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Development of the Pneumatic Engine
The development of the pneumatic engine was largely inspired by the pneumatic control circuit at  Eric Brok's website. The implementation of the concept into a practical application was carried out in stages as follows:

Stage I: Basic pneumatic control loop.
Stage II: Using the control circuit to control a model steam engine.
Stage III: Integrating the control circuit with the model steam engine itself.

Stage I: Basic pneumatic control loop

To test it out, two large pneumatic cylinders and two air switches were connected in the basic loop circuit. And it worked. When air was supplied, the pistons of the cylinders would move to-and-fro activating the air switches in the process. It was quite a sight to see the two cylinders moving automatically.
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Here's how  the air switch is linked to the piston of  the pneumatic device.

 Note the alignment of the air switch in relation to the pivot point of the pneumatic cylinder.
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The pneumatic tubings are cross-coupled from one air switch to the opposite cylinder. The two cylinder pneumatic control circuit in action.

Here is the connection diagram to help you.
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Here's a version of the 2 cylinder pneumatic control circuit using the small cylinders.

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Extension of the pneumatic control circuit

The basic pneumatic control circuit can be extended to 3 or more cylinders to carry out a sequence of pneumatic actions. The cylinders are connected to give positive feedforward except the last which is connected negatively to the first. When air is supplied, the cylinders will open in sequence and then close one after another. The cycle will be repeated automatically.
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4-cylinder pneumatic control circuit from above.

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4-cylinder pneumatic control circuit in action.

Stage II: Using the control circuit to control a model steam engine.

Encouraged by the success of Stage I, the next step was to connect the cylinders in the control block parallel to another set of cylinders of a model steam engine. Intuitively, we would expect that the cylinders need to be 180 ° out of phase. However, this didn’t work. After some experimentation, it was found that the pistons driving the flywheel must be offset by a right angle (90° phase difference). This is shown clearly by the alignment of the cross bar of the large pulleys.

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After this adjustment, when air was pumped the cylinders in the model were able to drive the flywheel (represented by the pulleys) round and round. However, don't expect whizzing speed as the Lego pneumatics parts are somewhat stiff but the fact that the engine worked at all, was satisfying.
Stage III: Integrating the control circuit with the model steam engine itself

The final stage was to integrate the control circuit into the model steam engine itself. That is, when one cylinder of the model moves the flywheel, it would also flip the air valve by way of a connecting link. This would cause the other cylinder to move and at the same time activate the air switch to the first cylinder. In this way, the cylinders would activate the air valves alternately. The net effect is to cause the flywheel to rotate continuously as long as air is supplied.

In Stage II, it had been established that the two cylinders must be offset by 90° phase difference. Note the alignment of the cams of the two cylinders in the following pictures.
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An additional mechanism was required to link the movement of the pistons to the air switches. It took quite a while to figure out the exact alignment of the linkages to the air switches to flip them over at the correct time to make the model work. Note the positioning of the lift arm in relation to the medium pulley and the cam in the following pictures.

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Several arrangements were tried but the final version using a medium pulley and a lift arm was the most simple and aesthetic. Sure enough, once the air valves were correctly linked and air was pumped, the engine worked as theory predicted. The engine encounters quite a lot of resistance to move the air switches, so it is best to provide the air supply by using the hand pump.

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A live steam model engine and the Lego model		 pic46-50.jpg (16642 bytes)
Pneumatic engine of a different kind
Here's another kind of  pneumatic engine which I built. This one runs literally on hot air - you've got to see it to believe it. Technically it's called the Stirling cycle engine. I got the kit of parts from Bailey Crafstman Supply. Bob Bailey, the proprietor, gives excellent before and after-sales service. The Stirling Engine site by Koichi Hirata is the coolest site I've come across on the subject of this most unusual and fascinating engine.
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