At concentrations above the upper explosive limit (UEL), the fuel has displaced enough air that what occurs?

• A. An inadequate level of oxygen is present to initiate a reaction
• B. There is excessive oxygen causing rapid combustion
• C. The mixture becomes nonflammable
• D. An ignition source is required for ignition

Answer: (A)

At the upper explosive limit, the mixture is too fuel-rich because the excess fuel has displaced much of the air, reducing the oxygen concentration below what’s needed to support a flame. Without enough oxygen, a reaction can’t be initiated even if an ignition source is present. That’s why the correct idea is that there is an inadequate level of oxygen to initiate combustion.