A fire piston consists of a hollow cylinder ranged in size from around 3 to 6 inches (7.5 cm to 15 cm) long, sealed at one end and open at the other, with a diameter of a quarter inch plus the width of the seal on the piston. A piston, about a quarter inch (ca 0.64 cm) in diameter, with some type of seal mounted in such a way that as the piston slides into the cylinder, an airtight seal is formed with the cylinder wall. The piston has a handle on the end to allow a firm grip to be applied to it, or a large enough surface area to 'whack' it on a hard surface without causing pain, and it can be completely withdrawn from the cylinder.
Native and modern versions are made from wood, animal horns, antlers and bamboo or wood. Lead was used to cast fire pistons in both modern and native versions. Other metals have also been used in modern versions. The piston has a notch or recess on or in its face, into which a piece of tinder is placed.
The compression of the air when the piston is quickly rammed into the cylinder causes the temperature to rise rapidly to 260 degrees Celsius, or 500 degrees Fahrenheit. This is hot enough for the tinder on or in the piston face to ignite with a visible 'flash' that can be seen if the cylnder is translucent or transparent.
The piston must be rammed quickly into the sealed cylinder (or vice-versa) with a single rapid stroke, or smacked down onto a hard surface (upside-down), and withdrawn almost as quickly, or the depletion of oxygen during the 'flash' will cause the tinder to go out just after it lights.
The tinder can then be withdrawn from the face of the cylinder, transferred to a larger 'nest' of fine kindling material such as hemp rope fibers, orbirch shavings, etc. It is then blown upon quite vigorously to create a flame, which can then be added to larger kindling, in place under largerand larger pieces of firewood, until built into a proper fire. The actual construction of the fire must logically precede use of the fire piston, else what is there to light with the ember?
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 leak away 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 10:1 for a gasoline engine. The piston of the firepiston is made deliberately narrow so that the force on the piston is not too great 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 fibers for example combust at 235 °C (455 °F) and will light in fire pistons.