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Part-time vs. full-time all-wheel drive benefits - myth or true?

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As long as I can remember myself I have only been driving full-time all-wheel drive cars, because I considered them the best option for our cold climate.

However over the years I´ve heard multiple time from various part-time all-wheel drive advocates, that part-time all-wheel-drive is a more fuel-efficient solution: following the lines that "when you do not need full-wheel drive, and this is most of the time, you just turn it off and dont pay for it / conserve fuel" etc.

I am not saying that this is incorrect: never really did or read an in-depth study or comparison. But I would be happy to read a well-grounded explanation for the statement that part-time all-wheel-drive is more fuel-efficient, than full-time, if this really is the case?

Because when I just start thinking of it, I cannot understand why that would / could be: how exactly would part-time all-wheel-drive help conserve fuel?

Basically, part-time all-wheel drive vehicles have all the mechanical parts full-time all-wheel drive vehicles have, which are required to propel the 4 wheels. Even if you temporarily disconnect some of those parts from your engine mechanically, they arguably still keep rotating, and the energy needed for this rotation is still delivered by that same engine - what changes is just the, so-to-say, energy delivery route: after such disconnection, such energy starts being delivered to those disconnected parts not directly via the drivetrain, but indirectly (unless you disconnect them in two points simultaneously - one on the drivetrain side, and the other - on the wheels side). So where is the gain?

In addition to that: when you switch the two wheels off, these extra parts required to propel those wheels do not lose their weight, and the car still has to carry this weight around, and not only that of these parts, but also of the additional device - the two wheels connection-disconnection device - a device a full-time all-wheel drive vehicle simply does not need. Inevitably, every additional device means more weight and less fuel economy.

Also, part-time all-wheel drive vehicles have to have a more strong (hence more heavy and less fuel-efficient) drivetrain part which delivers the engine power to the wheels which always stay connected to the engine - cause in two-wheels-drive mode this drivethatin part has to process 100% of the engine power output on a long-term basis. With full-time all-wheel drive vehicles, this is not the case - with them, the engine power is always distributed (in whatever proportion, but almost never 0/100%) between all the four wheels, hence, their drivetrain in general can be built more lightweight, hence more fuel-efficient...

To cut a long story short, I personally cannot wrap my head around why exactly part-time all-wheel drive vehicles can have any tangible fuel consumption related benefits when compared to full-time all-wheel drive cars, and would appreciate a meaningful explanation, Thanks in advance!

9 Answers
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With AWD there is almost double the rotating mass being run by the engine all the time and it's unnecessary except in low traction situations.  I would always get 4wd instead of AWD.

I like the manual 4WD on my old Jeep, and although the AMC Eagles I had in the past had dry pavement all-wheel-drive it could be switched off to run in rear drive only.

 

Another plus of manual 4WD is since you only engage it on slippery surfaces you don't need to replace all 4 tires if you manage to damage one beyond repair.

Dear Doc, thanks for your response, Could you please be more specific as to what mass does not rotate with partial-time 4WD and why? Thanks.

Doc is referring to systems with unlocking wheel hubs only. They are very uncommon now.

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So where is the gain?

You are absolutely right. There is no appreciable gain. It's still the same engine, pulling the same weight.

AWD will have a higher cost of maintenance, compared to 2WD, due to the additional driveline components, but will be the same as 4WD.

 

awd

(you can see here AWD makes 1MPG difference from 2WD. If there were a 4WD option, it would be the same as AWD)

 

 

Also, part-time all-wheel drive vehicles have to have a more strong (hence more heavy and less fuel-efficient) drivetrain part which delivers the engine power to the wheels ...

Usually on a part time system you will see a standard size 2WD driveshaft and rear axle, but the front components will be lighter duty since they do less work. They do not receive engine torque 100% of the time, and are assisted by rear axle.

