page_title: Fire piston
section_title: Principle of operation
hierarchical_section_title: Fire piston / Principle of operation
context_page_description: A fire piston, sometimes called a fire syringe or a slam rod fire starter, is a device of ancient Southeast Asian origin which is used to kindle fire. It uses the principle of the heating of a gas by rapid and adiabatic compression to ignite a piece of tinder, which is then used to set light to kindling.
context_section_description: 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 thermal equilibrium with them. If the compression is done quickly enough, then there is no time for thermal equilibrium to be achieved. The absolute temperature of the gas can suddenly become much higher than that of its surroundings, increasing from 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 as an oxidizer for the tinder fuel.
  The same principle is used in the diesel engine to ignite the fuel in the cylinder, eliminating the need for a spark plug as used in the gasoline engine. The principle of operation is closer to the hot bulb engine, an early antecedent to the diesel, since the fuel (tinder) is compressed with the gas, while in a diesel the fuel is injected when the gas is already compressed and at a 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 between 7:1 and 11.5:1 for a gasoline engine. The fire piston is made deliberately narrow so that unaided human strength can exert enough force to compress the air in the cylinder to its fullest extent. To achieve a high 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 can be sufficient to light other tinder, and in turn to light a larger fire.
  The tinders that ignite at a very low temperature work best.  Easily-combustible materials such as char cloth or amadou work well as tinder, and can also hold an ember. By contrast, cotton fibers ignite at 455 °F (235 °C) and will flash brightly but do not hold an ember. The bright flash of light is sometimes sufficient for demonstration purposes, but will not start a persistent fire.
  The construction of a hand-operated pump, such as an ordinary bicycle pump, is very similar except that the pump also has valves and a hose to deliver compressed air as an output. In the case of the pump, the heating of the compressed air is an undesired side effect. In both fire piston and pumps, the mechanism and lubricants must be chosen to resist high peak temperatures and pressures.
caption: A 19th-century glass-cylinder fire syringe with a metal piston to which the tinder is attached
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