Summer STEM Challenge: The Vacuum Engine
Image credit: Neil Downie
This week in our Summer STEM Challenge, Neil Downie provides enthusiastic engineering families a fun way to make a vacuum with a bike wheel.
STEM Challenge #48: The Vacuum Engine
“I sell what all the world desires: power.” So said Birmingham’s Matthew Boulton when he partnered with James Watt to make the world’s first powerful industrial engines. They were actually 'vacuum engines', with vacuum from condensing steam on one side of their pistons and atmospheric pressure on the other side. Here’s how to make a big working model of those pioneer engines, using a bike wheel, plastic plumbing, and vacuum cleaner power instead of steam.
Boulton and Watt’s enormous steam engines had a piston in a cylinder, pulling and pushing the flywheel via a crankpin and a valve to turn the vacuum on only when the piston is being sucked down and not otherwise. The valve needs to move with a quarter-turn lag after the piston.
A front bike wheel makes a good flywheel since it has ball bearings. The crankpin can be a long thin bolt through the spokes, positioned about 30mm from the axle. You’ll need some plastic (polypropylene) plumbing, with a bigger pipe (e.g. 40mm) for the cylinder and the outer case of the valve and a smaller pipe (e.g. 32mm) for the valve.
The piston is a cylinder of wood fitting 40mm pipe with a clearance of 0.5mm all round. The conrods, metal strips with holes at each end, can be attached by brackets to the piston and valve. The valve is a smaller version of the piston but runs up and down the straight part of a 40mm tee. This is fitted with 40mm tube ends with a 32mm tube glued inside. The piston valve covers and uncovers a hole in the 32mm tube to ‘let the vacuum in’ from the vacuum cleaner when the conrod is pulled furthest out. The tee arm is connected to the cylinder, by plumbing pipes or vacuum cleaner hose.
Mount the pieces on a wooden frame or baseboard, on blocks of wood that put everything in the right plane relative to the base. The bike wheel can be bolted to the frame with one end of its axle, while the other axle end can be shortened. All done? Time to hook up your vacuum cleaner and swing the flywheel to start the Vacuum Engine whirling around.
How fast does your Vacuum Engine go? At higher speeds, try a light gate, or use a hub dynamo bike wheel and measure the speed electrically. Changing the valve timing may improve your engine – letting the vacuum get through earlier or later, or for longer. How fast can you make it go?
You may wonder where the air gets in so the piston can return to its top position. The answer is that air simply leaks in! If you have made the piston and valve fit closely, then you’ll need a hole that lets air in when the valve is furthest into the tee.
With 200mbar negative pressure from the vacuum cleaner, 200mbar times the piston area gives a force, ~2kg, on the crank pin, turning the flywheel. If the flywheel turns at 300 rpm, and the force moves 5cm per rev, you get 5W of power. This isn’t much, but build a Vacuum Engine the size of a house with a piston the size of a small car as Boulton and Watt did and you’ll get a megawatt!
And finally… in ancient times, the ‘Fortuna Rota’, the Wheel of Fortune, was turned by the goddess Fortuna. She’s very busy these days, so what about a Vacuum Engine Wheel of Fortune? Attach numbers to the wheel and a big arrow on the side to point at them (see picture above). Use for raffle ticket prizes, bingo numbers, lottery tickets. If you make several, maybe you can open a Vacuum Casino?
If you liked this, you will find lots more fun science stuff in Neil Downie’s books, like ‘The Ultimate Book of Saturday Science’ from Princeton University, and for lots of other things (and a free copy of the ‘Exploding Disk Cannons’ book), visit www.saturdayscience.org. In line with this experiment, Neil’s current work includes developing a new ventilator system to support people with breathing difficulties – get more information on this great project here: Exovent.org.
There is a back catalogue of STEM-related challenges from the past year to choose from if you are looking for more options. The IET also has a host of resources that adults can use to engage children with the world of STEM.
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