As previously discussed, there are several ways in which fuel vapours can ignite: piloted ignition, auto-ignition or spontaneous ignition.

In Figure 41, the first image shows a cutting torch, which is the independent source of energy to which heat is added to ignite the fuel vapours by piloted ignition. Sometimes fuel-air mixtures will ignite through a process called auto-ignition, as is demonstrated in the second image in Figure 41. In this case, there is no direct contact between the flammable fuel mixture and the ignition source. Instead, the fuel-air mixture is heated indirectly and reaches a temperature at which it will ignite without the presence of an external source.

The final way a fuel-air mixture can ignite is through spontaneous ignition, as shown in the final image in Figure 41. Some fuels will begin to undergo exothermic reactions all by themselves, often through decomposition. The energy given off during these decomposition reactions will raise the temperatures in the local area and the reactions will increase in intensity. Gradually, enough fuel vapour is released, it mixes with air, and the mixture is raised to its auto-ignition temperature. When turpentine has soaked into a pile of rags, as is shown in the image on the right, the rags can trap the heat that is released naturally as the turpentine oxidizes. The rags can eventually become hot enough (this can take hours or even days) that enough flammable vapours are released, mixed with air and heated above the auto-ignition temperature, causing them to spontaneously ignite and burn.