Fuel economy, part-time vs. full-time AWD

Perc

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I was just reading a review about a new Subaru. The writer claimed that the "Symmetrical AWD" drivetrain layout hurt fuel economy because all axles are powered at all times. I kinda don't get it, though.

My Opel has a Haldex AWD system which means it's part-time. The rear axle kicks in when front wheel slip occurs. It's the same system as Volkswagen and Volvo among others have been using since around the turn of the century. Off the line, on snow, you can feel the front wheels losing grip for a split second before the Haldex clutch kicks in and sends power to the back... and sends the car away, slightly sideways with all four wheels spinning if you're giving it the beans. :p

Some people claim this is: 1) inferior because it's a reactive system and 2) widely used because it improves fuel economy, since the car is 2WD most of the time.

The thing is, where does the fuel saving come from? The coupling sits in the back between the prop shaft and rear diff. The gearbox keeps the propshaft spinning even if it isn't connected to anything. The half-shafts are always connected to the wheels, which means the entire rear axle is spinning while the car is moving.

Even Audi is moving away from full-time AWD with their "quattro ultra" which disconnects the rear axle when not needed. The difference here is that the computer brain decides when to disengage the rear, unlike Haldex where the rear is disengaged by default and the brain decides when to engage it.

The car makers are obviously going through great lengths to come up with different fuel-saving techniques for their AWD systems so there has to be something to be gained from disconnecting an axle, but I just don't get it. What am I missing here?
 
Fuel economy, part-time vs. full-time AWD

Rotational inertia and drag basically. When you are power all the wheels you have drag of all the tires, the mass of all the tires, the mass of all the propshafts and the drag of the same. Also in general powering rear wheels is less efficient than fronts because it?s a longer path. When it?s a part time the non driven wheels are basically along for the ride the engine doesn?t force the wheels to move so the drag is much less. Weight is still the same ofc because of all the extra components but those can be made pretty lightweight.
 
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In my brain, all the tires still have to be dragged because, well, they're on the ground. They're not powered via the drivetrain but "powered" nevertheless.
 
It's a difference of overcoming drag vs using drag, in the former case you have to make the tires moving first in the former the car moving makes them roll.
 
I believe Perc is correct. Drivetrain losses are equal for Part-time and Full-time systems, provided there is no physical disconnect from the secondary driveline. As long as all of your front and rear driveline components are spinning, you are encountering the same driveline losses regardless of where the power is being applied. IMO the major driver of reduced fuel economy in AWD vehicles is the additional mass of the secondary driveline.
 
Then why have a part time system at all?
 
Incorrect. Disconnects in the driveline reduce roadloads on the engine. The diff and rear halfshafts can be spinning all they want, but if they'er not connected to the engine, you're not burning fuel to try to get them to turn. Many manufacturers are looking into various application of this tech right now. I was at a demo event 2ish years ago where suppliers were demonstrating all sorts of variants - disconnects at each wheel end, disconnects at the trans, disconnects at the diffs, and don't be surprised when you start seeing that more and more.
 
I believe Perc is correct. Drivetrain losses are equal for Part-time and Full-time systems, provided there is no physical disconnect from the secondary driveline. As long as all of your front and rear driveline components are spinning, you are encountering the same driveline losses regardless of where the power is being applied. IMO the major driver of reduced fuel economy in AWD vehicles is the additional mass of the secondary driveline.

If that were the case then 2WD cars would have had similar fuel economy to AWD cars.

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Incorrect. Disconnects in the driveline reduce roadloads on the engine. The diff and rear halfshafts can be spinning all they want, but if they'er not connected to the engine, you're not burning fuel to try to get them to turn. Many manufacturers are looking into various application of this tech right now. I was at a demo event 2ish years ago where suppliers were demonstrating all sorts of variants - disconnects at each wheel end, disconnects at the trans, disconnects at the diffs, and don't be surprised when you start seeing that more and more.

