Why does torque plateau after a certain point?

Answer 1

I also asked the same question but could not find satisfactory answer, so I decided to try smacking this myself. Note that this discussion is limited to petrol engines. This may be a bit long but here it goes.

It's a bit theoretical but it is important to know "the basics" prior to further discussion. "Torque" is actually the amount of "work" done (in Newton.meter or Joule). It is a function of how much Newton of "force" being exerted on the engine's piston during expansion in "combustion stroke". If you refer to Otto / Diesel Cycle Temperature-Entropy (T-s) diagram, engine output can be increased at higher pressure ratio (volumetric energy from air density) or higher firing temperature (thermal energy by adding more fuel). So, more air + more fuel = more power..

What is power then? Well I simply view "power" as the "rate of doing work" (Joule/sec or kW). Hence, for the same amount of torque, the power increases at higher RPM (Power = Torque X Rotational Speed). Agree?

OK, now let's discuss on the power / torque vs RPM curve usually supplied by car manufacturers. In naturally-aspirated (NA) engines, initially the torque slowly increase, peaks at certain speed (say 4000 RPM) and started to decline beyond that point. As we've understand that torque and power are related, this torque dip reduces the power increase slope and eventually diminishes the power at a later speed (say 6500 RPM). This happens when the depleting of torque is more significant than increase in engine speed. Make sense, right?

In NA engines, I think the torque started to reduce because of 2 main reasons. Firstly, limitation is on the "intake valve lift" that controls the openings of air passage into the cylinder. At higher RPM, more air is required but the flow is "choked" by this constant opening area. This is the reason why some engines are designed with "variable intake valve lift" to cater for various RPM ranges i.e. small openings at low RPM (for fuel efficiency) and larger openings at higher RPMs (to sustain or even increase torque and power).

Secondly, the air into the cylinder is always at ambient pressure regardless of engine RPM. The amount of air drawn into the cylinder solely depends on the negative "suction" created by the downward movement of the piston during intake stroke. At higher RPM, constant atmospheric inlet pressure could not catch up with rapid cylinder volume expansion from faster piston movement. This results in lower air density available for combustion (although can be compensated with higher fuel consumption).

This is where forced-induction engines come into the picture. In turbocharged or supercharged engines, air pressure at the intake is compressed to slightly above atmospheric pressure, meaning there is a "pushing" of air into the cylinder to complement the "pulling" by the piston during intake stroke. This extra help enables peak torque to be achieved much earlier (say 1500 RPM). Another benefit is sufficient amount of presurized air can be supplied at a wide span of engine speed (say 1500 to 5000 RPM) and constant torque value can be achieved within this range. This flat torque profile is what you referred as "torque plateau". Using this technique, the power output rate can be rapidly increased across the RPM and "higher peak power" can be achieved with a relatively small engine capacity. all this just by playing with the torque value!

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Answer 2

Torque plateaus after a certain point because the engine's power output begins to decrease as engine speed increases. This is due to limitations in valve timing, airflow, and other factors that can no longer produce an increase in torque.