What is UEL?

• A. Upper explosive limit is the maximum amount of gaseous fuel that can be present for the air to ignite; above this limit, there is too much fuel and the air will not ignite.
• B. Lower explosive limit is the minimum amount of fuel for ignition.
• C. The point at which a vapor exceeds atmospheric pressure.
• D. The temperature for autoignition.

Answer: (A)

The concept being tested is the range of fuel concentration in air that can ignite. A flammable fuel–air mixture will only burn if its concentration lies between the lower and upper explosive limits. The upper explosive limit is the highest concentration of fuel in air that can still support ignition; beyond this point the mixture is too rich (not enough oxygen) for the flame to propagate. That’s why this statement matches UEL exactly: it defines the maximum fuel level that can ignite.

For context, the minimum concentration that can ignite is the lower explosive limit, the idea in one of the other options refers to vapor pressure rather than ignition limits, and autoignition temperature is the temperature at which ignition occurs without an external flame or spark.