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

 

[GregLocock]

"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

 

[ZoomZoom]

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?

 

[GregLocock]

Nearer 50 than 30 I suspect

Cheers

Greg Locock

 

[Metalguy]

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

 

[stingc]

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?

 

[Metalguy]

Your second reason is correct.

 

[peterhill]

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!

 

[GregLocock]

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

 

[stingc]

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

 

[GregLocock]

http://www.sae.org

Cheers

Greg Locock

 

[SBMotorsports]

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?

 

[SBMotorsports]

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.

 

[amorrison]

For some graphs see

http://www.swri.edu/3PUBS/BROCHURE/D03/trantch/trantch.HTM

 

[ivanheow]

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

 

[ivanheow]

oops ,pressed it twice ,numpty!!

 

[MKimagin]

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.

 

[GregLocock]

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.

 

[rob768]

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

 

[crysta1c1ear]

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.