brain teaser

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Makaveli_se7en

Member
The other problem with explaining surge is its a darn lengthy topic even to summarise. Explaining what surge is and how the compressor operates during surge is relatively straight forward but trying to explain the causes of surge is much longer as stall can happen at the entrance to the impeller, through the impeller passage, in the diffuser and even in the housing. Each has its own mechanism and cause and its often a combination of instabilities which leads to noticable "system" surge.

Having said that if people are genuinely interested I can write a bit of summary to post up.
 
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pulsarboby

Guest
haha its kind of what i was trying to explain to him but in leymans terms, but craig needs to know the ins and outs of a dogs arse so i begin to question my own sanity:lol:

it is very difficult to explain as eds wrote above, i understand the jist of it but have to say if im honest that all the mathematics of it are way over my head.

all i know is that the engine itself is the pump, the turbo is merely a secondary pump
small turbo =more effort at given boost
big turbo = less effort at given boost
more effort=more heat = more back pressure which slows turbine thus giving less power at same boost level

can anyone explain it simpler than that?
and ive just got back from the boozer:lol:
 
Fast Guy

Fast Guy

Moderators
Staff member
Makaveli_se7en said:
OK I'm cheating, I know I was banned (sorry Bob) but I can't help it (comes with being a know-it-all-engineer so the missus tells me)

Sorry Skiddus I see your most on the second page now, I lost it amongst all this talk of balloons. However I think the effect you were seeing with swapping the elbow is different to the effect of a bigger turbine. The elbow creates a back pressure on the turbine exit which reduces the expansion ratio over the turbine for a given flow (engine speed) and hence reduces the power the turbine supplies to the compressor for those conditions.

There are two effects happening when fitting a bigger turbo and people seem to have got caught up on the one with the lesser effect.

1. Most importantly bigger turbine = less pressure in exhaust manifold for a given flow rate (i.e. engine speed). Therefore if we consider 2 cylinders of an engine, which happen to have their inlet and exhaust strokes at the same time. If the inlet manifold is at a higher pressure than the exhaust manifold the inlet stroke is being pushed down by a higher pressure than the exhaust stroke has to push against to expel the exhaust gas. This decreases pumping losses across the engine, increasing efficiency and therefore power. So clearly the lower the pressure in the exhaust manifold the larger this effect becomes.

2. The second point can be summed up in two words which I’ve not seen yet in this thread (sorry to anyone if I’ve missed them): VOLUMETRIC EFFICIENCY

Part of the problem is that the name volumetric efficiency is misleading, however much air is in the cylinder it will fill it (gas will always expand to fill it container), so volumetrically the cylinder is always full. It really refers to the amount or mass of air that enters the cylinder on the inlet stroke compared to how much air is in the cylinder at BDC during static condition (for a given pressure and temperature)

As has been stated running a bigger compressor means its doing less work to the air and therefore heats it less, resulting in denser air and higher volumetric efficiency.

Another way of looking at it is to apply the following equation:

MAP = (M*R*T)/(0.5*N*VE*V)

Where:
MAP – is the required inlet manifold pressure (absolute)
M – is the mass flow rate required for a desired power output (calculated from assume BFSC, AFR )
R – Gas constant
T – temperature (in Kelvin)
N – engine speed
VE – volumetric efficiency
V – engine displacement

From here we can see that increasing the inlet temp (i.e. small or badly matched compressor) gives us a double hit as it also decreases VE meaning that to supply enough air to achieve the same power MAP has to increase.
Click to expand...
So in very simple terms are you saying
1. lower exhaust back pressure
2. Increased volumetric efficiency
 
Matt Evans

Matt Evans

Member
Wow very informative thread guys!

I'm not going to pretend I'm an expert on this matter (or any other!) as I'm certainly not, but it seems to me that people are getting a bit too caught up in the whole pressure thing.

I'm assuming that one turbo is larger than the other, so I would imagine the only purpose of this would be a larger flow rate of air, or in other words, over the same length of time the larger turbo would flow more air.

