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Drivetrain Loss
8

Drivetrain Loss

Drivetrain Loss

(OP)
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

RE: Drivetrain Loss

3

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

Neither.

Losses in hookes joints and cv joints tend to be small, and pretty much proportional to power.

Losses in gearboxes tend to be rather larger, and tend to be worse in percentage terms at both extremes of the power range.

Losses in diffs can be quite spectacularly large (10%), and again tend to be least at mid load.

Cheers

Greg Locock

RE: Drivetrain Loss

I was just pondering the same question yesterday.

So for example, you have a car dyno at 150 horsepower at the wheels and the manufacture states that on an engine dyno the engine will produce 180 horsepower. This means the drivetrain causes approximatley a loss of 30 horsepower, or 16.7%. Now you turbocharge the same engine and the dyno registers 250 horsepower at the wheels. Is the drivetrain loss still approximatley 16.7% (now 50 horsepower) or is it closer to the original figure of 30 horsepower?

I guess from Greg's answer it would be somewhere in between, right?

RE: Drivetrain Loss

Nearer 50 than 30 I suspect

Cheers

Greg Locock

RE: Drivetrain Loss

Agree.  Think (don't touch) about how hot a diff. case will get transmitting 250 HP for a while vs. 150.

RE: Drivetrain Loss

Are you saying that the temperature significantly affects the losses, or just that all the extra torque causes more gear friction (and therefore heat)? If its the first one, are diff coolers therefore useful for improving power even if not required for reliability?

RE: Drivetrain Loss

Your second reason is correct.

RE: Drivetrain Loss

There are two main causes of power loss in transmissions.

1: The nice simple textbook loss due to sliding contact of the gears.  Taken to be constant with speed, typically about 2% of power for spur gears and 3% for hypoid bevel.

2: The complex loss (never found in textbooks) due to oil fling and windage.  As the gear starts to rotate it picks up oil, has a large wetted area and the loss follows a normal V^3 drag power law.  As it picks up speed it tends to fling the oil and carve a groove in the oil bath reducing drag by entraining air.  As it flings oil the oil depth reduces, again reducing drag.  It moves to a loss approx proportional to speed regime.  I don't know what happens at very high speed when the oil level has been reduced as low as it can go or a larger gear on the same shaft is still flinging oil and a smaller gear runs clear of the oil bath.  Auto boxes main loss is due to pumping loss in the oil pump and hydraulic system as the gears are not dipped in oil.

Increased temperature will reduce viscosity and reduce drag.

My LSD diff has an oil cooler, not for the benefit of the gears but to maintain the oil at the working temperature of the Viscous LS unit.  It has a warning light to tell me when it is possibly an open diff.

Transmission loss is a black art known only in the makers dyno house and consultants like Ricardo or SwRI, there is very little published information.  Lots of people quote some % figure for loss but don't say what the power or speed was so the figure quoted is useless.  Even assuming it's for peak power and speed doesn't tell you what it will be at normal cruise.  In lots of engineering texts I have only ever found one graph of transmission loss against speed, it was in a book on dynometer testing of engines, 1936 reprinted in 1969!

RE: Drivetrain Loss

Well, oddly enough, all the figures I quoted for CVs, UJs and diffs are from publically available documents which specify test conditions and so on. It is no great secret. I agree, getting efficiencies for gearboxes is more difficult, but I have never needed to chase down public sources for them.

Cheers

Greg Locock

RE: Drivetrain Loss

Greg,
Where would one look to find documents like that? I'm not an engineer, so I don't know the usual sources. Thanks.

RE: Drivetrain Loss

Okay, so can anyone actually give a finite figure or even a simple "rule of thumb" to go by? I have heard people quote figures as high as 30% for a manual transmission but surely that can't be correct?
I would like to know in particular what is a rough figure to use for a front wheel drive manual vehicle, I am not after exact figures, I just need to be able to do a quick "guesstimation" on the spot. If anyone knows of a refernce table or has a similar text I would be interested to see it, anyone?

RE: Drivetrain Loss

Sorry, I forgot to put an example to my quoted figure.
For the 30% example, if you had a 100kW engine with a drivetrain loss of 30% then the "at the wheels" figure would be ~66kW.
I just need to know a real world figure to use, front wheel drive and rear wheel drive figures are required please.

