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Heat Engines

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Heat Engines

Youve probably been on a journey inside a car, a plane, or a steam train at some point in your life. You might have wondered what process makes these vehicles move. The answer is the heat engine.

This article about heat engines goes a bit beyond the scope of what you would be expected to know at GCSE level, but this will be very useful in your understanding of thermodynamics and its real-life applications. In thermodynamics, a heat engine is a system that converts the flow of thermal energy (heat) into mechanical work.

Heat Engines in Thermodynamics

Heat is the transfer of thermal energy from higher to lower temperatures. In heat engines, this is achieved by having the heat flow from a hot reservoir to a cold reservoir. Petrol engines, diesel engines, jet engines, and steam turbines are all examples of heat engines.

Heat Engines, Diagram showing the heat flow from hot reservoir to the cold reservoir, StudySmarterHeat Flow between a Hot Reservoir and a Cold Reservoir, adapted from image by Mike Run CC-BY-SA-4.0

Incredibly, the first recorded heat engine was invented by Heron of Alexandria in 50 AD but was only considered a novelty or a toy at the time. It wasnt until the industrial revolution that heat engines were made into useful devices. The steam engine was made useful in the 18th century and was quickly put to use as a power source. The internal combustion engine followed in the late 19th century, which in many ways was an improvement over the steam engine. Without the heat engine, many of the comforts and technologies of our modern world would be impossible.

Heat engine types

Heat engines can be categorised into two types. The first is the external combustion engine, where the combustion of fuel transfers heat to an external liquid, which then generates useful work by its movement as it expands. An example of this is the steam engine. Here, a fuel source such as coal or wood is combusted to heat water (external liquid) in a boiler. This produces steam that can then do useful mechanical work to power the engine.

In an internal combustion engine, the burning of fuel occurs inside the system. Internal combustion engines are generally more efficient than external combustion engines, as they directly convert the heat energy of the fuel into mechanical work. For instance, the heat engine in a car ignites petrol or diesel using a spark plug to generate useful mechanical work.

Examples of heat engines

In this section, we will discuss some examples of real-world applications of the heat engine, from ancient antiquity to the modern era, for both internal and external combustion engines.

External combustion engine

To understand the basics of how a heat engine works, it might be a good idea to start from the beginning and take a look at Heron of Alexandrias first steam engine. Heron named it the aeolipile or wind ball. The design was simple: he positioned a cauldron of water (which acted as the hot reservoir) over a fire. The water soon boiled into steam when heated. The steam then rose up through two pipes into a hollow sphere on top, where two bent nozzles on the sphere allowed the steam to escape. The expelled steam generated thrust like a rocket, forcing the sphere to spin. The entire external environment, in this instance, acted as the cold reservoir for the heat to flow into.

Heat Engines, An animation of Heron's heat engine, StudySmarterHeron's heat engine forcing the sphere to spin, commons.wikimedia.org

Steam engine locomotives have been made widely obsolete by electricity and the internal combustion engine. Steam trains, for example, are now relegated to heritage transport or tourist attractions. However, steam is still widely used on an industrial scale to produce electricity. Water is heated from a heat source in a boiler (hot reservoir), which turns the water into steam, which is then used to spin a turbine as it rises. This is an example of a heat engine, in which thermal energy is converted into mechanical work. The spinning turbine then drives an electric generator, which produces electricity for our use.

Heat Engines Diagram showing a steam turbine used to generate electricity StudySmarterDiagram of a typical steam turbine used to generate electricity, commons.wikimedia.org

The steam is then cooled back into water inside a condenser (cold reservoir) after driving the turbine. This is advantageous for two reasons. First, the larger the difference in temperature between the hot and cold reservoirs (boiler and condenser), the faster the heat will flow between them. This means the steam will travel faster and, therefore, drive the turbine faster, generating more electricity. Secondly, by condensing the steam back into water, we can reuse that water for the boiler. Both of these points vastly improve the efficiency of the heat engine.

Geothermal power plants work similarly to coal power plants. However, while a geothermal powerplant is a heat engine, it is neither an internal nor an external combustion engine because the hot geothermal fluids used to heat the boiler come directly from the Earth and not through burning fuels.

