Drive Losses question
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Posted by: Gavin ® 11/08/2003, 13:49:33 Author Profile Mail author Edit

When figuring drive-line losses are they a fixed amount or are they on a sliding scale? For instance A 215 HP rated engine register's about 176 rear wheel HP on a dynamometer. The drive-train losses can be assumed to be about 18% or 38 HP. If one were to increase the original 215 HP by 70 HP to 285 HP, shouldn't the dynomometer now read 246 HP? or should it read 246 HP less 18%? I would think that the drive-line losses are fixed, 38 HP in this example. Of course I am assuming that the torque and HP were developed at the same RPM and on the same dyno with the same conditions. Please explain if I am in error. This topic is a discussion on another website and I cannot seem to get my point across. Also please forgive my spelling and any fauxpas I may have made, this is my first post here. Retired old guy ;)

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RE: Drive Losses question
Re: Drive Losses question -- Gavin	Post Reply	Top of thread	Forum

Posted by: RKimball ® 11/08/2003, 15:31:55 Author Profile Mail author Edit

If you know the % of drive line losses, then you know the % of drive line loss, ... as long as you stay well within the drive line's designed load bearing capacity. If you get to either extreme of the DR's capacity you are going to introduce other load and friction problems. Fun Extreme Example #1; ...a 5 HP motor is not going to be very efficent working a load through an Allison truck transmision designed to be driven by an engine that develops 1000 ft lbs of torque. The motor would be doing good to handle the tansmission alone. Fun Extreme Example #2, ...a 300 HP engine pulling a 6000 LB truck off the line in a drag race using a old Model A transmision is going to get into some serious binding forces in the gears and bearings just before it shears teeth, twists shafts and splits open. -randy- ... not retired.. but bought the motor coach. ... wink. ** The worst suggestion of your lifetime may be the catalyst to the grandest idea of the century, don't fail to listen to suggestions. -randy- Modified by RKimball at Sat, Nov 08, 2003, 15:36:46

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Re: RE: Drive Losses question
Re: RE: Drive Losses question -- RKimball	Post Reply	Top of thread	Forum

Posted by: iskit4iam ® 11/12/2003, 02:38:37 Author Profile Mail author Edit

The answer is neither of your answers may be correct.  You  have to look at what is causing the energy loss.  Part of the loss is due to moving the lubrication fluids around.  The energy to move the fluid is not a function of power through the system.  Part of the loss is due to frictional forces.  Frictional forces are proportional to the transmitted force, which is proportional to the transmitted power.  So friction forces will be a percentage of transmitted power and fluid movement forces will be fixed.  And there are lots of other factors making up your losses.  So, the 285HP motor may not lose 18% but is will certainly lose more than 38HP.  For the exact same drivetrain at the same rpm.  (experience tells me fixed losses are small compared to proportional losses so the practical answer is you lose 18%)

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