How does over boosting occur?

• A. Temperature of the fuel-air mixture will rise above the safe limit and detonation may occur
• B. Exhaust temperature increases
• C. Fuel mixture becomes too rich
• D. Turbine speed decreases

Answer: (A)

Over boost happens when the boost pressure in the intake system goes above what the engine is designed to safely handle, pushing more air into the cylinders than intended. That extra air is compressed to a higher pressure and, as a result, the fuel-air mixture reaches a higher temperature. When the mixture temperature rises above safe limits, it can autoignite before the spark plug fires, causing detonation or knocking and risking engine damage.

Exhaust temperature increasing isn’t the mechanism for overboost; it’s more about what happens in the exhaust stream or afterturbine. A fuel mixture becoming too rich describes a fuel–air ratio issue, not the pressure surge that defines overboost. Turbine speed decreasing would reduce boost rather than cause it to exceed safe limits.