Ignition

Ignition is the initiation of combustion in a fuel-oxidiser system. It can produce a flame, glowing combustion or smouldering, depending on the fuel and exposure. Initial combustion may be brief or may persist after external heating is removed. Its onset depends on fuel production, mixing with oxygen, temperature, reaction rate and heat loss.

An ignition source and an ignitable material are therefore not sufficient as separate labels. Their interaction must produce suitable conditions in the same place and at the same time. A spark can fail near a poorly mixed vapour cloud, while a weaker source can succeed within a suitable mixture.

Ignition is also a stage within fire development rather than a complete description of the later fire. The initial reaction does not determine subsequent flame spread or the involvement of an entire enclosure. Persistent flaming requires continuing fuel and heat feedback, while transient ignition can produce a flame that disappears when external heating is removed. Prevention, detection and suppression depend on which of these conditions has been reached.

Piloted, non-piloted and self-induced ignition

Piloted ignition uses a local hot object or energy source such as a flame or spark to initiate combustion in a prepared fuel-air mixture. The pilot is not necessarily responsible for heating the entire solid or producing all its fuel gases. Its principal role is to start a reaction that can then propagate.

Three ignition modes with different initiating conditions. Based on Australian Building Codes Board et al. (2005), p. 2.4-4.

Non-piloted ignition occurs when the fuel-vapour and oxygen mixture reaches a condition in which it ignites without that local pilot. Temperature and composition remain essential, and heat losses must not prevent the reaction from accelerating. The absence of a visible flame before ignition does not mean that no energy was supplied.

Self-induced ignition begins with oxidation inside a solid material. These reactions release energy, increasing decomposition and temperature until the critical condition is crossed. The process is often associated with heat being retained by poor conduction through the material or its container.

Self-heating describes this internal rise caused by chemical heat generation exceeding heat loss. Larger bodies and higher ambient temperatures can favour criticality by reducing relative heat loss. The result depends on material chemistry, size, shape and initial temperature rather than on one universal ambient threshold.

Fuel gases, liquids and solids

For gaseous fuels, composition and mixing are immediate concerns. Fuel and oxidiser must occur within a flammable range, and the ignition source must exceed the mixture's requirements. The flame can then propagate through the connected mixture rather than remaining only at the source.

Liquids usually have to evaporate before flaming combustion. The vapour above the surface mixes with air and can flash when piloted. Sustained burning requires enough heat feedback to maintain evaporation after the initial flash. Liquid bulk temperature, volatility, ventilation and container geometry all affect this sequence.

Many solids require Pyrolysis before they can support a gas-phase flame. Heating decomposes the material and releases volatile products, often leaving char. These volatiles mix with oxygen in the boundary layer and ignite when their composition and thermal condition are suitable.

The solid's thermal response therefore forms only one part of ignition. Kanury groups the problem into production of combustible gases, their mixing with oxidant and induction of a sufficiently rapid, self-supporting reaction. Material properties, geometry and gas flow influence all three stages.

Smouldering and glowing are different combustion modes. Smouldering involves subsurface or surface oxidation in a porous fuel, while flaming occurs in the gas phase. A fire scenario should not assume that every ignition immediately produces a visible flame.

Heat exposure and ignition time

Critical heat flux for ignition can be estimated by extrapolating ignition-test data towards an indefinitely long ignition time. It is a parameter of the fitted model, distinct from an experimentally determined minimum flux. A surface can absorb energy while simultaneously losing heat through conduction, convection and radiation.

At the start of radiant heating, energy arriving at a solid is divided between conduction into the interior, reradiation and convective heating of nearby gas. Moisture evaporation and later pyrolysis change this partition. The surface and interior consequently follow different temperature histories.

Energy partition during heating and ignition. Based on Kanury (2002a), p. 2-232.

Time to ignition is the exposure duration needed to cross the ignition condition. It depends on heating intensity, material response, thickness, initial temperature and whether transient or persistent ignition is sought. Longer exposure at lower intensity cannot always be traded directly against shorter exposure at higher intensity.

Kanury's discussion of Martin's map distinguishes nonignition, glowing ignition, transient ignition and persistent ignition regions for the tested cellulosic arrangements. Thin specimens can ignite quickly without producing persistent flaming, while thick specimens can lose energy into their interior. At very high exposure, surface ablation can occur without ignition in the mapped conditions.

The 2005 International Fire Engineering Guidelines note that direct flame contact with a second item is sometimes treated conservatively as immediate ignition. This simplification ignores the time actually required for pyrolysis and heating of decomposition products. It is a modelling choice for that context, not a universal physical observation.

