IN internal combustion engines, when the explosion in the chamber occur producing power this power appears as an increase in torque and RPM, what confuses me is that how I could raise the RPM while the torque and power is decreasing after the point where the power peaks. from my understanding the combustion in the chamber become less efficient at higher speed . but what i could not figure out is how the rpm increase when the amount of power I could get from the combustion decrease. What factors are dependent on the others

  • Its all in the camshaft profile where horespower-torque peaks. There are other less factors such as compression, fuel octane and volumetric efficiency. – Moab Apr 8 '20 at 17:29

when the explosion in the chamber occur producing power this power appears as an increase in torque and RPM


Combustion only creates torque. If this torque is greater than the counter torque created by internal friction, (air) drag and the slope of the street, then the car will accelerate, and RPM will rise. If both are equal, the speed / RPM stays constant. And if created torque is less than counter torque, speed and RPM will drop.

A normal petrol car has its max torque somewhere at 3000RPM, that is, the difference between created torque and counter torque is big, and allows for high acceleration.

At much higher RPM, torque is much lower, but as long as it's still greater than the counter torque, the car still accelerates. Since air drag increases and produced torque decreases, there is a point where both torques cancel each other out, and where the car reaches a constant, maximum speed.

Now, I did not mention power. Power is torque times RPM times (constant factor). If you look at a typical power/torque vs. RPM chart, power increases with RPM almost the entire range. That is because torque decreases slowly with RPM, so the product still increases over a wide range. It is only next to the red RPM line, where torque decreases fast enough to let the power drop.


The time available for the expanding charge to deliver force on the piston is reduced as the engine speed increases.

Other things to consider is that as the engine speed increases the engine requires more air ie a higher flow rate but the losses caused by the air filter etc also increase and the time the inlet valve is open is shorter as well.

You can think about all the parameters wrt time and consider the effects.

  • the problem with me is that how to imagine the sequence of the process , the sequence through which the parameters interact, what confuses me is what happen first ? – A.ELAK Apr 8 '20 at 17:26
  • how the engine speed increase in the first place , how the rpm is raised? – A.ELAK Apr 8 '20 at 17:36
  • it has noting to do with the time available for expanding. (of course there are technical limits for piston speed) but other than that it depends on how well you can fill your cylinder to get the maximum possible pressure since torque is the pressure of the explosion x surface area of your piston (bore) x leverage of that force on the crankshaft (stroke) – Daniel Apr 11 '20 at 0:08
  • @Daniel power (in watts) is energy (Joules) multiplied by time and you say time is not relevant... – Solar Mike Apr 11 '20 at 3:44
  • Torque is not Power. Power in Watts/KW/HP is Torque multiplied by rpm<-(Times that torque per minute). Power is theoretically greater the higher the rpm, provided you can still make the engine function with decent torque. – Daniel Apr 11 '20 at 12:46

Its just a simple mathematical function. Power (HP) = Torque (lb-ft) x RPM ÷ 5252 So for example if your torque at say 3000 rpm is 200 lb-ft, you have about 114 HP. If your torque at 4000 rpm drops to 140 lb-ft you only get 91 HP.

Think of torque as a force trying to rotate the crankshaft. The expanding gases of your explosion pushing the piston down.

Your motor only runs optimal in a specific rpm-band. This depends on how a lot of the components are sized to get optimal flow speeds to fill up the cylinder to the maximum possible in the time the valves are open. Newer motors sometimes try to optimize this with variable valve lift etc. but it is always a compromise. Your street car is optimized for lower end torque to get good driveability in typical stop-and-go scenarios, while a race car usually only has to start once and is often optimized for higher end power.

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