So instead of using the common method of boost pressure e.g. 1bar, 1.2bar etc. would it not be more accurate to work out how much air a particular turbo flows @ any specific pressure?

Pressure seems to me to be only a factor of the equation, like saying "My vehicle has 1000bhp!" but without knowing the weight of said vehicle, stating the power is pointless, just like stating boost pressure without flow rate.

If we go back to the very basic principle of how an internal combustion engine works, its only 3 elements (Fuel, Air, Source of Ignition) that make it all happen.

Once again I have to point out that I'm not an expert so please someone correct me where I am wrong. :)

So I'll assume we all know that for maximum efficiency the Air/Fuel mixture has to be just right and that an increase in one has to be proportionate to the other, but the basic principle is that the more air and fuel you can pump into the engine and ignite over any given time = more power.

Ok back to turbos. So if a car is set to run at 1bar of boost on a standard T28 turbo, the turbo is actually flowing X amount of air. If however, the car was running a larger turbo the same volume of air flow would be achieved at a lower pressure due to the same amount of air being passed through a larger surface area.

If this is true then upping the pressure back to the original 1bar would mean you'd need more air to flow through the larger turbo to achieve the 1 bar. So purely as an example and please don't take these volumes literally because I'm just going to totally make them up as an example. If you need say 1 cubic metre of air per second to achieve 1bar of boost with a T28, you may need 1.5 cubic meters of air per second to achieve 1bar on the larger turbo.

So what I can understand is that there are 2 main reasons for getting a bigger turbo. 1. Is to be able to flow a higher volume of air, thus allowing a higher volume of fuelling which would equal = more power. 2. Is realiability. As stated earlier, the larger turbo doesn't have to work as hard to achieve the same flow of air as the smaller turbo and doesn't have to spin as fast which I guess would potentially increase the units life span. Not to mention that air heats up under compression so the lower pressure of the big turbo would mean lower charge temps which is obviousy good all round.

Well thats my take on it anyway, but please realise that contrary to how I've written this post, I'm sort of asking it as a question, not trying to make out I know it all. Indeed, I would like clarification where I'm right and wrong on this subject as I find it very interesting. :)
 
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skiddusmarkus

Active Member
I only have a basic understanding of how they work.VE/compressor maps go over my head as I'd need someone to explain it 1 to 1 with me really.I think you're wrong on the your last bit though Matt.
Lets assume the small turbo has a 2" outlet and the big turbo has a 4" outlet.All the pipework is 2".If you need 1 cubic metre of air per second then thats it whatever turbo.A bigger one will squash the air coming out of the inlet into the pipework and make it with less effort(double the diameter of a circle=quadruple the volume, although obviously inside the compressor isn't the same dimension).A smaller turbo might have similar dimensions to the pipework and so would have to work harder to achieve the same.
If you needed more air to flow through it to make the same pressure, it would either be escaping somewhere or just increase in pressure so wouldn't be 1 bar anymore.The air has to go somewhere, if its into the engine then you are squashing more weight of air into the same area, so increasing the pressure.Imagine a car tyre.You want to put 1 cubic metre of air into it.You do and the pressure is 1bar.Now if you put 1.5 cubic metres of air into it, you are squashing all the air inside even more so the pressure goes up also.
 
Matt Evans

Matt Evans

Member
Thanks skiddusmarkus, I'm still not much wiser on the subject but still find it interesting. I don't know why I'm that bothered though, I've only got a GT28R. lol
 
Trip

Trip

New Member
Matt Evans said:
Wow very informative thread guys!

I'm not going to pretend I'm an expert on this matter (or any other!) as I'm certainly not, but it seems to me that people are getting a bit too caught up in the whole pressure thing.

I'm assuming that one turbo is larger than the other, so I would imagine the only purpose of this would be a larger flow rate of air, or in other words, over the same length of time the larger turbo would flow more air.

So instead of using the common method of boost pressure e.g. 1bar, 1.2bar etc. would it not be more accurate to work out how much air a particular turbo flows @ any specific pressure?

