Lewis Cross of Balclutha asks :-
In my observation I cannot ignite petrol using the red-hot sparks from an angle grinder without using an intermediary such as dried grass, paper, or flaking paint. Why is this when flint sparks or electrical sparks will ignite petrol?
Colleen Wade, a fire scientist with the Building Research Association of New Zealand (BRANZ), responded.
To ignite petrol, the following three conditions must all be satisfied.
There must be combustible material present in the flammable range. For liquid fuels, the liquid itself will not burn but the vapours will if their concentration, when mixed with oxygen from the air, is within the flammable range. In the case of petrol, a flammable vapour heavier than air forms over the surface of the liquid at room temperature, therefore this condition is easily met. Dried grass, paper, and flaking paint would allow the transport of petrol by capillary action, significantly increasing the surface area of the petrol and decreasing the localized volume of petrol. This works just like a candle wick and leads to more evaporation compared to the same liquid volume in a “pool” with no wicking. This decreases the energy required to ignite the petrol (i.e. makes it easier to ignite). The wicking material can also be an intermediate source of ignition.
There must be an ignition source present. Sparks are a source of ignition, but there are different kinds of sparks. An angle grinder produces mechanical sparks comprising hot metal fragments burning as they move through the air, quickly cooling and forming a layer of ash on their surface as they travel. So even though the piece of metal must be approximately 500 – 1000 degree Celsius to look red, the mass of metal is very small and the total energy it can impart is small. Whereas a rod of metal heated to a red colour with an oxy-acetylene torch would have sufficient energy to ignite petrol - This is not advised. Electrical sparks involve a discharge of electrical current through air. The localized energy (and temperature, for example 1000s of degrees for a spark plug) produced by an electrical discharge is much higher than that from a hot metal fragment.
Thirdly, the ignition source must have sufficient energy to raise the temperature of the fuel at some localized point to higher than the fuel ignition temperature. The minimum ignition temperature of petrol vapour is about 280 degree Celsius. Established flames, electrical sparks (or static) or even the very small hot sparks generated by flint on steel produce localised temperatures higher than this and therefore ignition readily occurs. Grinder sparks comprising metal fragments flying through the air can sometimes ignite flammable vapour directly but its much harder. However the ash flakes may be hot enough and large enough to ignite dried grass etc, producing a small flame that then easily ignites the petrol vapours.