Engine efficiency
Encyclopedia
Engine efficiency of thermal engines is the relationship between the total energy
contained in the fuel
, and the amount of energy used to perform useful work. There are two classifications of thermal engines-
Each of these engines has thermal efficiency
characteristics that are unique to it.
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 gasoline
, about 70-75% is ejected as heat from the exhaust, as mechanical sound energy, 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 pumps and electrical generator
), and only about 25-30% of the fuel 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 throttle 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 Focus, maximum torque of 133 foot-pounds is obtained at 4,500 RPM, and maximum engine power of 136 bhp is obtained at 6,000 RPM).
Therefore, in the past 3-4 years, GDI (Gasoline Direct Injection) increased the efficiency of the engines equipped with this fueling system up to 35%. Currently the technology is available in a wide variety of vehicles ranging from affordable cars such as Mazda, Ford and Chevrolet to expensive cars such as BMW, Mercedes-Benz, VAG.
Modern turbo-diesel engines are using common-rail fuel injection, electronically controlled, that increases the efficiency up to 50% and with the help of geometrically variable of the turbo-charging system, also increases the engines' torque in low revs (1200-1800rpm)
. Most gasoline engines have a ratio of 10:1 (premium fuel
) or 9:1 (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 octane
value, which inhibits the fuel's tendency to burn nearly instantaneously (known as detonation or knock
) at high compression/high heat conditions.
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 displacement
.
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 diesels
. 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 friction
. 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 cam
's lobes used to operate the inlet and outlet valves (the valves' inertia
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 fuel
and the oxygen
are consumed. Mixtures with slightly less fuel, called lean burn
are more efficient, whilst slightly rich mixtures, with lower air fuel ratios produce more power at the expense of higher fuel consumption. The combustion
is a reaction which uses the air's oxygen
content to combine with the fuel, which is a mixture of several hydrocarbon
s, resulting in water vapor
, carbon dioxide
, and sometimes carbon monoxide
and partially-burned hydrocarbons. In addition, at high temperatures the air's oxygen tends to combine with the air's nitrogen
, forming oxides of nitrogen
(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 elements
, is what we see in the exhaust
.
is approximately 21% oxygen
. If there is not enough oxygen
for proper combustion, the fuel will not burn completely and will produce less energy. An excessively rich air fuel ratio will increase 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. Finally, the carbon monoxide burns to carbon dioxide. This last stage produces most of the power of the engine. If all of the oxygen
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 pollutants
such as nitrogen oxides (NOx
), 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 compressor
can additionally be used to force a larger charge (forced induction) into the cylinder to produce more power. The compressor
is either mechanically driven supercharging or exhaust driven turbocharging. Either way, forced induction increases the air pressure exterior to the cylinder inlet port.
There are other methods to increase the amount of oxygen available inside the engine; one of them, is to inject nitrous oxide
, (N2O) to the mixture, and some engines use nitromethane
, 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 engines and turbines operate on the Rankine cycle
which has a maximum Carnot efficiency of 63% for practical engines.
The efficiency of steam engines is primarily related to the steam temperature and pressure and the number of stages or expansions. Steam engine efficiency improved as the operating principles were discovered, which lead to the development of the science of thermodynamics
. See graph:Steam Engine Efficiency
In earliest steam engines the boiler was considered part of the engine. Today they are considered separate, so it is necessary to know whether stated efficiency is overall, which includes the boiler, or just of the engine.
Comparisons of effiency and power of the early steam engines is difficult for several reasons: 1) there was no standard weight for a bushel of coal, which could be anywehre from 82 to 96 pounds. 2) There was no standard heating value for coal, and probably no way to measure heating value. The coals had much higher heating value than today's steam coals, with 13,500 BTU/pound sometimes mentioned. 3) Efficiency was reported as "duty", meaning how many foot pounds of work lifting water were produced, but the mechanical pumping efficiency is not known.
The first piston steam engine, developed by Thomas Newcomen
, was slightly over one half percent efficient. It operated with steam at near atmospheric pressure with the condensing steam, cooled by a spray of water into the steam filled cylinder, causing the pressure of the atmosphere to drive the piston. Improvements made by John Smeaton
to the Newcomem engine increased the efficiency to over 1%.
