Engineering food for thought (4wd and Supercharged content)

[boost monkey 0]

Yes, that's right. I am going to put my university musings up on the interweb! ^_^

 

First thoughts were about 4wd. In my mind, it drives all four wheels and thus more power is lost from engine through the transmission. I'm 90% confident this is because of the friction/mass/drag of the system. If someone made lightweight props and driveshafts would this loss be reduced? Obviously lightweight magnesium alloy wheels will help reduce the mass and increase rotational acceleration, but can the 4wd systems be improved greatly....what is the new Haldex system like?, I hear it's electronically controlled as opposed to the old syncro / UR Quattro mechanical systems that I had in mind.

 

On a sidenote, does anyone know if there is a roadgoing 4wd standard/high-performance (Evos, Subarus etc) transmission system with equal to or less than 20% of engine power needed to drive it?? Hope i'm wording this right, it makes sense in my head !

 

 

 

With regards to superchargers, do they leech power due to the increase in air pressure they are generating or is the leech purely mechanical drag? If it's purely mechanical, could someone make a carbon fibre pulley and a lightened woven-kevlar ribbed belt to overcome these issues? Do you see where I'm going with this?

 

If it is purely mechanical drag too, surely the charger insides could be lightened / cast from different materials. If anyone has information from racing / championship supercharged engine backgrounds, I would be more than happy to listen!

 

Hope this is a good read,

 

Jon :D

 

 

I am all about efficiency!

[ziderapple 0]

The power leeched from running a supercharger is almost purely mechanical, the pulleys are made of ally and are very light and belts weigh sod all, as for the internals being cast or machined from lighter materials, that could help matters but its the internal frictions which are fairly high. Holding a supercharger in your hands if you turned the pulley you would feel quite a resistance, as soon as you stop turning the pulley the impellor blades will stop moving. With a turbo you can very easily spin the impellors using your finger and they'll carry on spinning.

 

Not clued up on 4WD. :(

[corozin 0]

You'll always get bigger friction losses with 4WD, all you can do is to try and minimise it, but that isn't a particularly cheap thing to do.

 

The 4WD conversion has been done a number of times in the UK. Normally the route is to basically use the Golf Rallye running gear, mated to a Golf Rally rear floorpan (to replace the Corrado one), but the kit itself is old now, and Golf Rallye rear 'pans are no longer available new as far as I know. Once you've got that done, you also have to resolve the issue of the rear suspension setup, as the rear beam will foul the 4WD kit if you try and run the car at a sensible ride height.

 

Nowadays using the Haldex kit probably makes more sense as it's newer kit. Although it's not full time 4WD, it will take more power than the Rallye systems do, and it can be rechipped to be more responsive than standard.

 

If you havn't seen it you should go and look for BiggerBigBen's thread in the gallery section, as he's done this conversion. Word of warning before you get your angle grinder out though - Ben's car is the result of almost 5 years of pain, hard graft, cold garage nights and head scratching.

 

If you're desperate to run 4WD reliably and in a sensible timeframe, you'd probably be better off saving for an Evo-6. Doing the conversion to a Corrado is very much the mission of a lifetime, and it won't necessarily make the car a lot quicker in most situations.

 

HTH,

John

[Toad 0]

4wd setups also never drive the wheels at the same speed, there is always a tiny difference, I believe the front wheels always rotate faster than the rears, this is where a lot of the loss comes from.

[junkie 0]

4wd setups also never drive the wheels at the same speed, there is always a tiny difference, I believe the front wheels always rotate faster than the rears, this is where a lot of the loss comes from.

 

Not heard that 1 before. Interesting.

 

Is this a good thing for drivability regarding oversteer/understeer?

[dr_mat 0]

4wd setups also never drive the wheels at the same speed, there is always a tiny difference, I believe the front wheels always rotate faster than the rears, this is where a lot of the loss comes from.

 

Not heard that 1 before. Interesting.

 

And I don't think it's true, either. Anyone who designs a 4wd system where the wheels don't turn at the same speed wants to lose their job pretty sharpish. You'd chew tyres in no time, the car would buck and twitch while the transmission flexed ..

[Toad 0]

4wd setups also never drive the wheels at the same speed, there is always a tiny difference, I believe the front wheels always rotate faster than the rears, this is where a lot of the loss comes from.

 

Not heard that 1 before. Interesting.

 

And I don't think it's true, either. Anyone who designs a 4wd system where the wheels don't turn at the same speed wants to lose their job pretty sharpish. You'd chew tyres in no time, the car would buck and twitch while the transmission flexed ..

 

The idea is that the front is pulling slightly, otherwise the back could potentially try to overtake. Besides, even on a system that is perfectly balanced, new tyres at one end and old ones at the other are going to create this sort of effect. On a road car that is capable of 150 mph the difference in gearing is going to be tiny to stop windup in the transmission.

[dr_mat 0]

Even if it is true (seems highly unlikely to me), not many 4wd systems have a central diff that will fully lock anyway, so any difference in gearing would just be lost in the diff.

[junkie 0]

 

Not heard that 1 before. Interesting.

 

And I don't think it's true, either. Anyone who designs a 4wd system where the wheels don't turn at the same speed wants to lose their job pretty sharpish. You'd chew tyres in no time, the car would buck and twitch while the transmission flexed ..

