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A fire piston, sometimes called a fire syringe or a slam rod fire starter, is a device of ancient origin which is used to kindle fire. It uses the principle of the heating of a gas (in this case air) by rapid and adiabatic compression to ignite a piece of tinder, which is then used to set light to kindling.
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Description and use
A fire piston consists of a hollow cylinder ranging in length from about 3 to 6 inches (7.5 to 15 cm), having a bore about 0.25 inch (6-7mm) in diameter, sealed at one end and open at the other. A piston with an airtight circular seal is fitted into the cylinder. 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to strike it sharply without causing pain while the cylinder is braced against a hard surface, and it can be completely withdrawn from the cylinder. The piston generally 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 interior temperature to rise sharply to 500 °F (260 °C). 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 cylinder is made of translucent or transparent material. The piston is then quickly withdrawn, before the now-burning tinder depletes the available oxygen inside the cylinder. The smouldering tinder can then be removed from the face of the piston and transferred to a larger nest of fine tinder material. The ember is then fanned or blown upon vigorously to create a flame, at which time various stages of larger kindling can be added until built into a proper fire.
Ancient and modern versions of fire pistons have been made from wood, animal horns, antlers, bamboo, or lead. Other metals have also been used in modern versions.
History
Fire pistons have been used in South East Asia and the Pacific Islands as a means of kindling fire for years. They are found in cultures where the blow pipe is used as a weapon and this suggests they may have developed out of blow pipe construction. Their use has been reported from Burma, the Malay Peninsula, Indo-China, Borneo, Sumatra, Java, Kalimantan, Sulawesi, the Philippines, Madagascar and South India.
An 1876 New York Times article reported the discovery of the earliest date of its use in the west. It reports an address by a Professor Govi that claimed a book written by Father Boscovich, of Rome in 1755, De Litteraria Expeditione per Pontifican Ditionem, (The Clever Mechanism) makes the claim that the fire piston was invented in 1745 by Abbe Augustin Ruffo. This report also claims that the modern fire piston was reinvented independently in the west through experiments with the air gun and not modeled after Asian designs.
It is recorded that the first fire piston made its wider debut in front of scientists in 1802 and was patented in 1807 simultaneously in both England and France. Fire pistons, or fire syringes as they were called then, were popular household tools throughout Europe during the early nineteenth century until the safety match was invented in 1844.
The fire piston may have inspired Rudolf Diesel in his creation of the diesel engine around 1892.
As a scientific curiosity
A fire syringe is a piston-and-cylinder device used to demonstrate compression ignition. A typical fire syringe is a thick-walled transparent cylinder fitted with a metal piston which can travel to within a short distance of the bottom of the cylinder.
Compression ignition is demonstrated by placing a matchhead or other piece of tinder at the bottom of the fire syringe and then plunging the piston forcefully into the cylinder. The tinder then bursts into flame due to the rapid rise in temperature which accompanies the sudden reduction in volume (and increase in pressure) of the air beneath the piston. The energy provided by the arm muscles working to compress the air is transferred into the much reduced volume of the air during compression and manifests itself as heat energy sufficient to ignite the tinder.
How it works
Rapid compression of a gas 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 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 455 °F (235 °C) and will light in fire pistons.