Engine efficiency of thermal engines is the relationship between the total
energyIn physics, energy is a scalar physical quantity that describes the amount of work that can be performed by a force, an attribute of objects and systems that is subject to a conservation law...
contained in the
fuelFuel is any material that is burned or altered to obtain energy and to heat or to move object. Fuel releases its energy either through a chemical reaction means, such as combustion, or nuclear means, such as nuclear fission or nuclear fusion...
, and the amount of energy used to perform useful work. There are two classifications of thermal engines-
- Internal combustion (gasoline, diesel
The Diesel cycle is the thermodynamic cycle which approximates the pressure and volume of the combustion chamber of the Diesel engine, invented by Rudolph Diesel in 1897. It is assumed to have constant pressure during the first part of the "combustion" phase , v2 to v3 in the diagram...
and gas turbineA gas turbine, also called a combustion turbine, is a rotary engine that extracts energy from a flow of combustion gas. It has an upstream compressor coupled to a downstream turbine, and a combustion chamber in-between. Energy is added to the gas stream in the combustor, where air is mixed with...
, ie., Brayton cycleThe Brayton cycle is a thermodynamic cycle that describes the workings of the gas turbine engine, basis of the jet engine and others. It is named after George Brayton , the American engineer who developed it, although it was originally proposed and patented by Englishman John Barber in 1791. It is...
engines) and
- External combustion engines (steam piston
A steam engine is a heat engine that performs mechanical work using steam as its working fluid.The idea of using boiling water to produce mechanical motion has a long history, going back about 2000 years...
, steam turbineA steam turbine is a mechanical device that extracts thermal energy from pressurized steam, and converts it into rotary motion. Its modern manifestation was invented by Sir Charles Parsons in 1884....
, and the Stirling cycleThe Stirling cycle is a thermodynamic cycle that describes the general class of Stirling devices. This includes the original Stirling engine that was invented, developed and patented in 1816 by Reverend Dr...
engine).
Each of these engines has
thermal efficiencyIn thermodynamics, the thermal efficiency is a dimensionless performance measure of a thermal device such as an internal combustion engine, a boiler, or a furnace, for example. The input, , to the device is heat, or the heat-content of a fuel that is consumed. The desired output is mechanical...
characteristics that are unique to it.
Gasoline (petrol) Engines
Modern
gasolineGasoline or petrol is a petroleum-derived liquid mixture, primarily used as fuel in internal combustion engines...
engines have an average efficiency of about 25 to 30% when used to power a car. In other words, of the total heat energy of
gasolineGasoline or petrol is a petroleum-derived liquid mixture, primarily used as fuel in internal combustion engines...
, 70 to 75% is ejected (as heat) in the exhaust or consumed by the motor (friction, air turbulence, heat through the cylinder walls or cylinder head, and work used to turn engine equipment and appliances such as
water and oil pumpsA circulator pump is a specific type of pump used to circulate gases, liquids, or slurries in a closed circuit. They are commonly found circulating water in a hydronic heating or cooling system...
and electrical
generatorAn alternator is an electromechanical device that converts mechanical energy to alternating current electrical energy. Most alternators use a rotating magnetic field but linear alternators are occasionally used...
), and only about 25% of energy moves the vehicle. At idle the efficiency is zero since no usable work is being drawn from the engine. At slow speed (i.e. low power output) the efficiency is much lower than average, due to a larger percentage of the available heat being absorbed by the metal parts of the engine, instead of being used to perform useful work. Gasoline engines also suffer efficiency losses at low speeds from the high turbulence and head loss when the incoming air must fight its way around the nearly-closed throttle; diesel engines do not suffer this loss because the incoming air is not throttled. Engine efficiency improves considerably at open road speeds; it peaks in most applications at around 75% of rated engine power, which is also the range of greatest engine torque (e.g. in the 2007
Ford FocusFord Focus can refer to two models of a car manufactured by the Ford Motor Company:* Ford Focus * Ford Focus , the model sold in the United States and Canada....
, maximum torque of 133 foot-pounds is obtained at 4,500 RPM, and maximum engine power of is obtained at 6,000 RPM).
Diesel Engines
Engines using the Diesel cycle are usually more efficient, although the Diesel cycle itself is less efficient at equal compression ratios. Since diesel engines use much higher compression ratios (the heat of compression is used to ignite the slow-burning diesel fuel), that higher ratio more than compensates for the lower intrinsic cycle efficiency, and allows the diesel engine to be more efficient. The most efficient type, direct injection Diesels, are able to reach an efficiency of about 40% in the engine speed range of idle to about 1,800 rpm. Beyond this speed, efficiency begins to decline due to air pumping losses within the engine.
Compression Ratio
The efficiency depends on several factors, one of which is the
compression ratioThe compression ratio of an internal-combustion engine or external combustion engine is a value that represents the ratio of the volume of its combustion chamber; from its largest capacity to its smallest capacity...
