How A Turbo System Works

4ACES · December 21, 2005

Yeah hey, bored at work so I thought I might post this up incase people have questions and prob get a decent convo going about things.

The Basics of How A Turbo System Works.

Ok, air comes in through the filter, then through the AFM (which measures the amount of air coming in so the ECU can calculate the amount of fuel required).

The air is then sucked into the turbo, where the compressor wheel increases its pressure and sends it to the engine. On the way there's an intercooler to lower the air temp, because a by product of the turbo is quite a lot of heat.

Before the air gets into the engine it passes through the throttle body, which is what your accelerator pedal is connected to.

The throttle valve restricts airflow into the engine so that you can vary the power it makes.

When it's open the engine tries to produce maximum power, and when it's closed (almost closed) the engine will be at idle.

So the path of air is: filter -> AFM -> compresor (turbo) -> intercooler -> throttle -> engine.

The exhaust gas flow from the engine is roughly proportional to the amount of power it's making, and that flow is what spins the turbo's turbine, which in turn drives the compressor.

So, the more air going in, the more power, and hence more exhaust coming out, which means more boost, which means more air, hence even more power. It's a feedback loop which is kept under control by the throttle input from the driver.

To keep the maximum boost at a safe level there's the wastegate, which is just a valve allowing exhaust to bypass the turbo, and therefore not spin the turbine any faster, or create more boost.

It works in a very simple way. The valve is held closed (all gas goes through the turbine) by a spring, until it is forced open by a diaphragm driven by boost pressure.

When the appropriate boost is reached the wastegate starts to open, and the turbo will stay at the set boost level while the exhaust flow can still increase as the engine revs rise.

All of this works quite simply under constant acceleration:

Air comes in, is compressed to the set boost level, intercooled, used by the engine, and the exhaust keeps the turbo spinning.

It becomes more complex when you start changing throttle settings.

If the throttle is wide open, and you're at full boost accelerating, then close the throttle (for a gear change, or back off) suddenly the turbo is pushing against a closed throttle, and at the same time the engine has stopped producing exhaust gas, so the turbo starts to slow down.

Now that there's a whole intake system full of compressed air with nowhere to go, and the compressor is not being driven by the turbine - so it can't hold the same air pressure that it did while spinning flat out. The air will start to flow the other way, which means going backwards through the still spinning compressor.

This is where compressor surge comes into play. The compressor is still trying to pump air, but at the speed it's going it can't provide as much pressure as there already is in the pipe.

This causes the compressor to "stall". Stall is a misleading turn used here, because it doesn't mean stall as in to stop spinning. It's the aerodynamic stall, like a plane that's tried to climb too steeply, or has slowed down too much to maintain lift.

The smooth air flow through the compressor blades is broken up, and some of the air will flow back out through the compressor, making a hissing noise.

Centrifugal compressors work on a "squared" relationship for speed vs flow / pressure, so if the turbo slows down to half speed it will only be able to hold a quarter of the boost pressure.

The "chopping" or "fluttering" sound is caused by the accoustic effects in the intake system, determined by the size and length of the piping / cooler.

Rather than air constantly flowing back out of the turbo it tends to come out in bursts, triggered by pressure waves in the piping hitting the compressor.

Generally though the flutter is the sound of the pressure wave cause by the throttle closing bouncing back and forth between the turbo and the throttle plate (the longer the pipes the slower the flutter).

Every time the wave hits the turbo it causes the compressor to cavitate, which makes the noise.

It's not the blades chopping the air making the flutter. Since the blades are spinning at 80000+rpm you wouldn't be able to hear the individual "chops"

If you listen closely you'll actually hear a more constant hiss sound at higher boost levels and rpm when backing off. (ie, a "whooosh" rather than a "chop chop chop")

The loudest flutter tends to occur at lower rpm and boost levels of only slightly above atmospheric pressure (0psi). This is partly because the sound is "chopped" up more noticeably, and partly because the rest of the car is making less noise at lower speeds. This is also where it's least damaging.

Car manufacturers don't like weird noises from the intake system, so they use restrictive airboxes designed to muffle the sound, and more recently (SR20 onwards) used a Blow Off Valve.

The blow off valve provides a new path for air to get out past the compressor. Instead of forcing its way back through the spinning blades it is piped out of the intercooler piping and back into the intake piping before the turbo.

Obviously it could be vented straight out to the atmosphere, but the AFM has already measured that air coming in, and if it doesn't reach the engine the ECU will be injecting too much fuel in its absence.

(Also, venting to atmosphere makes a bit much noise for conservative owners and manufacturers)

The BOV works a bit like the wastegate, in that it's held closed by a spring, and controlled by a pressure operated diaphragm.

This time though the boost pressure helps keep it shut rather than open it.

The control air pressure for the BOV comes from the plenum, which is the chamber between the throttle and the engine.

When the throttle closes the engine creates a vacuum (negative pressure) in the manifold / plenum as it tries to suck air past the closed throttle.

The vacuum is used to "pull" open the BOV, to vent the excess air in the intake system, preventing "flutter" and other noises.

A BOV is a compromise device because, to eliminate all fluttering it will have to open at very low boost levels, and very quickly - which means that it must have a very weak spring. This can cause some leaking of boost due to the valve not staying sealed properly, and also means that every time you back off the throttle, all the pressure in the intake system is vented. When you open the throttle again the turbo will have to build up all that pressure again, whereas if there was no BOV there'd be more pressure remaining as it's a lot harder to get out through the compressor.

Most aftermarket BOVs won't open until you reach 6psi or so of boost (because with an atmo venting BOV if it leaks boost you'll have major problems - whereas a slightly leaking plumb back type won't cause bad running, just a very slight loss of power)

When you start running well over stock boost, and have larger intercoolers / intake piping obviously there's a lot more air in the intake system, and it will be at higher pressure - so when you back off the throttle there's a lot of air trying to escape through the compressor. This means it will be slowed down quite quickly and violently, which puts large loads on the turbo bearings, wheels, and shaft. Repeated hammering by high boost backoffs can harm the turbo, and the amount of slowing each time causes a lag when the turbo spools back up after each backoff / gearchange.

The best compromise setup is to have a BOV which will open when you back off at over say 6 psi, and vent back into the intake to prevent the rich running problems of atmo BOVs. If it's too loose you'll lose boost response due to too much presure being lost on gearchanges, and if it's too tight you'll lose response due to the turbo being slowed down too much - and potentially damaged by the rapid deceleration.

Why does it flutter when my gauge isnt reading any boost?

The reason it flutters even when you can't see boost on the guage is that you're looking at the pressure in the plenum (after the throttle), but the turbo can actually be producing a bit of boost in the cooler and intake piping.

yeah hope it helps....*shrugs*

Edit: spelling