RE: Drivetrain Loss

having worked with my ram 12 rolling rd on this prob ,and having many convo, bout it with suns original installation and setup engineer,i can give this data .losses on a rolling rd are  created largely by the tyre to roller interface .eg ,my tvr 250 bhp at 4500 speed 115mph 20bhp losses  high pressure tyres ,low sidewall deflection .tyres cold .thin engine oil in box . my turbo 500 bike ,with sprayed tyes to increase traction ,losses at 135 bhp,20 bhp not sprayed ,37 bhp losses sprayed hss wurth grease.this at 120 mph.1969 cooper formula one car 'unfinished project 'from south africa ,3.1 litre alfa v8 ,straight cut box high tyre pressure 10 bhp losses at 280bhp .120mph.it is possible to create out of thin air a fair bit of bhp on a inertia dyno simply by pumpimg up the tyres .this also means that the tyres geting hot can change the readings .as their pressure increases and so they deform less .using a percentage for a loss is futile.running at about 120 mph , 2wd cars with normal rd tyres tend to have about 30bhp losses ,irrespective of the bhp up to a point,if manual gearbox. autos tend to have about double.running a dodge charger 69 i found 180 at wheels and 225 at crank using the ram 12 losses coast down method .having also run a bike on a set of rollers up to 125 mph and then ridden it off the rollers at that speed ,i can also say that the losses system on the r r  is still reading 6 bhp when there is no load on the rollers.. i put this down to mainly windage in the cooling fan component of the absorber.
regards
robert

RE: Drivetrain Loss

oops ,pressed it twice ,numpty!!

RE: Drivetrain Loss

Does this mean if you build hybrid car with out (even with out batteries - no recovery from braking), just engine, power generator and electric motors with direct drive you will got 25~30% gain in efficiency (assuming the the electric component are at ~95% efficiency)
So why all hybrid cars today have additional gears box?
I only so fare see one car (bus) that do not used gears box hybrid approach => E-traction.
http://www.e-traction.com/
Very surprising results.

  

RE: Drivetrain Loss

Because wheel motors have their own set of problems. Having said that 95% is entirely feasible, so I agree it would be sensible.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

RE: Drivetrain Loss

Guys,

in marine installations (propulsion installation of any kind) we MEASURE losses of up to 3% for older installations. Keep in mind that power loss in general means loss being transmitted as heat (fritional loss). You haved to loose the energy somewhere, and some losses mentioned above are just not possible. I'd say the power is never fully generated

RE: Drivetrain Loss

FAO MKimagin (Electrical)
These are the thoughts issued last year in a document from an automotive transmission company.

Electric Traction Systems

In principle, transmissions could be replaced by an all-electric drive system consisting of an engine-driven generator sending power to a motor or motors driving the wheels. Capable of developing maximum torque from a standing start, such motors would not necessarily need a change-speed transmission system. However, with the existing systems the loss of efficiency from the conversion into electrical energy and back again is seen as a disadvantage of a pure electric system.


Renault has been looking at infinitely variable transmission made from planetary gears with motors and generators, not using the CVT arrangement of the Toyota Prius, but having an electrical path within the transmission rather than using the generator's power downstream of the transmission. I believe they concluded that the most efficient ratios occured when power was transferred entirely mechanically, but that doesn't exclude the possibility that there would be a fuel saving at the intermediate ratios due to engine characteristics.

RE: Drivetrain Loss

they made a prototype hummer (the real kind) with direct to wheel electric drive, as i recall there was a lot of comparative data published on HP speed efficiancy etc. might be an interesting search.

RE: Drivetrain Loss

2
Seeing as most of us are pretty much stuck with what we already have, all we can really do is measure oil temperature rise. The losses all end up as heat anyway, whatever the cause.

I have never tried this myself, but the type and grade of oil you use may have a significant and measurable effect on transmission losses. A lot depends on what the gears are doing.

In a high Rpm low torque situation, (highway cruise) most of the losses will be windage and pumping losses, so much thinner oil may possibly lower those losses.

In a very high torque situation (diff gears on the dyno) most of the losses might be due to high pressure oil shear. A thicker high pressure lubricant may work better and show a lower temperature rise.

Lowest possible transmission loss might require totally different oil in a mileage miser, to a high horsepower endurance race car. Even if the gearboxes and diffs are otherwise absolutely identical.

But I believe measured oil temperature rise is trying to tell you what the losses are. If different oil lowers the operating temperature in your application, that has to be a step in the right (efficiency) direction.

RE: Drivetrain Loss

The only rig I have ever seen that measures the efficiency of differentials uses the temperature rise to measure the power absorbed.

Neat!

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

RE: Drivetrain Loss

kradicke:
One more thing to keep in mind,  when comparing numbers that someone throws out concerning horsepower "at the crank" vs horsepower at the wheels is this:

When on an engine dyno, an engine is running in a considerably different set of conditions than when it's installed in a chassis:  different exhaust backpressure,  different intake air temperature, and often, different water temperature. Different parasitic losses too. All these things have an effect on output, so the results in a car could be very different than on an engine dyno.

