Why do 4wd vehicles use more fuel?

Physics

think you pretty much nailed it.

Driveline drag, extra weight, friction etc.

I would say thats why the VW 4Motion system is pretty good, FWD until you need AWD, so whilst you get the weight penalty, but without the drag during regular driving conditions.

Baby Robins.

They are optimised for a general use profile that is different to that for a hatchback. They are likely to have more aaero-drag and mechanical friction but also tyres optimised for good grip rather than low rolling resistance, lower gearing for towing etc

Weight and more things needing to be driven/powered...

I imagine these days it's mostly weight, size and shape, and possible engine choice. Most '4x4' these days don't have a permenant 4x4 drive train, but have a system that only engages all 4 wheels in an event where the two front wheels start to slip - which might be never for most users. A lot of haldex type 4x4s the system can competely fail and the driver would be non the wiser. Whether the redundant half of the 4x4 system has any extra drag or friction I'm not sure

Bigger frontal area because they tend to be too big. Plus the rest of the things mentioned.

The 4wd drivetrain has more friction and also adds weight. Off-road vehicles are built strongly so are heavy. They also have higher ground clearance so they sit up higher and tend to be boxy so they are less aerodynamic. If you fit off-road tyres, they will be draggier than road tyres. I the old days, they also tended to be lower geared so the engines work harder at highway speed.

Thanks, I'm specifically looking at differences in the same model e.g.

https://www.honestjohn.co.uk/realmpg/bmw/3-series-touring-2012/320d-xdrive-automatic 44.9 mpg
https://www.honestjohn.co.uk/realmpg/bmw/3-series-touring-2012/320d-automatic 48 mpg

I would have thought that the difference would be bigger TBH

There are a lot of 2wd and 4wd variants of the same cars about now - and they'll be part-time 4x4 systems but otherwise the same shape and engines. Given the conditions / instances where the 4x4 is engaged are so rare I'd expect the fuel consumption figures will reflect them running exclusively in 2wd. So comparisons with between 4x2 and 4x4 versions of say a VW Tiguan would highlight only the weight and drag in that unused bit of drivetrain and everything else will be pretty much equal.

I'm not sure how much difference weight makes on its own. The additional drive train components are only going to be a a couple of dozen kilos - equivalent to a passenger maybe at most. Do you notice a difference at the pump when you have passengers in the car? Theres a 1.5ton difference between my van been laden and unladen and it doesnt really seem to alter the distance I can cover with a tank of fuel

So it may all be friction - a 2wd car the undriven wheels are just free-wheeling. On a 4x4 running in 4x2 the undriven wheels are turning an undriven axel and differential

Weight, aerodynamics, drive train losses, larger engines.

I don't really understand how part time 4wd helps (although it does seem to). There aren't clutches at both sides of the driveshafts etc, so they'll still all be spinning (pushed by the wheels) even if the transfer box isn't engaged. The only thing I can think is it reduces wind up etc as you go round corners but if you have an open diff in the middle that shouldn't be an issue either

@oldtennisshoes

Thanks, I’m specifically looking at differences in the same model e.g.

https://www.honestjohn.co.uk/realmpg/bmw/3-series-touring-2012/320d-xdrive-automatic /a> 44.9 mpg https://www.honestjohn.co.uk/realmpg/bmw/3-series-touring-2012/320d-automatic /a> 48 mpg

I would have thought that the difference would be bigger TBH

AFAIK mpg was chosen as an industry standard for fuel efficiency as it makes differences sound bigger than they actually are. ie the real world experience between a 44.9mpg and 48mpg car will be barely noticeable.

In the case above, it'll be drivetrain losses and weight.

Doesn't the Haldex type system use some kind of fluid coupling, which would have drag even when 4wd is not engaged?

The additional drive train components are only going to be a a couple of dozen kilos

I think I recall seeing a figure of 100 kg a couple of decades ago. It'll vary though, depending on whether the base model is front or rear wheel drive. You need to add a center differential and transfer case, another differential, two half-shafts, plus a long driveshaft to the back if it's a front-wheel drive car.

probably down to the type of driver they attract. 😉

i thinks it's because you're having to turn things that don't directly involve pushing the car forward (even if the 4wd isn't permanent).