Internal combustion engine

Lets discuss the internal combustion engine you will probably be most familiar with, the petrol car. The internal combustion engine inside the car burns the petrol directly inside the combustion chamber (hot reservoir). Some of the energy from combustion is then converted into useful work done. Most petrol engines are four-stroke engines, meaning that four piston strokes are needed to complete a full cycle of the engine.

Heat Engines, Diagram showing the different strokes of a heat engine, StudySmarterThe four-stroke cycle of an internal combustion engine, commons.wikimedia.org

First, during the intake stroke, the inlet valve is opened to allow fuel and air from the fuel tank to enter the working cylinder. The next step in the process is the compression stroke. Both valves close to trap the air-fuel mixture inside, and the piston moves up to compress the mixture into a small volume. Then, during the ignition stroke, an electric spark from the spark plug ignites the fuel, causing it to expand rapidly and pushing the piston back down. Finally, during the exhaust stroke, the exhaust valve opens, which allows the expanded gases from combustion to escape and then repeat the cycle again.

The expansion and exhaust of the mixtures inside the combustion chamber force the pistons to move up and down. The movement of these pistons attached to piston rods then rotates the crankshaft. Ultimately, a system of gears in the cars powertrain will drive the vehicle’s wheels, causing motion.

There is also such a thing as a reverse heat engine. Instead of using thermal energy to produce useful work, reverse heat engines use mechanical work to reverse the flow of heat. The mechanical work usually comes from an external power source like the national grid. Air conditioners and refrigerators are common examples of reverse heat engines. Imagine the inside of your fridge as the cold reservoir. The reverse heat engine forces the heat out of the fridge using a pump (mechanical work).

Heat engine equation

Energy and fuel are premium resources in our modern world, and we must find ways to reduce energy consumption as much as reasonably possible. When energy transfer occurs between energy stores (such as thermal to kinetic energy in a heat engine), not all of the energy produced is transferred into useful energy. When energy is transferred into an unwanted store, it is called waste energy.

The efficiency of a system is given by the following equation:

Using the principles of thermodynamics, heat engines have been designed to produce as little energy wastage as possible. Different heat engines have different efficiencies depending on a range of factors, such as their type, design, fuel source, etc. Energy is wasted due to unwanted sound produced by the engine, friction between moving parts, and waste heat that isnt converted into useful work done.

For example, to improve efficiency and reduce friction between an engines moving parts, engineers and mechanics add lubrication. Furthermore, thermal insulation can be used to reduce the wasteful loss of an engines thermal energy into the surrounding environment.

Heat Engines, Diagram showing the factors that play a role in the efficiency of a heat engine StudySmarterThe efficiency of a Heat Engine, adapted from image by Guy Vandegrift CC BY-SA 4.0

The thermal efficiency of a heat engine is given by:

Internal combustion engines are nearly always more efficient than external combustion engines. In general, combusting fuel directly into mechanical work is a more efficient process because external combustion engines have an extra step of energy transfer, which always ends up having more inefficiencies.

There is a limit to the potential efficiency of any heat engine. Carnot’s Theorem states that even an ideal, frictionless engine cannot convert close to 100% of its produced heat into useful work. The factors that limit efficiency are the temperatures at which the heat enters the engine and the temperature of the surrounding environment in which the engine exhausts its waste heat.

A heat engine does 6.3 kJ of work. 19.9 kJ of energy is wasted in the surrounding environment. What is the efficiency of this heat engine as a percentage?

Total energy produced by the engine is the sum of the work done and the energy lost to the environment.

An internal combustion engine has an efficiency of 42%. It produces 38 MJ of energy from the combustion of 1 litre of diesel fuel. How much useful work done is produced by 1 litre of fuel?

Rearrange the efficiency equation to make useful work done by the engine the subject.

Rearrange into:

Remember to convert the percentage efficiency back into decimal form.

