Turning moment diagram is the graphical representation of the torque behavior throughout the cycle. The turning moment diagram for four stroke engine is as following
In this diagram has 4 strokes comprised of 2 revolutions. In the first stroke, internal pressure of the cylinder is low than atmospheric pressure so air fuel mixture enters in the combustion chamber. This work is done on the engine so in the turning moment diagram, it is represented by negative area and is shown below the x-axis.
In the next stroke this mixture compresses and in the compression process, large amount of energy is consumed. Therefore the portion of this stroke is much larger compare to other strokes in which work is done on the system.
In the third stroke, mixture burns and large amount of energy is produced which is used to run the engine. The maximum portion of the loop is used to drive the engine in other strokes and the remaining energy is delivered to produce some useful work. After that exhaust gases leaves cylinder which also demands some energy.
In the whole mechanism there are two positives loops exists one is made in suction stroke and the other one is made in exhaust stroke, both are produced due to inertia.