. Most gasoline engines have a ratio of 10:1 (
premium fuelThe octane rating is a measure of the resistance of gasoline and other fuels to detonation in spark-ignition internal combustion engines. High-performance engines typically have higher compression ratios and are therefore more prone to detonation, so they require higher octane fuel...
) or 8:1 (regular fuel), with some high performance engines reaching a ratio of 12:1 with special fuels. The greater the ratio the more efficient is the machine. Higher ratio engines need gasoline with higher
octaneThe octane rating is a measure of the resistance of gasoline and other fuels to detonation in spark-ignition internal combustion engines. High-performance engines typically have higher compression ratios and are therefore more prone to detonation, so they require higher octane fuel...
value, which inhibits the fuel's tendency to burn nearly instantaneously (known as
detonation or knockKnocking in spark-ignition internal combustion engines occurs when combustion of the air/fuel mixture in the cylinder starts off correctly in response to ignition by the spark plug, but one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front...
) at high compression/high heat conditions.
It should be noted that at lower power outputs, the effective compression ratio is less than when the engine is operating at full power, due to the simple fact that the incoming fuel-air mixture is being restricted. Thus the effective engine efficiency will be less than when the engine is producing its maximum rated power. One solution to this fact is to shift the load in a multi-cylinder engine from some of the cylinders (by deactivating them) to the remaining cylinders so that they may operate under higher individual loads and with correspondingly higher effective compression ratios. This technique is known as
variable displacementVariable displacement is an automobile engine technology that allows the engine displacement to change, by deactivating cylinders, for improved fuel economy. The technology is primarily used in large, multi-cylinder engines...
.
Diesel engines have a compression ratio between 14:1 to 25:1. In this case the general rule does not apply because Diesels with compression ratios over 20:1 are
indirect injection dieselsA diesel engine is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber during the final stage of compression...
. These use a prechamber to make possible high RPM operation as is required in automobiles and light trucks. The thermal and gas dynamic losses from the prechamber result in direct injection Diesels (despite their lower compression ratio) being more efficient. An engine has many parts that produce
frictionFriction is the force resisting the relative lateral motion of solid surfaces, fluid layers, or material elements in contact. It is usually subdivided into several varieties:...
. Some of these friction forces remain constant (as long as applied load is constant); some of these friction losses increase as engine speed increases, such as piston side forces and connecting bearing forces (due to increased inertia forces from the oscillating piston). A few friction forces decrease at higher speed, such as the friction force on the
camA camshaft is a shaft to which a cam is fastened or of which a cam forms an integral part.-History:An early cam featuring a camshaft was built into Hellenistic water-driven automata from the 3rd century BC. The camshaft was later described in Iraq by Al-Jazari in 1206...
's lobes used to operate the inlet and outlet valves (the valves'
inertiaInertia is the resistance of any physical object, to a change in its state of motion. It is represented numerically by an object's mass. The principle of inertia is one of the fundamental principles of classical physics which are used to describe the motion of matter and how it is affected by...
at high speed tends to pull the cam follower away from the cam lobe). Along with friction forces, an operating engine has
pumping losses, which is the work required to move air into and out of the cylinders. This pumping loss is minimal at low speed, but increases approximately as the square of the speed, until at rated power an engine is using about 20% of total power production to overcome friction and pumping losses.
A gasoline motor burns a mix of gasoline and air, consisting of a range of about twelve to eighteen parts (by weight) of air to one part of fuel (by weight). A mixture with a 14.7:1 air/fuel ratio is said to be stoichiometric, that is when burned, 100% of the
fuelFuel is any material that is burned or altered to obtain energy and to heat or to move object. Fuel releases its energy either through a chemical reaction means, such as combustion, or nuclear means, such as nuclear fission or nuclear fusion...
and the
oxygenOxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
are consumed. Mixtures with slightly less fuel, called
lean burnLean burn refers to the use of lean mixtures in an internal combustion engine. The air-fuel ratios can be as high as 65:1, so the mixture has considerably less fuel in comparison to the stoichiometric combustion ratio ....
are more efficient, whilst slightly rich mixtures, with lower air fuel ratios produce more power at the expense of higher fuel consumption. The
combustionCombustion or burning is a complex sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat or both heat and light in the form of either a glow or flames, appearance of light flickering.Direct combustion by atmospheric oxygen is a reaction...
is a reaction which uses the air's
oxygenOxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
content to combine with the fuel, which is a mixture of several
hydrocarbonIn organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. With relation to chemical terminology, aromatic hydrocarbons or arenes, alkanes, alkenes and alkyne-based compounds composed entirely of carbon and hydrogen are referred to as "pure"...
s, resulting in
water vaporWater vapor or water vapour , also aqueous vapor, is the gas phase of water. Water vapor is one state of the water cycle within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice...
,
carbon dioxideCarbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state...
, and sometimes
carbon monoxideCarbon monoxide is a colorless, odorless and tasteless gas, yet very toxic to humans. It consists of one carbon atom and one oxygen atom, connected by a covalent double bond and a dative covalent bond...
and partially-burned hydrocarbons. In addition, at high temperatures the air's oxygen tends to combine with the air's
nitrogenNitrogen is a chemical element that has the symbol N and atomic number 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless and mostly inert diatomic gas at standard conditions, constituting 78% by volume of Earth's atmosphere.Many industrially important...
