“knocking”的概念、定义、翻译、参考文献-科学参考

knocking

Physics

The metallic sound produced by a spark-ignition petrol engine under certain conditions. It is caused by rapid combustion of the unburnt explosive mixture in the combustion chambers ahead of the flame front. As the flame travels from the sparking plug towards the piston it compresses and heats the unburnt gases ahead of it. If the flame front moves fast enough, normal combustion occurs and the explosive mixture is ignited progressively by the flame. If it moves too slowly, ignition of the last part of the unburnt gas can occur very rapidly before the flame reaches it, producing a shock wave that travels back and forth across the combustion chamber. The result is overheating, possible damage to the plugs, an undesirable noise, and loss of power (probably due to preignition caused by overheated plugs). Knocking can be avoided by an engine design that increases turbulence in the combustion chamber and thereby increases flame speed. It also can be avoided by reducing the compression ratio, but this involves loss of efficiency. The most effective method is to use high-octane fuel (see octane number), which has a longer self-ignition delay than low-octane fuels. This can be achieved by the addition of an antiknock agent, such as lead(IV) tetraethyl, to the fuel, which retards the combustion chain reactions. However, lead-free petrol is now preferred to petrol containing lead tetraethyl owing to environmental dangers arising from lead in the atmosphere. In the USA the addition of lead compounds is now forbidden. New formulae for petrol are designed to raise the octane number without polluting the atmosphere. These new formulae include increasing the content of aromatics and oxygenates (oxygen-containing compounds, such as alcohols). However, it is claimed that the presence in the atmosphere of incompletely burnt aromatics constitutes a cancer risk.

Chemistry

The metallic sound produced by a spark-ignition petrol engine under certain conditions. It is caused by rapid combustion of the unburnt explosive mixture in the combustion chambers ahead of the flame front. As the flame travels from the sparking plug towards the piston it compresses and heats the unburnt gases ahead of it. If the flame front moves fast enough, normal combustion occurs and the explosive mixture is ignited progressively by the flame. If it moves too slowly, ignition of the last part of the unburnt gas can occur very rapidly before the flame reaches it, producing a shock wave that travels back and forth across the combustion chamber. The result is overheating, possible damage to the plugs, an undesirable noise, and loss of power (probably due to preignition caused by overheated plugs). Knocking can be avoided by an engine design that increases turbulence in the combustion chamber and thereby increases flame speed. It also can be avoided by reducing the compression ratio, but this involves loss of efficiency. The most effective method is to use high-octane fuel (see octane number), which has a longer self-ignition delay than low-octane fuels. This can be achieved by the addition of an antiknock agent, such as lead(IV) tetraethyl, to the fuel, which retards the combustion chain reactions. However, lead-free petrol is now preferred to petrol containing lead tetraethyl owing to environmental dangers arising from lead in the atmosphere. In the USA the addition of lead compounds is now forbidden. New formulae for petrol are designed to raise the octane number without polluting the atmosphere. These new formulae include increasing the content of aromatics and oxygenates (oxygen-containing compounds, such as alcohols). However, it is claimed that the presence in the atmosphere of incompletely burnt aromatics constitutes a cancer risk.

Chemical Engineering

The premature explosion of an air–fuel mixture within a piston cylinder of an internal combustion engine. It is due to the over-compression of the mixture and gives a characteristic metallic knocking sound. When the fuel and air mixture is injected into the cylinders of the engine, the mixture is combusted by the use of spark ignition. However, as the flame travels from the sparking plug towards the piston, it compresses the unburnt fuel. Ignition of the unburnt fuel occurs before the flame reaches it, resulting in shock waves that travel up and down the cylinder. The consequence is noise, a loss of power, and damage to the sparking plugs. Knocking is avoided by reducing the compression ratio, although this also results in a loss of efficiency. Antiknocking agents such as tetraethyl lead added to the fuel that increase the octane number and reduce the tendency to detonate are known to cause air pollution and environmental damage, and their use is either much reduced or banned in some countries. New fuels have been designed to have a higher octane number with a higher aromatic content. However, the presence of unburnt aromatics in the atmosphere has an adverse effect on human health.