 

I personally cannot wrap my head around why exactly part-time all-wheel drive vehicles can have any tangible fuel consumption related benefits when compared to full-time all-wheel drive cars,

 

There is no straight answer. It depends which two particular systems you are comparing. How much do they weigh, are they mechanically coupled, fluid coupled, with or without center differential, etc.

If there is a difference, I doubt it will be significant. Certainly not enough to be a deciding factor when buying a vehicle. Also never seen a model of vehicle that comes with the option of either AWD or part-time 4WD. So it really comes down to your intended application. Fuel consumption will never be a consideration.

That’s a sexy looking van you got there.. Pity if it had an illuminated C E L..

@inthrustwetrust rub it in why don't ya
Bruise

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There are actually many different systems of four-wheel drive and all wheel drive. You would have to study each one in each individual make and model is there a many variations

Sounds like AWD and 4WD are not specific techical terms with precize definitions, but rather marketing BS?

yes, mostly marketing

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Rotating and carrying along mass causes more fuel consumption than just carrying it along. To keep it simple, follow the formula for kinetic energy, K=1/2×m×v^2. If you double the speed of something, you need 4x the energy to get it going that fast. This is also the case with rotational motion, but that equation's nasty to write in text only. Smile Basically, when the front wheels are spinning a similar sized differential and drivetrain as the rear, you need at least 2x the gas to do it. The transfer case and other stuff only adds a few hundred pounds to the truck as weight, which is significantly less than the percentage increase in mass you're spinning.

An excellent example I can provide is in my 4×4 Ford Ranger, which is part-time 4WD. When I originally bought the truck, the front wheels were permanently locked into the front axle, so when the rear wheels would drive the truck, the front tires wouldn't just be "along for the ride", they were connected to the CV axles, front differential and the front drive shaft. That means they were spinning with the wheels, increasing drag on the engine without getting power through the transfer case. I could feel the resistance when I'd take my foot off the gas on the highway, it would feel like I was pulling something and I'd slow down a bit quicker than normal. I'd average 14.5 MPG or so. 

 

As Chuck mentioned, most modern 4×4 vehicles don't have manual hubs. The '99 Ranger was able to be retrofitted with manual hubs, so I did the conversion. I unlocked the hubs and the truck gets 18.5 MPG. I would get more if I didn't have to tote around the whole front drivetrain, but that's still a boost of over 25%. 

 

The question comes down to, will you use a part-time 4×4 enough to justify the expense? I live in the country and move furniture and such. I do a lot of driving through fields. I use the part-time 4×4 to climb inclines in the yard without the rear end tearing up the grass and digging ruts. 

If you don't drive off-road and only drive in the snow, FWD is probably fine for you. Hopefully this isn't too technical of an explanation to not be understandable Smile I love explaining theoretical stuff. Haha 

 

 

This post was modified 4 days ago 2 times by Justin Shepherd

Yup. Unlocked hubs reduce drag. I think that makes sense to everyone. But there aren't many of those left on the road.

What about the majority of part time systems now, which don't have those ... I don't think they have any economy advantage. Maybe modern drivelines have improved efficiency to the point where the cost and complexity of locking hubs aren't worth it anymore. Look at the example economy numbers I posted ... 1mpg difference.

I'm curious how they did that as well, to be honest. The number I mentioned as far as approximating the front drivetrain was a conservative estimate, but the physics is always true. They have to reduce the rotating mass to maintain the efficiency. They may be getting cute and using lighter weight drive shafts and alloys in the front half of the powertrain to reduce the drag, or they've implemented some sort of auto hub that actually works reliably, unlike the crummy vacuum hubs in the turn of the century Rangers. They could use some sort of lock-up clutch in the transfer case I suppose, but that seems unnecessarily complex. I'd like to put a Go-Pro under the front end of a modern 4×4 F-150 and see if the front drive shaft actually spins all the time or not.