Pressure seems to me to be only a factor of the equation, like saying "My vehicle has 1000bhp!" but without knowing the weight of said vehicle, stating the power is pointless, just like stating boost pressure without flow rate.

If we go back to the very basic principle of how an internal combustion engine works, its only 3 elements (Fuel, Air, Source of Ignition) that make it all happen.

Once again I have to point out that I'm not an expert so please someone correct me where I am wrong. :)

So I'll assume we all know that for maximum efficiency the Air/Fuel mixture has to be just right and that an increase in one has to be proportionate to the other, but the basic principle is that the more air and fuel you can pump into the engine and ignite over any given time = more power.

Ok back to turbos. So if a car is set to run at 1bar of boost on a standard T28 turbo, the turbo is actually flowing X amount of air. If however, the car was running a larger turbo the same volume of air flow would be achieved at a lower pressure due to the same amount of air being passed through a larger surface area.

If this is true then upping the pressure back to the original 1bar would mean you'd need more air to flow through the larger turbo to achieve the 1 bar. So purely as an example and please don't take these volumes literally because I'm just going to totally make them up as an example. If you need say 1 cubic metre of air per second to achieve 1bar of boost with a T28, you may need 1.5 cubic meters of air per second to achieve 1bar on the larger turbo.

So what I can understand is that there are 2 main reasons for getting a bigger turbo. 1. Is to be able to flow a higher volume of air, thus allowing a higher volume of fuelling which would equal = more power. 2. Is realiability. As stated earlier, the larger turbo doesn't have to work as hard to achieve the same flow of air as the smaller turbo and doesn't have to spin as fast which I guess would potentially increase the units life span. Not to mention that air heats up under compression so the lower pressure of the big turbo would mean lower charge temps which is obviousy good all round.

Well thats my take on it anyway, but please realise that contrary to how I've written this post, I'm sort of asking it as a question, not trying to make out I know it all. Indeed, I would like clarification where I'm right and wrong on this subject as I find it very interesting. :)
Click to expand...

I've been trying to explain the same thing. At least someone is on my same page :thumbsup:
 
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pulsarboby

Guest
If we go back to the very basic principle of how an internal combustion engine works, its only 3 elements (Fuel, Air, Source of Ignition) that make it all happen.
Click to expand...
exactly matt all those 3 are constants in any engine more air/fuel = more power but here we are merely discussing the air volume and what craig originally asked myself
which is "why would one smaller turbo producing say 1 bar boost achieve less power than a bigger turbo producing the same1 bar boost"

and in a nutshell i would say the answers on the first page were pretty much correct, but trying to explain those answers to someone that doesnt really understand is very difficult to do without the need for all the mathematical equasions that ed had put up.

craig used this as his example to me

a simple compressor that you may find in your garage
a small 1hp compressor set at say 100psi (constant needed to run air tool) with the line connected to an impact gun, will run the impact gun for say 5 seconds before the impact gun uses up the air in the tank then the motor will not be able to supply air fast enough to keep gun running, the gun in this case being the engine!!! but that gun initially had 100psi pressure and worked for a few seconds.
now if you put the gun on a dyno:lol: it would have initially produced the same bhp as a compressor fitted with say a 2hp motor, the only difference being that the 2hp motor will keep a constant cfm so the gun/engine would work efficiently whilst its recieving air

now my answer to this was smacking him over the head:frusty: :lol:
as a compressor is using air as a driving force so effectively THE AIR/COMPRESSOR IS THE MOTOR/ENGINE and not the gun!
in a car the engine is the driving force of the turbo and the more efficient the engine can burn the air and fuel ratio the stronger the gas flow will be hence the more efficient the turbo will be.

another scenario is a dodgy knock sensor......the cars running the same boost level but retard ignition or the ecu going into safe mode will make the engine slower as its now working less efficiently so less power will be produced on the rollers

really dont know how else to put it to him to make him understand so il print this whole thread off for him to read
 
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skiddusmarkus

Active Member
If you want to make it even more complicated have a read up on what water/methanol injected pre-compressor does to turbo maps.
 
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