Understanding Steam Engines with Electrical and Hydraulic Analogies

Before you read, this let me give you a sense of why you may want to read this. If you’ve ever tried to read the explanation of how a steam engine or sterling engine works, you may have gotten bogged down in details and miss the big picture. This is about the fundamentals, so you can understand at a high level how any steam engine works. You will probably not fully appreciate this until you start asking yourself how on earth scientists can claim what the maximum possible theoretical efficiency of an engine is. How can they know that without having built it?

What if there is some clever way of doing it that they didn’t think of?

Intro

I have always found that analogies have helped me a lot in understanding various concepts within physics. Electricity did not really click for me until I was introduced to the hydraulic analogy for understanding electrical circuits.

This is an analogy comparing the flow of electricity to the flow of water. It is useful because we can have an intuition about water since we can see it. Electricity is harder because we cannot see it with the naked eye.

The hydraulic analogy explains well e.g. why electric current cannot flow without forming a circuit. Many wrongly think of an electric battery as an analogy to a flask of water. You pour out water onto say a water wheel to do some work. In this analogy it is not obvious why we need a circuit. Why do you need a positive and negative pole?

In reality an electrical battery is more like a water pump. You can imagine a water tower. The height the water drops down, represent the potential energy of the water and is an analogy to electrical potential, how many volts your battery supplies. Without a height difference, water cannot flow. Without a potential difference electric current cannot flow either.

Once the water in the tower is empty, the water can no longer flow and drive motors or other equipment. However if you have a pump, which can pump water up to the water tower you can keep going as long as there is some power to drive the water pump.

By forming a circuit, you can reuse the water the dropped down from the water tower. You just feed it to the pump, which pumps it up again.

A battery is similar. It is not a surplus storage of electrons. The electrons are already present in all the electrically conducting material. However the electrons are not moving, because there is no potential difference. The battery expends chemical energy to pump the electrons through the circuit. The electrons will keep going around until all the chemical energy in the battery has been expended.

Another analogy you can imagine is a pneumatic analogy: a circuit with pressurized air. Air will flow from a high pressure area to a low pressure area. This flow can drive machinery. At some point however the pressure is equalized and there is no more flow. That is the completion of one cycle. A compressor or pump can keep it going though. A pump will increase pressure in one area and reduce it in another area.

Heat Engines

An electric motor is powered by electric current due to a potential difference. A pneumatic motor is powered by an air pressure difference. Steam engines, Sterling engines and internal combustion engines in contrast do work due to a flow of heat.

This is not heat in the every day sense of the word though. In physics, heat is the flow of energy from a high temperature area to a low temperature area. In physics heat does not exist when there is no flow. This is similar to how there is no electric current when there is no flow.

Let me clarify with some analogies. Electric current is the flow of electric charge per second. This can be compared to the hydraulic equivalent of a volume of water flowing per second. Volume of water is thus analog to electric charge.

Comparing with thermodynamics, heat is similar to volume of liquid or electric charge. Thus the thermal equivalent of an electric current is heat transfer rate. Heat is measured in Joules, while heat transfer rate is Joules per second.

Potential in Heat Engines

You can probably guess by now, what is the equivalent of potential in a heat engine. It is temperature. For hydraulics of pneumatics it is pressure, and for electric circuits it is electric potential (measured in volts).

This gives you a clue of why you cannot make a steam engine do work without cooling the steam (condensing it to water). Nor can you get a sterling engine to do work, without cooling it’s working gas.

Think about this in hydraulic terms. No matter how far up you lift water, it cannot do useful work until it falls down. It is the height difference between where the water falls from and to which matters. Likewise no matter how high pressure of gas you got inside a tank, it will not go any work, if there is no lower pressure to move to.

Thus it is not merely about how making something really hot but also about being able to cool that hot thing. It is the temperature differential that matters.

In one cycle of a steam engine, heat flows due to temperature differential. This flow gets work done. However eventually the temperature difference is equalized and thus there is no more heat flow.

Thus we need cooling and heating to start the cycle over again. All steam engines are about various clever ways of building up a temperature difference over and over again at the end of each cycle. One tries to find ways of expending as little energy as possible accomplishing this.

There are lots of ways we can imagine our efforts get thwarted. Imagine our hot and cold areas are really poorly insulated. Then a lot of heat we add to our high temperature area gets lost, because heat travels away in every direction. We want the heat to only travel in the direction of our low temperature area where we have put something in place to get work done.

Newcomen Steam Engine

Newcomen devised one of the simplest possible steam engines. It is in fact not the simplest. Heron, of Alexandria, made a simpler one in ancient times. However it does not make much sense to talk about it as a steam engine, because it was so extremely inefficient that it was completely impractical to use.

In Newcomen’s engine a cylinder if filled with steam by boiling water with burning coal. Manufacturing technology was actually not very good back then, so they did not make steam at very high pressure. The steam is really just made to equal out atmospheric pressure. This would cause the other side of the pump to go down due to gravity.

Water has very high heat capacity (specific heat). Newcomen utilized this by injecting cold water into the steam filled cylinder at the end of the cycle. High specific heat means a substance is able to absorb a lot of heat without changing temperature. That means the water could absorb the energy of the steam and causing it to condense to water. When the water condensed to water the volume rapidly declines. This creates a vacuum in the cylinder. The atmospheric pressure above causes the piston in the cylinder to be pushed down. The piston pulls the pump, doing the actual work.

It may not seem like the atmosphere will do much work, but atmospheric pressure is in fact very strong. You just don’t notice because you have equal pressure inside and outside of yourself.

James Watt Engine

James Watt made numerous improvements to Newcomen’s steam engine. The primary improvement was to use a separate condenser. A condenser is basically a tank where steam gets cooled. Instead of cooling the cylinder itself, James Watt would lead the steam into a separate tank through a valve. Here cold water would condense the water. Thus the metal casing of the main cylinder would not have to get heated up again at the beginning of each new cycle.

Recap

A steam engine follows these steps:

1. Coal, wood, oil, concentrated solar or whatever is used to heat water until it boils and creates steam.
2. The steam pushes a piston up.
3. Now we got to get the piston down again. The valve letting in steam to the cylinder gets closed off. Another valve is opened leading the steam into a tank, called a condenser. Here the steam is cooled in some manner. I could be by injecting cold water or using a heat exchanger containing cold water pumped through it.
4. The condensation of steam creates a vacuum sucking in the rest of the hot steam in the cylinder and pulling the piston down.
5. The valve to the condenser closes and the valve leading hot steam into the cylinder is opened again. The cycle repeats. Goto step 1.

Important thing to always keep in mind: you need a temperature differential. The larger this differential is, the more efficient your steam engine can theoretically get. Thus modern highly efficient steam engines create very high temperature steam.