Would you want to completely disconnect the shafts and diffs? I would think that you want them moving around and spreading the lubrication around. In fact I often wonder if I'm hurting the 4WD system in the X by having it mostly in RWD (basically if it ain't snowing it aint in 4WD)
 
Incorrect. Disconnects in the driveline reduce roadloads on the engine. The diff and rear halfshafts can be spinning all they want, but if they'er not connected to the engine, you're not burning fuel to try to get them to turn.

What's making them spin, even when disconnected? The dino juice going boom in the engine.
 
Would it really make that much of a difference? I'm averaging 7.2 l /100km (with a downward trend) on my A5 with permanent AWD.
 
Would it really make that much of a difference? I'm averaging 7.2 l /100km (with a downward trend) on my A5 with permanent AWD.

It must, seeing as how automakers are all switching to part time.

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What's making them spin, even when disconnected? The dino juice going boom in the engine.

In pedantic terms yes, but the way the engine does it is what makes a difference.
 
What's making them spin, even when disconnected? The dino juice going boom in the engine.

Exactly my point. The only time this isn't true is in an oldskool 4x4 system where you can put the transfer case in RWD and disengage the locking hubs.


Would it really make that much of a difference? I'm averaging 7.2 l /100km (with a downward trend) on my A5 with permanent AWD.

That's way better than I'm doing, with 50% more displacement too. I guess I'm doing more stop and go driving than you.
 
In pedantic terms yes, but the way the engine does it is what makes a difference.

Pedantic terms are the only that matter. The engine makes everything spin in your car, whether it's directly driven or by increasing the resistance the rear wheels need to spin - both sips fuel.

To put it in extreme terms, if you lock up your rear brakes in a FWD car - does that not cost fuel, because the engine doesn't drive the rear wheels?



Disengaging still helps - by not having load on the shaft, diff, etc. you reduce internal friction.
 
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That's way better than I'm doing, with 50% more displacement too. I guess I'm doing more stop and go driving than you.
Living in a small town with few traffic lights, mostly roundabouts and mostly long-ish distance driving, yeah.
 
Aren't the rear driveline components (rear driveshaft, rear diff, rear half-shafts) in part-time systems usually built from lighter components, because the forces those components experience are much smaller than they are in the full-time systems.
Part-time systems only come into play when there is front wheel slip, so pretty much only in low grip situations, or if the front wheels slip in high grip situations, the manufacturer most likely has limited how quickly the rear is connected and thus limits the forces the rear end components experience. But in full-time systems, those rear components have to withstand the forces from launching the car from standstill in high grip situations and thus need to be made stronger.

Lighter components -> less rotational mass -> less energy needed to change the rotational speed of the component -> less fuel needed.
 
Pedantic terms are the only that matter. The engine makes everything spin in your car,

If you want to be pedantic, engine doesn't make the starter motor spin :p

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Aren't the rear driveline components (rear driveshaft, rear diff, rear half-shafts) in part-time systems usually built from lighter components, because the forces those components experience are much smaller than they are in the full-time systems.
Part-time systems only come into play when there is front wheel slip, so pretty much only in low grip situations, or if the front wheels slip in high grip situations, the manufacturer most likely has limited how quickly the rear is connected and thus limits the forces the rear end components experience. But in full-time systems, those rear components have to withstand the forces from launching the car from standstill in high grip situations and thus need to be made stronger.

Lighter components -> less rotational mass -> less energy needed to change the rotational speed of the component -> less fuel needed.

I hadn't thought of that consideration but I think you are right, I remember the test with RAV4 or a CR-V where with full front wheel slip on "ice" rears never engaged and manufacturer said that it was due to how they limit the amount of torque they are willing to send to the rear.
 
I hadn't thought of that consideration but I think you are right, I remember the test with RAV4 or a CR-V where with full front wheel slip on "ice" rears never engaged and manufacturer said that it was due to how they limit the amount of torque they are willing to send to the rear.

