brain teaser

[skiddusmarkus]

The source of air is your mouth, the balloon is whats being blown into ie the engine .

If we do it how you say, that a balloon twice as big is going to pump twice as much air in then it would need to be making 2bar.Both turbos can sustain 1 bar, the smaller one doesn't empty out like the balloon in your anaology.

 

[Trip]

The source of air is your mouth, the balloon is whats being blown into ie the engine .

If we do it how you say, that a balloon twice as big is going to pump twice as much air in then it would need to be making 2bar.Both turbos can sustain 1 bar, the smaller one doesn't empty out like the balloon in your anaology.

The source of air for the combustion to burn is the baloon. Unless you don't want to blow pure oxygen from your mouth direct on a naked flame

 

[skiddusmarkus]

How did the balloon fill up though ?

Put a restrictor on that bigger balloon so that it can only flow 1 bar of pressure and it won't make any more of a flame than the smaller balloon.Of course as the balloons empty the smaller balloon won't be able to keep up the pressure as long but that isn't the case with turbos as they are being fed by the exhaust.
If you want that bigger balloon to make the flame any more potent it would have to be flowing more air than the smaller balloon so it would be more than 1 bar.Hence, 1 bar on a smaller turbo is the same as 1 bar on a bigger turbo except that the air from the big tubo would be cooler and suffer from less back pressure.

 

[Trip]

Forget heat.. Its not heat which generate CFM.. The balloon can be filled up from any source. The balllon was just an example of a source generating pressure for a combustion to burn, Irresepctive of how it got its pressure in the first place.

forget the baloon... Look at turbo maps.. you have pressure on the vertical axis and CFM on the horizontal axis. Just compare a simple GT28 Vs a GT35 same pressure and you find your answer.

 

[skiddusmarkus]

Ok Trip I am not too hot on compressor maps so I won't pretend I am.The original question though was about which turbo is going to make more power on the same boost(1.2bar)

Well you said "its all in the MASS(volume). Think of it getting hit by a Smart car at 40Mph Vs getting hit by a 10ton lorry at the same speed (or even lower). "Well if the air being forced into the engine is the same on both turbos ie 1.2bar then your smart car and lorry analogy doesn't work.Pressure = force divided by area.The area that the turbo is pumping into is fixed so the only variable is the force.If both tubos are pushing 1.2 bar into the engine then both turbos must be making the same force ie volume of air.
The bigger turbo is certainly capable of making more boost and thus forcing more air into the engine but then it wouldn't be doing so at 1.2 bar.

 

[olliecast]

 

[Trip]

In my anology i was comparing pressure as speed and weight of the vehicle as the mass. both pressure are the same (speed) but the mass is different.

 

[Trip]

with your reasoning, why does a 1.2bar standard turbo generate 320ish bhp and a gt3076 at 1.2bar generate close to 400bhp. Because the bigger turbo can flow more air (volume) at the same boost pressure.

 

[skiddusmarkus]

Its already been agreed earlier in the thread.Its because a standard turbo will be heating up,spinnig its tits off at that boost and heating up the air in the compressor.A bigger turbo won't need to spin as fast and will have a bigger compressor so not heating up the air as much.
The bigger turbo will have a lot less backpressure which is a performance killer on turbo engines.This is why there wouldn't be much more point in going much over 1.2 bar or so with a standard turbo and why people fit bigger ones.Otherwise we'd all run 2bar on a T2 and have 350bhp and no lag.

 

[skiddusmarkus]

Also, the bigger turbo cannot flow more air at the same boost pressure.It can only flow the same amount of air, albeit cooler.For it to flow more air, it would have to also increase the pressure.

Edited to add, it could ofc flow more air if it was blowing into the atmosphere but as its not, its blowing into a fixed volume pipe and then into a fixed volume engine its restricted by physics and as Scottie said, "Ye cannae change the laws of physics Cap'n".

Edited again to add I think I might know where you are getting confused.You see an ickle outlet of the t28 and a massive one on the big turbo.If you measured the flow at this then at 1.2 bar the big turbo would be flowing more air but we don't measure pressure there, its manifold pressure hence they are both having to force air into the same are ie the mainfold.If your 2 turbos were to be connected up to a ballon and run at 1.2 bar, the big one would fillt it faster because the balloon can expand and isn't a fixed volume.The engine cannot.If it were measured like this then a 4" outlet of a big turbo would be forcing into the engine 4.8bar compared to the 1.2bar of the 2" T28 and you'd be picking bits of conrod out of the trees.