Yes that would be interesting. I know that modern transfer cases for example are made of strong lightweight magnesium alloys ,where they used to be cast iron. Cast iron axle housings have been replaced by aluminum. You can see from this page that Ford used a wide variety of systems: https://www.blueovaltrucks.com/tech-articles/fullsize-ford-truck-transfer-cases/

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Well of course you get worse gas mileage with full all-wheel drive. Basically depends on what you want to buy and what's available in the model as you're looking at. Many people waste their money on all wheel drive when they don't need it at all front wheel drive works quite well in the snow

Dear Scotty, with all due love and respect, I am afraid that you did not get my point. You are indeed right with what you said regarding AWD vs front wheel drive, but I was trying to compare full time AWD/4WD with part-time AWD/4WD. I am so sorry that my learnt English I am so often failing to deliver the essense. And I truly beg your pardon for being so hard-to-get.

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With selective 4wd, power is not being directed to the front wheel drive system through the transfer case.  Suffice it to say that if you live in Antarctica and drive on ice and snow every day, get AWD.  If not, get 4WD. 

I've had multiple AWD vehicles, and I beg to differ. They consume marginally more fuel. A sacrifice that is easy to make. And they offer very balanced traction both on-road and light offroad. Driving is completely transparent to the driver. AWD handles a lot better on-road.

Get part time 4WD if you like playing with your vehicle: rock crawling and playing in the mud.
Get AWD if you like to focus on getting where you're going, not on what your vehicle is doing. It's good enough for most people.
Smile

One man's ceiling is another man's floor. To each his own.

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Have you tried searching the forum?

https://carkiller.com/scottykilmer/qa/awd-3/

My bad, I did not think of it. Thank you for pointing this out for me, and for the link full of useful videos I am looking forward to watching!

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My understanding of AWD vs 4WD used to be that AWD is four-wheel drive which requires zero user interaction, while 4WD does require some. Is this correct?

That is one aspect yes. But mainly, AWD can be used all the time whereas 4WD is not. AWD is aimed more towards on-road driving where you might encounter inclement conditions. 4WD is more aimed towards off-road use.

 

The car also has a low gear for rough terrain which can only be engaged manually. Does this mean that this car has 4WD and not AWD?

No. Again, low range is aimed at the offroad user. Which I find strange, because lets face it no Audi driver is going offroad. This leads me to think it's just marketing gimmick.

But low range has nothing to do with the AWD/4WD distinction. There are some AWD drivetrains with a low range function (like your AMG)

Pressing this button is also some sort of user interaction. Does this also mean that this car has 4WD?

If my understanding is incorrect - what are the real differences between 4WD and AWD?

 

Now you are starting to see that there is no clear distinction between the titles AWD and 4WD. They are blurry and meaningless to any deeper understanding.

 

To make the most of the traction systems on your particular vehicle, forget the names, and read the owner's manual about how to use them. For a deeper understand about how they work, find out the model number (often stamped into the housing or on a nameplate) of the components (transfer case, center differential, power transfer unit, etc.). Then do a search for that model number, including keywords like "exploded diagram", "power flow", "operation mode/description", etc.)

 

Good luck.

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Topic starter

My understanding of AWD vs 4WD used to be that AWD is four-wheel drive which requires zero user interaction, while 4WD does require some. Is this correct?

If yes, and if you think of it in such strict terms, just about all cars I know have 4WD and not AWD.

Example: 
My 5-cyl. turbo C4 Audi Avant AAN Quattro has full-time all-wheel drive, and has a rear differential lock button. Pressing this button is user interation. The car also has a low gear for rough terrain which can only be engaged manually. Does this mean that this car has 4WD and not AWD? 

Another example:
My 8-cyl. ML 55 AMG has full-time all-wheel drive, and has a low gear button, which also engages some sort of a special off-road traction control mode. Pressing this button is also some sort of user interaction. Does this also mean that this car has 4WD?

If my understanding is incorrect - what are the real differences between 4WD and AWD? Thanks.

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