What occurs if liquid is present in the cylinder of an engine during propeller rotation?

When liquid is present in the cylinder of an engine during propeller rotation, the situation leads to a condition known as "hydraulic lock." This occurs because liquids are incompressible, and if a piston encounters a liquid in the cylinder, it cannot compress it like it can compress air or a gas. As a result, the presence of the liquid creates a situation where the engine experiences resistance to movement.

When the propeller is turned, the engine requires abnormal force to rotate due to the liquid being trapped in the cylinder. This means that the propeller will not turn freely and can even cause mechanical damage if excessive force is applied. The abnormal effort required indicates that there is a blockage preventing the piston from moving as expected, leading to severe operational issues.

Understanding the mechanics of engine operation reinforces the importance of ensuring that cylinders are clear of liquids before attempting to start or rotate the propeller, as doing so helps prevent potential engine damage and ensures safe operation.