Physics question - sucking 'n blowing
Discussion
For some reason, i keep having a silly question buzzing around in my head that I need clarifying.
Right, if you have a pipe, let's say a turbo inlet pipe. Now, no matter how much that turbo sucks, the maximum amount of air it will be sucking in will be determined by the atmospheric pressure, so a maximum of 1 bar at sea level?
To increase the airflow into the turbine will need a larger inlet diameter.
Right, if you have a pipe, let's say a turbo inlet pipe. Now, no matter how much that turbo sucks, the maximum amount of air it will be sucking in will be determined by the atmospheric pressure, so a maximum of 1 bar at sea level?
To increase the airflow into the turbine will need a larger inlet diameter.
I would have said the airflow would be determined by the pipe itself, and the atmospheric pressure. However the atmospheric pressure doesn't change wildly so is generally ignored.
The only way to increase overall flow volumes into the engine will be with less of a restriction (assuming your turbo inlet is the main restriction), as even if you speed up the flows through the restirction, the pressure will drop.
That is, as far as I know.
The only way to increase overall flow volumes into the engine will be with less of a restriction (assuming your turbo inlet is the main restriction), as even if you speed up the flows through the restirction, the pressure will drop.
That is, as far as I know.
The lower the pressure on the 'downstream' side of the pipe the more volume flow you will get, but the lower the density will be that you get out. You get an effect called a 'sonic choke' where the speed through the restriction reaches the local speed of sound, and reducing the downstream pressure beyond that point does not increase the mass flow rate. Obviously, the downstream pressure can never go below absolute zero i.e. -1 bar gauge pressure.
Bigger inlet will give more airflow, but leads to bigger compressor wheel etc. which will lead to more lag. Turbos are sized to be as small as possible whilst achieving the airflow required at full load.
As an aside, I see Porsche have introduced variable geometry turbines onto the 997 turbo - this has been around on diesel cars for a while, so I wonder how long it will take to filter down to more affordable petrol turbos...
As an aside, I see Porsche have introduced variable geometry turbines onto the 997 turbo - this has been around on diesel cars for a while, so I wonder how long it will take to filter down to more affordable petrol turbos...
Dont forget, it could suck hard enough to create a vacuum in front of the turbocharger, which would effectivley be 2 bar.
But its not so much pressure on the inlet, its quantity of air.
Production cars turbos are sized for specific reasons. That doesnt mean you cant improve on them everywhere. You can buy pretty big turbos these days that arent laggy at all.
As for VNT...Ive spoke to a few people about them. Their opinion, and they have done testing, is that if you spec the turbo right in the first place, VNT isnt much of a benefit.
But its not so much pressure on the inlet, its quantity of air.
Production cars turbos are sized for specific reasons. That doesnt mean you cant improve on them everywhere. You can buy pretty big turbos these days that arent laggy at all.
As for VNT...Ive spoke to a few people about them. Their opinion, and they have done testing, is that if you spec the turbo right in the first place, VNT isnt much of a benefit.
aceparts_com said:
For some reason, i keep having a silly question buzzing around in my head that I need clarifying.
Right, if you have a pipe, let's say a turbo inlet pipe. Now, no matter how much that turbo sucks, the maximum amount of air it will be sucking in will be determined by the atmospheric pressure, so a maximum of 1 bar at sea level?
To increase the airflow into the turbine will need a larger inlet diameter.
Well, to be picky, the turbo doesn't 'suck' per se. All it can do is create a lower pressure area that atmospheric pressure will push air into.
People I've worked with in the diesel trade have related stories of runaway engines and blocking the intake with 1/4" sheet steel, to shut the motor down, and how the motor 'sucked' the steel into a bowl shape. All bollox, as the most it can ever 'suck' is 1 atmosphere per square inch. 14.5psi
Back to the point, to increase the flow into said turbo to maximum you can use bellmouths, but, as you say, it'll never go above 1 atmosphere.
aceparts_com said:
Thank you. Mind is now settled. Now, can anyone tell me why we have a tax free limit of only £4700 (ish) on earnings and why burglars and thieves don't get fixed penalties like us law abiding motorists do? 6 months for robbery, 1 year for mugging etc!
Sorry mate. I only deal in facts, common sense and reality, non of which have any say in the sort of Mickey Mouse jail sentences courts are handing out today
aceparts_com said:
For some reason, i keep having a silly question buzzing around in my head that I need clarifying.
Right, if you have a pipe, let's say a turbo inlet pipe. Now, no matter how much that turbo sucks, the maximum amount of air it will be sucking in will be determined by the atmospheric pressure, so a maximum of 1 bar at sea level?
To increase the airflow into the turbine will need a larger inlet diameter.
The flow along a pipe will be choked when the velocity reaches the speed of sound.
As an exmaple of this my Fiat Coupe 20VT consumes approxiamtely 300g of air per second at maximum power. To choke this flow at standard temperature and pressure you would have to feed it along a pipe 10mm in diameter.
In practice if you start trying to force gas down pipes at such high velocity they will loose pressure and consequently choke in an event larger pipe (increasing the pressure of a gas allows a greater mass flow to pass down a smaller pipe).
Gasses will incure greater losses if they are forced to travel at higher mach numbers so using a greater diameter inlet pipe will reduce mach number along that pipe and therefore reduce pressure loss and allow you engine to flow more gas. However you will find this somewhat of a diminishing return.
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