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Cycles are a big deal in engineering. Today we’ll explain what they are and how they’re used in heat engines, refrigerators, and heat pumps. We’ll also discuss phase diagrams and the power of using renewable energy resources

Crash Course Engineering is produced in association with PBS Digital Studios:


Çengel, Yunus A., and Michael A. Boles. Thermodynamics: An Engineering Approach. 8th ed., McGraw-Hill Education.


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CC Kids:
To succeed in engineering, you have to master the art of repetition.

That applies to repetition in your own work – so that you’re always designing, prototyping, testing, and then designing again. But repetition is also key to the processes that you use as an engineer.

I’m talking here about cycles. They can be pretty complex, but at their core, each one is just a sequence of events or steps that repeat themselves in the same order. And that’s exactly what’s happening in some of the most commonly used devices that have ever been engineered, like the heat pump that’s heating your home and the refrigerator that’s keeping your food from spoiling.

So, let’s learn, and then repeat. [Theme Music] In engineering, we say that a system has undergone a cycle if it returns to its initial state at the end of the process. So, its initial and final states should be the same. And cycles are important because, they allow us to run through the same process again and again, instead of limiting us to doing something only once.

With the right resources, we can keep running through cycles until we get enough of what we want, like the distance traveled in a car. We can also keep making cycles for as long as we need, say, to keep food cold in a fridge at a constant temperature. The process that your fridge uses today is based on the work of 19th century American inventors Oliver Evans and Jacob Perkins.

In 1805, Evans came up with a closed vapor-compression refrigeration cycle, but he never actually built a refrigerator. Then, in the 1830’s, Perkins used Evans’ ideas and actually did. Perkins’ system didn’t succeed commercially at first, but it was the first step towards the modern refrigerators that we use today.

So how do refrigeration cycles work? Well, it’s easier to understand if we start with something similar that we already know a bit about: heat engines. Last time, we learned that heat engines are machines or systems that convert heat into other forms of energy.

A basic heat engine can do this by taking in heat at a higher temperature, from say, solar energy or a furnace, and then converting part of that heat to work, usually by rotating a shaft. The engine then releases wasted heat at a lower temperature, maybe into its surroundings or a water supply, and then readies itself to start over again. So, like so many other things, heat engines operate on a cycle.

And cycles achieve some goal, like heating or cooling a room, by circulating what’s known as a working fluid through a series of operations. This working fluid will absorb and release energy, change from liquid to vapor and back again, and continue to circulate through the cycle as part of the system’s operation. So, let’s look at a heat engine that uses water as its working fluid.

It goes through four main stages. In the first stage, we’ll add heat to our system by bringing in an energy source, QH. The water will absorb this heat through a boiler, which will cause it to become compressed steam.

In stage two, that steam will enter a turbine, expand, and cause the turbine shaft to turn, which gives us an output of work, Wout, which was converted from some of the heat energy in our fluid. Remember, not all of the heat energy will convert to work. We’re going to have excess heat, which needs to be released from our system.

We do this in stage 3 by condensing the steam in a condenser, which releases that excess heat into an energy sink, QL. For the fourth and final stage, our fluid needs to be re-pressurized. To make this happen, we’ll send it through a pump, which will need work as an input.

We’ll then send the re-pressurized water back to the boiler at the beginning to start the process all over again! That’s just one cycle. For each one that we do, we should have an output of work.

And if we look at the heat engine as a closed system, then the total changes in the kinetic energy, potential energy, and internal energy are all 0 through the cycle. So, per the first law of thermodynamics, the changes in work and heat should equal themselves out! Now, another way of looking at this cycle is by using a phase diagram.

Phase diagrams compare different properties to show what state or phase a substance is in. For this example, we’ll compare entropy to the temperature of the heat engine’s fluid using the Rankine cycle, which is the ideal cycle for vapor power plants. If we take a look at the diagram, all the material to the left of the curve is in a liquid phase, while all the material to the right is in a gaseous state.

Everything under the curve is a mixture of gas and liquid. This plot lets us easily see what phase our fluid is in as it goes through each stage of the heat engine cycle. Now, not only is the heat engine a great way to turn heat energy into work, but with a few small changes, we can turn it into a very different type of system.

Instead of trying to get work as our output, what if we tried to get heat? Well, we do this every day when we try to heat or cool our homes! We use heat pumps to add heat to a system when we’re feeling chilly, and refrigerators to remove heat from a system when we want to keep things cool.

In either case, we put work into the system, rather than trying to get it out. What’s interesting for refrigerators and heat pumps is that, since we’re aiming for an output of heat, it’s possible to get a 100% conversion from work, which we know from the second law of thermodynamics. However, it’s important to note that even though all of our work can be converted into heat, it may not all be the exact heat we want, because we're still going to have two different temperature levels.

