Carburetor heat introduces heated air into the carburetor to prevent icing. Warmer air is less dense, so for the same volume of air the engine receives less mass. With the throttle set, this reduced mass flow lowers the pressure in the intake manifold, i.e., manifold pressure drops. The path through the carb heat system adds resistance, creating a greater restriction to airflow, and the heated air is less dense than the air it replaces. The combination of lower ram air pressure behind that restricted path and the lower air density explains the drop in manifold pressure when carb heat is selected.

The system works by the pilot setting the desired manifold pressure with the throttle, and then a sensor such as an aneroid capsule provides the real-time pressure signal to the boost-control mechanism. This signal drives the wastegate actuator, which automatically moves the wastegate to regulate exhaust flow and maintain the set manifold pressure. In other words, the waste gate is the component whose position is adjusted to achieve the required turbocharging.

Moisture control in aircraft fuel tanks is the key idea. When the tanks are not full, the air inside contains water vapor. As the aircraft temperature drops overnight, that air cools and the water vapor can condense into liquid water, which settles at the bottom of the tank and can be drawn into the fuel system or cause icing at altitude. By filling the tanks at the end of the day, you reduce the amount of air in the tank, leaving less opportunity for condensation to form, and any moisture can be found and drained from the sump. Evaporation is a less significant factor here, and the other options aren’t related to how moisture behaves in cool-down conditions, so the primary purpose is to minimize condensation of water vapor as temperatures fall.

The exhaust stroke is the one that clears the cylinder of spent combustion products. As the piston moves upward, the exhaust valve opens and pushes the burnt gases out through the exhaust system. This clearing is essential so that the next induction stroke can draw in a fresh air–fuel charge. The other strokes have different roles: induction brings the charge in, compression tightens it for efficient combustion, and the power stroke uses that combustion to push the piston down. None of those stages are fundamentally about expelling the exhaust gases.