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
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
"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
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
Cheers
Greg Locock
RE: Drivetrain Loss
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RE: Drivetrain Loss
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
Cheers
Greg Locock
RE: Drivetrain Loss
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
Cheers
Greg Locock
RE: Drivetrain Loss
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
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
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regards
robert
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RE: Drivetrain Loss
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
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Drivetrain Loss
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
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
RE: Drivetrain Loss
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
Neat!
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Drivetrain Loss
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
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
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
RE: Drivetrain Loss
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
RE: Drivetrain Loss
Ken
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RE: Drivetrain Loss
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 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
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
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.
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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
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
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
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
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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
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
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.