Yes, the current generation CR-V is known for being all out of ideas when it's on an incline with the front wheels on rollers. I've never driven one so I don't know how it behaves but the videos I've seen is not confidence inspiring. I've also been behind many older generation CR-V's where you can see the rear half shafts, sized like drumsticks.

I can tell you though that my car with Haldex Gen IV has no problems transferring torque to the rear, nor does it have anything against going sideways on snow when you give it a bootful. It requires a bigger boot than a typical FWD before the tail comes out, though. And the half shafts aren't drumstick sized. :p
 
One way to explore this is to look at 4WD systems. For the sake of this discussion, I'm not going to worry about 4-low or 4WD on dry pavement; I'll only look at power distribution, power loss, and causes of resistance for the sake of illustration.

Full Time 4WD/Symmetrical AWD

In this case engine power is being sent to all the wheels all the time, to varying degrees. Power loss occurs due to the resistance and rotational inertia of those driveline components - rear drive shaft, rear differential, power takeoff unit/transfer case.

2WD Converted AWD

Depending on the system, this runs in FWD with no power to the rear wheels unless it is needed. Then only a small amount, often 20-30% is sent to the rear until traction is regained. For most driving, the fuel economy will pretty much just suffer the added weight of those extra components and the resistance of spinning the rear end as the wheels are pulled along behind the drive wheels. There is more resistance due to the extra "parasitic" drive components that don't have power most the time. Additional power loss may happen at the PTU (power take off unit) if it is sending small amounts of power to the rear in cornering or on loose surfaces (such as wet leaves or gravel).

4WD with disengaging hubs

Primarily RWD unless the power is manually sent to the front and the hubs are engaged. The hubs allow the front wheels to disconnect from the front diff, half-shafts, front prop shaft, and transfer case. The only added resistance here is weight when in 2WD with hubs disengaged.

4WD without disengaging hubs

The front wheels are pushed along by the powered rear wheels. The motion of the car spins the front wheels, front half-shafts, front diff, and front prop shaft in 2WD, but the front end gets no engine power unless engaged at the transfer case. This is functionally similar to the 2WD AWD conversion, except it is manually operated. You still have the resistance of spinning all those drive line components, even if they are not getting power. You also have the weight to account for.
 
A bit of Bro science perhaps, as I haven't looked all that much in to this, but the way I understand it, the issue with having permanent drive to all wheels is the speed difference between wheels. As soon as you move from straight line, all diffs start a battle of directional control.

Centre diff is the worst, as it will try to keep axle speeds level. When the drive is cut from the centre, even the rear diff can give way more freely, as it can unload some of the speed difference to the free spinning prop shaft. This is quite apparent in more crude setups, like in my Pajero. Engaging awd will actually increase turning circle noticeably compared to keeping it in rwd. Engaging lower gear will of course make turning even more difficult, as it locks the centre diff. :lol:
 
A bit of Bro science perhaps, as I haven't looked all that much in to this, but the way I understand it, the issue with having permanent drive to all wheels is the speed difference between wheels. As soon as you move from straight line, all diffs start a battle of directional control.

Centre diff is the worst, as it will try to keep axle speeds level. When the drive is cut from the centre, even the rear diff can give way more freely, as it can unload some of the speed difference to the free spinning prop shaft. This is quite apparent in more crude setups, like in my Pajero. Engaging awd will actually increase turning circle noticeably compared to keeping it in rwd. Engaging lower gear will of course make turning even more difficult, as it locks the centre diff. :lol:

You are confusing 4WD and AWD, AWD uses an LSD center diff that allows for a certain speed differential between fronts and rears, enough so that you wouldn't notice during driving. Front and rear diffs are typically open, in more expensive cars they are also some type of an LSD though most these days use brakes to force torque shift.

What you are talking about with Pajero is a 4WD system, those were always meant to keep all 4 wheels spinning at the same speed, which is why they are used only in low traction situations like heavy rain, off road or snow.
 
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