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Of a combustion engine, the speed, torque, and power is measured at the crankshaft. For a measured pair of values for speed $N$ and torque $M$, can there be more than one value for power $P$? In other words, is the power defined by the torque and speed?

There are opinions trying to convince me that it depends on the engine's mode of operation, which depends on various factors such as internal temperatures and pressures. Such changes between modes of operation are intended to prolong the engine's life and/or reduce fuel consumption.

I know, this kind of question again... But please, I am trying to fight an "argument from authority" here and nothing really answers my question with regards to combustion engines.

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  • $\begingroup$ By speed, do you just mean linear speed of the vehicle? $\endgroup$
    – JMac
    May 16, 2020 at 15:19
  • $\begingroup$ Engine speed, revolutions per time interval $\endgroup$
    – jkbx
    May 17, 2020 at 6:12

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Any combustion engine is capable of operation at a variety of RPM's and torque output combinations that depend on the nature of the load that the engine is driving. Since delivered power is the product of torque and RPM, this means that there are many different combinations of the two across the engine's operating regime that will yield the same net power output.

For example, an engine connected to a light load and running at a high speed can deliver exactly the same power as the same engine connected to a heavy load and "lugged down" to a low RPM.

This characteristic gives the operator of the engine a lot of flexibility in choosing where they want to be on the engine's operating curve, but note the following: If the engine is being operated for maximum power, then there will be only one combination of torque and RPM available to maximize their product, which occurs when both torque and RPM are numerically identical.

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  • $\begingroup$ OK, but can there be two different power outputs for the same torque and speed combination? For example 3000 rpm and 200 Nm resulting in two different power values, depending on how the engine is operated? $\endgroup$
    – jkbx
    May 17, 2020 at 6:12
  • $\begingroup$ nope. if the torque and speed are both the same, the engine is operating at the same point and the power will be exactly the same. power = torque x rpm $\endgroup$ May 17, 2020 at 17:26

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