Heat Engines - Key takeaways

  • In thermodynamics, a heat engine converts the flow of thermal energy (heat) into useful mechanical work.
  • Heat flows in a heat engine due to the difference in temperature between a hot and a cold reservoir.
  • In external combustion engines, the fluid in the hot reservoir is heated by an external fuel source. The movement of the heated fluid can then be used to produce useful work. An example of this is the steam engine.
  • In internal combustion engines, the combustion of fuel occurs directly inside the hot reservoir. They directly convert the thermal energy of combustion into useful work. Examples of this include the petrol or diesel engine.
  • In some heat engines, the external environment can act as the cold reservoir.
  • The greater the difference in temperatures between the hot and cold reservoirs, the faster the heat will flow between them, ultimately generating more useful mechanical work.
  • Internal combustion engines are generally more efficient than external combustion engines because external combustion engines have an extra step of energy transfer.
  • A heat engines type, design, fuel source, and a range of other factors all affect its efficiency.
  • Energy is wasted by unwanted sound, waste heat, and friction between moving parts of the heat engine.

Frequently Asked Questions about Heat Engines

A heat engine converts the flow of thermal energy (heat) into useful mechanical work. This is achieved by having heat flow between a hot reservoir and a cold reservoir within the engine.

Heat engines are used primarily for electricity generation in power plants and to make our vehicles move. In general, they are used to convert thermal energy into useful work.

Five examples of heat engines are:


  1. Petrol Engine.
  2. Diesel Engine.
  3. Steam Engine.
  4. Air Conditioners.
  5. Refrigerators.

The formula for calculating the efficiency of a heat engine is:


Efficiency = Useful Work Done by Engine / Energy produced by Engine burning fuel

Internal and external combustion engines, such as the steam engine or petrol engine, do use combustion. However, geothermal power plants do not, as their thermal energy comes directly from the earth.

Final Heat Engines Quiz

Question

In which direction does heat flow?

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Answer

Hot to Cold

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Question

Which one of the following is an example of an external combustion engine?

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Answer

Steam Engine

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Question

Which of the following types of engine is generally more efficient?

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Answer

Internal Combustion Engine

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Question

Which of the following is not an example of a fuel source?

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Answer

Water

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Question

Write a short description of how an external combustion engine works in 3 sentences or less.

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Answer

The combustion of fuel is used to heat an external fluid in the hot reservoir. The fluid can then generate useful work as it expands and moves. 

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Question

Heron of Alexandria's aeolipile (wind ball) was the first recorded steam engine. What was the cold reservoir in his heat engine?

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Answer

The external environment.

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Question

Name one advantage of employing a condenser in a steam engine used to generate electricity.

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Answer

Any one of the following:

  • Cooled water in the condenser (cold reservoir) will have a bigger temperature difference than between the boiler and the external environment, generating more heat flow and therefore more useful mechanical work.
  • By condensing the steam back into the water, the water can be reused in the boiler instead of being wasted.

Show question

Question

Geothermal power stations are heat engines. What type are they?

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Answer

Neither

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Question

The majority of cars in the UK use petrol as a fuel source in a four-stroke engine. Name the four strokes in order.

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Answer

  1. Intake Stroke
  2. Compression Stroke
  3. Ignition Stroke
  4. Exhaust Stroke

Show question

Question

In one sentence, describe what happens during the ignition stroke in a four-stroke internal combustion engine.

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Answer

During the ignition stroke, an electric spark from the spark plug ignites the fuel causing it to expand rapidly, pushing the piston down.

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Question

In one sentence, describe what happens during the compression stroke in a four-stroke internal combustion engine.

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Answer

Both valves close to trap the air-fuel mixture inside, the piston moves up to compress the mixture into a small volume ready for ignition.

Show question

Question

Due to inefficiencies, energy is always lost in heat engines. List two ways in which this energy is wasted.

Show answer

Answer

Any two of the following:

  • Unwanted sound
  • Friction between moving parts
  • Waste heat not converted into useful work done

Show question

Question

What can engineers and mechanics use to improve efficiency by reducing the friction between moving parts of a heat engine?

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Answer

They can add lubrication

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Question

What was invented first?

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Answer

External Combustion Engine

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