sorry for posting such a basic question...but does it get triggered at a specific RPM or does it work "in the background all the time"?

I understand (from youtube video) the mechanism (i.e. hot exhaust spins turbine which compresses air, etc). But I how should it "feel" like?

 

The higher the rpm the more exhaust spins the turbo....

How should it feel? It tends to be a little bit of power at first and then builds in the higher rpms. Think "surge" or exponential curve. Each engine design is a bit different. Some have a lag between when you press the pedal and the turbo spools up and gives you the power you want. Some seem to do nothing and then build power rapidly between 3500-6000 rpms.

 

Car engines produce energy by the combustion of gasoline. 1/3rd of the energy is used by friction of the moving parts of the drive system, 1/3rd is transferred to the road by the wheels, and 1/3rd is sent out the tailpipe as heat.

In the A4 the two liter engine can only produce a certain amount of power at normal atmospheric pressure, 14.7 lb/sq in, or 1 bar. We can get more power out of the engine if there were more oxygen in the cylinder. We can operate a compressor to make that additional oxygen available. Some cars do have a compressor (supercharger) that is driven by the crankshaft. This of course uses more fuel and takes some Hp from the engine to run it. Turbochargers have a shaft from the compressor to a turbine that is in the exhaust path. The gas in the exhaust does not turn the turbine, the heat does. The turbo recovers about 80% of the one third of the energy that usually goes out the tailpipe.

In order to compress the air necessary the turbine spins over 100,000 rpm. The act of compression heats the air which is undesirable, so an intercooler cools the gas before it passes the throttle valve. The design of the intercooler has a large effect on how fast you feel extra horsepower. The A4 intercooler is quite good. The lag is really not noticeable to most drivers.

The way to feel the difference would be to drive one of the VW's with a two liter engine that is normally aspirated. Without the turbo the power of the car is greatly reduced.

 

Thanks Moviela and Allroad,
I have a better understanding now...I have never own a car with turbo before so this is new to me.
I guess I am "mislead" by arcade racing games where you push the "turbo" button and the car goes into hyperdrive. Hence, I have always thought of turbo as being discrete instead of continuous (as Allroad suggested a exponential curve).

 

The relatively small size of the turbo makes it hard to feel when its starts to work. It is actually producing boost much more often than you would think. Here are the main parts that control the turbo.

Wastegate-Is normally closed so that all exhaust is routed thru the turbo to spin it. It has a spring which applies preload to keep it closed and air pressure is used to open it; allowing gas to bypass the turbine to control overspeed/overboost.

Diverter valve/Blow off valve-In VW/Audi it is on the turbo itself but can be anywhere from the compressor side of the turbo to the throttle plate.

Charge Air Pressure sensor- simple systems just use a vacuum tap but VW/Audi uses a sensor.

The first 2 are controlled by the ECU the last sends info to the ecu along with the throttle plate sensor and gas peddle position sensor.

At idle the wastegate is closed the turbine is spinning but not at a high enough rate to produce boost. I'm unsure but the DV valve may be actuated as well to prevent any boost. The ECU senses no load using the gas pedal sensor.

When you press the gas ECU senses this and will allow the DV to close (the turbine is already spinning) and allow charge pressure to build. The wastegate is still closed. The ECU monitors the intake charge pressure. The charge pressure it targets will be based on the ECU; more throttle equals more demand. The boost level will be controlled once it reaches the desired level.

The same air pressure the turbo is making is used to control it. A small tap on the compressor sends charged air to a solenoid controlled valve to modulate the wastegate. The ecu controls this solenoid and it allows the wastegate to be opening in variable amounts as needed. It will open slightly to maintain a desired turbine speed/ boost pressure or it may open fully to bring it back down to turbine idle speed.

Since the turbine is spinning at a high rate and has mass intake pressure can't shut off without some help. The DV (solenoid controlled by ECU) basically opens a port and allows the air to escape and near instantly depressurize while the turbine spins down. Audi sends the air back into the intake before the turbo, some cars sent it to atmosphere (hence blow off valve).

The ECU has preloaded parameters to set limits preventing overspeeding of the turbo, keeping boost in limits and providing the fueling and ignition timing to match. ECU tuning tweaks these parameters.

Its neat to see how often the turbo is actually working, even cruising up a hill at 65MPH in 6th (or 8th now) the turbo is often putting out some boost. Its not only used at full throttle.

If I have any wrong info please let me know I'll edit this.

 

That would typically be the "Nitro" button and you weren't misled. Nitrous oxide works by supercooling air, increasing its density and providing instant power boost. Somewhat similar principle as turbo and supercharger but far less practical.

 

Depending on the size of the turbo boost can be felt almost instantly when you press the gas pedal (bigger engine smaller turbo) but often your power curve will start to flatten off sooner as you go through the RPMs.

Smaller engines with bigger turbos will produce lag. Lag is the time it takes the turbo to spool up and reach the maximum boost allowed.

On most vehicles you will "feel" the turbo kick in when it hits maximum allowed boost.

At this point torque is at its highest and the vehicle will accelerate much more quickly.

This "feeling" is much more noticeable when the increase of torque is very high and that usually comes as you make more boost. More boost = more horsepower and torque.

If you install a boost gauge, you an see exactly when the turbo hits maximum allowed boost and when it first starts to build pressure.

I hope this helps!

Jason