The lack of general mechanical ability, including skilled mechanics, machine tool
s, and manufacturing methods, limited the eficiency of acutal engines and their design until about 1840.
James Watt
made several improvements, the most significant of which was the external condenser, which prevented the cooling water from cooling the cylinder. Watt's engine operated with steam at slightly above atmospheric pressure. Watt's improvements increased efficiency by a factor of over 2.5.
Higher pressures engines were developed by Oliver Evans
and independently by Richard Trevithick
. These engines were efficient enough and had high enough power-to-weight ratio to be used for powering locomotives and boats.
The centrifugal governor, which had first been used by Watt to maintain constant speed, worked by throttling the inlet steam, which lowered the pressure, resulting in a loss of efficiency on the high (above atmospheric) pressure engines. Later control methods reduced or eliminated this pressure loss.
The improved valving mechanism of the Corliss steam engine
(Ptd. 1849) was better able to adjust speed with varying load and increased efficiency by about 30%. The Corliss engine had separate valves and headers for the inlet and exhaust steam so the hot feed steam never contacted the cooler exhaust pots and valving. Valve timing was controlled by the governor.
Others before Corliss had at least part of this idea, but machining technology was not advanced enough to produce the degree of precision required.
Compound engines gave further improvements in efficiency. By the 1870s triple expansion engines were being used on ships. Compound engines allowed ships to carry less coal than freight. Compound engines were used on some locomotives but were not widely adopted because of their mechanical complexity.
The most efficient reciprocating steam engine design (per stage) was the uniflow engine
, but by the time it appeared steam was being displaced by diesel engines, which were even more efficient and had the advantage of requiring less labor for coal handling and oil being a more dense fuel displaced less cargo.
is the most efficient steam engine and for this reason is universally used for electrical generation. Steam expansion in a turbine is nearly continuous, which makes a turbine comparable to a very large number of expansion stages. Steam fossil fuel power stations operating at the critical point
have efficiencies in the low 40% range. Turbines produce direct rotary motion and are far more compact and weigh far less than reciprocating engines and can be controlled to within a very constant speed.
and Air-Hybrid Engine with its split-cycle design, thermodynamic redesign to fire after top-dead centre and air hybrid modes of operation could increase fuel efficiency up to 50 percent and reduce NOx emissions by 80 percent. In January 2011, preliminary tests from the Southwest Research Institute indicated that the engine consumed up to 36 percent less fuel through simulations on a 2004 Chevy Cavalier. Currently, the Southwest Research Institute is conducting additional simulations on a 2011 Nissan Sentra.
is most efficient at maximum power output in the same way reciprocating engines are most efficient at maximum load. The difference is that at lower rotational speed the pressure of the compressed air drops and thus thermal and fuel efficiency drop dramatically. Efficiency declines steadily with reduced power output and is very poor in the low power range - the same is true in reciprocating engines, the friction losses at 3000 RPM are almost the same whether the engine is under 100% load or not having any useful output on the driveshaft. The inertia of high speed gas turbine together with the low air pressure under low speed cause it to have a significant lag which many drivers are unwilling to cope with. Today the gas turbine is not used for automobiles and trucks because the usage patterns dictate varying loads, including idling speeds. General Motors
at one time manufactured a bus powered by a gas turbine, but due to the economy where crude oil prices rose exponentially (1970's) this concept was abandoned. Driving comfort was good, but overall economy lacked due to reasons mentioned above. This is also why gas turbines can be used for permanent and peak power electric plants. In this application they are only run at or close to full power where they are efficient or shut down when not needed.
Gas turbines do have advantage in power density - gas turbines are used as the engines in heavy armored vehicles and armored tanks and in power generators in jet fighters.
One other factor negatively affecting the gas turbine efficiency is the ambient air temperature. With increasing temperature, intake air becomes less dense and therefore the gas turbine experiences power loss proportional to increase in ambient air temperature.
where,
is the heat absorbed and 
is the work done.
Please note that the term work done relates to the power delivered at the clutch or at the driveshaft.
This means the friction and other losses are subtracted from the work done by thermodynamic expansion. Thus engine not delivering any work to the outside environment has zero efficiency. Engine delivering less than maximum power at its current speed will have less than the maximum efficiency at that speed as the internal losses do not change with load, but with engine speed. The reason to this is that any engine needs power to run itself. In order to not consume fuel engine needs to be stopped.
