Experiment: What do I lose with AWD?

[roflwaffle]

Originally Posted by Frank Lee

Back in the day I read a comment from an Audi engineer saying the AWD doesn't hurt fe because a powered wheel has less rolling resistance than an unpowered wheel and that pretty much cancelled out the extra weight and mechanical transmission losses. They made it sound like an fe wash.

How does that work? I could buy a negligible difference, since a decent axle is at what, ~97-99% efficiency, and an extra ~50lbs won't do much, but I've never heard of a powered wheel having less RR.

[Frank Lee]

I don't know how that works. That's what the dude said, back in the '80s and he didn't expound on it IIRC.

During testing, Audi also found another benefit: tyres generate less rolling resistance when driven gently than when rolling freely, more than enough to offset the additional frictional losses in the four-wheel-drive transmission. Prototypes delivered higher maximum speeds when driven by all four wheels than when the rear driveshafts were removed.

http://www.pistonheads.com/doc.asp?c=52&i=10065

Ahhh- here we are:

http://books.google.com/books?id=Ldk...stance&f=false

[roflwaffle]

The only thing I can come up with is that by spitting power between two axles the axle losses are minimized over some operating range, so it may look like having a driven wheel minimizes RR, but in fact it's that axle efficiency isn't linear and can be optimized to some extent.

Edit- Jeez , I've had five beers and I'm talking about axle losses. I should just get a degree in ME and call it a day.

[rick3090]

One thing that hypermiling can gain is eco friendly good habits but when you are picking a car to set up hypermiling it may be best to look at all of the aspects first , and dont fall into the old trap that any body can hypermile with any car , for example if you have a big van you may sacrifice any fuel gain due to the shear size of the vehicle . awd is also a factor in picking a vehicle as well ,why ? well take for example a subaru the awd is mounted in the rear and is always turning while the car is moving , the separation point is in the transfer case so that means that everything up to that point is turning , redneck thinking is this way k.i.s.s. keep it simple stupid (not calling anyone stupid) but by looking at all the factors of saving gas or diesel all factors are concidered, think of it this way..... can you get out and push your car down the street a ways? i know for a fact that a subaru is a tough push for one person , that said now you know the engine will still work hard even cruseing . so what im saying is that hypermiling can work but if you really want it than you have to think outside the box and disregard some of the rules . more later

[bgd73]

I run AWD.. with a 3 main boxer. 90hp.
3 modes to transmission..
4wd hi == lsd rear,
4wd lo == full diff lock, and reduction
2wd == open front, no rear.

no difference at all bewteen 4hi and 2wd in gas consumption.
could stay there all day...in maine, I have many days.
there is even a momentum, after speed achieved..and no difference in throttle getting to a speed...but once at speed, it seems as though, I am even less toes on the throttle..a momentum is happening in the 4wd. I did learn this similar drivetrain from an inline four, and it was not the same.

But inline fours are notorious for completely dying to the word called "work", there is no torque, until it gets over itself. the boxer is just simply there at all times...every single last rpm...all the way to bogging like a diesel, and still keeps going.

vw AWD synchro van, and old subarus...similar results.

[shovel]

Extra weight is extra weight, and extra spinning weight is extra spinning weight.


I'd be surprised if there was any consistent difference in mpg whatsoever between an AWD car in AWD mode and an AWD car with all of its drivetrain bits spinning but only 2 wheels actually receiving torque from the engine. The wheels don't care if they are connected to the engine by a shaft or by a planet whose surface links the front wheels to the rear, the net result is the same parts spinning and causing drag.

As a side note, early (at least through 2001) 5-speed Foresters (and probably other soobs) directly transmit engine torque to the front wheels and only utilize their viscous coupling to link the front wheels to the rear. What this means is on those vehicles you can simply unbolt the rear diff & drive shafts and remove them entirely from the vehicle with no consequence other than loss of AWD. The output yoke on the transmission will just spin and the front wheels will continue to receive 100% engine torque.

[AeroModder]

In Subarus, cars with automatics have a 90/10 torque split while cruising to offset the dragging of the rear wheels (electronically controlled transmission control unit), but the manual models have a 50/50 (Center differential with a viscous coupling device).

Subaru's All Wheel Drive System - AutoWorld.Com

I was actually thinking about looking for an Impreza for my next project, since there's so many more aftermarket parts out there than there are for Tempos.

[Mifunego]

Originally Posted by Frank Lee

I don't know how that works. That's what the dude said, back in the '80s and he didn't expound on it IIRC.


Audi quattro

Ahhh- here we are:

Popular Mechanics - Google Books

Sounds interesting, but I'm not familiar with any Audis that can flesh out that theory in terms of FE, at least according to the EPA (and my limited VAG experience). Of the models that I'm familiar with, available both in FWD and quattro (or haldex), the FWD always is rated at a better mpg. Maybe that wasn't the case with the early 80's models? Go figure.

[shovel]

Originally Posted by AeroModder

In Subarus, cars with automatics have a 90/10 torque split while cruising to offset the dragging of the rear wheels (electronically controlled transmission control unit), but the manual models have a 50/50 (Center differential with a viscous coupling device).

Subaru's All Wheel Drive System - AutoWorld.Com

I owned an 01 Forester with manual transmission and personally had it apart. The engine (through the transmission) drives the ring gear of the front wheels directly and a jack shaft drives the rear output flange to the rear axle through the viscous coupling. When I disconnected the rear driveshaft and attempted to verify this by crawling up a curb, and by dumping the clutch on flat dry pavement, it behaved like a front wheel drive car with 100% engagement.

Hypothetically if you removed the front axle shafts from the same vehicle and tried to drive it as a rear wheel drive, it would only be able to deliver half of its torque to the drive axle and would likely wear itself out by freewheeling the front flanges any time the engine supplied more torque than the coupling wanted to send the rear wheels (such as acceleration/climbing/etc)

[micondie]

I remember reading years ago about an AWD Can-Am race car that somebody was building and their conclusion was that since frictional losses were a percentage of the power being transmitted, splitting the power between the front and back axles cut the losses to each axle in half resulting in the total losses being about the same. This would explain why cutting power to the rear axle in an AWD vehicle but retaining all the moving driveline parts would reduce MPG because you would be using just as much power to move the vehicle and extra power to turn the extra driveline parts. A vehicle with locking hubs such as the Suizuki would be able to eliminate the movement of these parts and show some improvement.