Fuel efficiency

Fuel efficiency is a form of thermal efficiency, meaning the ratio from effort to result 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 fuel efficiency, especially fossil fuel power plants or industries dealing with combustion, such as ammonia production during the Haber process. Fuel efficiency is a form of thermal efficiency, meaning the ratio from effort to result 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 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 economy is the energy efficiency of a particular vehicle, given as a ratio of distance traveled per unit of fuel consumed. It is dependent on engine efficiency, transmission design, and tire design. Fuel economy is expressed in miles per gallon (mpg) in the USA and usually also in the UK (imperial gallon); there is sometimes confusion as the imperial gallon is 20% larger than the US gallon so that mpg values are not directly comparable. In countries using the metric system fuel economy is stated as 'fuel consumption' in liters per 100 kilometers (L/100 km). Litres per mil are used in Norway and Sweden. Fuel consumption is a more accurate measure of a vehicle’s performance because it is a linear relationship while fuel economy leads to distortions in efficiency improvements. Weight-specific efficiency (efficiency per unit weight) may be stated for freight, and passenger-specific efficiency (vehicle efficiency per passenger). Fuel efficiency is dependent on many parameters of a vehicle, including its engine parameters, aerodynamic drag, weight, AC usage, fuel and rolling resistance. There have been advances in all areas of vehicle design in recent decades. Fuel efficiency of vehicles can also be improved by careful maintenance and driving habits. Hybrid vehicles use two or more power sources for propulsion. In many designs, a small combustion engine is combined with electric motors. Kinetic energy which would otherwise be lost to heat during braking is recaptured as electrical power to improve fuel efficiency. Engines automatically shut off when vehicles come to a stop and start again when the accelerator is pressed preventing wasted energy from idling.