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
contained in the fuel
Fuel
Fuel is any material that stores energy that can later be extracted to perform mechanical work in a controlled manner. Most fuels used by humans undergo combustion, a redox reaction in which a combustible substance releases energy after it ignites and reacts with the oxygen in the air...
, and the amount of energy used to perform useful work. There are two classifications of thermal engines-
- Internal combustion (gasolineOtto cycleAn Otto cycle is an idealized thermodynamic cycle which describes the functioning of a typical reciprocating piston engine, the thermodynamic cycle most commonly found in automobile engines....
, dieselDiesel cycleThe 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...
and gas turbineGas turbineA gas turbine, also called a combustion turbine, is a type of internal combustion engine. It has an upstream rotating compressor coupled to a downstream turbine, and a combustion chamber in-between....
, i.e., Brayton cycleBrayton cycleThe Brayton cycle is a thermodynamic cycle that describes the workings of the gas turbine engine, basis of the airbreathing 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...
engines) and - External combustion engines (steam pistonSteam engineA steam engine is a heat engine that performs mechanical work using steam as its working fluid.Steam engines are external combustion engines, where the working fluid is separate from the combustion products. Non-combustion heat sources such as solar power, nuclear power or geothermal energy may be...
, steam turbineSteam 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 cycleStirling 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 efficiency
Thermal efficiency
In thermodynamics, the thermal efficiency is a dimensionless performance measure of a device that uses thermal energy, such as an internal combustion engine, a boiler, a furnace, or a refrigerator for example.-Overview:...
characteristics that are unique to it.
Gasoline (petrol) Engines
Modern gasolineGasoline
Gasoline , or petrol , is a toxic, translucent, petroleum-derived liquid that is primarily used as a fuel in internal combustion engines. It consists mostly of organic compounds obtained by the fractional distillation of petroleum, enhanced with a variety of additives. Some gasolines also contain...
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 gasoline
Gasoline
Gasoline , or petrol , is a toxic, translucent, petroleum-derived liquid that is primarily used as a fuel in internal combustion engines. It consists mostly of organic compounds obtained by the fractional distillation of petroleum, enhanced with a variety of additives. Some gasolines also contain...
, about 70-75% is ejected as heat from the exhaust, as mechanical sound energy, 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 pumps and electrical generator
Alternator
An alternator is an electromechanical device that converts mechanical energy to electrical energy in the form of alternating current.Most alternators use a rotating magnetic field but linear alternators are occasionally used...
), and only about 25-30% of the fuel 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 throttle 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 Focus, maximum torque of 133 foot-pounds is obtained at 4,500 RPM, and maximum engine power of 136 bhp is obtained at 6,000 RPM).
Therefore, in the past 3-4 years, GDI (Gasoline Direct Injection) increased the efficiency of the engines equipped with this fueling system up to 35%. Currently the technology is available in a wide variety of vehicles ranging from affordable cars such as Mazda, Ford and Chevrolet to expensive cars such as BMW, Mercedes-Benz, VAG.
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.Modern turbo-diesel engines are using common-rail fuel injection, electronically controlled, that increases the efficiency up to 50% and with the help of geometrically variable of the turbo-charging system, also increases the engines' torque in low revs (1200-1800rpm)
Compression ratio
The efficiency of internal combustion engines depends on several factors, one of which is the compression ratioCompression ratio
The '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 fuel
Octane rating
Octane rating or octane number is a standard measure of the anti-knock properties of a motor or aviation fuel. The higher the octane number, the more compression the fuel can withstand before detonating...
) or 9:1 (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 octane
Octane rating
Octane rating or octane number is a standard measure of the anti-knock properties of a motor or aviation fuel. The higher the octane number, the more compression the fuel can withstand before detonating...
value, which inhibits the fuel's tendency to burn nearly instantaneously (known as detonation or knock
Engine knocking
Knocking 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.The...
) at high compression/high heat conditions.
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 displacement
Variable displacement
Variable displacement is an automobile engine technology that allows the engine displacement to change, usually 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 diesels
Diesel engine
A 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...
. 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 friction
Friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other. There are several types of friction:...
