Mitchy260

This may sound like a silly question but if i never ask ill never know. I would normally search for the answer, but its near impossible to on the 'new' scoobynet.

On N/A cars the more cold air through the induction the better, this goes straight into the throttle body and into the cylinders........correct???

Now on to the turbo car, and this is where im having a little problem understanding. To my knowledge and i may be wrong, air goes in through the induction, then into the turbo, turbo compresses and pressurises air and passes it through the intercooler, through the wastegate and into the cylinders............is this correct???

If this is the correct route of air flow, then why the need for cold air passing through the induction stage, this air will then get hot passing through the turbo, until cooled again by the intercooler. The turbo surely pressurizes the air to a certain pressure so max airflow isnt a problem, with the knowledge that cold air is denser, so the denser the air, the more air you'll have. Surely the turbo should work exactly the same no matter what kind of hot/cold airflow comes in as in the end this air is cooled by the IC.

Like i said, i may be totally wrong and not understanding things correctly, but if i dont ask i dont get.

Cheers for your help.

greasemonkey

Correct.

Not quite. Air goes through the induction system (where it passes the airflow meter), through the turbo where it's compressed, through the intercooler and into the throttle body. The wastegate is after the turbo, not before it, and the air doesn't pass through the dumpvalve (which is before the turbo) unless the throttle is shut.

No. The hotter the air is before it goes into the turbo, the hotter the air will be when it comes out. Conversely, the cooler the air is before it goes into the turbo, the cooler it'll be on the way out...

You're thinking along the right lines, it's just that cool intake air is just as beneficial (maybe moreso) on a turbocharged car as it is on a normally aspirated engine.

Mitchy260

My thinking about it, when the air is compressed, surely the air is compressed to a certain pressure, which constitutes a certain temperature.....no??? The IC will then cool this air down, but surely the difference between 25 degree inlet air and 10 degree inlet air is negligible as airflow across the IC is going to be at a constant temperature.

What kind of compressor temperatures are we looking at, ive always imagined compressed turbo'd air to be scaulding hot, 50, 100.....500.....degrees??? but then surely the IC can not cool temperatures this hot for a cold supply into the cylinders. help please!!!

Another point....the air goes through the turbo, and then through the wastegate, then into the throttle body.....this is correct isnt it?
Is the DV in the compressor line before the wastegate to get rid of the excess backpressure when the wastegate is shut??


Cheers for your response, im just starting to get to grips with the way my car actually works.

BedHog

This is my first turbo car and I'm trying to understand how it works too.

As I see it, when the air is compressed by the turbo the temperature of the air is raised (probably minimally though) due to friction. As it passes through the IC the temperature is dropped (but not the pressure, but again maybe slightly due to pressure drop through the pipework) thus allowing a higher density of oxygen into the cylinders for the same pressure producing a bigger bang.

You are right, except with colder/denser air there is more oxygen. Presumably the IC will reduce the temp by a fixed amount no matter what the weather i.e lets say 2 degrees (guess!!) so air into the cyliders will be 2 deg less than ambient (plus what it was heated up by the turbo)

Nope - think logically - if you put air into the IC at 80deg - and air in at 20deg, theres no way the output will be the same temp. It will be reduced by the same amount but still a 60 deg difference

FMIC's and Water sprays help reduce the temperature even more giving yet more oxygen.

Feel free to correct me GM, I'm an engineer but know little about turbo's.

Frankly I'm suprised that an IC would make a noticeable difference, anyone know what it is? If so weather will affect your performance, and a rolling road test will be inaccurate due to the IC being effectively unused...

john banks

Compressor, turbine and intercooler are only typically 60-80% efficient. You've got a lot of heat to sort out, as well as the nasty effects of exhaust manifold pressure which pollute the fresh charge with end gases.

BedHog

Forgive my ignorance here, but surely the end gases are chucked out of the exhaust while the inlet air is sucked from the induction at the front. Shouldn't be a problem unless your going pretty quick backwards...???

greasemonkey

Erm, no. The temperature rise will be a virtual constant. In other words, if the inlet temp is 0 degrees celsius and the turbo raises the temperature by 20 degrees, the charge temp will be 20 degrees. If, however, the inlet temp is 20 degrees, the charge temp (given the same turbo and boost pressure) will be around 40 degrees.

On the contrary. It'll result in a 15 degree difference in the charge temperature as well.

The airflow into the IC isn't at a contstant temperature, that's the point.

Hell no. The greater the boost, the greater the heating effect, and the bigger the turbo, the less the heating effect. Thus, if you try and screw too much boost out of too small a turbo, it wouldn't be unusual to see charge temps in the 50 celsius range, but not into three figures.

This is why bigger turbos are said to produce "kinder" boost. Because they're bigger, the air doesn't get heated as much for a given boost/flow level, and so the charge will also be cooler when it hits the cylinders. The cooler the charge is, the less it'll be inclined to det.

No. As stated previously, the air doesn't go anywhere near the wastegate until after it's been through the engine.

Assuming that both dumpvalve and wastegate are shut, the air path is: Air filter - air flow meter - compressor - intercooler - throttle body - inlet manifold - cylinders - exhaust manifold - uppipe - turbine - downpipe - exhaust.

