Fire Piston

You may have noticed that if you pump up a bicycle tyre your pump gets hot, we find out why, and do a slightly more extreme version

What you need

We used a large "fire piston" this is an ancient fire lighting device from SE Asia. The one we used is basically a large transparent bicycle pump, but with nowhere for the air to get out at the bottom.

Some cotton wool is teased out and then placed in the bottom of the cylinder. The plunger is then pushed down increadibly hard.

What may Happen

When the plunger is pushed down very fast the cotton wool gets immensely hot and bursts into flames. This was used in the Burma, Malaysia Borneo region as a way of starting fires. These were made of horn or bamboo and would light a small piece of tinder. It is thought that Rudolf Diesel saw one of these and it gave him an idea..

What is going on?

It takes a large amount of energy to compress the air in the cylinder. This energy is transferred to the air, the only way a gas can hold this energy is as heat, so the gas heats up. The gas inside the cylinder is compressed to about 1/20th of it's original size and theoretically could be heating up to about 400°C although it is probably a little less than this.

This high temperature and the fact that the oxygen in the air is at a much higher pressure means that the cotton wool can start reacting with the oxygen in the air, and burn. The cotton wool has a large surface area so burns increadibly quickly and is ash by the time the piston is back to the beginning.

What has this got to do with Rudolf Diesel?

Rudolf Diesel invented (possibly unsurprisingly) the Diesel engine. This works similarly to a petrol engine, it mixes air and fuel in a cylinder, ignites it and uses the resulting explosion to move a vehicle along.

A diesel engine uses the same principle as the fire piston to ignite the fuel.

 Why does the gas get hotter when you compress it?

All gasses are made up of molecules, these are small groups of atoms. In a gas they are flying around at high speed, colliding with each other and the walls. Normally when they collide with a wall, on average they will leave it with the same energy as they hit it with. But what happens when that wall is moving?

If the wall is moving towards the gas molecule (like when the piston is  compressing the gas), it will tend to bounce off faster than it approached the piston (just like how a tennis ball will bounce off a racket faster when the racket is moving towards it.)

So the molecules bounce off faster, and therefore have more energy and the gas heats up.

Similarly if the piston is moving away from the gas molecule it will bounce off more slowly and loose energy.

This is why an aerosol can gets cold when you spray it (the gas is expanding agianst the air and getting colder) and the principle most fridges work on.

Written by Dave Ansell