Figuring Out The Stirling Engine

-PURPOSE

The experiment is to show people why a stirling engine works. Show how the temperature difference plays a mayor role in the energy creation by the engine. And how the energy output may be increased.

This will be showed by, testing:

-First with just hot water cup on the bottom of the Stirling engine.
Here we can see how with hot water as a heat source under and the room temperature acting as a “cold” source the engine is able to run.

-Then  with hot water as a heat source under and 3 ice cubes on top of the engine.
Here we’ll see how by adding 3 ice cubes on top, the RPM’s increase.

 

APPARATUS

1. Stirling engine–

-For our experiment we used the common, Displacer type engine.

-Instead of having two pistons, one constantly heated and the other constantly cooled, a displacer-type engine has one piston and a displacer.

2.Strobeligth–

-We used a Strobeligth to measure the RPM’s created by the engine

-A strobe light flashing at the proper period can appear to freeze or reverse a cyclical motion.

 

3. Cup of 325ml

-Which we used to put the hot water in, to later place the engine on top.

4. Thermometer (ranging from -10 degrees celcious to 100 degrees celcious)

-To be able to record the different temperatures, from hot water and ices

5. Stop-watch

-Which we used to: keep  timing of when to take measurements of RPM’s, when to stop the engine, etc.

 

 

-PROCEDURE

 

So for the procedure, we did two sets of trials:

-First with just hot water cup on the bottom of the Stirling engine.
Here we can see how with hot water as a heat source under and the room temperature acting as a “cold” source the engine is able to run.

-Then we run a second trial, with hot water as a heat source under and ice cubes on top of the engine.
Here we’ll see how by adding 3 ice cubes on top, the RPM’s increase.

 

From this to sets of trials we obtained two types of results:

-From the first set of trials, we can draw that as time passes and the temperature of water decreases the Rpm’s slow down. Less energy is being created.

-From the second set of trials, we can see how the first measurement  RPM’s is pretty approximate to the one of the first set of trials. But with the incorporation of the ice cubes, the 2nd and 3rd measurements of RPM’s of this second set of trials are higher than those in the first set.

 

Guided steps:

– Get 1 cup of boiling water.

-Stick the thermometer in the water and wait until the thermometer reads 80 degrees Celsius, once the temperature reaches 80 degrees Celsius, place the stirling engine on the cup of hot water.

-Let it sit for one minute.

– Then place three ice cubes on the top of the stirling engine, making sure that they do not interfere with the spin of the wheel.

– Let the engine sit for one minute, after one-minute touch the wheel of the engine to start it moving.

– When the wheel starts moving, using the strobe, try to match the rotations of the wheel with the flashes of the strobe.
The way this is done is that when the light flashes, you should be seeing the black spoke of the wheel in the same location every time. To make this even easier is to match up the sound of the flash of the bulb with the tick of the stirling engine. 

– Record the RPMs in the table below.

-Wait two and a half minutes.

-Take another measurement using the strobe machine, record the RPMs in table below.

-Wait two and a half more minutes.

-Take another measurement using the strobe machine and record the RPMs in table below.

-Remove engine from hot water cup and let it cool.

 

– Get 1 cup of boiling water and 3 ice cubes.

Stick the thermometer in the water and wait until the thermometer reads 80 degrees Celsius, once the temperature reaches 80 degrees Celsius, place the stirling engine on the cup of hot water.

Let it sit for one minute.

– After one-minute touch the wheel of the engine to start it moving.

– When the wheel starts moving, using the strobe, try to match the rotations of the wheel with the flashes of the strobe.
The way this is done is that when the light flashes, you should be seeing the black spoke of the wheel in the same location every time. To make this even easier is to match up the sound of the flash of the bulb with the tick of the stirling engine. 

-Record the RPMs in the table below.

-Then place three ice cubes on the top of the stirling engine, making sure that they do not interfere with the spin of the wheel.

-Wait two and a half minutes.

-Take another measurement using the strobe machine, record the RPMs in table below.

-Wait two and a half more minutes.

-Take another measurement using the strobe machine and record the RPMs in table below.

 

–ANALISYS

 From this results we can, clearly see :
-How the stirling engine creates energy.
(When heat source heats the gas and the pressure built’s inside the engine making the pistons work)

But also, if we pay more attention to detail, we can also see:
-How by adding ice cubes to the top the RPM’s increase.
(This increase it not only due to the temperature difference created again, it also has to do with the compression of the gas)

 

See, theres 2 ways of increasing the energy output created by a Stirling engine: 

-By increasing the heat source on the bottom, which creates more pressure inside the engine, which makes it work more, which translates into more energy out put.

-And by, making the process of gas compression easier for the engine. Which basically happened when we placed the ice cubes on top. By making the top colder, its easier for the engine to compress the gas, which makes increases its efficiency when creating energy.

 

-THEORY BEHIND IT

The Stirling engine is a heat engine that is very different from the combustion engine in your car. Stirling engines are used only in some very specialized places, as in submarines or auxiliary  generators for yachts, where quiet operation is important.

Instead of having two pistons, a displacer-type engine has one piston and a displacer. This type of Stirling engine is mostly used in classrooms. You can even buy a kit and try it out, just as we did!! (www.stirlingshop.com)

In order to run, the engine above requires a temperature difference between the top and the bottom. In this case, the difference between the temperature of your hand and a couple of ices is enough to run the engine.

In the figure , you can see two pistons:

-The power piston: This is the smaller piston at the top of the engine. It is a  piston that moves up as the gas inside the engine expands.

-The displacer: This is the large yellow piston in the middle. This piston is very loose the cylinder, so air can move easily between the heated and cooled areas of the engine as the piston moves up and down.

The displacer moves up and down to control whether the gas in the engine is being heated or cooled (there’s two positions for it).

1. When the displacer is near the top of the cylinder, the gas inside the engine is heated by the heat source and it expands, causing the pressure to build up inside the engine, pushing the power piston up.

2. When the displacer is near the bottom of the cylinder,  the gas inside the engine cools and it contracts. Causes the pressure to drop, making it easier for the power piston to move down and compress the gas.

The engine  heats and cools the gas continuously, obtaining energy from the gas expansion and contraction.

 

-DISCUSSION/CONCLUSION

 Before deciding on our final experiment, we tried many different ideas. First we wanted to test the different combinations of hot and cold on the stirling engine. First we wanted to put the hot source on the bottom and then put the cold source on the top and take measurments, and then we wanted to flip that and put the hot source on the top and the cold source on the bottom and take some more measurements. However, when we actually purchased and started using the stirling engine, we realized that flipping the components rendered the engine useless. The wheel would not spin and therefore not generate any power or RPMs.

Then we wanted to see if the engine could run simply off of the heat of the table that it would sit on by placing a few ice cubes on the top. Again, when we tried to actually test this experiment, our result was not what we wanted, as the wheel refused to spin once again. Finally we decided to compare using two components with just one, and this was able to work. We noticed with just one of the components the RPMs were significantly less than with both, which was the result that we were expecting.

We also ran into some snags with the strobe light. The process of figuring out how to measure RPMs with the strobe light was a challenging one. It took us around three hours to figure out how to get a measurement from the light. However, once we did it was pretty easy to measure quickly and accurately. Something that we were concerned about was communicating how to get a measurement with the strobe to the students doing our experiment, but I feel that after a few test runs they were able to get an accurate measurement. We also feel that the use of the strobe light was a very fun and interesting part of the experiment.