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

[DaBoom]

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.

I'm well aware of this. But still, the point is when wheels are connected they will try to maintain even speed and turning will upset that balance. As you said the amount of extra effort depends on types of drivetrain or diffs used, but it's still added drag. Having some form of lsd in the centre to handle torque split seems to be the worst scenario, consumption-wise. Pre x-drive awd BMWs had a viscous unit in the centre. That converts forward drive to heat whenever you're turning (I know as I have one). :lol: And while torsen handles speed differences with better efficiency, it's a heavier drivetrain to run.

 

[prizrak]

I'm well aware of this. But still, the point is when wheels are connected they will try to maintain even speed and turning will upset that balance. As you said the amount of extra effort depends on types of drivetrain or diffs used, but it's still added drag. Having some form of lsd in the centre to handle torque split seems to be the worst scenario, consumption-wise. Pre x-drive awd BMWs had a viscous unit in the centre. That converts forward drive to heat whenever you're turning (I know as I have one). :lol: And while torsen handles speed differences with better efficiency, it's a heavier drivetrain to run.

Yeah my Subaru had a viscous and Audi had a torsen, both were set for a 50/50 default with open diffs at either end. I'm curious how much realistic drag there is from that type of diff in the first place.

 

[narf]

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

Of course it does (ignoring plug-ins). The engine makes the starter motor spin, the battery is just temporary storage for the dino juice going boom.

 

[Blind_Io]

I'm well aware of this. But still, the point is when wheels are connected they will try to maintain even speed and turning will upset that balance. As you said the amount of extra effort depends on types of drivetrain or diffs used, but it's still added drag. Having some form of lsd in the centre to handle torque split seems to be the worst scenario, consumption-wise. Pre x-drive awd BMWs had a viscous unit in the centre. That converts forward drive to heat whenever you're turning (I know as I have one). :lol: And while torsen handles speed differences with better efficiency, it's a heavier drivetrain to run.

That is why a differential exists, it manages the speed differential between two outputs. The center diff does this for the total power going to front and rear axles, the axle diffs then split the power again to go left and right - but they don't split it equally. It doesn't add any stress to the system because the total power output all equals 100%

Left front + Right front + Left rear + Right Rear + mechanical power loss and heat = 100% of the engine work output. You can move the numbers around regarding the individual wheels all you want, but it will all equal in the end.

 

[prizrak]

Of course it does (ignoring plug-ins). The engine makes the starter motor spin, the battery is just temporary storage for the dino juice going boom.

Not necessarily, you can put a brand new battery in that engine never charged

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That is why a differential exists, it manages the speed differential between two outputs. The center diff does this for the total power going to front and rear axles, the axle diffs then split the power again to go left and right - but they don't split it equally. It doesn't add any stress to the system because the total power output all equals 100%

Left front + Right front + Left rear + Right Rear + mechanical power loss and heat = 100% of the engine work output. You can move the numbers around regarding the individual wheels all you want, but it will all equal in the end.

I think his point was more that if you are not disconnecting 2 wheels in an AWD system engine is fighting against whatever resistance there is in the diffs.

 

[Matt2000]

TL,DR for me here but you'll always have parasitic losses with a 4WD system, hence locking (or more importantly unlocking) hubs that stop the front axle turning in any 4WD. In a full-time system with a centre diff you'd have to lock the diff with the front wheels unlocked to go anywhere but you'd still see an improvement in fuel economy.

Leave a centre diff locked or a part-time system in 4WD and you'll get the same result in both cases, twisted (or broken in the worst case) shafts in the transfer box. That's beside the point though. The advantage of a full-time system is that on a surface with enough traction and the wheels can maintain contact with the road, you have traction from all 4 wheels while still being able to negotiate corners with no fear of damaging the transmission. You can't do that in a part-time system.

I'm pretty familiar with the viscous centre diffs too, those are great until they fail, then they become a massive load and the fuel economy goes through the floor.