Ultra_Nexus

No, i meat what i meant

You are trying to find an ultimate power/stress level for the engine. By this I mean where by you can run the engine at a consistent stress level you know will offer up some longevity.

As the revs rise, the stress on the engine becomes increased so to compensate, you reduce boost. However, you are still flowing the same amount because you are revving the engine higher.

Have a look at ideal gas laws and it'll make sense

MB

You aren't reducing boost as such, you are bleeding it off to maintain a reasonably fixed pressure. The more revs you have, the more exhaust gas flow and therefore the higher flow the compressor puts out.

Were it not for the wastegate, you would develop very high boost and your flow would lower to the point of surge. As the wastegate opens to keep boost down, it bleeds off pressure allowing the compressor to flow more air.

I would look at fan laws rather than gas laws.

Ultra_Nexus

Yes, but you haven't read what I have posted properly.

Exhaust flow would remain the same because power remains the same, irrespective of whether you are at 3000rpm or 6000rpm.

Yes, this doesn't happen in real life, but this is the ultimate goal of a turbo engine

Here is a CRUDE diagram trying to explain:

lower

[QUOTE=Ultra_Nexus,Nov 27 2010, 06:00 PM] No, i meat what i meant

You are trying to find an ultimate power/stress level for the engine.

Ultra_Nexus

Yes, but I am on about designing an engine for a STRESS LEVEL.

At lower RPM, you can run more boost because stresses are less.

At higher RPM, stresses are increased so you have to turn the boost down to compensate.

MB

That's not really the way to look at it imo Ron.

I agree with Lower on this.

Exhuast flow at 3000 is not the same as at 6000! Your pistons are pushing more air at a linear rate.

At a lower rpm you want to get more boost in to aid spool up, but you cannot do this easily with a fixed geo impeller because the speed is lower at lower rpm, due to lower gas velocity. Therefore you get lower boost.

There are always sacrifices with turbo / compressor design.

lower

But why would you?

In your scenario your engine would produce constant power throughout the rev range. But why on earth would you want that. Engines are good at producing more power with revs.

I you wanted to design an engine for constant stress you'd design an engine that only ever ran at one speed and used a constantly variable transmission ratio gearbox to vary the output speed,

Ultra_Nexus

Yes, but you are forgetting the lesser boost rate.

Seriously, have a good think about out - think outside of usual parameters and you will see what i'm getting at.

Rich is right with the CVT comparisson though.

As a last ditch attempt, i'll add some numbers in to see if it helps

Lets say you have the F20C, and it is stress rated for 500bhp at 9000rpm.

This equates to 1bar of boost.

This stress number becomes 'X' (And I said i'd use numbers....)

Any more stress than X would mean the engine would fail and the only way to get more power would be to raise the strength of the engine, thus increasing X

Now, as it stands with your 'conventional' principles, stress at 4500rpm would be considerably less than X, and power would be considerably less than 500bhp.

However, with a turbo, you can cheat! You can add more boost so that you are now boosting at 2bar, producing 500bhp and are at stress level 'X'.

Instantaneously, you have produced a better engine that now has a better operating rev range.

Exhaust flow would be the same because you are still making the same power (because power is work done per unit time).

However, the exhaust per stroke would be different because you are obviously getting more air into the cylinder at 4500rpm than you are at 9000rpm.

Make any sense now?

MB

Ron, I work with aero gas turbines and large compressors on a daily basis, it's not a case of making me understand

I just think your point is unusual and not sure how relevant it is to this thread? (no offence)

You can't just have whatever boost you want at whatever point. So you can't have 2 bar at 4500 rpm and a bar at 9000. Well you could, but that would be one odd sized compressor wheel and you would have to be bleeding off a lot of boost up top, which is not efficient.

I do see your point, but having a flat 500 bhp through most of the rev range just isn't practical and not easy to achieve.

MB

To elaborate on why you can't achieve the same bhp at 4500 as 9000, especially on a small capacity engine.

It's flow of air into the cylinders, and exhuast gas out basically.

At 4500 you just aren't pumping enough exhuast gas to get your turbo to spool to give you your 2 bar required boost (in your scenario)

It's a viscious circle, becuase one depends on the other.

Clearly the more rpm builds, the more exhuast gas there is to provide more motive force to the turbo and then compressor wheel, which then pumps more volume and pressure into the cylinders and so on...