Autoignition and critical conditions

Autoignition temperature is associated with ignition of a gas or vapour mixture without a pilot under specified test conditions. It differs from the temperature of a hot surface or from the bulk temperature of every reacting solid. Test geometry, pressure and exposure can affect the measured result.

Self-heating bodies have a critical condition representing a watershed between stable subcritical behaviour and thermal runaway. Chemical reaction rates can rise steeply with temperature, while heat loss changes more gradually. Once generation exceeds loss sufficiently, internal temperature can accelerate upwards.

Critical ambient temperature is used when body size is fixed, while critical size or diameter is relevant when ambient temperature is fixed. Preheated materials introduce a further stacking-temperature consideration. These variables show why one material cannot be assigned a universal spontaneous-ignition temperature in every storage arrangement.

The location of runaway is often internal. Gray describes charring or ash at the centre while the exterior remains comparatively intact in some spontaneously ignited bodies. Such deep-seated combustion can be difficult to extinguish and can reignite after apparent suppression.

Flash point describes a liquid temperature at which the vapour-air mixture just above the surface supports a momentary flash under a specified piloted test. The flame can sweep through the mixture and then go out. It does not yet demonstrate persistent combustion of the liquid.

Fire point instead requires self-sustained burning. This distinction mirrors Kanury's broader separation of transient and persistent ignition. An engineering assessment should therefore state whether it concerns the first observable reaction, a propagating flame or a fire that continues after the source is removed.

Smouldering can also precede later flaming. Transition requires a flammable pyrolysis-gas mixture and sufficient heat to ignite it at the same location and time. Enhanced oxygen supply, reduced heat loss and local geometry can promote this change, but it is not an automatic stage of every smouldering fire.

Ignition sources and the fire triangle

An Ignition source supplies the local energy or reactive condition needed to initiate combustion. Examples include a small flame, heated wire, electric spark or incandescent particle. The source's effectiveness depends on the mixture it contacts and on heat lost to the surroundings.

The Fire triangle places ignition source within the wider combination of fuel, oxygen and heat. Most building environments already contain fuel and oxygen, so qualitative prevention often focuses on separating them from open flames, sparks and sufficiently hot surfaces. The model remains a checklist rather than an ignition-time calculation.

For gases and vapours, avoiding flammable concentrations is a principal strategy. The 2005 guidelines recommend keeping concentrations well below the lower flammable limit rather than relying on a rich mixture remaining above the upper limit. Dilution can move a rich mixture back into the ignitable range.

Ignition can also pass from one object to another. NUREG-1805 uses the example of a burning wastebasket igniting a chair, which can then ignite further fuel. The initial igniter and subsequent burning objects become sources in a sequence, so the fire history is not limited to the first device.

Measurements and material data

Ignition tests impose controlled radiant or convective exposure and observe the response. The measured temperature, heat flux and time depend on apparatus, specimen thickness, backing and pilot arrangement. Results should retain these conditions instead of being reduced to an unqualified material label.

The 2005 guidelines list grouped ignition-temperature ranges for several solid categories. Wood, paper and cotton are listed at 250 to 365 °C for piloted ignition and 250 to 400 °C for autoignition. These older grouped values illustrate variability and should not be used as universal thresholds for every product and exposure.

Grouped ignition-temperature ranges in the 2005 IFEG. Based on Australian Building Codes Board et al. (2005), p. 3.3-4.

Exposure duration changes the appropriate data. The guidelines note that many wood studies used exposures of 20 minutes or less and provide a lower heat-flux result from a day-long exposure. Long-term self-heating requires a different treatment again, including the size of the stored aggregation.

Material properties also change during heating. Moisture leaves, char forms and some polymers melt before decomposition. These changes affect heat transport and volatile release, so an ignition model based on unchanged properties can apply only within its stated assumptions.

Prevention and engineering assessment

Prevention can address fuel condition, oxidiser concentration, ignition energy and heat retention. Keeping work areas free of unnecessary combustibles, controlling flammable atmospheres and separating hot surfaces from fuel each acts on a different part of the ignition problem. Their suitability depends on the scenario.

For subsequent items, radiant exposure depends on the burning item and separation distance. The 2005 guidelines describe a simplified pre-flashover method in which radiation from the upper layer and room surfaces is neglected and only flame radiation from the first item is included. That assumption must not be transferred to a developed compartment fire.

Probabilistic studies can use fire statistics for ignition frequency by building class and ignited material. The 2005 guidelines state that general quantitative methods for predicting ignition potential were not available in that document. Statistical frequencies support analysis but do not replace scenario-specific assessment of sources and fuels.