. 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 cam
Camshaft
A camshaft is a shaft to which a cam is fastened or of which a cam forms an integral part.-History:An early cam was built into Hellenistic water-driven automata from the 3rd century BC. The camshaft was later described in Iraq by Al-Jazari in 1206. He employed it as part of his automata,...
's lobes used to operate the inlet and outlet valves (the valves' inertia
Inertia
Inertia is the resistance of any physical object to a change in its state of motion or rest, or the tendency of an object to resist any change in its motion. It is proportional to an object's mass. The principle of inertia is one of the fundamental principles of classical physics which are used to...
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 fuel
Fuel
Fuel is any material that stores energy that can later be extracted to perform mechanical work in a controlled manner. Most fuels used by humans undergo combustion, a redox reaction in which a combustible substance releases energy after it ignites and reacts with the oxygen in the air...
and the oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
are consumed. Mixtures with slightly less fuel, called lean burn
Lean burn
Lean 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 combustion
Combustion
Combustion or burning is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species. The release of heat can result in the production of light in the form of either glowing or a flame...
is a reaction which uses the air's oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
content to combine with the fuel, which is a mixture of several hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
s, resulting in water vapor
Water vapor
Water vapor or water vapour , also aqueous vapor, is the gas phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Under typical atmospheric conditions, water vapor is continuously...
, carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
, and sometimes carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
and partially-burned hydrocarbons. In addition, at high temperatures the air's oxygen tends to combine with the air's nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
, forming oxides of nitrogen
Nitrogen oxide
Nitrogen oxide can refer to a binary compound of oxygen and nitrogen, or a mixture of such compounds:* Nitric oxide, also known as nitrogen monoxide, , 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 elements
Atmospheric chemistry
Atmospheric 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 exhaust
Exhaust system
An 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
AirEarth's atmosphere
The atmosphere of Earth 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% oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
. If there is not enough oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
for proper combustion, the fuel will not burn completely and will produce less energy. An excessively rich air fuel ratio will increase 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. Finally, the carbon monoxide burns to carbon dioxide. This last stage produces most of the power of the engine. If all of the oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
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 pollutants
Air pollution
Air pollution is the introduction of chemicals, particulate matter, or biological materials that cause harm or discomfort to humans or other living organisms, or cause damage to the natural environment or built environment, into the atmosphere....
such as nitrogen oxides (NOx
NOx
NOx is a generic term for the mono-nitrogen oxides NO and NO2 . They are produced from the reaction of nitrogen and oxygen gases in the air during combustion, especially at high temperatures...
), 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 compressor
Gas compressor
A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume.Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas...
can additionally be used to force a larger charge (forced induction) into the cylinder to produce more power. The compressor
Gas compressor
A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume.Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas...
is either mechanically driven supercharging or exhaust driven turbocharging. Either way, forced induction increases the air pressure exterior to the cylinder inlet port.
There are other methods to increase the amount of oxygen available inside the engine; one of them, is to inject nitrous oxide
Nitrous oxide
Nitrous oxide, commonly known as laughing gas or sweet air, is a chemical compound with the formula . It is an oxide of nitrogen. At room temperature, it is a colorless non-flammable gas, with a slightly sweet odor and taste. It is used in surgery and dentistry for its anesthetic and analgesic...
, (N2O) to the mixture, and some engines use nitromethane
Nitromethane
Nitromethane is an organic compound with the chemical formula . 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 as a cleaning solvent...
, 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 engine
-
- See also: Steam engine#Efficiency
- See also: Timeline of steam powerTimeline of steam powerSteam power developed slowly over a period of several hundred years, progressing through expensive and fairly limited devices in the early 17th century, to useful pumps for mining in 1700, and then to Watt's improved steam engine designs in the late 18th century...
Steam engines and turbines operate on the Rankine cycle
Rankine cycle
The Rankine cycle is a cycle that converts heat into work. The heat is supplied externally to a closed loop, which usually uses water. This cycle generates about 90% of all electric power used throughout the world, including virtually all solar thermal, biomass, coal and nuclear power plants. It is...
which has a maximum Carnot efficiency of 63% for practical engines.
