Most efforts to improve the efficiency of heat engines goes into increasing the temperature of the heat source, but at very high temperatures the metal of the engine begins to go soft.
The efficiency of various heat engines proposed or used today ranges from 3 percent (97 percent waste heat) for the OTEC ocean power proposal through 25 percent for most automotive engines, to 45 percent for a supercritical coal plant, to about 60 percent for a steam-cooled combined cycle gas turbine. All of these processes gain their efficiency (or lack thereof) due to the temperature drop across them.
The least efficient, OTEC, uses the temperature difference of ocean water on the surface and ocean water from the depths, a small difference of perhaps 25 degrees Celsius, and so the efficiency must be low.
The most efficient, the combined cycle gas turbine burns natural-gas to heat air to nearly 1530 degrees Celsius, a large temperature difference of 1500 degrees Celsius, and so the efficiency can be very large when the steam-cooling cycle is added.
People mostly use heat engines where the heat comes from a fire that expands a working fluid (usually either water or air) and the heat sink is either a body of water or the atmosphere as in a cooling tower.
Familiar ones that use the expansion of heated gases include: the steam engine, the diesel engine, and the gasoline (petrol) engine in an automobile.
The Stirling engine is much rarer but is found in small models which can run off the heat of a hand.
One kind of toy heat engine is the drinking bird.
A bimetallic strip is a device that converts temperature into mechanical movement and is used in thermostats to control temperature. It is a heat engine that does not use a liquid or gas.
Thermodynamic cycle
A thermodynamic cycle is a series of thermodynamic processes which returns a system to its initial state. Properties depend only on the thermodynamic state and thus do not change over a cycle. Variables such as heat and work are not zero over a cycle, but rather depend on the process. The first law of thermodynamics dictates that the net heat input is equal to the net work output over any cycle. The repeating nature of the process path allows for continuous operation, making the cycle an important concept in thermodynamics.
If the cyclic process moves clockwise around the loop, then it represents a heat engine, and W will be positive. If it moves counterclockwise then it represents a heat pump, and W will be negative.
Two primary classes of thermodynamic cycles are power cycles and heat pump cycles. Power cycles are cycles which convert some heat input into a mechanical work output, while heat pump cycles transfer heat from low to high temperatures using mechanical work input.
Thermodynamic power cycles are the basis for the operation of heat engines, which supply most of the world's electric power and run almost all motor vehicles. Power cycles can be divided according to the type of heat engine they seek to model. The most common cycles that model internal combustion engines are the Otto cycle, which models gasoline engines and the Diesel cycle, which models diesel engines. Cycles that model external combustion engines include the Brayton cycle, which models gas turbines, and the Rankine cycle, which models steam turbines.
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Christoph Metzelder
Christoph Metzelder (born 5 November 1980 in Haltern, Germany) is a German football player. He plays for Real Madrid In the Germany national team he has played in 47 matches and scored no goal.
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Thermodynamic system
In thermodynamics, a thermodynamic system, originally called a "working substance", is defined as that part of the universe that is under consideration. A real or imaginary boundary separates the system from the rest of the universe, which is referred to as the environment or surroundings (sometimes called a reservoir.) A useful classification of thermodynamic systems is based on the nature of the boundary and the quantities flowing through it, such as matter, energy, work, heat, and entropy. A system can be anything, for example a cylinder, a solution in a test tube, a living organism, or a planet, etc.
Treat
Treat could mean:
Piston
In general, a piston is a lubricated sliding shaft that fits tightly inside the opening of a cylinder. Its purpose is to change the volume enclosed by the cylinder, to exert a force on a fluid inside the cylinder, to cover and uncover ports, or some combination of these. A rubber seal is sometimes used to keep the lubricate within the shaft. Due to the constant motion of the machine this seal wears quickly and should be replaced with every servicing. If the seal should break during usage there can be disastrous long lasting consequences for the machine.
There are two ways that a piston engine can transform combustion into motive power. These are two-stroke and four-stroke.
A single cylinder two-stroke engine produces power every crankshaft revolution, while a single cylinder four-stroke engine produces power once every two revolutions. Older designs of small two-stroke engines produced more pollution than four stroke engines. However, modern two-stroke designs, like the Vespa ET2 Injection use fuel-injection and are as clean as four-strokes. Large diesel two-stroke engines, as used in ships and locomotives, have always used fuel injection and produce low emissions. One of the biggest internal combustion engines in the world, the Wärtsilä-Sulzer RTA96-C is a two-stroke; it is bigger than most two-story houses, has pistons nearly 1 metre in diameter and is one of the most efficient mobile engines in existence. In theory, a four stroke engine has to be larger than a two stroke engine to produce an equivalent amount of power. Two stroke engines are becoming less common in developed countries these days, mainly due to manufacturer reluctance to invest in reducing two-stroke emissions. Traditionally, two stroke engines were reputed to need more maintenance. Even though the simplest two stroke engines have fewer moving parts, they could wear out faster than four stroke engines. However fuel-injected two-strokes achieve better engine lubrication, and cooling and reliability should improve considerably.
A steam engine is another type of piston engine. In most steam engines, the pistons are "double acting": steam is alternately admitted to either end of the cylinder, so that every piston stroke produces power.
Pinyin
Pinyin is the Mandarin word for "spelling", writing letters showing the sound of a word in place of a Chinese character. Today, it almost always has the sense of Hanyu Pinyin, the way of writing Mandarin sounds which has been used by the People's Republic of China from the 1950s. Other countries like the United States and Taiwan used other ways of spelling Chinese (such as the Wade-Giles way) for a long time, but in the late 20th century started to use Hanyu Pinyin as well. It is the standard of China as well as the United Nations. It is the most common romanization system for Chinese.
Pinyin uses Roman letters in a special way for certain Chinese sounds. For example, the pinyin letter C is pronounced as /ts/ as in the English word "cats" or the Japanese word "tsunami", and does not sound at all like /k/ or /s/ by itself. Pinyin uses special marks to show the four Chinese tones, which are very important to using Mandarin clearly.
b p m f w
d t n l
g k ng h
j q x y
z c s
a e i o u ü
ai ei ao ou
an en ang eng ong
There are five kinds of tone in Pinyin:
ā, á, ǎ, à, a
"Geyin fuhao" is used after the syllables starting with vowels "a, o, e", for example: pi'ao.
We should divide Pinyin text by words and write syllables connectedly, such as "I am a foreigner" should be written as "Wo shi waiguoren" in Pinyin.
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