Fire Piston - How IT Works

How It Works

Rapid compression of a gas (known as adiabatic compression) increases its pressure and its temperature at the same time. If this compression is done too slowly the heat will dissipate to the surroundings as the gas returns to equilibrium with them. If the compression is done quickly enough then there is no time for equilibrium to be achieved and the absolute temperature of the gas can suddenly become several times that of its surroundings, increasing the original room temperature of the gas to a temperature hot enough to set tinder alight. The air in the cylinder acts both as a source of heat and an oxidizer for the tinder fuel.

The same principle is used in the diesel engine to ignite the fuel in the cylinder rather than the spark plug used in the gasoline engine. It is closer, however, to the hot bulb engine, an early antecedent to the diesel, since the fuel (tinder) is compressed with the gas, while in a diesel it is injected when the gas is already compressed and at the high temperature.

Fire pistons have a compression ratio of about 25 to 1. This compares with about 20:1 for a modern diesel engine and 7:1 - 11.5:1 for a gasoline engine. The piston of the fire piston is made deliberately narrow so that the force on the piston is not so great as to make it impossible for unaided human strength to compress the air in the cylinder to its fullest extent. To achieve the compression ratio, the final compressed volume of the tinder and air must be small relative to that of the length of the piston tube. These two factors together mean that only a tiny amount of tinder can be lit by a fire piston, but this is sufficient to light other tinder, and then to light a larger fire.

Easily combustible materials such as char cloth or amadou work well as tinder in the fire piston. The tinders that work best in the fire piston combust at a very low temperature. Cotton fibres for example combust at 235 °C (455 °F) and will light in fire pistons.