Fire Piston

Rationale for this lesson

Learn useful fluid mechanics knowledge.
Learn how engineers approach design.
Have fun ==> fire pistons are cool!

The fire piston

Fire piston: A piston/cylinder that uses compression ignition to ignite paper or other materials.


  • Buy a fire piston at Amazon
  • Many website explain how to build. Here is an explanation on YouTube
  • You can find many other websites that give useful information.

Applications. The ideas of the fire piston show up in many engineered products. For example

  • refrigeration system work based on expansion (cooling) and compression (heating) of fluids.
  • diesel engines use compression ignition
  • air compressors require cooling (because the compression of gas causes the gas to heat up).

How Engineers Approach The Design Process

A smart person with no engineering training can easily design and build a FP.

An engineer would do everything a smart layperson would do, but would approach the problem

  • apply a math model to predict what will happen; to understand cause and effect
  • take data on the experiment (experimental validation)
  • understanding ==> why, ….

Math Modeling

Math model = one or more equations that can be used to predict or explain occurrences in the physical world.

Math model of a fire piston ==> 2 Eqns ==>

  • IGL
  • Eqn. for a polytropic process

Let's start building knowledge for math modeling

Units for Pressure

pressure is ratio of normal force of a fluid to area at a point

3 categories of units

  • force/area: Pa, psi, pst
  • height of a fluid column
  • atmospheric pressure: atm, bar

Pressure values to memorize
1 atm = 101.3 kPa = 1.013 bar = 14.7 psi = 10.3 m (33.9 ft) of water = 760 mm (29.92 in) of mercury

How to convert from one pressure unit to another

Gage Pressure, Absolute Pressure, and Vacuum Pressure

  • p_abs = p_atm + p_gage (Eq. 3.3a)
  • p_abs = p_atm - p_vacuum (3.3b)

Why gas gets hot?

macroscopic description: explain w/o resorting to molecules
microscopic description: explain based on what the molecules are doing

Temperature Units

Kelvin & Celsius
Ranking and Fahrenheit


Resource: IGL NETMAD


  • list three categories of pressure
  • covert pressure units
  • convert between gage and absolute pressure
  • explain what macroscopic and microscopic mean
  • convert temperature units (K, deg-C, deg-F, Rankine)
  • carry and cancel units using the ideal gas law
  • define a mole, a lbm-mole, and a kg-mole
  • define an ideal gas
  • apply 3 forms of IGL in traditional and SI units
  • apply the IGL to relevant calculations
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