Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Does friction increase with rpm in an engine?

  1. Mar 25, 2007 #1
    me and my friend are having an arguement about this. I say that because frictional force is less than or equal to the coefficient of friction times the normal force, and that neither the coefficient of friction nor the normal force can be increased while an engine is running(until it's oil is overheated or too cold), then frictional force remains constant in a running engine. While he says that "since more heat is generated at higher rpms than lower rpms, then it must take more power (power = work/time) to increase rpms at higher rpms rather than lower rpms." But I'm pretty sure, not positive, that this doesnt mean more power has actually been lost. who is right in this arguement?
    Last edited: Mar 25, 2007
  2. jcsd
  3. Mar 25, 2007 #2
    The friction in an engine is due to the bearings which have thin film oil layers. It will change as the temperature affects the viscosity and shear stresses.

    ....I dont know what your talking about when it comes to normal forces. Normal forces where? That implies dry friction. What part of an engine has dry friction?
  4. Mar 26, 2007 #3


    User Avatar
    Science Advisor
    Gold Member

    The reason for higher heat rejection at higher RPMs is due in main to the more frequent combustion events. Note that your friend doesn't refer to anything about the load condition, which has a huge effect on heat rejected.

    Cyrus, I also didn't know what 'normal forces' were being talked about.

    To answer your actual question, it sounds like neither of you are right.
  5. Mar 26, 2007 #4
    lol well can u shed some light on the subject then? is there a wet friction formula? why do trannies get hotter than engines if its all about the combustion? I was thinking since more distance is traveled there is more heat, since more heat can be made with the same -force by increasing the distance
    Last edited: Mar 26, 2007
  6. Mar 26, 2007 #5


    User Avatar
    Gold Member

    I've never taken any engineering courses, only physics courses, so I've really only studied the very general cases, but I think the friction they're talking about is fluid friction which is generally proportional to velocity or velocity squared and has nothing to do with normal forces.

    For instance, when you hold your hand out the window while the car is driving, you feel a lot of pressure pushing your hand back, but as the car slows down, the resisting force becomes smaller and smaller until it's negligible.
  7. Mar 26, 2007 #6
    hmm, i really dont think that the crank/gears spin fast enough to make any noticeable loss.
  8. Mar 26, 2007 #7


    User Avatar
    Science Advisor

    Remember they're spinning in oil. From what I've heard, these losses are actually the dominant ones in most drivetrains (outside of the engine). They're also very hard to understand analytically, since gears are usually not in a uniform bath of oil. The fluid is sitting in a pan, and is kicked up as things rotate. Regardless, these losses are not strongly load-dependent. They do depend on rotational speed, of course.

    There are also load-dependent effects in gears, though. Even though there's lubrication, gear teath are sliding against each other. Thrust bearings also end up taking different loads, which likely changes their angular losses.
  9. Mar 26, 2007 #8


    User Avatar
    Science Advisor

    Just to cloud the issue, one other thing that changes drastically in an engine is component clearances as temperatures change. What frictional forces exist when the engine is cold will, most likely, decrease due to thermal expansion of components. Of course, in some areas, the frictional forces could increase with temperature. I don't think that there is a way to really answer this question in the general sense.
  10. May 12, 2007 #9
    I think the increasing in temp is due the shortage in time to release the heat....so you will not have the enough time to release the heat because of the high velocity!! so heat will build up!!!

    so you feel that it is hotter...

    that is mean, no more losses in energy is happing actually!!!! it is the same as the low speed lost energy ......

    actually, force dose not increase by increasing velocity...it increases only by acceleration increasing or mass increasing...

    of course increasing force will increase the friction force and so the heat will be produced..

    heat may increase also if the surface changed (more roughness)... this usually happens when the surface get contaminated

    at the end, i think you are the one who is right
    Last edited: May 12, 2007
  11. May 13, 2007 #10
    That's what I was trying to say in my 2nd post, but you explained it much better lol.

    And now I am a little bit more experience in this subject so I'll state how my views have changed. The gears inside a transmission will spin at a maximum of 2000rpms. While the engine's crank shaft spins at 12,000 rpms in your average engine (2 rpms crank shaft = 1 rpm). So the fluid friction can't be the cause of the transmission's heat. The engine would still create more heat via fluid friction than the transmission.

    I have 2 reason's that transmissions are hotter than engines. Coolant is run through an engine, not in a transmission. And there is more surface area of contact in a transmission. However an engine would get hotter exponentially until the clearances were = 0, if there wasn't a thermostat regulating the temperature (temperature is always the same in a warmed up engine). So reason #1 contributes much more, and also makes it impossible to tell us if friction is being increased with velocity with a real world situation.

    This IS because there are explosions going on inside the engine.

    However I still think if more heat is being created, it doesn't necessarily mean more friction force. It could also mean more velocity like you said.
    Last edited: May 13, 2007
  12. May 16, 2007 #11
    NO 2 rev per power stroke but the tack's reads actual rev's not power strokes so 6k avg NOT 12k

    drag/friction is more from piston rings and valve springs and other the odd side forces on bits like cams then the oil feed bearings and the crankshaft but flowing oil and it's pumps add some friction too
    as does water pumps and flow even as the water removes heat
    and all those forces go up with more RPM so more speed = more friction

    normal power loss is about 10% to 20% thru a transmission gear train, that power is converted to heat, and friction in a trans is also dependant on load, thats why a car towing should have a add on trans cooler, as higher loads = higher heat
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Does friction increase with rpm in an engine?