Calculation of the efficiency of an engine

In summary: This is the most useful definition of efficiency for engines. In summary, efficiency (η) in an engine is defined as the ratio of net output work to heat input, taking into account the work done to compress the gas. This is a human measure that is necessary for practical purposes, as it distinguishes useful work from energy that is not used for the intended purpose.
  • #1
vroy
6
0
We know efficiency (η) = Woutput/Winput

Now in any cycle, for instance, the Otto cycle, let the total work output be WO, total heat added be WH & the total work done on the gas to compress it be WC.

Then, will efficiency be equal to (η) = (WO-WC)/WH ?
Or will it be equal to (η) = WO/(WH+WC) ?
 
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  • #2
There are lots of internal things that happen in an engine. Compression is one. Friction is another.

The only useful definition of efficiency is useful work out divided by work in. Out and in refer to things external to the engine. Useful is also necessary, because for example heat leaves the engine and warms up the environment. That's energy out, but it is not useful.

In other words, efficiency is a human measure that you define to be useful for your purposes. In that respect, it is less about physics, and more about your opinions.
 
  • #3
vroy said:
We know efficiency (η) = Woutput/Winput

Now in any cycle, for instance, the Otto cycle, let the total work output be WO, total heat added be WH & the total work done on the gas to compress it be WC.

Then, will efficiency be equal to (η) = (WO-WC)/WH ?
Or will it be equal to (η) = WO/(WH+WC) ?
It's neither. Work output already does not include the compression energy. You are subtracting it twice.
 
  • #4
Defining the input power in the case of a liquid or gas fuel is not always straight forward. Here in the UK we have gas home heating boilers that are more than 100% efficient because of the way the input power was defined historically.
 
  • #5
For an Otto cycle, there is no work input. There is heat input (Q_h) from the fuel. Work and heat are thermodynamically different forms of energy.

Taking the work of compression as W_c and the work of expansion as W_e the thermal efficiency is (W_e - W_c)/Q_h.

In words, it is the net output work divided by the heat input.
 

Related to Calculation of the efficiency of an engine

1. What is the efficiency of an engine?

The efficiency of an engine is a measure of how well it converts the energy from fuel into useful work. It is typically expressed as a percentage and can vary depending on the type and design of the engine.

2. How is the efficiency of an engine calculated?

The efficiency of an engine is calculated by dividing the useful work output by the total energy input. This can be expressed as: efficiency = (useful work output / total energy input) x 100%. The useful work output can be measured in various units such as horsepower or kilowatts, while the total energy input is typically measured in units of energy, such as joules or calories.

3. What factors affect the efficiency of an engine?

The efficiency of an engine can be affected by various factors such as the type and quality of fuel, the design and condition of the engine, and operating conditions such as temperature and pressure. The efficiency can also be affected by external factors such as friction and heat loss.

4. Can the efficiency of an engine be improved?

Yes, the efficiency of an engine can be improved through various methods such as regular maintenance, using high-quality fuel, and optimizing the design and operating conditions of the engine. However, the efficiency of an engine is also limited by the laws of thermodynamics, which state that no engine can be 100% efficient.

5. Why is it important to calculate the efficiency of an engine?

Calculating the efficiency of an engine is important because it provides valuable information about its performance and can help identify areas for improvement. It also allows for comparison between different engines and can aid in making informed decisions about their use and maintenance.

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