What is thermal efficiency?
Thermal efficiency is a measure of output energy divided by input energy in the system. It must be between 0% and 100%. The thermal efficiency of 100% would mean that all energy inserted into the system will come out even if in a different form. Thermal motors and refrigerators have related thermal efficiency, even if they try to achieve the opposite goals. The thermal efficiency in the real world generally dropped significantly below 100% for various reasons.
In the petrol engine, the input energy is stored in the chemical bonds of hydrocarbon fuel. The hydrocarbon molecule consists exclusively of hydrogen and carbon. When these molecules are combined with oxygen, they can react chemically and form carbon monoxide and water; Basically, the hydrocarbon molecule is divided and combined with oxygen atoms. However, part of this reaction, which is useful for the engine, is released. The heat released from gasoline combustion is the relevant input energy of the vteperal efficiency.
Output energy when calculating the thermal efficiency of the engine is not heat but mechanical worke. In physics, the work of energy transmitted by a force acting at a distance is in physics. Shifting the box through the carpet to a certain distance requires the final amount of work; This equals the distance product of the distance moved and the average force developed. Likewise, the gasoline engine works when it moves the wheels of the car.
In the case of a refrigerator or air conditioning, the heat work relationship turns. The required result in this situation is to remove heat from the system and throw it into the external environment. Available input is therefore mechanical work, often provided by an electrically driven compressor. However, the calculation of thermal efficiency still requires the distribution of output energy by input energy. Of course, the different from the petrol engine is that the output is warm and the input is to work.
A typical automotive engine has a thermal efficiency of less than 35%. This number seems to be low for two important reasons. First of all, there isThe theoretical upper limit of the thermal efficiency of any thermal engine, which has to do with the system temperature compared to the environmental temperature. The higher the difference in temperature, the higher the maximum thermal efficiency, the engine without friction can achieve. This is called Carnot efficiency.
The second reason why car engines apparently have low efficiency is that the engines cannot be done in such a way as to behave in an ideal way. The friction between the moving parts constantly tends to slow down the engine. Some heat escape from the combustion chamber and become unnecessary to the engine. Fuel does not always burn at the highest temperature, which reduces the amount of heat released. For these reasons, the thermal efficiency in the L -wor equipment is a tendency to be significantly below 100%.