced from centralised generation of electricity is emitted at a distant power station, rather than "on site". Some railways, such as the french SNCF and Swiss federal railways derive most, if not 100% of their current, from hydroelectric or nuclear power stations, therefore atmospheric pollution from their rail networks is very low. This was reflected in a study by AEA Technology between a Eurostar train and airline journeys between London and Paris, which showed the trains on average emitting 10 times less CO2, per passenger, than planes, helped in part by french nuclear generation which, however, creates its own radioactive waste which air flight does not. So only comparing CO2 is misleading. . This can be changed using more renewable sources for electric generation.
In the future hydrogen cars may be commercially available. Powered either through chemical reactions in a fuel cell that create electricity to drive very efficient electrical motors or by directly burning hydrogen in a combustion engine (near identically to a natural gas vehicle, and similarly compatible with both natural gas and gasoline); these vehicles promise to have near zero pollution from the tailpipe (exhaust pipe). Potentially the atmospheric pollution could be minimal, provided the hydrogen is made by electrolysis using electricity from nonpolluting sources such as solar, wind, or hydroelectricity. One advantage of fuel cell vehicles is that they can electrolyze water using their own fuel cells, operating in exactly the same closed-loop fashion as any other rechargeable electric battery.
In any process, it is vitally important to account for all of the energy used throughout, i.e., cradle-to-grave. Thus, in addition to the energy cost of the electricity or hydrogen production, we must also account for transmission and/or storage losses to support large-scale use of such vehicles. For this reason the use of the idea "zero pollution" should be avoided.