Now, with all that in mind, let’s go back to the refrigerator. If we’re talking about the fridge in your kitchen, the inside stays cool because of what’s happening on its rear exterior wall. This is where our cycle will take place.

Just like with the heat engine, we can break this cycle down into four stages, again with a working fluid that’s circulating through all the stages. In your kitchen fridge, this working fluid is a hydrochlorofluorocarbon chemical that’s usually referred to generically as Freon. The first stage of the cycle is the evaporator, which removes heat from the inside of the fridge.

It starts out with liquid fluid that’s colder than the inside of the fridge, which is the result of the last stage of the previous cycle. But we’ll get to that. So, this liquid is really cold, but its boiling point is also really low.

In fact, the liquid is just about at the temperature where it’s ready to boil. And when a liquid changes to a gas, it absorbs heat. So when the liquid in the evaporator boils, it absorbs heat from the refrigerator at the same time.

But even though it’s absorbing heat, its temperature doesn’t actually change. All that heat energy is going into changing the liquid into a gas, not raising its temperature. Stage two is the compressor.

Its job is to raise the pressure of the gas, which also raises its temperature — and its boiling point. After stage two is complete, the gas is really hot — hotter than the air outside the fridge. But because its boiling point increased too, it’s still around the temperature where it’s ready to condense into a liquid.

Which brings us to stage three: the condenser, which is basically the opposite of the evaporator in stage one. In the condenser, the gas turns into a liquid, a process that releases heat. Since the refrigerant is now hotter than the air outside, heat can flow from inside the condenser to the surrounding air.

But, like in the evaporator, the temperature of the refrigerant stays constant in the condenser. Finally, in stage four, an expansion valve throttles the liquid, lowering its pressure — and therefore, lowering both its temperature and boiling point. It’s the opposite of what happens in the compressor.

You end up with cold liquid refrigerant at a lower pressure, ready to enter the evaporator and start the process all over again, absorbing more heat from the fridge as it boils. So basically, the food inside your fridge stays cold because we’re taking heat out from the inside of your fridge. And we can see this all a bit more clearly if we take another look at a phase diagram, this time for a refrigeration cycle.

While similar to the phase diagram for the heat engine, we’ll see a few differences in what phases our fluid is in at the different stages. The fluid spends more time in a gaseous state and less time as a liquid than our fluid did for the heat engine. Now, full cycles like these are great, but sometimes we’ll need to have an incomplete cycle with a little outside help to get what we want.

That’s because sometimes we’re limited by our environments and what we have available. So, for example, refrigerators often need electricity, but that kind of power isn’t always available. So, with a little bit of problem-solving, we can design ones that don’t need it!

That was the idea behind the zeer pot. The zeer pot is a simple refrigerator made from one earthen pot set inside another, with a layer of wet sand in between them. It was made famous by Nigerian inventor Mohammed Bah Abba in the 1990’s, but similar devices may date back all the way to Egypt around 2500 BCE.

So how does the zeer pot work? Well, as the moisture from the sand evaporates, it cools the inner pot by “pulling” out heat. It’s a great way to have a refrigeration system in a hot climate when you have very limited resources.

But while it’s pretty awesome, the zeer pot isn’t quite a cycle. Without recapturing the evaporated water, we’re going to need some outside work to make our sand wet again if we want to continue the cooling process. So, sometimes we need to forgo a perfect cycle for the sake of practically.

But all that being said, let’s say we do have the resources for a refrigerator that can run on a cycle. How can we improve this process even more? Well, one way is by using a renewable energy resource to fuel our system and produce the work that we need!

Solar energy is a great example. Rather than taking electricity that was made from a typical source, we can use solar-powered photovoltaic panels to convert the sun’s rays into electricity by exciting the electrons within the panel’s cells. The electricity that we’d get from this energy could replace the work that we needed for the cycle, thus making the cycle itself more reusable!

Which is great, because a big goal for us as engineers is to find ways to improve our processes, even when something is already working. We can always improve on our designs in one way or another. We can always take another step forward.

The refrigerators that we make now are far better than the ones that Perkins made back in the 1800’s. Our heat engines are getting more and more efficient as time goes on. You see, the journey of an engineer is both discovery and optimization.

And we’re just getting started. So today, we learned all about cycles, what they are, and some of the systems that use them. One of the biggest ones was heat engines.

Not only did we learn how heat engines work, but we also saw that with a few small changes, we can create other systems too, like refrigerators and heat pumps. We also learned about phase diagrams and the power of using renewable energy resources. I’ll see you next time, when we’ll learn about fluid mechanics and momentum transfer.

Crash Course Engineering is produced in association with PBS Digital Studios. You can head over to their channel to check out a playlist of their amazing shows, like America from Scratch, Hot Mess, and Eons. Crash Course is a Complexly production and this episode was filmed in the Doctor Cheryl C.

Kinney Studio with the help of these wonderful people. And our amazing graphics team is Thought Cafe.