Remember back to your College days, the "fire triangle", which says, that all 3 sides of the triangle, must be present, in sufficient amount, for a fire or explosion to occur - FUEL, OXYGEN and HEAT. If you eliminate any one of the 3, no fire can occur.
So, if oxygen content is lowered, eventually there will be a point, where the composition of a mixture will not have enough oxygen to support fire or explosion, even if otherwise sufficient amounts of fuel and heat are available. That point, below which there is insufficient oxygen to support combustion is sometimes called the LIMITING OXYGEN INDEX or (LOI) or it is sometimes also called the Maximum Oxygen Concentration (MOC), depending on the source of the data. So, the threshold where fire or explosion can occur is above the LOI or MOC; and below the LOI or MOC, no fire or explosion can occur.
The value of the LOI or MOC differs somewhat, depending on the particular type of fuel and the temperature range, but several useful generalizations are valid.
For most hydrocarbon fuels, such as methane (natural gas), propane, gasoline, kerosene, LOI or MOC values lie in a fairly narrow range between approximately 9 and 12% oxygen. These types of fuels are made up of molecules containing only carbon and hydrogen atoms in their chemical structures.
For some fuels and fuel additives, which have oxygen atoms as part of their chemical structure, such as ethyl alcohol (ethanol), the LOI or MOC value is slightly lower, approximately 8% oxygen. This is understandable, since the oxygen atoms in the chemical structure can contribute towards combustion.
Higher temperatures generally lead to lower values of LOI or MOC for a particular fuel.
Good accurate data on MOCs is hard to come by, in part because most of the available technical information on flammability of fuels has historically only considered the source of oxygen to be regular air, which has about 21% oxygen content. The attached article is among the best general summary of MOC values, although it is a bit too detailed to be easy to use for a wide audience. This article is from a German university and it discusses the flammability data in light of European regulatory agencies. However, the information is the most up to date and relevant for discussion purposes.
The article shows in Table 1, MOC values for several representative chemicals or fuels, including methane, ethanol and hexane (a major component of gasoline). Note that the MOC values are shown for 20C (68F) and 100C (212F) temperatures. Also note, that since this article is from the technical literature, it points out that there are some minor disagreements among values reported by different literature publications. This is common for experimentally determined data.
Note also, that some other information, important to understanding fire hazards, is also given in Table 1. The LEL (lower explosion limit) and UEL (upper explosion limit) values are shown in Table 1, in addition to the MOC values. The LEL and UEL values are the limiting fuel concentrations in a mixture with regular air. Explosion or fire can only occur when the fuel concentration is BETWEEN the LEL and UEL, for air present at its normal oxygen concentration of about 20.9% oxygen. So, the LEL and UEL values are useful to know, if normal air may be present with a particular concentration of the fuel vapor. But, for our purposes, the MOC is what we want to focus on, for situations where you have oxygen concentrations lower than in normal air, because we are altering the oxygen concentration in your tires.
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