21.2:
Heat Engines
A heat engine is a machine used to extract heat from a source and then convert it partly into work in the form of mechanical energy.
The components of a heat engine are a hot reservoir acting as the source, a working substance for heat transfer during cyclic operation, and a cold reservoir acting as the sink.
When the working substance extracts heat from the source, only a significant part of the heat is utilized to do some useful work, and the remaining heat is transferred to the sink.
In an ideal reversible heat engine, the change in the internal energy of the system is zero.
Using the first law of thermodynamics, an expression for work extracted from the heat engine is obtained.
The thermal efficiency of the heat engine is given as the ratio of this work to the amount of heat absorbed from the source.
In practical heat engines, this efficiency is always less than unity as it is impossible to convert all the heat into work; there is always some heat loss.
21.2:
Heat Engines
A heat engine is a device used to extract heat from a source and then convert it into mechanical work used for various applications. For example, a steam engine on an old-style train can produce the work needed for driving the train.
Whenever we consider heat engines (and associated devices such as refrigerators and heat pumps), we do not use the standard sign convention for heat and work. For convenience, we assume that the symbols Qh, Qc, and W represent only the amounts of heat transferred and work delivered, regardless of what the givers or receivers are. Whether heat is entering or leaving a system, or work is done to or by a system, the symbols are indicated by the proper signs in front of them and additionally by the direction arrows.
We assume that a heat engine is constructed between a heat source (high-temperature or hot reservoir) and a heat sink (low temperature or cold reservoir). The engine absorbs heat Qh from a heat source (hot reservoir) of Kelvin temperature Th, uses part of this energy to produce useful work W, and then discards the remaining energy as heat Qc into a heat sink (cold reservoir) of Kelvin temperature Tc.
Power plants and internal combustion engines are examples of heat engines. Power plants use steam produced at high temperatures to drive electric generators, while releasing heat to the atmosphere or a nearby body of water (in the role of the heat sink). In an internal combustion engine, a hot gas-air mixture is used to push a piston, and heat is released to the nearby atmosphere in a similar manner.
The most crucial measure of a heat engine is its efficiency (e), which is “what we get out” divided by "what we put in" during each cycle. The efficiency is always less than unity. Ideally, we wish to convert all the heat into work, but there is always some heat lost; hence Qc can never be zero.
Suggested Reading
- OpenStax. (2019). University Physics Vol. 2. [Web version]. Retrieved from https://openstax.org/details/books/university-physics-volume-2; section 4.2; page 148-150.