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I have been trying to understand the torque curve and power curve of an engine. From what I have read and understood I know that the torque curve and power curve is has a peak at a particular RPMs(RPM is different for torque curve and power curve). I also understand that when the engine is running at a particular RPM it will be producing a particular torque(drive torque) and if we increase the load on the engine(increase the load torque) then the RPM will start to decrease. To keep the RPM from decreasing, we can open up the throttle( which will increase the fuel burnt and so the energy produced at the power stroke) and so, increase the drive torque being produced to match the increased load torque.

What I am not able to understand is if the torque value at a particular RPM shown in the torque curve is the maximum possible torque at that RPM (i.e throttle valve fully open at that RPM) or if that torque curve is given for a particular external load(i.e we can still increase the load and to keep the RPM constant we can still open up the throttle valve). I am guessing that the curve shows the maximum possible torque at a particular RPM but I am not sure.

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to produce a torque curve, the throttle is opened, the load increased, the throttle is opened more, the load is increased more, etc., etc. until the throttle is wide open- then the torque measurement is made. then the load is decreased slightly, which causes the RPM to increase slightly, and the torque is measured again. the process of decreasing the load incrementally and measuring the torque after the engine's RPM has increased is continued until the engine's redline RPM is reached. then the process is reversed: the load is increased, which brings the RPM down, torque is measured; then load is increased some more to lower the RPM some more and the torque is measured, and so on until the engine is finally lugged down to the minimum speed at which it can still run smoothly. This produces a torque curve at every point of which the throttle is wide open.

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  • $\begingroup$ So basically throttle opening is not a factor in drawing the throttle-curve as the throttle is always fully open(in automotive engine terms accelerator is fully depressed) when drawing the curve? $\endgroup$
    – GRANZER
    Aug 1, 2018 at 16:27
  • $\begingroup$ as far as I know, that is right. note here that for any operating point on that torque curve, reducing the throttle will reduce the torque which will pull the RPM's down, so as to produce a different curve that lies below the full-throttle case. a whole family of curves can be thus plotted, each corresponding to a different throttle setting. $\endgroup$
    – niels nielsen
    Aug 1, 2018 at 17:02
  • $\begingroup$ "... until the throttle is wide open- then the torque measurement is made. then the load is decreased slightly, which causes the RPM to increase slightly, and the torque is measured again. the process of decreasing the load incrementally and measuring the torque after the engine's RPM has increased is continued until the engine's redline RPM is reached..." As the load is decreased every time before the torque measurement..woudldnt this give a downward sloping 'torque vs RPM' curve instead of the inverted V type of curve? in starts at low torque at low RPM reaches a peak then low. $\endgroup$
    – GRANZER
    Aug 2, 2018 at 3:56
  • $\begingroup$ @granzer, think of it this way: it is the pressure inside the combustion chamber which gets converted into a torque by the connecting rod and the crankshaft. in the ideal world, that pressure would be independent of the rotating speed of the engine, so we would expect the torque curve for an engine to be flat versus RPM. But at high RPM, that pressure falls off because air friction in the inlet system starves the cylinder for air. The peak in the torque curve comes about because of the design of the cam(s) that operate the valves (continued) $\endgroup$
    – niels nielsen
    Aug 2, 2018 at 4:26
  • $\begingroup$ is chosen to produce peak torque at the engine's design speed. below that speed, the exhaust valve (which begins opening before the piston reaches bottom) significantly vents the combustion pressure to the exhaust and decreases the torque. $\endgroup$
    – niels nielsen
    Aug 2, 2018 at 4:30
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Yes the engine would have its throttle wide-open (or whatever position gives the highest torque reading). Each rpm point on the torque curve shows the maximum torque resistance that the engine can maintain and NOT result in a drop in rpm. Once rpm starts to drop, that indicates the engine cannot overcome the amount of torque resistance that is being applied at that time.

The torque curve does not show how much torque the engine is producing at a particular rpm in normal driving situations (or in neutral). The curve shows how much torque can be produced at each rpm point when the engine is running under maximum sustainable load.

Maximum vehicle speed would be obtained by having the final drive ratio adjusted so the rpm where the torque is at the maximum can be reached but not exceeded. Speed is mostly limited by the ability of the engine to overcome the 'drag' on the vehicle, and drag increases with speed. So the top speed is obtained when the engine can produce both 1) its maximum torque and maximum rpm that produces that torque. If the drive ratio permits rpm to increase beyond the 'max torque rpm', that means that less than max torque is actually being produced. if the drive ratio prevents rpm from reaching the 'max torque rpm', that also means that less than max torque is being produced.

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