Explanation:

The 4-stroke internal combustion engine has four strokes, which are Intake stroke, Compression stroke, Power stroke, and Exhaust stroke. During the compression stroke, the piston moves from bottom dead center (BDC) to top dead center (TDC), compressing the air-fuel mixture in the combustion chamber. The turning moment during the compression stroke is the torque produced by the engine to rotate the crankshaft.

Positive Turning Moment:

The turning moment during the compression stroke is positive throughout. This means that the torque produced by the engine is in the same direction as the rotation of the crankshaft. The reason for the positive turning moment is that the air-fuel mixture in the combustion chamber is compressed, increasing the pressure and temperature. This increase in pressure and temperature causes the molecules of the air-fuel mixture to move faster and collide more frequently, producing a force that acts on the piston and rotates the crankshaft.

Negative Turning Moment:

If the turning moment during the compression stroke is negative, it means that the torque produced by the engine is opposite to the direction of rotation of the crankshaft. This is not possible in a 4-stroke internal combustion engine because the engine needs to compress the air-fuel mixture to ignite it during the power stroke. If the turning moment during the compression stroke is negative, it means that the engine is not compressing the air-fuel mixture, and the combustion will not take place during the power stroke.

Conclusion:

In conclusion, the turning moment during the compression stroke of a 4-stroke internal combustion engine is positive throughout. The positive turning moment is due to the compression of the air-fuel mixture, which increases the pressure and temperature, producing a force that acts on the piston and rotates the crankshaft.