|02-09-2001, 02:46 PM||#1|
Join Date: Aug 2000
Location: Mission Viejo, CA, USA
Q. What is a Turbo?!? Answer!!
I just thought this might make for some entertaining reading.
A turbocharger is a gas driven gas pump -- it utilises gases to pump around other gases.
It does not provide free power as has been claimed.
The idea is to use the exhaust gases to drive a compressor which compresses the air going into the engine -- making it denser, which means there are more oxygen molecules per unit of volume -- so it makes a bigger bang when ignited with more fuel. Hey presto! More power. One way of looking at it is that a turbocharger enables a 2 litre engine to burn as much air and fuel as a normally aspirated 3 litre engine, or even more.
Exhaust gas which is being pushed out of the cylinders is forced through a restriction and round a snail-shaped housing called the turbine housing. The snail shape causes a speed increase as the gas turns through 360°. At the centre of the turbine housing the gas spins a turbine wheel. This wheel is a high temperature steel alloy on the Impreza and 95% of all turbocharged cars, but can be a lightweight ceramic as used on the Nissan Skyline for example.
You can see from the configuration of the inlet and exhaust turbine wheel that by varying the number of vanes on both the inlet and exhaust wheels it is possible to alter both the spin up speed and air flow characteristics.
This is a complex process which the manufacturers do an excellent job of combining good response and air flow.
IHI have the best reputation in the world of making highly responsive and high flow turbos. It is all too easy to fit a larger flow turbo and reduce response and therefore torque.
This turbine wheel is connected via a shaft to the compressor side of the turbo. This is inside another housing called a compressor cover. The compressor's job is to suck in air through the air filter and accelerate it and compress it before forcing it into the engine. This creates a positive pressure (i.e. a pressure greater than atmospheric pressure which in it's natural state is 1 bar or 14.7 psi). However before the turbo spins up to speed the compressor acts as a slight hindrance to the flow of the air coming into the engine. The size and shape of the turbine housing, wheel, shaft and compressor cover dictate airflow in and out of the engine, and therefore power and response.
The shaft between the turbine and compressor runs in a bearing housing which is water cooled. There is not actually a bearing in the bearing housing but a guide channel through which oil passes. This pressurised oil becomes the bearing and takes up the clearance between the shaft and the housing. This is why when you feel the shaft movement on a turbo when the engine is not running it always feels as if the shaft bearing is worn out.
The bearing housing on the Impreza Turbo (and 98% of all turbo cars on the road today) is water cooled. Water cooling of the bearing housing is something developed in the '70s when oil was not as good as it is now. It was developed to keep the bearing housing cooler after engine shutdown and avoid oil burning on the shaft. It is still used today by all manufacturers but these days modern oils do not burn on the shaft even after hot shutdowns. But it would be a brave manufacturer who admitted they had fitted a non-water cooled turbo to their latest super, quad turbo, five valve, V12, tarmac eating stonkermobile.
If you are using the car extremely hard then just allow a few minutes for it to cool down or use a turbo timer. It is not vital but it helps.
So this is a turbocharger.
If bigger turbos make more power, I want a bigger one.
Bigger turbos also mean more lag.
What is lag?
Lag is the time taken for the exhaust gas to spin the shaft fast enough for the compressor wheel to start to generate some positive pressure. Bigger turbos take longer to spin up so there's more lag.
Great, now I want a smaller turbo.
Smaller turbos spin up faster so there's less lag but they flow less air so deliver less power.
Now I'm completely confused, I think I'll take the damn turbo off.
No don't do that they are great fun. It is just that the rules governing them are not straight forward.
The game engine designers play involves balancing exhaust gas flow with compressor air flow volume. Add to this equation shaft speed, shaft weight, compressor and turbine housing flows and turbine wheel tip speeds and you can see how difficult things are. (And I haven't even mentioned inlet and exhaust cam design, exhaust primary and secondary pulsed extractor lengths, variable turbo inlets and wastegates!)
So what is a wastegate?
A turbocharger has a very limited shaft speed band during which it is efficient. But an engine works over a huge range of gas volume output (rev range to you sir).
Consider a diesel turbo for a truck. Most diesel turbos are matched to the engine flow. Therefore they get up to their efficient rpm and start pumping positive pressure. Then as engine revs rise so does boost pressure so roughly speaking max boost is at max revs. No problem on a diesel because they have such a limited rev range.
Take your Impreza. You want boost at low speed and boost at high rpm too. So the turbo is designed to spin up quickly and provide positive pressure at say 2500rpm. It will then make more boost until the turbo shaft reaches maximum speed at say 3500rpm. At that speed if the turbo goes any faster two things will happen very close together. Firstly the turbo will overspeed and start to explode (the tips of the blade will actually go supersonic and the resulting sound shock wave will start to break the tips off of the turbine wheel!), secondly it will generate rapidly rising boost of 2-3 bar (30-45 psi) at which point your engine explodes too.
So instead a valve opens inside the turbo which allows exhaust gas to escape without going around the turbine housing. This slows the turbine wheel and reduces boost. Then the valve closes and more gas goes through the turbine housing therefore increasing boost until the valve opens again, etc, etc.
This wastegate valve is controlled by a pneumatic actuator (although the Ford Focus World Rally Car has a hydraulic one) which itself is controlled by an electric solenoid valve which is controlled by the ECU which is controlled by the software which can be re-programmed by Power Engineering. Clever eh?
So do you understand about turbochargers now?
No, but please don't explain any more.
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|02-09-2001, 04:57 PM||#3|
Join Date: May 2000
Location: Phoenix, AZVehicle:
1999 Outback Sport
Steel Blue Mica
Great write up Dante. Well done.
[ edit - can't spell, even with a spell checker ]
[This message has been edited by kastle (edited February 09, 2001).]
|02-09-2001, 05:58 PM||#5|
Join Date: Nov 2000
Location: S/E TX USA
i already read "maximum boost" by corky bell.. thats nothing new.
just kiddin! thats a good "quick" summary of what a turbo is.
|02-09-2001, 09:53 PM||#6|
Join Date: Aug 2000
Location: Mission Viejo, CA, USA
I can't take credit for the write up, it is actually from one of my UK vendors. But I thought everyone here would like it.
|02-10-2001, 06:29 AM||#7|
Join Date: Aug 1999
2012 J train
thanks I just had my breakfast while reading that....I think I am learning stuff and its only 9:30 in the morning! BAhhh..I think I am gonna have to go to sleep now
|02-10-2001, 11:35 AM||#8|
Join Date: Sep 2000
Location: Pepperell MAVehicle:
8Sti | 07 allez
White | Comp
next... Intercoolers please. Thanks, very informative.
|02-10-2001, 11:53 AM||#9|
Join Date: May 2000
Location: San Juan, Puerto RicoVehicle:
2005 Legacy GT 5MT
Satin White Pearl
wow my first Powertrain post!!
ill probably be seeing more of you guys when i get the wrx
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