Carburetor heat results in heated air being less dense than ambient air.

• A. True
• B. False
• C. Depends on humidity
• D. Depends on altitude

Answer: (A)

Carburetor heat warms the air entering the engine. When air is heated, its density drops (at roughly constant pressure) because the molecules move apart and occupy more volume. So the air coming into the carburetor after heating is less dense than the cooler ambient air at the same altitude and pressure. This is why carb heat is effective for preventing ice, but it also reduces engine power since less dense air means less mass of air (and fuel) can enter the cylinders. Humidity or altitude don’t change the basic relationship—the density decrease comes from the temperature rise caused by the heat.