Heat Engines

You’ve 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.

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 wasn’t 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

A reconstruction of Heron's aeolipile, wikimediacommons.org

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 Alexandria’s 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.

A Victorian steam engine.

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.

Diagram 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.

Internal combustion engine

Let’s 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.

The 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 car’s powertrain will drive the vehicle’s wheels, causing motion.

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:

$\text{Efficiency}=\frac{\text{Total useful energy produced}}{\text{Total energy supplied}}$

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 isn’t converted into useful work done.

The 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.