, forming
oxides of nitrogenThe term nitrogen oxide typically refers to any binary compound of oxygen and nitrogen, or to a mixture of such compounds:* Nitric oxide , nitrogen oxide* Nitrogen dioxide , nitrogen oxide...
(usually referred to as
NOx, since the number of oxygen atoms in the compound can vary, thus the "X" subscript). This mixture, along with the unused nitrogen and
other trace atmospheric elementsAtmospheric chemistry is a branch of atmospheric science in which the chemistry of the Earth's atmosphere and that of other planets is studied. It is a multidisciplinary field of research and draws on environmental chemistry, physics, meteorology, computer modeling, oceanography, geology and...
, is what we see in the
exhaustAn exhaust system is usually tubing used to guide reaction exhaust gases away from a controlled combustion inside an engine or stove. The entire system conveys burnt gases from the engine and includes one or more exhaust pipes...
.
Oxygen
The
airThe Earth's atmosphere is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...
is approximately 21%
oxygenOxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
; if there is not enough
oxygenOxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
for proper combustion, the fuel will not burn completely and will produce less energy. An excessive rich air fuel ratio will cause an increase of pollutants from the engine. The fuel burns in three stages. First, the hydrogen burns to form water vapour. Second, the carbon burns to carbon monoxide. Lastly, the carbon monoxide burns to carbon dioxide. This last stage produces most of the power of the engine. If all of the
oxygenOxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
is consumed before this stage because there is too much fuel, engine's power is reduced.
There are a few exceptions where introducing fuel upstream of the combustion chamber can cool down the incoming air through evaporative cooling. The extra fuel that is not burned in the combustion chamber cools down the intake air resulting in more power. With direct injection this effect is not as dramatic but it can cool down the combustion chamber enough to reduce certain
pollutantsAir pollution is the introduction of chemicals, particulate matter, or biological materials that cause harm or discomfort to humans or other living organisms, or damages the natural environment, into the atmosphere....
such as nitrous oxides, while raising others such as partially-decomposed hydrocarbons.
The air-fuel mix is drawn into an engine because downward motion of the pistons induces a partial vacuum. A
compressorCompressor may refer to:*Gas compressor, a mechanical device that compresses a gas *A device used to apply Video compression to a video signal*A device used to apply Audio data compression to an audio signal...
can be used to force a larger charge into the cylinder to produce more power. In practice this is achieved either by belt driven supercharging or exhaust driven turbocharging. Also, two-stroke diesel engines have forced induction, where a supercharger moves air into the engine or the crankcase so that the cylinder will be filled with air as soon as the inlet port is uncovered.
There are other methods to increase the amount of oxygen available inside the engine; one of them, is to inject nitrous oxide, (
nitrousNitrous oxide is a chemical compound used as an oxidizing agent to increase an internal combustion engine's power output by allowing more fuel to be burned than would normally be the case.-Nitrous and NOS:...
) to the mixture, and some special engines use
nitromethaneNitromethane is an organic compound with the chemical formula CH3NO2. It is the simplest organic nitro compound. It is a slightly viscous, highly polar liquid commonly used as a solvent in a variety of industrial applications such as in extractions, as a reaction medium, and...
, a fuel that provides the oxygen itself it needs to burn. Because of that, the mixture could be 1 part of fuel and 3 parts of air; thus, it is possible to burn more fuel inside the engine, and get higher power outputs.
Steam
Piston steam engines are relatively inefficient (about 8% overall efficiency) which is why there are no longer any
steam locomotiveA steam locomotive is a locomotive powered by steam. The term usually refers to its use on railways, but can also refer to a "road locomotive" such as a traction engine or steamroller....
s in commercial use. Large output steam turbines equal or exceed the efficiency of the Diesel, which is one reason they are used for electric utility
generatingElectricity generation is the process of creating electricity from other forms of energy.The fundamental principles of electricity generation were discovered during the 1820s and early 1830s by the British scientist Michael Faraday...
plants (the other reason is the greatly reduced maintenance requirement). The Stirling cycle engine has the highest efficiency of any thermal engine but it is more expensive to make and is not competitive with other types for normal commercial use.
The gas turbine is most efficient at maximum power output. Efficiency declines steadily with reduced power output and is very poor in the low power range. This is one reason, among several, why the gas turbine is not used for automobiles and trucks where much of the operating cycle is at idle and low to intermediate speeds. Detroit (
General MotorsGeneral Motors Company, often known as simply GM, is a United States based automaker with headquarters in Detroit, Michigan. GM was the world's 18th largest corporate entity and third largest automaker as ranked by 2008 revenues on the Fortune Global 500. Ranked by global unit sales for 2008, it...
) at one time tried to make a gas turbine for an automobile and gave up. This is also why gas turbines can be used for peak power electric plants. In this application they are only run at full power where they are efficient or shut down when not needed.
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