In the figure attached showing the throttle position versus an IC engine produced torque, I didn't understand why does the torque value at a specific rpm vary for different throttle positions. My knowledge for these curves is that the torque actually increases due to more air-fuel mixture entering the chamber. now if we took the 20% throttle position from the graph, or any other percent, we can see that the torque is increasing;but how does that happen when the amount of air-fuel mixture is held fixed (ie : the throttle opening is fixed at (20% for example); and thus the air-fuel mixture entering the combustion chamber is fixed and predetermined by this percentage. Then how would the torque value increase? unless of course we vary the throttle position?
why does the torque value at a specific rpm vary for different throttle positions
This is the difference in acceleration when you fully open the throttle versus open it part-way. For a spark-ignition engine, if you open the throttle more, you let in more air, and so you can also apply more fuel. A carbuerator does this automatically by adding more fuel according to the air flow through it. A modern electronic engine measures the air flow and injects proportionally more fuel. More fuel equals more energy, more torque, and more power. A compression ignition (diesel) engine doesn't actually have a throttle, the air charge is the same and more fuel is applied as you press the accelerator pedal.
now if we took the 20% throttle position from the graph, or any other percent, we can see that the torque is increasing;but how does that happen when the amount of air-fuel mixture is held fixed
The air isn't really fixed, only the throttle valve position is fixed. The throttle valve is just a block to the inlet of the engine. Wide open throttle means minimal blockage. A higher RPM engine pulls more vacuum at the intake which pulls in more air (and thus more fuel is added). Combustion produces more and more energy at higher RPM, but losses add up until they overcome the increase in torque, and less is then available to do work.
Regardless of the position of the throttle if the torque demand ( total torque load on the engine, eg, torque from the wheels and drive train sum of frictions), is less than the available torque, the engine will rev up and more RPM means more fuel, more torque!
Vice versa, if the torque demand is more than available torque the RPM will decrease, regardless of the throttle position, and the engine torque will decrease!
just my opinion it has to do with inertia opening the throttle allows air to fill the manifold and slug the closed valve and reverse direction when the valve opens it is fighting the inertia of the reverse charge and still has to fill the cylinder partial throttle angle vs rpm allows the runner to fill until the intake valve opens and the inertia helps fill the cylinder until piston speed increases like a merry go round if you get it spinning it has inertia so if you pull hard it gets moving and after its moving its storing energy that throws you and breaks your spine and everyone laughs