RE: Drivetrain Loss

West Texas answer.
It takes a lot of power to stir all that oil and slide those gears. But not nearly as much as it would take without all that oil.
Have a happy day.
Pancholin

RE: Drivetrain Loss

Sometime back I read about the efficiency of Diesel-electric railroad locomotives vs. Diesel locomotives with gearboxes and hydraulic couplings. The conclusion was that the geared transmissions were usually more efficient, but the electric transmissions had a place on the largest freight locomotives.

But even if electric transmissions are less efficient (which I don't concede), they'd likely be cheaper in cars. Are wheel motors then really the sticking point? If vibration is an issue, it seems rational to mount the high-torque motors near the centerline of the car, attached to the wheels with axles.

RE: Drivetrain Loss

This has to be a no-brainer with real dyno sheets to back it up all say long? Anyone have any or seen any?

RE: Drivetrain Loss

I've seen the dyno sheet for a 98.4% efficient wheel motor.

Will that do?

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

RE: Drivetrain Loss

"I've seen the dyno sheet for a 98.4% efficient wheel motor." Nope...lol

RE: Drivetrain Loss

never having seen one of these wheel motors, i imagine they  cant be light....  not good for unsprung mass

Ken

RE: Drivetrain Loss

What I got from Greg Locock's last post is that wheel motors are NOT the sticking point in transitioning from mechanical to electrical drivetrains.

RE: Drivetrain Loss

Quote:

Sometime back I read about the efficiency of Diesel-electric railroad locomotives vs. Diesel locomotives with gearboxes and hydraulic couplings. The conclusion was that the geared transmissions were usually more efficient, but the electric transmissions had a place on the largest freight locomotives.

Though not quite as reliable, the Diesel-Hydraulic locomotives that many countries used for a while were also more efficient than Diesel-Electrics.

Electric motors are efficient but they really only represent a little more than the short block of a gas engine. For the most part, they have no throttle means whatsoever and supplying proper current to them is a fairly monumental task.

RE: Drivetrain Loss

Efficient high power electric motors and generators really only have one problem... weight.  They work particularly well in locomotives and ships where high weight can actually be an advantage..

Efficient PWM control of a very high power electric traction motor is not as difficult as Fabrico suggests. Suburban trains in the electrified rail network have solved all the problems long ago, and they usually employ regenerative braking too. Very high voltage gate turb off thyristors easily operate at megawatt levels, with very high efficiency and precise control of power in both directions.

RE: Drivetrain Loss

Our power supply for the motor upconverted the voltage and supplied the correct phasing (in essence the motor is a 3 phase switched DC motor with electronic commutation). It is just about 99.4% efficient.

If we had fitted more FETs it would have improved the efficiency.


Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

RE: Drivetrain Loss

Quote:

Efficient PWM control of a very high power electric traction motor is not as difficult as Fabrico suggests. Suburban trains in the electrified rail network have solved all the problems long ago, and they usually employ regenerative braking too. Very high voltage gate turb off thyristors easily operate at megawatt levels, with very high efficiency and precise control of power in both directions.

I have not kept pace with the latest advances in automotive electric drives. But, up until a short time ago, 3-phase A-C motors were considered among the most efficient direct drive motors in existence. But the controllers were bulky and extremely complex. Most needed water cooling. Trains have solved nothing for automobiles.

Unless something completely new has come along, frequency control, rheostats, multiple windings, moving brushes on a commutator, and throttling an engine/generator unit, are all effective but inefficient means of motor speed control for automotive use. Motor controllers have been just about as much a stumbling block as the batteries.


RE: Drivetrain Loss

Electronic motor speed control is a mature technology. The key to control system efficiency is using a sufficiently high operating voltage to reduce the current.

The problem will always be the sheer size and weight of the batteries and motor, the power electronics to control it is simply not a problem.



  










RE: Drivetrain Loss

Yes. That motor controller had an unaoidable voltage drop of 0.6 V (basically one FET). To get the high efficiency we used a bus voltage of around 200V.

I doubt that it is a coincidence that the Prius uses a ~200V battery

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

RE: Drivetrain Loss

Quote:

Unless something completely new has come along.....

Seems it has, the brushless DC motor, or "BLDC". Speed control through electronic commutation. As far as efficiency, it's claim to fame is during zero to moderate output. At high output it is about the same as a brush motor. I hope they turn out better than the brushless automotive generators. Dependable, but very bulky with low output.

RE: Drivetrain Loss

An electric motor may have 99% efficiency at some point on a power/speed curve, but average efficiency is far lower.
What little testing I have done and the few graphs I have found puts average efficiency less than 90% and as low as 70%. I have never seen any regeneration efficiency graphs.

RE: Drivetrain Loss

   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.

RE: Drivetrain Loss

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.

RE: Drivetrain Loss

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

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

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