At the Upper Explosive Limit, which condition prevents ignition due to composition?

• A. There is too much fuel relative to oxygen.
• B. There is too much oxygen relative to fuel.
• C. Vaporization is occurring too slowly.
• D. Ignition source is unavailable.

Answer: (A)

At the upper explosive limit, the mixture is too fuel-rich for the available oxygen. Combustion needs a balance of fuel and oxidizer; when there is too much fuel, there aren’t enough oxygen molecules to react with all of it, so an ignition source cannot produce a sustained flame. The other factors—excess oxygen relative to fuel (a lean condition), slow vaporization, or simply no ignition source—do not describe this upper-limit situation, which is defined by too much fuel relative to oxygen.