Drivetrain Loss 8

[kradicke]

I did not find anything in the archives, so I have to ask this rather simplistic question:

Is drivetrain loss a linear loss as engine power increases or rather a constant that does not change for a particular drivetrain combination?

With that generalization out of the way, does anyone know what kind of drivetrain loss Laycock style overdrive units contribute to power figures as measured at the wheels?

Thanks... I'll put this question in "Auto Engineering for Dummies" Chapter 1 ;-)

Kai

 

[Kmelthratter]

This is what I understand from what I have read in magazines, seen in the real world etc. drivetrain loss (using an accurate chassis dyno, which are almost none are) with FWD is generalized at 14% for FWD 18% for RWD and 20-21% for AWD. This is by no means computed but it is what I have seen between manufacture flywheel horsepower specs and the amount that is measured on a chassis dyno, I am sure there is a big variece on these numbers but they are a good guess to the midrange of drive train loss. in E.X. a 96-01 Acura Integra LS
is advertised as 140 fylwheel horsepower. Typical measurments
from car to car for the manual transmission are 118-121whp. Also at the shop I used to work for we had a dyno day in which
10 or more Infiti G35's came in. I believe they are advertised at either 298 or 300 flywheel horsepower for the 06 6 speed, of which serveral attended. They varied by one or two horsepower but they were right at 238 horsepower. However the dynojet we had is also known to read lower than other local dynos, my guess is 5-8% low. Also automatic transmissions seem to read much much lower than manual transmissions
None of this is fact and there are an increadible amount of variables involved (including the dyno) but it should shed some light on actual drivetrain loss for high speed (fourth gear to redline on a 5-speed manual) wide open throttle.

 

[Warpspeed]

There is another issue here that nobody speaks about.

A RWD transmission will have to push the car along against the rolling resistance of the front tyres. Likewise a FWD car will need to drag those rear tyres around.

Hysteresis in a pneumatic tyre can soak up a surprising amount of energy, and they are nowhere near as efficient as steel wheels running on steel rails for example. Another clue is the higher power reading obtained on a correctly calibrated hub dyno, against a correctly calibrated roller dyno.

The point here is that the supposedly higher losses of 4WD transmission do not seem to manifest themselves as higher fuel consumption or serious loss of on road performance.

Perhaps if the undriven wheels were externally powered, and that power subtracted from the power available at the driven wheels, 4WD may show up as having a very similar (or possibly even lower) overall loss than 2WD.

I have never seen published figures on this, but my own experience of converting a FWD car to 4WD suggests that combined transmission and rolling losses may actually reduce with 4WD. Certainly tyre wear rates are much reduced, and that may be trying to tell us something as well.

 

[GregLocock]

Audi claimed much the same when they introduced the Quattro.

Bear in mind that the rolling resistance of a tire on a small roller, as used in a typical chassis dyno, is an order of magnitude greater than seen on the road.



Cheers

Greg Locock

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