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What is meant by thermodynamic efficiency of an engine? and what are the factors that affect the thermodynamic efficiency of a diesel engine?

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3 Answers 3

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It's basically:

$$ \eta = \frac {Brake power (kW)}{\frac{dm_f}{dt} \cdot LCV}$$

Where m_f is the fuel mass that enters the engine and its temporal derivative the mass flow [kg/s]

LCV is the Low Calorific Value [kJ/kg], it depends on the fuel used.

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The thermodynamic efficiency of an engine is the energy output divided by the energy input. For a car engine for example, the output is the shaft horsepower (torque times RPM) and the input is the heat added by burning fuel inside the engine. A low efficiency engine burns a lot of fuel and produces a little horsepower (it wastes the heat input); a high efficiency engine burns a little fuel and produces a lot of horsepower (it makes good use of the heat input).

Different operating cycles (otto, brayton, stirling, etc.) exhibit different thermodynamic efficiencies depending on how they are designed and operated, but all of these are limited to being less than the theoretical maximum possible thermodynamic efficiency of any heat engine, which is called the Carnot efficiency.

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  • $\begingroup$ It's the useful output ie the power and torque, otherwise you include all the heat losses as they are output as well... $\endgroup$
    – Solar Mike
    Apr 20, 2019 at 19:29
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Thermal engines take in Q amount of heat energy out of a fuel and convert part of it as mechanical work and part of it as wasted heat back to the environment.

$$ Q_{in}=W_{out} +Q_{out}$$ The thermal efficiency is the percentage of heat energy taken in to work put out. $$ \eta= \frac{W_{out}}{Q_{in}}=\frac{Q_{in}-Q_{out}}{Q_{in}}$$

$$ \eta=1-\frac{Q_{out}}{Q_{in}}$$

The efficiency of even the best heat engines is low; usually below 50% and often far below. So the energy lost to the environment by heat engines is a major waste of energy resources, Since a large fraction of the fuels produced worldwide go to powering heat engines, perhaps up to half of the useful energy produced worldwide is wasted in engine inefficiency, ..

Quoted from Wikipedia page engine efficiency.

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