Interestingly, the NEWER polestar EV completely disengages the front motor when you're not pinning it, making it almost 'true' 2wd (in terms of efficiency) when in 2wd mode.

DrP

Pretty much a transverse FWD differential absorbs about 5% of the power.

The differential longitudinal in a RWD car absorbs about 10%.

A 4WD has effectively 1x of the first (the transfer box) and 2x of the 2nd. So it's losing about 20% more of it's power before it's even made it to the wheels.

Then there's the inherently "4x4" bits. Lots of bolted on trim disrupting the airflow, the large frontal area, the large space underneath the body meaning the underside creates more drag too.

Then there's the "GET OUT OF MY WAY, IM MORE IMPORTANT THAN YOOOOOOOOOOOO" nut behind the wheel using the pedals in a binary fashion.

Haldex uses many thin clutch plates that are forced together by oil pressure from an electric pump. Pump is switched by ecu when differential speed/slip seen between axles and can lock drive to rear in a fraction of a second. 

3 series x drive sits higher than a rwd 3 series so will be less aerodynamic as well as heavier.

I believe they also don't handle as well as the rwd if you are in to that kind of thing

Above - also 4 x 4s tend to be the higher models. More kit that eats mpg, bigger wheels, bigger body kit etc

I would say thats why the VW 4Motion system is pretty good

Haldex, it's in loads of cars.
The losses in 2wd mode are mostly in the power take off section of the gearbox (spinning an extra couple of bits of metal. inc the prop).

Extra weight is slightly heavier gearbox (need to add the powertake off hardware), an entire propshaft, the haldex unit itself, rear diff (quite a few have them in the same housing now), rear halfshafts and a few other bits that will be heavier/reinforced. From memory, the entire package adds about 150 kilos in a mid size (CD segment) car. Generally no centre diff.

The only "standard" thing is they tend to have slightly shorter final drive (3-5%) to overcome the slightly higher driveline drag and losses, all the taller/bigger tyres/more grip/more drag/more ground clearance is just as applicable to 2wd/AWD/4WD.

Thanks, I’m specifically looking at differences in the same model e.g.

https://www.honestjohn.co.uk/realmpg/bmw/3-series-touring-2012/320d-xdrive-automatic 44.9 mpg
https://www.honestjohn.co.uk/realmpg/bmw/3-series-touring-2012/320d-automatic 48 mpg

I would have thought that the difference would be bigger TBH

The AWD xDrive BMWs use a completely different system with a transfer case, clutches, diffs and so on. More in common with a Land Rover. Well, that's actually where they got the idea from.

It takes more energy to drive 4 wheels than 2.

The BMW xDrive is one of the better systems when it come to weight penalty but its still exists, this combined with drivetrain losses from the additional driven wheels is why the MPG is lower

I seem to recall that when Audi produced the first generation Quattro they were surprised just how efficient it was and they put down to having driven wheels all round being better than having two non driven wheels with their rolling resistance. The drivetrain was also much simpler and lighter than conventional 4WD systems (centre diff was from a Polo!).

Thanks, I’m specifically looking at differences in the same model e.g.

https://www.honestjohn.co.uk/realmpg/bmw/3-series-touring-2012/320d-xdrive-automatic 44.9 mpg
https://www.honestjohn.co.uk/realmpg/bmw/3-series-touring-2012/320d-automatic 48 mpg

I would have thought that the difference would be bigger TBH

I suspect the difference here will be narrower than expected due to the standard BMW drivetrain. Being RWD as standard they already have some of the extra weight in place getting the power to the rear. There will certainly still be some extra weight adding the transfer to front wheels, but not as much as doing it in - say - a golf.

Where does the benefit come from disengaging the 4x4 in haldex type systems. If the engine is driving all 4 wheels then you've got those losses at the various differentials. Disengage via the clutch and.... all those wheels are still turning and all those differentials are still turning - but instead of being pushed along by the engine they're being pulled along by the undriven wheels. How is that better?

It's lighter (a centre diff is heavy and complicated), especially in a transverse engined car. So AWD only has two diffs.
If you have permanent AWD without a centre diff, you'll tend to go straight on at every corner (i've driven a few test cars with the haldex permanently closed, it's not much fun.)

