Heat Engine - SKengineers
WHAT IS HEAT ENGINE?
A heat engine is a device that converts heat to work. It
takes heat from a reservoir then does some work like moving a piston, lifting
weight etc and finally discharges some heat energy into the sink. Schematically
it can be represented as:
In thermodynamics and engineering, a heat engine is a system
that converts heat to mechanical energy, which can then be used to do
mechanical work. It does this by bringing a working substance from a higher
state temperature to a lower state temperature. A heat source generates thermal
energy that brings the working substance to the high temperature state. The
working substance generates work in the working body of the engine while
transferring heat to the colder sink until it reaches a low temperature state.
During this process some of the thermal energy is converted into work by
exploiting the properties of the working substance. The working substance can
be any system with a non-zero heat capacity, but it usually is a gas or liquid.
During this process, some heat is normally lost to the surroundings and is not
converted to work. Also, some energy is unusable because of friction and drag.
In general, an engine converts energy to mechanical work.
Heat engines distinguish themselves from other types of engines by the fact
that their efficiency is fundamentally limited by Carnot's theorem.[3] Although
this efficiency limitation can be a drawback, an advantage of heat engines is
that most forms of energy can be easily converted to heat by processes like exothermic
reactions (such as combustion), nuclear fission, absorption of light or
energetic particles, friction, dissipation and resistance. Since the heat
source that supplies thermal energy to the engine can thus be powered by
virtually any kind of energy, heat engines cover a wide range of applications.
Heat engines are often confused with the cycles they attempt
to implement. Typically, the term "engine" is used for a physical
device and "cycle" for the models.
What is
the Function of a Heat Engine?
The function of a heat engine is to convert the heat energy
into useful mechanical work. This can be done by taking a working substance.
First, it is heated at a high temperature and cooled at another stage. Like
this, we can get benefited from the heat engine. Though the type of heat engine
and cycle varies, the main function is to convert the heat energy into useful
mechanical work.
Types of Heat Engine -
There are mainly two types of heat engines – external
combustion engine and internal combustion engine.
External
Combustion Engine -
Here the fuel gets burned outside the engine or at a far
distance from the engine by which it can produce force and motion. A very good
example of an external combustion engine is a steam engine.
Internal
combustion engine -
Here the fuels are burned inside the chamber. Car engine a
very good example of this type of combustion engine.
The heat produced due to combustion is given to the engine
and the work is done. It is also called 4-stroke because it takes 4 strokes for
the piston to complete one combustion cycle.
So, if we consider the two types of heat engine, Internal
combustion engines are more efficient than external combustion engine. This is
because no energy is wasted in an internal combustion engine by transferring
heat from the boiler to the cylinder as in an external combustion engine.
Everything happens in one place.
Working
of Heat Engine -
As we look earlier, a heat engine contains basically a heat
reservoir, engine and a cold sink. The heat that we produce as internal
combustion or external combustion is given to the engine where the movement of
the piston takes place. The power generated is given to the machine that
connected to the engine and hence the work is done. Excess heat is given to the
sink where the temperature will remain the same at both reservoir and sink.
Heat engine like automobile engines operated in a cyclic
manner. They will add energy in the form of heat in one part of the cycle and
use this energy to do useful work from another part of the cycle.
Heat Engine PV Diagram -
Heat engines can be typically illustrated on a PV diagram,
Heat Engine PV Diagram
Pressure-Volume (PV) diagrams are the basic tool for the
study of heat engines that use gas as the working substance. PV diagram will be
a closed-loop for a cyclic heat engine. The area of the loop is the
representation of the amount of work done during the cycle.
The idea of the efficiency of an engine cycle can be
obtained by comparing the PV diagram with the Carnot cycle, which is the most
efficient type of heat engine.
Explanation of PV diagram -
The fluid changes from liquid to vapour isothermally if the
source is at a high temperature. This vaporization process occurs at constant
pressure and increasing volume.
At the turbine end, the gas expands reversibly and
adiabatically and it follows the equation of state for an adiabatic and
reversible process.
The fluid changes from a gas to liquid isothermally if the
source is at a low temperature. This condensation process occurs at constant
pressure and decreasing volume.
At the compressor end, the liquid is compressed reversibly
and adiabatically by increasing its pressure to the original point.
Examples
of Different Engines -
Steam
Engine -
A steam engine is a heat engine that gives mechanical work
using steam as working fluid. It uses the force produced by the steam pressure
to move the piston forward and backward inside the chamber. This pushing force
is transferred to work. This is a type of external combustion engine in which
working fluid is burned out of the chamber giving the steam to the source using
a pipe.
Car
Engine or Gasoline(petrol) Engine -
This is a type of internal combustion engine where
combustion takes place internally. The main purpose of a gasoline car engine is
to convert gasoline into motion so that your car can move. The easiest way to
produce this motion is to burn the gasoline inside the engine. Here the air is
also used to burn the working fluid.
