Combustion Pump

We have made a combustion pump! It's a spectacular experiment where compressing the air inside the pump burn a piece of cotton. Perfect for teachers to impress students with the power of physics. In this instructable we will explain how you can make one for yourself. Technical drawings and assembly instructions will be provided, but you are of course allowed to deviated from them. However, always keep safety in mind. The pressure and temperature isn't very dangerous, but make sure everything is secure.

The following materials and tools where used to create the device:

Materials (check bronnen)

• PMMA cylinder (H x outer diameter x inner diameter = 300mm x 20.20mm x 16.00mm)
• Round PVC cap (H x diameter = 16mm x 30mm)
• Massive piece of piece of wood/plywood (210mm x 210mm x 40mm)
• Stainless rod (H x diameter = 320mm x 14mm) 
• Teflon piece (H x diameter = 30mm x 16mm)
• Spherical knob (diameter = 40mm)
• Two component glue
• Cotton (approx. 0.5 gram per usage)
• 50 ml Vaseline

Equipment

• Turning lathe
• Drill press
• Handsaw
• Sander
• Tapping threads (10mm)

You can deviate from these materials when making your own pump. For safety use a tube with a thickness of at least 2 mm. Also we recommend keep the diameter as small as possible. This makes the tube stronger and have less resistance. For the other parts make sure the fit is correct and the connections strong. Also make sure the bottom part is fixed strongly to the tube. When increasing the pressure in the tube this part has the tendency to come lose very violently.

The pump head is going to compress all the air. For this it needs to be air tied, but also still move. In the project it was a balance between sealing and speed. There will always be some air that escapes, but if you push fast enough the required temperature can be easily achieved.

The pump we made out of PTFE. These shape and dimension can been seen in the pdf below. Like you can see there is a grove in the middle. This is so the PTFE has some place to expand to and to lower the resistance. We let the part be made with a lathe and taped the screw thread afterwards. The shape can be easily clamped inside the chuck.

The rod for the pump was a bit trickier then one might expect. Because the air pressure is quite large in at the end, the rod needs to be strong and as straight as possible. Therefore, we used a stainless steal rod.

To make the stainless steal rod ready to fit the pump head as well as the knob, use the turning lathe and starting from the ends reduce the diameter from 14 to 10mm up to up to 20mm from the ends. Next use the 10mm screw taps to tap a screw thread onto both ends. In the pdf below the technical drawing can be found.

To have a comfortable grip on the pump you need a good handle/nop. We have chosen a round nop of around the size of a billiard ball, but you can of course use something else. It might be a bit more comfortable and easier to produce to use a flatter nop. However, this was sufficient for our pump.

To make the knob which is used to press down onto the cylinder. Start drilling a hole in one side of the spherical knob of about 20mm deep and 10mm in diameter with the drill press (make sure it’s secured well and is not able to move). After that is done, tap a screw thread down the same hole, with the 10mm screw taps. In the pdf below is a technical drawing.

The combustion chamber is the most important part of the design. This holds all the pressure for the pump so it's essential that it doesn't fail. We recommend at least following the step we went through to get to this design for safety. The two parts that need to be taken into account are: the tube walls and the adhesion between the tube and the bottom part.

The bottom part (the black part) is made with the lathe. It's a part of a plastic rod. The hole in the middle is made to fit the tube exactly. In the pdf below the technical drawings can be found for producing you own.

The tube walls are thick enough to hold the pressure, but always check the specifications of the tube and make sure that there're no noticeable imperfections like cracks. For the connection with the bottom part we used a resin based glue. Use plenty to make sure no air will be able to escape between the gaps.

The last step is the baseplate. If you have completed the previous steps correctly, pump is now usable. You could leave it like that, but we found it nice to have a heavy baseplate so the pump doesn't move while pumping or damage the surface it stands on.

To make the baseplate grab a large piece of layered plywood and cut a piece of around 200x200 mm. Then drill a hole in the middle of the width of the bottom part of the combustion chamber. In our case this was a bit more then 30 mm diameter. Make sure that this fit is as tight as possible by drilling a bit to small as a start and sand it down until it just fits.

As a last step we recommend rounding to corner or if you are really fancy, you can varnish the wood. There is also a technical drawing in the pdf below, but you probably won't need that for this part.

Using the pump can be a bit difficult. Keep your body relaxed while pushing down and try to get as much speed as possible. We recommend using your fingers more and not the palm of your hand since that will hurt quite a bit. Our pump was able to ignite cotton, but if it doesn't work, you can try using a bit of unrefined sunflower oil. Just put a little cotton on the bottom of the tube with and push it down with a stick that doesn't damage the tube wall, like wood.

This was the explanation on how to make the combustion pump. Have fun!