In a four-stroke cycle aircraft engine, when does the intake valve close?

The intake valve closes at the end of the intake stroke, just before the beginning of the compression stroke. This timing is crucial in the operation of a four-stroke engine, as it ensures that the cylinder is sealed properly to compress the fuel-air mixture during the compression stroke.

In the context of the options given, the correct answer reflects that after the intake valve has allowed the mixture to enter the cylinder, it closes as the piston moves upward, preparing for compression. This closure is essential because it maintains the integrity of the combustion chamber, allowing for efficient compression and subsequent ignition during the power stroke that follows.

The other options do not accurately describe the timing of the intake valve's operation in the four-stroke cycle. For instance, while the exhaust valve does open during the exhaust stroke, this does not relate to the timing of the intake valve closing. Similarly, stating that the intake valve closes on the intake stroke misrepresents the sequence, as it actually closes at the end of this stroke, not during it. Lastly, the intake valve opening on the power stroke is incorrect because the valve must be closed during this phase to maintain pressure for combustion.