Heat Engine: Efficiency, Carnot Engine, Types, Parts

Heat Engine is a system that converts heat to work. Examples of Heat Engines are air conditioners, refrigerators, and heat pumps that are run in reverse. They use work to take heat energy at a low temperature and increase its temperature in a more efficient way than the simple conversion of work into heat (either through friction or electrical resistance).

Key Terms: Heat Engine Efficiency, External Combustion Engine, Internal Combustion Engine, Heat Pump, Carnot Engine, Stirling Engine

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What is Heat Engine?

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Heat Engine is a system that converts the chemical energy of a fuel into thermal energy which is used to do other work. A heat engine will convert the locked energy in the fuel into force and motion. Fuels used are coal, gasoline, natural gas, wood, and peat. When burnt in an engine, these release stored energy utilized to power locomotives and factory machinery. 

Carnot Engine

A Carnot Heat Engine is a heat engine that operates on Carnot Cycle. Carnot engine is the most efficient heat engine which functions on the absolute temperatures of the hot and cold heat reservoirs between which it operates.

Carnot engine calculates the greatest potential efficiency of a heat engine during the conversion of heat into work and, conversely, working between two reservoirs. Carnot Theorem states that “any system that operates between hot reservoir and cold reservoir will never be more efficient than the Carnot engine that operates between identical reservoirs. The efficiency of this engine is unaffected by the working substance and is only determined by temperature of the reservoirs.

Carnot Engine Video Explanation

Stirling Engine

Stirling engines are energy conversion devices that can be used as prime movers, refrigerating engines, or heat pumps. These engines are used commercially as cryogenic cooling systems and are being developed to manufacture lesser noise-producing and low-emission automotive engines.

Class 11 Physics Thermodynamics: Important Questions

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Heat Pump

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Heat Pump extracts heat from one piece and transfers it to another. A heat pump transfers heat by circulating a substance called refrigerant through a cycle of evaporation and condensation.

Examples of heat pumps are refrigerators and air conditioners.  A refrigerator is an insulated box that has a connected heat pump system. An evaporator coil is placed inside the box, in the freezer compartment. Heat is absorbed from this location and transferred outside, usually behind or underneath the condenser coil’s unit.

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Functions of Heat Engine

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A heat engine's job is to transform heat energy into useful mechanical work.

This can be accomplished by taking a working substance. It is first heated to a high temperature before being cooled at a later stage. We can profit from the heat engine in this way. Regardless of the kind of heat engines and cycles, the only fundamental function of the heat system is to transform heat energy into useful mechanical work.

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Types of Heating Engines

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Heating systems are of two kinds, the first being Internal Combustion engines, and other External Combustion engines.

Internal Combustion Engine

Internal Combustion Engine comprises the burning of fuel within the system, i.e. these engines operate when the fuel is burned in the engine or where the combustion of fossil fuels happens. The internal combustion engine works on Otto Cycle’s principle and the apparatus required during this procedure is generally a piston.

Internal Combustion Engine

• Piston is responsible for drawing a fuel-air combination into the heat engine, which is the very first step of the process. These pistons scroll vertically within the cylinders of the heat engines. This single upward or downward movement of a piston inside the cylinder is known as a stroke. 
• In the second step, the mixture is extremely adiabatically compressed. 
• In the third stage, it passes through the ignition process, which dramatically raises the amount of pressure and temperature.
• With its growing temperature, Piston clears the gases once again as the process expands.

External Combustion Engine

These heat engines operate where the fuel is burned outside the engine or where the fuel combustion happens outside the engine. Here, the internal working fluid is heated by external combustion via the engine wall. By expanding and working on the engine's mechanism, this results in fluid generated motion and usable work.

• Like an Internal Combustion engine, here also the piston is responsible for drawing a fuel-air combination into the heat engine and will send it into the first cylinder.
• The gas in the first cylinder is heated to a high temperature, resulting in increased pressure. It is once again gathered by the Piston, which transforms it into work. It is then moved to a different cylinder.
• The gas is distinct from the previous level and also straightforward.
• Now, the cooling operation will be performed on the gas here. The cold reservoir aids in cooling the gas before it is compressed.
• Finally, the compressed gas is returned to the original chamber, where it is collected and converted into productive work by the Piston.

