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

 

[carnage1]

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.

 

[Warpspeed]

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.

 

[GregLocock]

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.

 

[Rob45]

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.

 

[Pancholin]

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

 

[mackerm]

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.

 

[RobHughes]

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

 

[GregLocock]

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.

 

[RobHughes]

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

 

[kenre]

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

Ken

 

[mackerm]

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.

 

[Fabrico]

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.

 

[Warpspeed]

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.

 

[GregLocock]

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.

 

[Fabrico]

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.

 

[Warpspeed]

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.

 

[GregLocock]

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.

 

[Fabrico]

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.

 

[EdDanzer]

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.

 

[fueliefan]

regeneration curves

http://www.uqm.com/products/specsheets/PowerPhase100.html