How does the combination of intake heat and turbocharged air influence engine knock and fueling strategy?

• A. Hot intake air increases density and reduces knock.
• B. Hot intake air decreases density and reduces knock tendency.
• C. Intake heat has no effect on knock.
• D. Hot intake air reduces density and increases knock tendency; fueling must be adjusted to avoid overly rich mixtures that reduce efficiency.

Answer: (D)

Intake charge temperature directly affects how much oxygen is available and how hot the charge will ignite. Hot intake air is less dense, so for the same volume you get fewer oxygen molecules. In a turbocharged system, the air is also pressurized, which increases the potential for high in-cylinder temperatures; the result is a higher tendency for premature ignition or knock. Because the engine’s fuel metering is tied to the amount of air entering the cylinders, hot, less-dense air means that if you keep the same fuel quantity, the mixture becomes richer than intended. That not only wastes fuel but can reduce efficiency and performance. To maintain the proper combustion, fueling must be adjusted to match the actual air mass entering the cylinders, avoiding an overly rich mixture that hurts efficiency.