On a standard Impreza, the dumpvalve (when open) allows boost air to jump from the intercooler back into the system between the AFM and the compressor.

The wastegate (when open) allows air to bypass the turbine and travel straight from the up-pipe into the downpipe.

The dumpvalve is plumbed into the intercooler, and opens when you shut the throttle, venting off excess boost pressure in the intercooler (which would otherwise have nowhere to go other than back into the compressor blades).

A hell of a lot! If you ever have your intercooler off, try blowing a hair dryer through it. You'll find that the air will be stone cold by the time it gets out the other side!

Exactly. The weather does have an easily noticeable effect on the performance of any car, but especially a turbocharged one. The colder, denser air results in more power.

Exactly, rolling road results do vary significantly according to the ambient temperatures prevailing at the time. This is why RR printouts usually list the temps. Your comment about the effectiveness of the I/C is also spot-on, and it's the reason why there's normally a big fan in front of the car.

BedHog

Seems my only error was in underestimating the effectivness of the IC. Applying the logic I mentioned earlier I have to admit that the fact the IC is used must be testiment to it's effectiveness. Maybe I will consider a FMIC after all

Thanks for the Wastegate explanation, I wasn't 100% sure what it did. Again that makes sense and explains how the boost can be bought in at a certain rpm.

greasemonkey

While the wastegate can certainly affect the spool, this isn't its primary function. The spool characteristics are primarily determined by the physical design of the turbo itself, from the diameter, pitch and number of blades on the turbine, to the size of the exhaust housing, and the diameter and shape of the nozzle that blows the exhaust gas onto the turbine blades.

The wastegate certainly can influence the spool characteristics, but its main purpose is to regulate the turbo rotation speed (and hence airflow/boost pressure) after the turbo has spooled and is into its boosting phase.

BedHog

So the RPM at which the turbo starts to boost is purely down to the design of the turbo? That's interesting and something I hadn't considered. Seems like it's a more complex device than I thought.

Thanks for the explanation greasemonkey, you must do this many times a week to newbies like me and I admire your patience.

Mitchy260

Thanks for your help grease monkey. Like someone has said, you must do this explanation thing frequently so thanks for your time.
We all learn something new.

Dazza's-STi

Q for GM...

Is it the waste gate that gices rally cars the distingtive chirpy flutter sound when hey lift off...???????

And how could you achieve it on a roadgoing scoob????

hawkeye

my belief is that it is waste gate chatter you hear

but ive been led to believe to get that effect on a road going car would be costly..... because of the damage done to the engine & turbo ... i think the chatter is the wastegate shutting the turbo down?? when off boost or something like that... i think its something along those lines

hawk

greasemonkey

No, it's air pulsing back past the compressor blades. WRC cars don't have dumpvalves, so when the throttle butterfly shuts the air has nowhere to go but back from whence it came. On a WRC car it's also exaggerated by the anti-lag systems that keep the turbo spinning - and hence keep the boost pressure in the pipework and intercooler.

Remove the dumpvalve! You'll get an almost identical sound on lift-off (and better throttle response on reapplication), but be aware that doing this puts a lot of stress on the turbo bearings, shaft and compressor blades, and thus the turbo working life will be shortened.

How much so is impossible to say, the MHI TD series turbos seem to cope relatively well with it, but IHI turbos don't tend to react so favourably (although some are better than others). It also depends how much boost you're running - the greater the boost pressure, the greater the shock to the turbo will be when the air starts to pulse back.

greasemonkey

The wastegate can be used to take the edge off a turbo who's spool would naturally be too aggressive, and changes to the exhaust system will affect spooling, but yes, the design of the turbo itself is the primary factor in determining the spooling and flow characteristics.

Smaller turbos will inherently accelerate and decelerate quicker than bigger ones, because smaller means lighter rotating components, and lighter means less inertia. Of course while you can mitigate against it by using advanced lightweight components wherever possible, if all things are equal smaller is always better as far as response is concerned, while bigger is usually better for efficiency. Balancing these two opposites is always a compromise, and it's one that the WRC teams in particular are finding very novel ways to work around.

Even altering one single aspect of the turbo's design in isolation will have a noticeable effect. A smaller diameter turbine nozzle will increase the velocity of the exhaust gas as it hits the turbine blades, promoting earlier spool. However, once the turbo is boosting, the small nozzle causes a greater and greater flow restriction as engine speeds (and gas volumes) increase, with resultant (and problematic) increases in back pressure and temperature.

Turbochargers may all look broadly similar from the outside, but the design has to be carefully optimised for its intended installation. If you looked at, for example, a turbo used in a large diesel generator (which is designed to run at constant speed), you'd see a considerably different detail design than you would in a car's, which has a totally different set of priorities and compromises to meet.

Graz

The weather does make a difference, that's why your car feels faster on cold days - cold dense air at the intake and cold air over the IC.

The IC is used during a rolling road test, they have a massive fan blowing air through it, otherwise things might get a bit too hot, cause detonation, and blow up your engine!