I've always wondered this, one of those random thoughts. If you're cruising on the motorway, why can't you switch the turbo off. In the interests of economy... Or wouldnt it make any difference?

Presumably you could take the same argument if you had a v6 or v8 engine. Why when peak power isnt required use all the cylinders?

Im sure there's good reasons...

Dan

 

There is in a way, if you have a standard dump valve it opens at cruise taking the load off the compressor wheel and reducing the exhaust back pressure.

The switch is the throttle, don't press it hard and you don't use so much fuel

 

It would be incredibly useful if you could switch between maps, say a rediculously fuel efficient one for driving long distances on the motorway, a normal one and a sport one.

 

That does not sound quite right.

Surely when the dump valve is open, the compressor wheel would have a route back from the outlet to the inlet of the compressor.

With a centrifugal compressor, as the discharge pressure is proportional to wheel speed, load is proportional to flow. Ie at no flow, the loading is frictional only, at full flow, the loading is 100%. Basically energy is being used to move the mass of air.

Of course, when the throttle is partially open, there is insuficient exhaust flow to spin the turbo at high speed which is why compressor speed reduces.

 

You could rig up solenoid of sorts to open the waste gate which will divert some of the exhaust gasses away from the turbine but this wont stop the turbo completely.

Don't forget the compression ratio on turboed cars is very low, this will mean the fuel economy will be horrendous when the turbo is not producing any boost, so on the motorway your car would be drinking fuel without the turbo.

 

You do have a switchable map, and yes it's switched by the throttle.

At motorway speeds you're doing reasonable revs and very light throttle, when you're pressing on you're doing lots of throttle and lots of revs.

The fuelling is controlled by a map with throttle and revs as inputs and at these two points is very different.

 

i used to have a starlet gt turbo a few years ago and they all come standard with a high-lo boost switch on the dash..was very handy for saving juice..i think power went from 133bhp down to 110bhp at the touch of a button...

 

Old school.. The same kind of method as a bleed valve except it was simply a solenoid that opened a preset valve. The problem is the starlet also had an EFi and probably had a mechanical boost advance module on the side of the dizzy.

 

That would be the case with a fixed speed ie electrically driven compressor, but not with a turbo. The turbo is spinning at slower speeds with zero head and low flow. The load by creating flow is more than offset by the friction induced on the compressor wheel, by the (almost)static air at pressure on the outlet side, which is trying to act as a brake.

 

blimey thats a lot of replys

i figured if you were cruising that it wouldnt make much difference...

there's always the pshychological advantage tho. when going down country lanes if you could turn it off, then you may proceed at a more leisurely pace! After all once that boost comes in who can resist?

 

Ah; I didnt know that. Thats precisely the reason i was looking for...

Thanks!
Dan

 

If you've got a turbo big enough to stay off boost at motorway speeds then you save a lot of fuel. For example I can get 400miles to a tank in a classic at 70mph by staying off boost.

Dave

 

there are a couple of ways to acheive this.
1) remap the ecu, as mentioned when pressing on at full chat the ecu is in entirely different zones to when cruising or idle, the map on my own car is leaned right out to 15.5:1 at cruise and idle, very good economy.
2) an electronic boost controller like an AVCR allows you to have two user set boost maps and one default boost map (3 in total), again, on my own car these are set to 0.7bar, 1.3bar and 1.65bar.
simply select which boost limit you want and wahay

 

Ive got an avcr and I only have 2 settings, A & B. Where is the third?

 

Hum, interesting.

So the turbulent friction at the compressor vanes trailing edge is greater than the energy required to provide flow ?.

Why do WRC cars make do without dump valves and allow the turbo to go into stall ? I thought the stalled compressor while exposed to a shock load, spins longer in stall than if pumping through a discharge route ?

 

jaytc2003.

<B> 1.65bar
<A> 1.30bar
<OFF> 0.70bar

its there alright

 

Intersting read guys

 

not really
if you bought a subaru for fuel economy then your slightly adrift from reality

 

Too True

 

indeed i know that! it was just an idle curiosity anyway, not a serious plan!
Saying that if the EU get their way about minimum mpg, then subaru and everyone else may well have to figure out a way to do something about it.

 

we have! map the ecu at idle and cruise to a lean afr.
theyre not going to send the gestapo round and test each car at all load zones on the map.

 

Quote of the year

 

WRC cars are in a whole different situation, I don't think they're all to worried about fuel economy at cruise, add to that anti-lag and it's not really a comparison. Opening the throttle and retarding the ignition far enough to avoid engine acceleration is not an efficient way of keeping the turbo up to speed.

Rather than get into fluid dynamics how about this. Blank the DV and make a quick upshift at full power, try the same with a dv and you'll notice straight away the increased response from the turbo, particularly with a big turbo. Do the same with anti-lag on, with no dv the response is instant, with one it's a little better than no anti lag.

Anyway that's beside the point. at cruise speeds the turbo is not spinning fast enough to provide enough flow to cause a pressure head, it wouldn't even register as a point on the compressor map (unless you had a stupidly over cooked map). At this point with no flow restriction it is spinning (pretty much) freely on the bearings. Blocking this flow provides resistance to the compressor wheel braking it. In this situation the turbo is not acting as a centrefugal compressor does as it is so far outside it's optimum working range.

I hope this reads in some sort of order I've been doing other things at the same time

Dave

 

I am trying to understand the dynamics of a compressor, only really have experience of constant speed hydraulic pumps and a little about 10MW gas circulators in a nuclear reactor.

I can't understand how a compressor wheel can draw more energy when not moving a mass of gas than it does when its pumping. Where is the energy going ?, is it into turbulance at the lip ?

 

It obvious really. I cant believe you cant grasp this simple premise.
Should this not be in the technical section as my head is starting to hurt.

 

Sorry Sgcooby, bit out of hand but you know how these threads go... if it's not a flaming of some poor charactor it's massively overcomplicating a simple issue

I don't really know how to explain this so sorry if this is a bit condacending.. you can always flame me for spelling

a turbo at low revs is working more like one of those windmills on a stick than a centrafugal pump it's spinning away merrily in the wind if you blow on it from the other way it stops pretty quickly and spins the other way.

This is the pressure difference between inlet and outlet, the less back pressure you have the less effective 'wind' you have.

As the turbo is being driven by exhaust gasses the compressor is spinning away and moving air very inefficiently. If you build a backpressure you're as much as blowing against the flow of the windmill effectively braking it. The load from pumping is relatively tiny as the compressor is way out of it's working speed range so can't develope the torque to brake the wheel, for example optimum speed may be 80K rpm and at cruise it's doing 3k rpm.

Even if both ways use the same energy with the speed with the recirc open is going to be higher than if it's shut hence less exhaust backpressure and more efficient enginer.

Does that make sense? I know it's not overly technical but the rules of centrifugal compressors don't entirely work until you get into a decent speed range for that particular compressor

 

A big thing to remember is ,an internal combustion engine is inefficient(something like around 20%) i'm sure someone will correct me ,so the initial purpose of a turbo is to improve engine efficiency and in turn gives better performance. Turning the turbo "off" so to speak would serve no real purpose.If your looking for better economy,then a smoother driving style and improved roadcraft would bring greater improvement.

 

Exactly - top notch spot-on advice.

The quoted MPG figures are not much better for a non-turbo Impreza - so I'm thinking that being able to turn off the turbo would be pointless and the MPG benefit so marginal. I didn't buy an Impreza WRX Bugeye for out and out economy - but given the type of car it is and potential performance, I think I do pretty well to have often achieved averages of 30-32MPG for a pretty mixed bag of driving during summer months.