Why is there a negative torque in TMD of 4 stroke engine?

[pravi]

In the torque moment diagram of 4 stroke engine, there is a negative loop during suction stroke. Many books state that it is due to the higher atmospheric pressure. But, I do not understand the concept. Please help me understand why there is a negative and positive loop during suction and exhaust stroke. I would also like to know the usage and working of the TMD. Please help me. Thanks.

 

[BvU]

Hello Pravi, it's high time you were welcomed to PF: welcome !

During the suction stroke, the volume above the piston increases and the piston has to pull in new fuel and air. If it's vertical and going down, then above the piston there is a low pressure and below atmospheric.

Did you look at a few tutorials, e.g here ?