Turbos

I'm not exactly a mechanical numpty (well not on the theory side, anyhoo)
but, as befits someone of my calibre (i.e. someone who does **** all each
day and occasionally lets their mind wander onto totally pointless stuff in
oder to take up a bit of the day), I was thinking about turbos today (obv
wasn't too clever to let the mind wander too much when the subject popped
into my head as i was driving my turbo (allegedly) diesel car and I was in
too high a gear and **** all happened when i stood on the 'go' pedal..).

I know how they work.

I understand the physics and mechanics of their operation *when_running*.

But I got thinking that I relly don't know how/why turbos start up at
certain rpm.

Is it a friction thing? I.e. does there need to be enough gas pressure to
start up the exhaust side?

Or is it an electronics thing? I.e. the gases are routed away from the
turbo until a certain rpm?

Or is it something else that prevents a turbo from acting like a
supercharger and providing a bit of boost at all rpm?

Just curious.

Dave

 

I'm probably wrong, but my understanding is they're actually an obstruction
in the airflow until they reach a critical mass point at which point they
then function as intended forcing the charge in.

 

It's because they're bugger all use under a certain rpm and act more
like a restricter than a booster. It's in the nature of the beast, being
a centrifugal impeller for light weight and fast spin-up, but until
there's a decent quantity of zorst gas banging past the blades, the
effect on the input is negligible.

 

Ta,

I (sort of) had guessed that bit - otherwise they *would* be used all the
time.

But I still want/need to know - what stops one from working below the
optimum start-up revs/gas-flow etc? *That* is the big question.

Dave

 

It's just like jumping off the kerb - and then jumping off
portland bill.

When you jump of the kerb there is not
enough energy to do you any harm when you
reach the road 6" below.

When you jump off Portland Bill on the other hand
there is more energy available when you reach the bottom.

At low speed there is not enough exgaust in the turbine
to make it do much. Of course as the power builds up
there is more gas which makes more power which makes
more gas which makes more power which makes
more gas which makes more power until .....

The engine melts.

Which is why 'big' turbo installations need a waste gate.

If the turbo is little compared with the size of the engine
then maybe the waste gate is not necessary.

I think:
One way to think of a turbocharged piston engine
is that it is really a Gas Turbine with some pistons
getting in the way as opposed to the usual model
which is of a gas turbine helping out a piston
engine a bit.

 

As per my other response. OK, so they aren't blanked off or anything. But
what stops the gas flow from the exhaust at, say, 3000rpm on a bike turbo
turning the exhaust side, thus (supposedly) improving airflow on the inlet
side (is it better for the blades to be completely still at low revs or
turning slightly on the inlet side?) when the pressure from a TDi car starts
the boost at as low as 1500 rpm (and possibly lower as I see FIAT make a
1300TDi that produces max torque at 1500rpm, so I guess the turbo starts up
earlier....)

Dave

 

Same thing as stops me working sometimes - inertia.

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As has been said, they get more efficient the faster they spin. They
can be tuned to a limited extent by varying the relative proportions
of the "snail shell" and the impeller and turbine. I can't remember
the details (I read it somewhere) but they can be tuned for faster
response at lowr revs or higher boost at high revs, but not both.

The reason that they lag is because at lower revs there simply isn't
the drive from the turbine to overcome the rotor's inertia. Remember
it has to vary by tens of thousands or RPM in order to keep up with
the engine.

Back in the days of turbo'd F1 cars, they apparently had a cunning
scheme to keep the turbo revs up.

I'm not totally sure, but I *think* a bypass could be opened when the
car slowed for corners; at the same time surplus fuel was injected
(somewhere) so that it sort of ran like a gas turbine, thus keeping
the revs up ready for accelerating out of the corner.

 

In communiqué <>, Pip Luscher

Bad form etc. but see:
http://www.rallycars.com/Cars/bangbang.html

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| Pete Fisher at Home: |
| Voxan Roadster Moto Guzzi Mille GT/Squire RS3 Gilera Nordwest |
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It's just like jumping off the kerb - and then jumping off

And recently VW have released a 1.4 engined car with both a supercharger and
turbocharger... the supercharger gives the car a boot at low revs and then
the turbo takes over at higher revs. Supposed to produce about 160bhp, not
bad for a modern car especially as they're putting it into a Golf chassis!

Chris

 

Between 140 and 170 depending on tune.

And it'll be in the next generation Polo/Fabia/Ibiza

 

All this talk of turbos made me remember the 500 Morini turbo prototype.

On that a 'distributor' allowed a single carb to feed the motor directly
below 2000 rpm and featured the only intercooler on a motorcycle turbo
setup of that era. Interesting article here:

http://www.motomoriniclub.nl/turbo.html

70 BHP from a 500 V twin in 1981 and 140 from a fuel injected 1200 V
twin in 2006 - 25 years of Lambertini logic.

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| Pete Fisher at Home: |
| Voxan Roadster Moto Guzzi Mille GT/Squire RS3 Gilera Nordwest |
| Gilera GFR Moto Morini 2C/375 |
+-----------------------------------------------------------------+

 

And it's a right **** if you stall the turbo, as the lag effect seems to
last forever.

 

That's what I wated to know.

They spin, but are shit below whatever revs they're designed to operate
at...

Ta

Dave

 

Why are you replying to me? I know all about leaping off pavements.

And snip sigs, ffs.

 

Exactly, and if you want useful air flow at lower speed you need a
bigger turbine [1]- then you run into problems of severe lag with it
having much more mass to spin up. Like so much else, it's a compromise.

[1] Yonks back I experimented with a 6" centrifugal blower run by an
electric motor and plumbed into an A-series lump with a pressurised
carb. Shit, it was - but it would probably have worked ok on a
stationary engine at a set speed.

 

It probably worked better than the Austin Metro Turbo.
That had no noticable lag but only because the turbo had no noticable effect.

 

Yes - if you are talking about improving all round performance and
reducing lag by using sequential turbos.


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| Pete Fisher at Home: |
| Voxan Roadster Moto Guzzi Mille GT/Squire RS3 Gilera Nordwest |
| Gilera GFR Moto Morini 2C/375 |
+-----------------------------------------------------------------+

 

At this point, I find myself I wondering why anyone'd bother with a
turbo when they could use a supercharger.

 

It's supposed to be more efficient. A supercharger takes power from the
crankshaft. A turbocharger takes power from the exhaust gas, which would
otherwise be wasted.