This is strictly theoretical, but can you guys explain something to me? I was driving home tonight pondering the great unknown questions of the universe and this gem occurred to me.

Here is the given: Our cars operate with two turbochargers. They feed into the engine and a boost gauge monitors boost levels. If everything is operating correctly, sequentially, that is, you should see a 10-8-10 boost pattern.

Here is the question: When the first turbo reaches 10 psi output, it is delivering 10 psi boost to the engine. When the second turbo comes fully on line, the combined boost is still 10 psi. Why then if the combined boost pressure is only 10 psi, do you need a second turbocharger, assuming the first turbo was putting out 10 psi in the first place? If the first turbo is putting out 10 psi and the second comes online, shouldn't we be seeing more than 10 psi with both of them spining at full output? I mean 10 +10 should equal 20, shouldn't it?

What am I missing?

 

As rpm goes up, more air volume is inhaled, thus another turbo is needed to maintain said 10 psi.

 

As the engine rpm rises it can consume more air, around 4500 rpm the first turbo is going to start running out of capacity to provide the higher volume the engine needs while keeping the boost pressure at 10psi.

If you have one garden hose that supplies 40 psi and you hook it up to another garden hose that is at 40 psi will you have 80 psi of water or twice as much water at 40 psi?

 

 

at high rpm one turbo would run out of juice. split the work and the 2 will stay in a more productive part of their compressor maps. spin the turbo too fast and alot of bad things happen high intake temps, back pressure failure etc.. with 2 you get low end boost response that a New car buyer is looking for and the drivability that the Average driver equates with performance

lol i am slowest poster evar....

 

I should have figured this out for myself. That explains why I could never get an A in my college physics classes. Thanks guys.

 

Wow...good analogy Jeff. Thanks guys...I've always wondered this myself...

 

Explain: when the engine turns faster, burning more fuel/unit of time, and producing more exaust force/unit of time there should be more boost available. The amount of exaust energy does not remain constant as the engine spools up. More fuel is burned more exaust is produced, at a greater rate/unit of time.

 

You are doubling the volume of the system.

pv=nrt

the pressure remains the same as does the volume (engine intake etc).

your outlet temperatures will be optimal if you keep the turbos in their efficiency range.

as stated earlier the single small turbo will quickly run out of breath and go off it's efficiency and expel hot air.

This same debate is applied to larger turbos such as a t78 at 10psi.

the intake of the engine (aka garden hose) is not any larger.

the gains are the decrease in back pressure and temperature. With a smaller T and the same pressure/volume the nr will be greater (number of molecules of oxygen) thus giving more power.

oxygen and fuel is where the power is at. Intercoolers cool air to get larger amount of oxygen thus increasing power

 

OMGWTFBBQ!!1!!!

someone finally got it! on the first page of the post no less.

 

I suppose you are generally asking "why can't the one little turbo keep up with engine since it will be spinning faster and faster as the engine revs increase?" The main reason is that the compressor loses efficiency.

Even if you could do it, spinning the turbo at 1 million RPM isn't going to flow 1000 CFM, for instance -- the compressor just isn't big enough. Instead, it will start to super-heat the air, and I am certain the wheel would fly apart long before you got to 1 million RPM.

-Max

 

Thanks for explaining that for those of us missing the math gene.
From what I read in my factory brochure the system was designed this way was to allow the first turbo to spin up quickly with little lag and then have the second turbo pre spool and add air as it's needed to maintain that boost.
Now if someone could just explain the thermodynamics of this beast in garden hose terms.

 

Garden hoses are usually best to describe fluid dynamics. Thermodynamics are often represented by cups of coffee!

-Rob

 

Another limitation of turbomachinery is acceleration. On diesels, I have seen speeds near 100,000 RPM. There is a real limit at which it will gernade, so the choice is one of three. 1) Re-size the turbo which effects performance 2) make the components out of trick material like titanium 3) Add a turbo

We run turbos into turbos (2 staging) which is to combat thin air at altitude (>10000 ft.). If you don't do that, the thin air allows turbo speeds to go through the roof. The turbine wheels end up coming out the exhaust pipe.

The turbo test lab at work has parts of turbos stuck in the ceiling due to overspeed tests. They can really come apart if you push them.

 

i heard that big rig diesel manufacturers are going to 2 turbos for low altitude driving, so they can cut their emissions. I read somewhere (or saw it on an automotive show on tv, can't remember) that the next generation of diesels are going to use 2 stage turbos pushing 100+ psi into the engine in an effort to cut emissions. have you heard anything about this?

 

lol
Fairly poor in comparison to many piston engines, unfortunately...

 

OK, when you have a single hose at 40psi and you drive it harder, it starts heating the water flow rather than increasing flow. Obviously it's a gradual - there is always heat added - but at some point the heating overwhelms the useful work.

That's what they mean when a compressor loses efficiency - the 'more backpressure = more compressor output' only applies roughly in a limited range. The further you go from the sweet spot the more you make heat instead of useful output.

On the FD, sequential turbos meet the needs of the engine intake at 10psi. Most engines don't have the RPM range to justify that.

On a related note, (I should search this), what is the volume of air (at 1atm of course) taken in by the 13b per revolution?

Dave

 

My flow is Linoleum and everybody knows the harder you drive your hose the hotter it gets.

I'm gettin one of these bad boys - http://autospeed.drive.com.au/cms/A_0237/article.html

 

1.3L are ingested per rotation of the eccentric shaft at 100% VE. Much like a 2.6L four-stroke piston engine.

-Max