The efficiency of steam engines is primarily related to the steam temperature and pressure and the number of stages or expansions. Steam engine efficiency improved as the operating principles were discovered, which lead to the development of the science of thermodynamics
Thermodynamics
Thermodynamics is a physical science that studies the effects on material bodies, and on radiation in regions of space, of transfer of heat and of work done on or by the bodies or radiation...
. See graph:Steam Engine Efficiency
In earliest steam engines the boiler was considered part of the engine. Today they are considered separate, so it is necessary to know whether stated efficiency is overall, which includes the boiler, or just of the engine.
Comparisons of effiency and power of the early steam engines is difficult for several reasons: 1) there was no standard weight for a bushel of coal, which could be anywehre from 82 to 96 pounds. 2) There was no standard heating value for coal, and probably no way to measure heating value. The coals had much higher heating value than today's steam coals, with 13,500 BTU/pound sometimes mentioned. 3) Efficiency was reported as "duty", meaning how many foot pounds of work lifting water were produced, but the mechanical pumping efficiency is not known.
The first piston steam engine, developed by Thomas Newcomen
Thomas Newcomen
Thomas Newcomen was an ironmonger by trade and a Baptist lay preacher by calling. He was born in Dartmouth, Devon, England, near a part of the country noted for its tin mines. Flooding was a major problem, limiting the depth at which the mineral could be mined...
, was slightly over one half percent efficient. It operated with steam at near atmospheric pressure with the condensing steam, cooled by a spray of water into the steam filled cylinder, causing the pressure of the atmosphere to drive the piston. Improvements made by John Smeaton
John Smeaton
John Smeaton, FRS, was an English civil engineer responsible for the design of bridges, canals, harbours and lighthouses. He was also a capable mechanical engineer and an eminent physicist...
to the Newcomem engine increased the efficiency to over 1%.
The lack of general mechanical ability, including skilled mechanics, machine tool
Machine tool
A machine tool is a machine, typically powered other than by human muscle , used to make manufactured parts in various ways that include cutting or certain other kinds of deformation...
s, and manufacturing methods, limited the eficiency of acutal engines and their design until about 1840.
James Watt
James Watt
James Watt, FRS, FRSE was a Scottish inventor and mechanical engineer whose improvements to the Newcomen steam engine were fundamental to the changes brought by the Industrial Revolution in both his native Great Britain and the rest of the world.While working as an instrument maker at the...
made several improvements, the most significant of which was the external condenser, which prevented the cooling water from cooling the cylinder. Watt's engine operated with steam at slightly above atmospheric pressure. Watt's improvements increased efficiency by a factor of over 2.5.
Higher pressures engines were developed by Oliver Evans
Oliver Evans
Oliver Evans was an American inventor. Evans was born in Newport, Delaware to a family of Welsh settlers. At the age of 14 he was apprenticed to a wheelwright....
and independently by Richard Trevithick
Richard Trevithick
Richard Trevithick was a British inventor and mining engineer from Cornwall. His most significant success was the high pressure steam engine and he also built the first full-scale working railway steam locomotive...
. These engines were efficient enough and had high enough power-to-weight ratio to be used for powering locomotives and boats.
The centrifugal governor, which had first been used by Watt to maintain constant speed, worked by throttling the inlet steam, which lowered the pressure, resulting in a loss of efficiency on the high (above atmospheric) pressure engines. Later control methods reduced or eliminated this pressure loss.
The improved valving mechanism of the Corliss steam engine
Corliss Steam Engine
A Corliss steam engine is a steam engine, fitted with rotary valves and with variable valve timing patented in 1849, invented by and named after the American engineer George Henry Corliss in Providence, Rhode Island....
(Ptd. 1849) was better able to adjust speed with varying load and increased efficiency by about 30%. The Corliss engine had separate valves and headers for the inlet and exhaust steam so the hot feed steam never contacted the cooler exhaust pots and valving. Valve timing was controlled by the governor.
Others before Corliss had at least part of this idea, but machining technology was not advanced enough to produce the degree of precision required.
Compound engines gave further improvements in efficiency. By the 1870s triple expansion engines were being used on ships. Compound engines allowed ships to carry less coal than freight. Compound engines were used on some locomotives but were not widely adopted because of their mechanical complexity.
The most efficient reciprocating steam engine design (per stage) was the uniflow engine
Uniflow steam engine
The uniflow type of steam engine uses steam that flows in one direction only in each half of the cylinder. Thermal efficiency is increased in the compound and multiple expansion types of steam engine by separating expansion into steps in separate cylinders; in the uniflow design, thermal efficiency...