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Ideal Heating System

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An ideal heating system is a system that performs all the processes cyclically. This means wherever it stops again, it finishes from that point itself. An Ideal Heating System contains all of the reversible processes. Carnot Engine is an example of an Ideal Heating System.

Ideal Heating Systems

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Heating Engine: Parts

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The heating engine's equipment consists of three major components, each of which serves a purpose in turning heat energy into mechanical work.

Parts of Heating Engine

Source (High Temperature)

It is the main component of the heat engine.

It is sometimes referred to as a hot body since the heat engine's primary function is to transform heat into mechanical work. As a result, the source is supplied in the device to collect the heat. It is always kept with an unlimited thermal capacity, allowing it to remain stable even after creating a significant quantity of heat.

Working Object

The amount of heat applied to the hot source will be gathered by this functioning material, which may vary. The function, but on the other hand, is to collect from the hot source and transfer it to the sink.

Sink of Heat (At Lower Temperature)

This is completely contradictory to the first section, which is the source. It just has a limited thermal capability and absorbs heat from the working substance in addition it also aids in the preservation of mechanical work gained from the working substance. However, It does not enable it to be influenced by outside temperatures.

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Heat Engine: Mechanism

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A basic heat engine is made up of a gas contained in a cylinder by a piston. As it is heated, the gas expands, thus pushing the piston. This is not conceivable in a real engine since motion ceases when the gas achieves equilibrium. 

Heat Engine Mechanism

In a functional engine, the piston travels back and forth in cycles. When the gas heats up, the piston travels upward, and when it cools, it moves downward. We can say that to drive the piston forward and backward, a cycle of heating and cooling is required.

Three key highlights of the whole cycle of heat engine:

1. Because heat is injected at a relatively high degree, it is referred to as Q_H.
2. Some part of the energy, known as added energy is used to perform the work/tasks.
3. Lastly, The left or unused energy is removed relatively at a cold temperature, referred to as Q_C.

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Efficiency Of Heat Engines

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The efficiency of a heat engine is an important metric because it reflects how much usable work we obtain as an output from the engine when we utilize the same quantity of heat energy as input. Let’s talk in mathematical terms.

Suppose the engine takes heat (Q1) from a hot reservoir at temperature(T1 )and releases heat (Q2 ) to a cold reservoir at temperature (T2) hereby, delivering work W to the surroundings.

The efficiency of heat engine: η = 1- Q₂/Q₁

Where η = W/Q₁

Note that :

• Q₁=heat input in 1 cycle
• Q₂=work done in 1 cycle.

Note: There is no such thing as a heat engine with 100% efficiency. There is always a certain loss of heat in the surroundings.

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Things to Remember

• Heat Engine is a device that converts heat into mechanical energy. 
• Heating Systems are of two types: Internal Combustion Engines and External Combustion Engines. 
• Internal Combustion Engine is where fuel is burned within the system. Engines operate when the fuel is burned in the engine.
• External Combustion Engine is where the fuel is burned outside the engine or where the fuel combustion happens outside the engine.
• Carnot Engine is an ideal Heat Engine that works on the principle of a reversible closed thermodynamic cycle. Its four successive operations include Isothermal Expansion, Adiabatic Expansion, Isothermal Compression and Adiabatic Compression
• The heating engine's equipment consists of three major components: Source (High Temperature), Working Object and Sink of Heat (Lower Temperature)
• A basic heat engine consists of gas contained in a cylinder by a piston. As it is heated, the gas expands, thus pushing the piston. In a functional engine, the piston travels back and forth in cycles. 
• Heat Engine Efficiency reflects how much usable work is obtained as an output from the engine when the same quantity of heat energy is utilized as input.

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Previous Year Questions

1. A deep rectangular pond of surface are A, containing water… [NEET 2019]
2. A carnot engine takes 300 calories of heat at 500 K and rejects… [UPSEE 2019]
3. A black body has maximum wavelength 1 m at… [NEET 1989]
4. If λm denotes the wavelength at which the radiative emission from… [KEAM 2004]
5. A metal rod of length 10cm and area of cross-section… [AP EAPCET]
6. Two spherical black bodies have radii… [MHT CET 2017]
7. A black body at 227^∘C radiates heat at the rate of… [NEET]
8. The two ends of a rod of length L and a uniform cross-sectional area… [NEET 2009]
9. A black body has wavelength… [NEET 2001]
10. Two bodies A and B are placed in an evacuated vessel maintained at… [BITSAT 2019]