Also the harder you drive a gear, the drag increases exponentially. Dragging the entire back end of the driveline with (virtually) zero load adds (virtually) zero drag. Forcing it around with a couple of hundred Nm, when you don't need to, will add loads of drag. There are also loads of direction changes, which add even more losses when under power.

centre diff was from a Polo!

There might have been Polo parts in there, but the magic part (centre diff) of Quattro is a Torsen differential, bought in from Gleason in the US.

Quattro adds 130kg to an AUDI.

For non-SUV vehicles it’s the weight and gearing.

For instance a manual Mk 7 Golf Gti (2WD) weighs 100kg less than the equivalent manualGolf Mk7 R  (Haldex 4WD)and has significantly higher gearing than the R. The Gti can average 45 mpg driven gently, the R 38 mpg driven in the same way. There are of course many other factors but those are real world figures from my family’s vehicles

There might have been Polo parts in there, but the magic part (centre diff) of Quattro is a Torsen differential, bought in from Gleason in the US.

Torsen diff was a later addition I believe (late '80s). Early Quattros had a non torque sensing centre diff with manual diff locks on the dash.

They have to transport the ego of the driver.

probably down to the type of driver they attract. 😉

Not sure about that, but when i had a couple of Impreza turbos they were such fun that it was almost impossible to drive them at any less than 8/10ths, and that didn't do much for consumption. A total gas tho!
Ahh, happy memories, when i was younger and less sensible. Though i could get the MTB in the boot with the back seats down.
Scared the bejesus out of have-a-go 3 series tourings back then who didn't have quite as much grip as they thought they did on wet roundabouts. Hence x-drive i imagine, that and losing sales to audi quattros

TheGingerOneFull Member
3 series x drive sits higher than a rwd 3 series so will be less aerodynamic as well as heavier.

I believe they also don’t handle as well as the rwd if you are in to that kind of thing

This is not true, on this market all xdrive 3 series on (F31) were delivered on sport suspension unless specified otherwise as factory order. Also with Audi,  VW and Merc can be ordered with different suspension heights with or without 4WD - the highest configuration is usually about 30 mm higher than standard. And Allroad / Alltrack versions are even higher.
There is some difference in handling or at least different feeling in steering when comparing rwd and xdrive 3-series.

Yeah I would have gone for an Alltrack type thing if it weren't for the fuel hit, since they'd be handy on a caravan site - plus I live on a hill.

Related to my other car thread - someone near here has a new CLS V6 diesel with 4-matic.  That would be my perfect car except for that it probably struggles to get above 40mpg. Also it's not electric obvs 🙂

Most 4WD/AWD cars have a worse turning circle than a RWD car on the same chassis.  It’s usually up to a meter.

How about 4WD v. FWD?

Bigger frontal area because they tend to be too big.

Particularly noticeable on a Fiat Panda 4x4 or a Golf 4x4, or an Octavia 4x4…

It’s usually up to a meter.

Parking, water, or electric?

Particularly noticeable on a Fiat Panda 4×4 or a Golf 4×4, or an Octavia 4×4…

Yes I've never heard of a Suzuki ignis being described as "too big" but while generalising. Plenty room for chips in the back of most of the "too big" vehicles.

On the xdrive its mechanical drag.

I can still see over 50mpg in my 530dxdrive though.

I was impressed in the small pocket of snow we had compared to a rwd bmw.

Most 4WD/AWD cars have a worse turning circle than a RWD car on the same chassis. It’s usually up to a meter.

Yes, converting a RWD car to AWD/4WD has a lot of compromises around the front steering and suspension, adding in the half shafts can impact on the steering rack (extra shafts and joints going though the suspension/hubs) that limits the lock.
Also have to muck around with the sump and bottom end of the engine to get the diff into place. (sometimes enough to change the required oil volume).

Owning a 4wd SUV is about showing the world how much you can afford to spend on your car, you don’t want that undermined by people thinking it’s cheap to run. Apologies if you actually live at the end of a long tough track.

Owning a 4wd SUV is about showing the world how much you can afford to spend on your car,

£500

TBH, can people not tell the difference between an SUV and a hatch, estate or saloon? (AWD/FWD/RWD/4WD or whatever).

I'd be gobsmacked if i could go from a Golf to an X7 and get the same fuel consumption even if it had the some drive configuration...

Though a Passat to a Passat 4Motion, that's actually sort of comparable.