单词	knocking
释义	knocking Physics The metallic sound produced by a spark-ignition petrol engine under certain conditions. It is caused by rapid combustion of the unburnt explosive mixture in the combustion chambers ahead of the flame front. As the flame travels from the sparking plug towards the piston it compresses and heats the unburnt gases ahead of it. If the flame front moves fast enough, normal combustion occurs and the explosive mixture is ignited progressively by the flame. If it moves too slowly, ignition of the last part of the unburnt gas can occur very rapidly before the flame reaches it, producing a shock wave that travels back and forth across the combustion chamber. The result is overheating, possible damage to the plugs, an undesirable noise, and loss of power (probably due to preignition caused by overheated plugs). Knocking can be avoided by an engine design that increases turbulence in the combustion chamber and thereby increases flame speed. It also can be avoided by reducing the compression ratio, but this involves loss of efficiency. The most effective method is to use high-octane fuel (see octane number), which has a longer self-ignition delay than low-octane fuels. This can be achieved by the addition of an antiknock agent, such as lead(IV) tetraethyl, to the fuel, which retards the combustion chain reactions. However, lead-free petrol is now preferred to petrol containing lead tetraethyl owing to environmental dangers arising from lead in the atmosphere. In the USA the addition of lead compounds is now forbidden. New formulae for petrol are designed to raise the octane number without polluting the atmosphere. These new formulae include increasing the content of aromatics and oxygenates (oxygen-containing compounds, such as alcohols). However, it is claimed that the presence in the atmosphere of incompletely burnt aromatics constitutes a cancer risk. Chemistry The metallic sound produced by a spark-ignition petrol engine under certain conditions. It is caused by rapid combustion of the unburnt explosive mixture in the combustion chambers ahead of the flame front. As the flame travels from the sparking plug towards the piston it compresses and heats the unburnt gases ahead of it. If the flame front moves fast enough, normal combustion occurs and the explosive mixture is ignited progressively by the flame. If it moves too slowly, ignition of the last part of the unburnt gas can occur very rapidly before the flame reaches it, producing a shock wave that travels back and forth across the combustion chamber. The result is overheating, possible damage to the plugs, an undesirable noise, and loss of power (probably due to preignition caused by overheated plugs). Knocking can be avoided by an engine design that increases turbulence in the combustion chamber and thereby increases flame speed. It also can be avoided by reducing the compression ratio, but this involves loss of efficiency. The most effective method is to use high-octane fuel (see octane number), which has a longer self-ignition delay than low-octane fuels. This can be achieved by the addition of an antiknock agent, such as lead(IV) tetraethyl, to the fuel, which retards the combustion chain reactions. However, lead-free petrol is now preferred to petrol containing lead tetraethyl owing to environmental dangers arising from lead in the atmosphere. In the USA the addition of lead compounds is now forbidden. New formulae for petrol are designed to raise the octane number without polluting the atmosphere. These new formulae include increasing the content of aromatics and oxygenates (oxygen-containing compounds, such as alcohols). However, it is claimed that the presence in the atmosphere of incompletely burnt aromatics constitutes a cancer risk. Chemical Engineering The premature explosion of an air–fuel mixture within a piston cylinder of an internal combustion engine. It is due to the over-compression of the mixture and gives a characteristic metallic knocking sound. When the fuel and air mixture is injected into the cylinders of the engine, the mixture is combusted by the use of spark ignition. However, as the flame travels from the sparking plug towards the piston, it compresses the unburnt fuel. Ignition of the unburnt fuel occurs before the flame reaches it, resulting in shock waves that travel up and down the cylinder. The consequence is noise, a loss of power, and damage to the sparking plugs. Knocking is avoided by reducing the compression ratio, although this also results in a loss of efficiency. Antiknocking agents such as tetraethyl lead added to the fuel that increase the octane number and reduce the tendency to detonate are known to cause air pollution and environmental damage, and their use is either much reduced or banned in some countries. New fuels have been designed to have a higher octane number with a higher aromatic content. However, the presence of unburnt aromatics in the atmosphere has an adverse effect on human health.
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