6

I was looking over the specs for a 98 Mazda 626 2L and noticed that it says the max power is at 5500 RPM.

Will the Max Power RPM also be the RPM at which the engine is most efficient assuming you're at a steady cruise?

  • 1
    As @Vini says "No" petrol engines produce maximum Efficiency at max torque but jet engines on the other hand produce max Efficiency at max power range. – Shobin P Aug 30 '15 at 12:34
7

The short answer is no. The engine is most efficient at the RPM when maximum torque is achieved. It's easiest to explain with a picture. Engine Torque and Power VS RPM

Power is a function of torque and RPM. Maximum torque is achieved because the engine is able to move the maximum amount of air and fuel in and out of the engine. The power continues to climb even when the torque starts falling because the RPM is climbing faster than the torque is falling. Eventually the torque begins to fall so fast because at the high speed the engine struggle to move the fuel and air in and out. Finally to torque drops of so drastically that the power also begins to fall.

  • The engine speed at which maximum torque is achieved is where maximum acceleration will be found. It has nothing to do with maximum fuel efficiency, which usually corresponds to a different speed. Some of Nissan's promotional videos on CVT gearboxes will illustrate this, where the engine is held at max. torque while the vehicle is accelerating, before it drops to a different RPM for maximum fuel efficiency. – Zaid Aug 30 '15 at 11:54
  • @Zaid Most cars have maximum fuel efficiency near the peak torque range. I am not able to understand your Nissan example .. Could you elaborate.. – Shobin P Aug 30 '15 at 12:32
  • @Anarach : I have tried to explain it in my answer. It is a trade-off between drag forces and torque. Search for any video with the terms "Nissan Altima CVT acceleration" and you should see what happens to the rev counter once the car stops accelerating – Zaid Aug 30 '15 at 12:42
  • Basically, the engine, taken alone, is most efficient at maximum torque, but we have to figure in the whole system, the car and it's wind resistance. – Robert S. Barnes Aug 30 '15 at 17:51
  • Per this discussion: mechanics.stackexchange.com/questions/25157/… HP and torque curves should meet at 5252 RPM because that is how they are mathematically related. I'm probably reading something wrong, any thoughts on why this example would not cross there? I thought because it was about the math that most other factors would not affect the crossing point. Just curious and trying to understand why this one's different.. – cdunn Apr 18 '16 at 19:06
5

No, the engine speed at which maximum fuel efficiency is obtained is different.

In fact, the faster you go in any given gear the worse the resultant fuel economy will be because it takes increasingly more power to overcome the vehicle's drag forces.

It worsens because the power required to overcome drag forces on a vehicle at a constant speed is proportional to the cube of the speed, so to drive at twice the speed, you need 8 times the power.

However, this doesn't mean that fuel economy is best at the lowest RPM, because the engine's fuel efficiency at that point is low (resulting in low torque).

Instead, there will be a sweet spot where the efficiency is best closer to the lower end of the RPM scale which represents a best compromise between the torque generated and vehicle speed (drag forces).

  • So how do you find that sweet spot for a particular car? – Robert S. Barnes Aug 30 '15 at 17:50
  • @RobertS.Barnes - If you don't have a means of data logging, the easiest way is through trial and error. If you have in car system which shows fuel efficiency, leave it on that and keep a close eye out. You'll soon discover where the better RPM's ranges are to drive. – Pᴀᴜʟsᴛᴇʀ2 Aug 30 '15 at 23:35
-2

No.. Max torque is not the max force you will have available to accelerate your vehicle at any vehicle speed. that point happens at max HP. Efficiency means how much you got out for how much you put in.. For most high performance engines, that points to the high HP levels, again, not max torque. At the point of max HP you are burning the most amount of air and fuel possible, the most potential energy consumption is at that point and the greatest rate of doing work (HP) happens at peak HP, not at max torque.

  • one other thing about CVTs... they operate at Max HP NOT max torque to have the greatest acceleration forces on the vehicle at any time. – zanick Apr 18 '16 at 18:44
  • I suggest you go look at some HP and torque curves. You IS statements are pretty much not in line with reality. i.stack.imgur.com/Y7lkM.gif as an example. Cheers! – DucatiKiller Apr 18 '16 at 19:17

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.