Engine efficiency
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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- (1) Internal combustion (gasoline, diesel and gas turbine, ie., Brayton cycle engines). (2) External combustion engines (steam piston, steam turbine, and the Stirling cycle engine). Each of these engines has thermal efficiency characteristics that are unique to it.
Modern gasoline engines have an average efficiency of about 25 to 30% when used to power an automobile. In other words, of the total heat energy of gasoline, 70 to 75% is rejected (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 pumps and electrical generator), and only about 25% of energy moves the vehicle. At idle the efficiency is zero by definition, since no useable 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 Focus, maximum torque of 133 foot-pounds is obtained at 4500 RPM, and maximum engine power of 136 brake horsepower (101 kW) is obtained at 6000 RPM).
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.
The efficiency depends on several factors, one of which is the compression ratio. Most gasoline engines have a ratio of 10:1 (premium 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 octane value, which inhibits the fuel's tendency to burn nearly instantaneously (known as detonation or knock) 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 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), with the most efficient mixture being a 14.7:1 air/fuel ratio. The combustion is a reaction which uses the air's oxygen content to combine with the fuel, which is a mixture of several hydrocarbons, 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.
The air has approximately 20% of oxygen; if there is not enough oxygen for combustion, the fuel will not burn and will not produce energy. It is senseless to introduce more fuel to the engine in order to make a richer mix (more fuel) of fuel and air, this will not produce more power, instead it will cause an increase of pollutants from the engine. 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 isn't burned in the combustion chamber cools down the intake air resulting in more power. With direct injection this effect isn't as dramatic but it can cool down the combustion chamber enough to reduce certain pollutants such as nitrous oxides, while raising others such as partially-decomposed hydrocarbons.
The air is drawn into the engine because of the vacuum produced by the motion of the pistons. But there are several engines that have a turbine that forces air to enter the engine; thus, the combustion process has more oxygen available, and it is possible to increase the amount of fuel. The result is a more powerful engine without increasing its size, this kind of motor is known as turbocharged. 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 opening is uncovered.
There are other methods to increase the amount of oxygen available inside the engine; one of them, is to inject nitrous oxide, (nitrous) to the mixture, and some special 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.
Piston steam engines are relatively inefficient (about 8% overall efficiency) which is why there are no longer any steam locomotives 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 generating plants (the other reason is the greatly reduced maintenance requirement). The Stirling 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 Motors) 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.