Self-heating requires particular caution in extrapolation. Gray warns that time to ignition is highly sensitive to material, size, initial temperature and distance from the critical condition. Hot stacked material can ignite much sooner than a simple ambient-temperature model predicts.

Calculating ignition under radiant heating

For a thermally thick solid under constant radiant exposure, a simplified correlation estimates the time needed for piloted ignition. Thermally thick means that the interior remains appreciably cooler during the relevant heating period. The model combines the external heat flux with two properties fitted from ignition tests. Both properties must belong to the same material and correlation.

The thermal response parameter represents resistance to surface heating. Subtracting the fitted critical heat flux from the external exposure represents the margin of heating above that model's threshold. In the Tewarson method reproduced in NUREG-1805, ignition time depends on the square of their ratio. The equation applies during transient heating before steady conditions develop.

$$t_{ig}=\frac{\pi }{4}{\left(\frac{\mathrm{T}\mathrm{R}\mathrm{P}}{{\dot{q}}^{\prime }{\text{'}}_e-\mathrm{C}\mathrm{H}\mathrm{F}}\right)}^2$$

Symbol	Description	Unit
$t_{ig}$	Calculated time to piloted ignition	s
$\mathrm{T}\mathrm{R}\mathrm{P}$	Thermal response parameter from the specified correlation	kW·s1/2/m²
${\dot{q}}^{\prime }{\text{'}}_e$	Constant external radiant heat flux	kW/m²
$\mathrm{C}\mathrm{H}\mathrm{F}$	Critical heat flux estimated from that correlation	kW/m²

For red oak, the report lists a critical heat flux of 10 kW/m² and a thermal response parameter of 134 kW·s^1/2/m². Choose a constant external exposure of 30 kW/m² for this illustrative calculation. Substituting the tabulated material values gives about 35 s to ignition. The result represents the stated model and exposure, rather than a measured ignition time for every oak product.

$$t_{ig}=\frac{\pi }{4}{\left(\frac{134}{30-10}\right)}^2=35.256\dots \mathrm{s}\approx 35\mathrm{s}$$

At a chosen exposure of 20 kW/m², the same calculation gives approximately 140 s. Halving the margin above the critical heat flux multiplies the predicted ignition time by four. This illustrates why a heat-flux value needs an exposure duration when assessing ignition. The unrounded calculation is retained until the final reported time.

The equation is used only above the fitted critical flux. At or below that value, substituting numbers does not yield a valid ignition-time prediction. Near the threshold, the estimate becomes highly sensitive to the flux and fitted properties. A positive result produced by squaring a negative denominator would have no physical meaning in this model.

The report explicitly distinguishes the extrapolated critical flux from the minimum flux observed under practical conditions. Other correlations fit the data differently and produce different effective properties. Substituting a thermal response parameter from one method into another can therefore destroy the connection with the tests. Variable heating, thin specimens and long-duration self-heating also require separate treatment.

An ignition calculation predicts a specified observation, such as the appearance of piloted flaming. It does not establish that the fire will persist after the source is removed or spread to another item. Forced pyrolysis may produce smoke before combustion, while smouldering can begin without a visible flame. Those different outcomes need their own observation criteria.

See also

• Firebrands
• Flame spread
• Smouldering-to-flaming transition

References

• Australian Building Codes Board, National Research Council of Canada, International Code Council, & Department of Building and Housing New Zealand. (2005). International fire engineering guidelines (2005 ed.). Australian Building Codes Board.
• Beyler, C. L. (2002). Flammability limits of premixed and diffusion flames. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed., pp. 2-172–2-187). National Fire Protection Association.
• Beyler, C. L., & Hirschler, M. M. (2002). Thermal decomposition of polymers. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed., pp. 1-110–1-131). National Fire Protection Association.
• Gray, B. (2002). Spontaneous combustion and self-heating. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed., pp. 2-211–2-228). National Fire Protection Association.
• Iqbal, N., & Salley, M. H. (2004). Fire dynamics tools (FDTs): Quantitative fire hazard analysis methods for the U.S. Nuclear Regulatory Commission fire protection inspection program (NUREG-1805). U.S. Nuclear Regulatory Commission.
• Kanury, A. M. (2002a). Flaming ignition of solid fuels. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed., pp. 2-229–2-245). National Fire Protection Association.
• Kanury, A. M. (2002b). Ignition of liquid fuels. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed., pp. 2-188–2-199). National Fire Protection Association.
• Ohlemiller, T. J. (2002). Smoldering combustion. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed., pp. 2-200–2-210). National Fire Protection Association.
• Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.