Refrigerator
or Heat Pump -
A normal refrigerator is an example of a heat pump and in
reverse, it is a heat engine. Here, work is used to create a heat differential.
Many more cycles can work in reverse to flow heat from the cold side to the hot
side. Internal combustion engine parts of these cycle are also not reversible.
Heat pump consumes work and is basically used for heating purposes.
Beam
Engine -
It is a type of external combustion engines. In olden years,
the machine that was used was very gigantic in size almost the size of a room.
These early devices had a cylinder and a piston attached to a large beam which
performed the working of machines.
Stirling
Engines -
Not all external combustion engines are huge and
inefficient. Stirling engine consists of two cylinders with piston powering a
single wheel. One cylinder is kept permanently hot and the other cylinder is
kept permanently cold. The engine works as the movement of gas between the
cylinders back and forth.
It is important to note that although some cycles have a
typical combustion location (internal or external), they often can be
implemented with the other. For example, John Ericsson developed an external
heated engine running on a cycle very much like the earlier Diesel cycle. In
addition, externally heated engines can often be implemented in open or closed
cycles. In a closed cycle the working fluid is retained within the engine at
the completion of the cycle whereas is an open cycle the working fluid is
either exchanged with the environment together with the products of combustion
in the case of the internal combustion engine or simply vented to the
environment in the case of external combustion engines like steam engines and
turbines.
Everyday
examples -
Everyday examples of heat engines include the thermal power
station, internal combustion engine, firearms, refrigerators and heat pumps.
Power stations are examples of heat engines run in a forward direction in which
heat flows from a hot reservoir and flows into a cool reservoir to produce work
as the desired product. Refrigerators, air conditioners and heat pumps are
examples of heat engines that are run in reverse, i.e. they use work to take
heat energy at a low temperature and raise its temperature in a more efficient
way than the simple conversion of work into heat (either through friction or
electrical resistance). Refrigerators remove heat from within a thermally
sealed chamber at low temperature and vent waste heat at a higher temperature
to the environment and heat pumps take heat from the low temperature
environment and 'vent' it into a thermally sealed chamber (a house) at higher
temperature.
In general heat engines exploit the thermal properties
associated with the expansion and compression of gases according to the gas
laws or the properties associated with phase changes between gas and liquid
states.
Earth's
heat engine -
Earth's atmosphere and hydrosphere—Earth's heat engine—are
coupled processes that constantly even out solar heating imbalances through
evaporation of surface water, convection, rainfall, winds and ocean
circulation, when distributing heat around the globe.
A Hadley cell is an example of a heat engine. It involves
the rising of warm and moist air in the earth's equatorial region and the
descent of colder air in the subtropics creating a thermally driven direct
circulation, with consequent net production of kinetic energy.
Phase-change
cycles –
In these cycles and engines, the working fluids are gases
and liquids. The engine converts the working fluid from a gas to a liquid, from
liquid to gas, or both, generating work from the fluid expansion or
compression.
Cycles
used for refrigeration -
A domestic refrigerator is an example of a heat pump: a heat
engine in reverse. Work is used to create a heat differential. Many cycles can
run in reverse to move heat from the cold side to the hot side, making the cold
side cooler and the hot side hotter. Internal combustion engine versions of
these cycles are, by their nature, not reversible.
Refrigeration
cycles include –
Air cycle machine
Gas-absorption refrigerator
Magnetic refrigeration
Stirling cryocooler
Vapor-compression refrigeration
Vuilleumier cycle
Evaporative
heat engines -
The Barton evaporation engine is a heat engine based on a
cycle producing power and cooled moist air from the evaporation of water into
hot dry air.
Mesoscopic
heat engines -
Mesoscopic heat engines are nanoscale devices that may serve
the goal of processing heat fluxes and perform useful work at small scales.
Potential applications include e.g. electric cooling devices. In such
mesoscopic heat engines, work per cycle of operation fluctuates due to thermal
noise. There is exact equality that relates average of exponents of work
performed by any heat engine and the heat transfer from the hotter heat bath.
This relation transforms the Carnot's inequality into exact equality. This
relation is also a Carnot cycle equality.
Efficiency
-
The efficiency of an engine is the percentage of energy
input that the engine can convert to useful work. The equation for this is η =
work output / energy input. The most efficient piston engines run at about 50%
efficiency, and an average coal-fired power plant runs at around 33% efficiency.
More recently built power plants are getting more than 40% efficiencies.
Smaller heat engines, like those in cars, have mechanical
power outputs measured in terms of horsepower. Larger heat engines, like power
plants, measure outputs in terms of MW. Of course the output can be measured in
any units of power, such as watts.
The input of a heat engine is also a power, often measured
in MW. With a power plant there is also an electric output power. In order to
distinguish between the two powers, the thermal power (input power) is measured
in megawatts thermal (MWt), while for electricity production the output power
is measured in megawatts electric (MWe). For heat engines that provide motion
instead of electricity, the output power would be mechanical power.
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