, but by the time it appeared steam was being displaced by diesel engines, which were even more efficient and had the advantage of requiring less labor for coal handling and oil being a more dense fuel displaced less cargo.
Steam turbine
The steam turbineSteam turbine
A 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....
is the most efficient steam engine and for this reason is universally used for electrical generation. Steam expansion in a turbine is nearly continuous, which makes a turbine comparable to a very large number of expansion stages. Steam fossil fuel power stations operating at the critical point
Critical point (thermodynamics)
In physical chemistry, thermodynamics, chemistry and condensed matter physics, a critical point, also called a critical state, specifies the conditions at which a phase boundary ceases to exist...
have efficiencies in the low 40% range. Turbines produce direct rotary motion and are far more compact and weigh far less than reciprocating engines and can be controlled to within a very constant speed.
Scuderi Engine
The Scuderi EngineScuderi Engine
The Scuderi Engine is a split cycle, internal combustion engine invented by Carmelo J. Scuderi . Scuderi Group, an engineering and licensing company based in West Springfield, Massachusetts and founded by Carmelo Scuderi’s children, is testing a working prototype of the engine that was officially...
and Air-Hybrid Engine with its split-cycle design, thermodynamic redesign to fire after top-dead centre and air hybrid modes of operation could increase fuel efficiency up to 50 percent and reduce NOx emissions by 80 percent. In January 2011, preliminary tests from the Southwest Research Institute indicated that the engine consumed up to 36 percent less fuel through simulations on a 2004 Chevy Cavalier. Currently, the Southwest Research Institute is conducting additional simulations on a 2011 Nissan Sentra.
Stirling engines
The Stirling cycle engine has the highest theoretical efficiency of any thermal engine but it is more expensive to make and is not competitive with other types for normal commercial use.Gas turbine
The gas turbineGas turbine
A gas turbine, also called a combustion turbine, is a type of internal combustion engine. It has an upstream rotating compressor coupled to a downstream turbine, and a combustion chamber in-between....
is most efficient at maximum power output in the same way reciprocating engines are most efficient at maximum load. The difference is that at lower rotational speed the pressure of the compressed air drops and thus thermal and fuel efficiency drop dramatically. Efficiency declines steadily with reduced power output and is very poor in the low power range - the same is true in reciprocating engines, the friction losses at 3000 RPM are almost the same whether the engine is under 100% load or not having any useful output on the driveshaft. The inertia of high speed gas turbine together with the low air pressure under low speed cause it to have a significant lag which many drivers are unwilling to cope with. Today the gas turbine is not used for automobiles and trucks because the usage patterns dictate varying loads, including idling speeds. General Motors
General Motors
General Motors Company , commonly known as GM, formerly incorporated as General Motors Corporation, is an American multinational automotive corporation headquartered in Detroit, Michigan and the world's second-largest automaker in 2010...
at one time manufactured a bus powered by a gas turbine, but due to the economy where crude oil prices rose exponentially (1970's) this concept was abandoned. Driving comfort was good, but overall economy lacked due to reasons mentioned above. This is also why gas turbines can be used for permanent and peak power electric plants. In this application they are only run at or close to full power where they are efficient or shut down when not needed.
Gas turbines do have advantage in power density - gas turbines are used as the engines in heavy armored vehicles and armored tanks and in power generators in jet fighters.
One other factor negatively affecting the gas turbine efficiency is the ambient air temperature. With increasing temperature, intake air becomes less dense and therefore the gas turbine experiences power loss proportional to increase in ambient air temperature.
Mathematical description
The efficiency of engine is defined as ratio of the useful work done to the heat provided.
where,
is the heat absorbed and is the work done.Please note that the term work done relates to the power delivered at the clutch or at the driveshaft.
This means the friction and other losses are subtracted from the work done by thermodynamic expansion. Thus engine not delivering any work to the outside environment has zero efficiency. Engine delivering less than maximum power at its current speed will have less than the maximum efficiency at that speed as the internal losses do not change with load, but with engine speed. The reason to this is that any engine needs power to run itself. In order to not consume fuel engine needs to be stopped.