Why does torque become positive at the end of suction and exhaust strokes?

[Athi Sankar]

TL;DR

Theoretical turning moment diagram for an four stroke engine has the positive turning moment at the end of suction and exhaust stroke.

Figure 1 shows the Turning Moment diagram of Single Cylinder four stroke Engine. Can anyone tell me, how the turning moment becomes positive at the end of suction and exhaust stroke.

Net force F = Force due to gas pressure - Inertia Force - Reaction Force due to friction

If the net force acting on the piston (F in fig (2) ) becomes positive then only the piston moves towards right side. e(i.e) During expansion stroke the torque will be developed at the crank shaft. Excess Energy stored in flywheel. This excess energy releases whenever the torque is negative to maintain constant output. But as per the fig (1) at the end of suction stroke there is some positive torque can you say the reason?

 

[Baluncore]

I believe you are seeing the momentum of the gas flow in the inlet and exhaust ports.

The induction stroke starts out drawing air through the inlet port into the cylinder. The velocity rises at first but when the piston slows down there is an over-pressure due to the momentum of the induced air.

During the exhaust stroke the gasses accelerate into and along the exhaust port. As the piston slows down, there is a partial vacuum drawn due to momentum of the escaping exhaust gasses.

By tuning the inlet and exhaust ports the momentum effect can be enhanced to draw in more air and to expel more exhaust. That gives more engine power and less mixing of exhaust and inlet gasses.

 

[Athi Sankar]

Thank you