 

The idea is that the front is pulling slightly, otherwise the back could potentially try to overtake. Besides, even on a system that is perfectly balanced, new tyres at one end and old ones at the other are going to create this sort of effect. On a road car that is capable of 150 mph the difference in gearing is going to be tiny to stop windup in the transmission.

 

Regarding the tyres, maybe thats why ppl say replace all at the same time.

[boost monkey 0]

The power leeched from running a supercharger is almost purely mechanical, the pulleys are made of ally and are very light and belts weigh sod all, as for the internals being cast or machined from lighter materials, that could help matters but its the internal frictions which are fairly high. Holding a supercharger in your hands if you turned the pulley you would feel quite a resistance, as soon as you stop turning the pulley the impellor blades will stop moving. With a turbo you can very easily spin the impellors using your finger and they'll carry on spinning.

 

Not clued up on 4WD. :(

 

Cheers for the info, that's awesome.

 

Any idea why the two sets of impellors spin (freely) with different frictions? I guess thig might be how a superchargers blades should spin at a governed and constant velocity, varying only with engine speed?

[boost monkey 0]

You'll always get bigger friction losses with 4WD, all you can do is to try and minimise it, but that isn't a particularly cheap thing to do.

 

The 4WD conversion has been done a number of times in the UK. Normally the route is to basically use the Golf Rallye running gear, mated to a Golf Rally rear floorpan (to replace the Corrado one), but the kit itself is old now, and Golf Rallye rear 'pans are no longer available new as far as I know. Once you've got that done, you also have to resolve the issue of the rear suspension setup, as the rear beam will foul the 4WD kit if you try and run the car at a sensible ride height.

 

Nowadays using the Haldex kit probably makes more sense as it's newer kit. Although it's not full time 4WD, it will take more power than the Rallye systems do, and it can be rechipped to be more responsive than standard.

 

If you havn't seen it you should go and look for BiggerBigBen's thread in the gallery section, as he's done this conversion. Word of warning before you get your angle grinder out though - Ben's car is the result of almost 5 years of pain, hard graft, cold garage nights and head scratching.

 

If you're desperate to run 4WD reliably and in a sensible timeframe, you'd probably be better off saving for an Evo-6. Doing the conversion to a Corrado is very much the mission of a lifetime, and it won't necessarily make the car a lot quicker in most situations.

 

HTH,

John

 

 

Cheers John, that's some quality information.

 

I was just thinking about it really. Ideally (and what an awesome if non-economical project...) I would like to play with a VR6 turbo grafted to 4wd system. But then this would be in the Jetta. New definition of boost-brick? It's all ideas anyways, I'm always thinking more than doing, especially with my Poverty Spec income! :(

 

I will check out BBB's 4wd post again, it does make a good read.

[boost monkey 0]

with the 4wd setups, would it be ridiculous to convert one to RWD by blanking the driveshaft hubs at the front and just running the prop out of the rears? I guess you'd still get a lot of power loss and the system prob wouldn't work that great?

 

I guess the mech one might be ok, but the Haldex might throw a fit with it's computer?

[dr_mat 0]

Any idea why the two sets of impellors spin (freely) with different frictions? I guess thig might be how a superchargers blades should spin at a governed and constant velocity, varying only with engine speed?

 

A turbo impeller has no gearing attached, it's just a couple of vanes on a spindle in a high quality lightweight bearing. A supercharger has a whole bunch of cogs and gears, which have physical mass, which have significant inertia of their own. Plus obviously a supercharger has more bearings, and *heavier* bearings because unlike turbos it has to sustain lateral forces (caused by the cogs meshing).

 

There's not really any magic way to reduce 4wd transmission losses other than using quality bearings and the lightest propshafts you can get away with (bearing in mind your torque transmission requirements of course).

 

Of course, the way trains do all-wheel drive is MUCH more efficient. Use the engine to run a generator, stick an electric traction motor on each wheel, and transmit the energy to them via high quality electrical cables. Very low mechanical losses, you just have to fight electrical losses instead (easier to control just by adding more or thicker cables).

[Toad 0]

with the 4wd setups, would it be ridiculous to convert one to RWD by blanking the driveshaft hubs at the front and just running the prop out of the rears? I guess you'd still get a lot of power loss and the system prob wouldn't work that great?

 

I guess the mech one might be ok, but the Haldex might throw a fit with it's computer?

 

You would still be driving the front diff, so as you say you'd end up with unfortunate power losses. and you'd need a custom haldex controller. as no front drive would probably make it sad. :)

[boost monkey 0]

fair enough, no barn modding of 4wd to be tail happy then! :D

 

@dr_mat, thanks for that update. Very interesting stuff! I am in the midst of buying textbooks for this year, but am hoping to get by on my olds and use the money to buy more interesting engine books. woot!

[Skimask 0]

A guy I know is a Volvo mechanic - he had a 4Wd one in for a new diff - it was trashed because the owner had used a new tyre and a part worn one on the same axle. The difference in diameter was enough to trash the diff.

[boost monkey 0]

:shock: :shock: :shock: that's crazy! Sometimes it's the little things that you don't even think about that make the biggest difference.

[dr_mat 0]

Sounds like idle speculation to me .. FWD cars have a diff too, and I never heard anyone blaming a dead diff on mismatched tyres across the axle there ..

 

Not that it's a smart idea to have badly mismatched tyres on the same car, let alone the same axle of course .. :)