 

[steve963]

i agree i think 1.2bar is 1.2bar whatever turbo,

the CFM just means that your turbo can supply air even at high revs and wont tail off like the stock turbo at 1.4 bar, which drops back to 1.2 or less at high RPM

 

[pulsarboby]

i agree i think 1.2bar is 1.2bar whatever turbo,

the CFM just means that your turbo can supply air even at high revs and wont tail off like the stock turbo at 1.4 bar, which drops back to 1.2 or less at high RPM

correct and wrong at the same time lol

1.2 bar is 1.2 bar fact
but its all down to the heat of the charge temp which makes the difference in power as skiddus has said.
the more air / fuel you get into cylinders the faster the engine will be, the turbo is merely an air pump but the harder the pump works the more heat it generates and thus higher charge temp which will reduce power at the crank (larger tubby will work more efficiently at same boost level)....the reduced power will then in turn make the turbo spool less at its driving point 'which is ultimately the exhaust gases.

its a big cycle you see, the engine is the main source of power to run the turbo and so that dictates in turn how well the turbo works.

its very difficult to explain in leymans terms without delving into mathematical equasions which is why my laddy craig asked the question in the first instance:lol:
and hense the reason for this thread to see if someone could explain it simpler than i did

 

[Makaveli_se7en]

I know I'm not allowed to answer but I will say it seems you're all only thinking about one side of the turbo ;-)

 

[skiddusmarkus]

I've mentioned back pressure about a billion times.

 

[Trip]

Bob.. Heat is a factor as you described but not all .. If you get a small tubby and boost it to 1.2bar and manage to cool the air to lets say 30deg, you still won't make power as much as a big tubby at 1.2bar and cooling the air tothe same 30deg.

Look at the efficiency islands of turbo maps. lets take a GT3071 and GT3076. if you plot 1bar (2bar absolute) . a 3076 will flow 32 lb/min and the 3071 will flow 27 lb/min . Turbo maps do not take into account heat.. and they are both plotted inthe best efficiency island.

Heat will reduce power thats for sure and a small turbo will generate more heat at high boost compared to a bigger one.

 

[Trip]

Another example. taking two very similar turbos. A GT2871R 48trim and a GT2871R 52trim. Exhaust side are identical so the other side of the turbo and back pressure is out of the equation here.

If both turbos use same boost level i can assure you that by the time the air reaches the engine via a good intercooler both will be of same temerature. The 52trim is rated to make slightly more power.

 

[skiddusmarkus]

Bob.. Heat is a factor as you described but not all .. If you get a small tubby and boost it to 1.2bar and manage to cool the air to lets say 30deg, you still won't make power as much as a big tubby at 1.2bar and cooling the air tothe same 30deg.

Look at the efficiency islands of turbo maps. lets take a GT3071 and GT3076. if you plot 1bar (2bar absolute) . a 3076 will flow 32 lb/min and the 3071 will flow 27 lb/min . Turbo maps do not take into account heat.. and they are both plotted inthe best efficiency island.

Heat will reduce power thats for sure and a small turbo will generate more heat at high boost compared to a bigger one.

Why would the same amount of air at the same temperature make more power on one turbo over another?Its exactly the same.I think you are taking what they are capable of flowing(ie the compressor map) as literal rather than what they are actually forcing into the engine.The air pressure inside the manifold tells you how much air is getting into the engine, not the compressor map.If you redesigned the whole intake so that your intercooler piping, end tanks, throttle body and ports into the engine were all the same (well 1/4 size for the ports) as the bigger turbo outlet,it would be flowing what you say it is, but there are restrictions in the system which don't allow this.

 

[skiddusmarkus]

Another example. taking two very similar turbos. A GT2871R 48trim and a GT2871R 52trim. Exhaust side are identical so the other side of the turbo and back pressure is out of the equation here.

If both turbos use same boost level i can assure you that by the time the air reaches the engine via a good intercooler both will be of same temerature. The 52trim is rated to make slightly more power.

Then I presume one has a bigger compressor?This means it will flow more air per revolution than the smaller wheel, so won't have to spin as fast to flow the same amount of air, which means it won't heat the air up as much.There may be some points in the rev range where the air temps would be the same but as the revs rise the one with the bigger wheel will start to mak emore power hence the higher rating.

 

[Makaveli_se7en]

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.

 

[red reading]

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.

I want to be there when you explain this to craig,bob (the formula):lol::lol::lol: