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Pressure vs flow

Pressure vs flow

Pressure vs flow

(OP)
I'm trying to understand why some say that on the same exact engine a turbocharger which has a high flow capacity at a relatively low pressure ratio actually flows more air than a supercharger at the same boost level?  I understand how it can produce more power as other variables come into play.  I understand that the pressure/density is generated in different manners (internal vs external compression) and this affects the volumetric and thermal efficiencies. However, everything else being equal (air temps in the manifold via intercooling, pressures, etc), how do you explain this statement?

RE: Pressure vs flow

I can't explain the statement and therefore I do not accept it until I see a satisfactory explanation.

I would expect that a turbo charger would heat the air less and therefore flow a higher mass of air at the same pressure, but if the temperatures and pressures were equal, WHICH THEY WOULD NOT ACTUALLY BE, then the airflow would be the same as it would be controlled by flow through the head. If anything the supercharger would flow more air as more boost would be lost through the exhaust valve at TDC overlap, as the pressure differential between the inlet and exhaust manifolds would be higher due to the exhaust pressure being lower. Therefore more air would be required from the supercharger to maintain the same temperature and pressure.

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RE: Pressure vs flow

As Pat already has pointed out, a supercharged engine runs with minimal exhaust back pressure, whereas a turbocharged engine will always develop considerable exhaust back pressure with which to drive the turbine. If you had two identical engines running with identical boost pressures and induction temperatures, the supercharged engine will always end up having higher mass airflow and less thermal stress.

Throttling the exhaust reduces engine flow just as effectively as throttling the induction.

Both engines run at a disadvantage. The supercharger subtracts mechanical drive power from the crank.

The turbocharged engine runs with very high exhaust back pressure which can be extremely detrimental to engine operation in a whole variety of ways.

RE: Pressure vs flow

One point to note is that, there should be very little or no valve overlap in this type of engine application.

Overlap is used more in NA engines, since a degree of scavenging is required to allow for higher VEs. In a FI engine there is no need for this as you have a higher pressure in the intake manifold to fill the chamber with.

In the comparison stated there should be equal amounts of torque (power) produced if all other variables remain equal. Especially:  
- actual engine load (airmass/stroke)
- spark timing
- AFR
- Intake air temp

As far as engine load goes, this is self compensating for the back pressure of the turbocharger/exh manifold. If this backpressure becomes great enough that there is significant amounts of internal EGR displacing the fresh charge then the actual airmass consumed will fall - in which case everything ceases to remain equal and you are comparing apples with oranges!

MS

RE: Pressure vs flow

As patprimmer already mentioned if pressure, temperature and humidity are the same so is the massflow (airflow). (The question is not what produces more power or is it not?)

The turbo has a centrifugal compressor which is adiabatically more efficient (less temperature increase and therefore higher air density at the same pressure (= higher massflow)) than a mechanical supercharger such as a roots blower (which was originally not intended to be used as a compressor anyway), G-charger (VW) or even a screw type compressor.  
However, if you were to use an axialflow compressor (supercharger) which has a higher adiabatic efficiency than a centrifugal compressor you'd end up with a higher massflow than you would with a turbocharger at the same boost level.
http://www.axialflow.com/

RE: Pressure vs flow

(OP)
I understand that a turbo is thermally more efficient, however, my original assumption is that the blower (or even a less efficient turbo) can recover the lost density via intercooling making an apples to apples comparison more realistic.  So, an environment at X temperature and Y pressure captured within a manifold of Z volume should have the same density regardless of what device created that environment.  However, as usual were not talking about a static condition.  Therefore, I was curious if anyone knew of some unique characteristic of a turbo charger that would allow it to deliver more mass given the same manifold conditions (at least as measured by our crude methods) than a blower under dynamic conditions.  Granted, boost is simply a bulk measurement of the average pressure in the manifold at any given time.  Could it be that the turbo has a greater ability to “respond” flow wise to transient conditions existing under actual operating conditions?  Or maybe the turbo creates mass flow of higher velocity and therefore can fill the cylinders faster.  Both would result in a higher VE without resorting to mechanical modifications (porting, bigger valves, etc) and therefore “flow” more at the same boost levels.  I’m stretching here, but the flow vs. pressure theory is touted by some pretty big names in the tuner business and I need knowledge to dispel any untruths.
These same “experts” will argue that the turbo of higher flow vs. pressure ratio will flow more mass than a smaller turbo at the same (or lower) pressure ratio on the same engine (thermal & volumetric efficiencies the same).  To me, if the smaller turbo produces enough flow to produce X pressure the larger turbo will produce the same numbers but it won’t be working as hard in terms of turbine speed.  I would expect an increase in HP simply due to less backpressure of the larger system (along with greater lag), but not due to more flow.  
    I base this on the model of an engine as a highly dynamic “variable” leak.  The forced induction device simply tries to “fill the leak”.  Any flow not consumed by the leak is manifested as excess pressure which increases the density of the air in the manifold which in turn increases the overall mass flow rate.  I’m still a bit unsure if the actual port velocities change much due to the increased pressure differentials (you would think so based on flowbench data at different inches of water), but to a degree it would depend on how quickly the pressure is recovered inside the cylinder at various piston speeds.  Anyway, exhaust backpressure aside (on turbos), one would think that the same engine will exhibit the same “leak” rates regardless of what’s feeding it.    Therefore if one were to measure actual intake flow pre FI device I would expect the numbers to be fairly equal given the same conditions in the manifold regardless of what FI method is used.  Where is the flaw in my logic?      

RE: Pressure vs flow

The intercooler can recover the lost air density but it can't recover the work the compressor put in the first place. So a less efficient compressor will either increase parasitic loss (supercharger) or increase backpressure (turbo) both will result in a loss of power even if you 'recover' the lost density with an intercooler.
Besides an intercooler is also a restriction in the airflow.

Regarding massflow and higher velocity:
If p*V=m*R*T (ideal gas) and if massflow is m/t this means
massflow = (p*V)/(R*T*t)
or in other words you cannot increase massflow without increasing pressure or reducing temperature. (The volume is basically given by the displacement and rpm of the engine.)

Well, that's how I understand it.

RE: Pressure vs flow

Fitting a higher flow rated centrifugal compressor cannot increase flow through the engine unless the air density at the engine rises somehow. Boost must increase or induction temperature fall, or both. Most likely it will just reposition the different map contours under the same operating point without any significant net change in flow.

I dispute that all turbocharger compressors are more thermally efficient than all superchargers. A given turbo may have a fairly high peak efficiency at one particular combination of flow and pressure, but everywhere else efficiency falls off rapidly. A screw supercharger can have similar adiabatic efficiency to a turbo, but over a far wider operating range which makes it more useful. For a practical road applications it would be far superior without intercooling. But as has already been said, a reasonable attempt at intercooling is a great leveler.

Saying a turbo compressor will respond faster to flow changes may actually be true, assuming it is already spooled up to the required compressor Rpm. That is a very big if.

How can a turbo already be up to the required operating speed BEFORE the throttle opens to gain this theoretical advantage ? While a supercharger will usually have to fill all the pipework on sudden throttle opening, at least the supercharger itself does not lag behind the engine.
 

RE: Pressure vs flow

Warspeed--
I don't question what you just said.
I just wonder why VW used this twin charging concept instead
of just one twin screw supercharger to supercharge the engine?
http://www.greencarcongress.com/2005/08/inside_vws_new_.html
They could have applied a miller cycle instead of using an additional turbocharger and all the added complexity.
Mazda reached 92 HP/litre at 5300 rpm without intercooling so I think 120 HP/litre doesn't appear to be completly out of reach for a miller-cycled and intercooled engine.
What am I missing?

RE: Pressure vs flow

2 Points about why compound forced induction makes sense.

1. The twin charging system allows a huge, efficent turbo to be used at high mass flows. With all of its inherit benefits in terms of recouping lost energy through the exhaust system.

2. A supercharger is used to allow the 'off boost' performance penalties to be negated because boost is available from idle onwards.

As for the use of the miller cycle in an automotive application - I have no real experience. However I would have thought that the precise air metering requirments will be unavailable. Condsidering you never know how much air is truly where.

Also, the fuel delivery will not be emissions optimal considering the amount of time that the inlet valve will remain open and and the homogenity of the charge (brownian motion!?!)

MS

RE: Pressure vs flow

Quite possibly the design goals were completely different.

The Miller cycle is really all about engine efficiency, particularly at small throttle openings. The high compression ratio and resulting higher expansion ratio can recover more heat energy from the fuel. If you want good specific power along with excellent fuel economy, the Miller cycle is an excellent way to get both.

If your goal is sheer power and performance from a small capacity engine, twincharging is hard to beat. A positive displacement blower in series with a turbo brings out the very best in both. The blower provides instant low rpm boost which really kicks the exhaust turbine. It will be very responsive. For sheer top end power, a turbo is going to maintain high airflow even when the Ve of engine and blower are starting to fall off.

If set up correctly boost pressure can be held just above total exhaust back pressure over a very wide Rpm range which gives wonderful exhaust scavenging with a zero overlap camshaft. The result is a very tractable responsive and powerful engine.

Several years ago I developed my own twincharge system on a small 4WD 1.6 Litre hatchback (Ford Laser in Australia). The results were beyond expectations. I can well believe VW are very pleased with their effort. Cost is the only real problem but VW were not the first to do it. Lancia and Nissan have built successful production twincharge engines.

RE: Pressure vs flow

>>Saying a turbo compressor will respond faster to flow changes may actually be true, assuming it is already spooled up to the required compressor Rpm. That is a very big if.

How can a turbo already be up to the required operating speed BEFORE the throttle opens to gain this theoretical advantage ?<<

Easy.....by placing the throttle in front of the compressor inlet. During part throttle cruise you can hear the turbine spooling up because the compressor isn't compressing much air. When the throttle is nailed, the boost gauge moves as fast as your foot moves the throttle.....zero lag/instanteous reponse.

RE: Pressure vs flow

How much boost does this give at idle ?

RE: Pressure vs flow

>>How much boost does this give at idle ?<<

None...I simply responded to your prior statement about turbo compressors and in it you did not specify off idle. In a subsequent post, you did however mention instant low RPM supercharger performance.

RE: Pressure vs flow

But idle may be from where you open the throttle, yes ?

If it is to have zero lag/instantaneous response as you claim, then it must be fully spooled up even at idle.

A supercharger will respond instantly, because it is mechanically coupled.

The engine may not, because of pipe and intercooler volumes that must first be filled, but those same (or similar) volumes can exist with a turbo too.

RE: Pressure vs flow

>>If it is to have zero lag/instantaneous response as you claim, then it must be fully spooled up even at idle.<<

Nope.....what part of part throttle cruise do you not understand? There is no "idle" involved in either the motor or the

turbo in my statement.

>>A supercharger will respond instantly, because it is mechanically coupled.

That can also be a problem with a supercharger.....a turbo does not suffer from the same fate of being mechanically

coupled/speed restricted, especially during part throttle cruise when the compressor is spooling up in a partial vacuum.

>>The engine may not, because of pipe and intercooler volumes that must first be filled, but those same (or similar)

volumes can exist with a turbo too.<<

In the real world, volumes are filled apparently much quicker than you imagine. The engine/turbo throttle response for all intents and purposes was pretty much as *instant* as it gets!

In 4th gear - 3.73 rear end - 12 psi

30 - 50 mph in slightly less than 2 seconds.

50 - 70 mph in slightly less than 2 seconds.

Photo circa 1984. Z28, 350 sbc, variable geometry manifold and Bosch K-Jetronic FI from a 6.9 litre Mercedes Benz.



RE: Pressure vs flow

(OP)
We’re getting off the topic with all this lag talk.  When I mentioned “transient” conditions I didn’t mean on-boost off-boost conditions.  What I meant was the dynamics of the intake process during steady state (engine rpm) conditions.   For example, if one could show for whatever reason unique to the greater “flow vs pressure ratio” that the mass flow created by a turbo in the manifold, intake ports or across the valves is somehow greater than the mass flow created by a blower under the same conditions (density, temp, pressure, valve lift, rpm, etc) I be interested in hearing it.   A good example of what I’m talking about is a recent article in a popular tuner mag where the express purpose was to keep upping the boost on a STOCK 4 cyl Honda eng until it blew and then figure out what broke (the weak link).  In this test the authors managed X HP at Y boost level with the “initial turbo” combo before the turbo showed signs of diminishing returns (but still within it’s max efficiency island).  Since the engine was still intact, they installed a huge turbo to achieve the higher boost pressures necessary to blow the engine and left everything else the same (except AF tuning of course).  The interesting part is that they achieved X+40 some HP at THE SAME Y boost level with the larger compressor (obviously with a much higher spool up rpm but that’s not the issue).   So the question is what accounts for the extra 40+HP at the same boost?  It is reduced backpressure/pumping losses?  Is it increased mass flow (at the same boost pressure and temps) and if so how?  Is it a combination and if so what’s the distribution?  These are the kind of questions I’m interested in being answered.
I’ll give you an off the wall example of what I consider a “smoking gun” kind of answer.  Since the turbo is a “non-positive” displacement device it has some amount of flow leaking from the pressure side back to the ambient side (carry back).  Possibly, during those brief milliseconds where the mass flow/pressure tends to lag cylinder pressure (initial crankshaft degrees following INT valve open when instantaneous VE is low, I just coined a new term!) this momentary drop in manifold pressure causes some or most of this leakage to cease and actually add to the overall flow.  I call this “turbo sprint capacity” (another new term!).  This would happen so quickly that a typical manifold pressure gauge wouldn’t even see it, but the result is more flow at the “perceived” same boost pressure.   A “blower” could not respond in kind since it has very little leakage (relatively speaking) and therefore no sprint capacity.  Again, the momentary drop in pressure is so quick that the boost gauge will still show full boost.  The bigger the compressor is on the turbo the larger the “sprint capacity” is with respect to a smaller turbo explaining why a larger flow vs pressure ratio compressor can outflow a smaller compressor under the same dynamic conditions (exact same engine) even if the larger one is not in its highest efficiency band and the smaller is (ie, lower then recommended pressure ratio for the large turbo). Anyway, that’s the kind of explanation I’m looking for if one exists.  
    BTY, the stock 4 cly Honda engine finally broke a ring land at 18 psi and 420 flywheel hp just in case you were wondering.   

RE: Pressure vs flow

Mark911 I can see what you are getting at (sort of).

The flow through a centrifugal compressor is not defined by compressor Rpm, but by the total restriction to flow. If throttle position suddenly changes, airflow will too, limited only by the inertia of the air.

But it assumes that the compressor Rpm lines on the flow map are horizontal, and more flow is available at the same boost at the same compressor Rpm. That may sometimes almost be true, or definitely not true, depending where you are on the flow map.

It would only work for reasonably small changes in throttle opening. For large changes boost would suddenly fall off until compressor speed could build back up, the dreaded lag demon.

iolair, throttle response most certainly IS altered adversely whenever the throttle is located further away from the cylinder head. Any serious race engine uses individual throttle bodies very close to the intake valves for that exact reason. A few feet of pipe can make a very big difference.

I can tell you for a fact that snapping closed the throttle for gear changes can cause engine Rpm to be very slow to fall off when there are excessive pipe volumes between throttle and engine. Throttle opening is subjectively not quite so bad as throttle closing.

That is the difference between an engineer and a race car driver. The engineer assumes a few milliseconds mean nothing. The race car driver will tell you in no uncertain terms it feels like crap to drive.

RE: Pressure vs flow

Mark,

Maybe the larger compressor also had a larger A/R Turbine housing therefore producing a more favorable delta/p between the compressor outlet and the turbine inlet pressure.....that would easily generate 40HP more at the same pressure ratio.......plus, it would make it much easier to explain. :)

RE: Pressure vs flow

>>Any serious race engine uses individual throttle bodies very close to the intake valves for that exact reason.<<

Warp,

Would you consider a Renault F1 and a BMW F1 serious race engine?



RE: Pressure vs flow

I think another major factor is the true nature of the term "Pressure" in this context.

Boost pressure is simply *static* pressure

If we compare 'small' and 'large' turbos we should not just level the output air temps, but also the total pressures (where total pressure = dynamic pressure + static pressure)

RE: Pressure vs flow

(OP)
Mark,

Maybe the larger compressor also had a larger A/R Turbine housing therefore producing a more favorable delta/p between the compressor outlet and the turbine inlet pressure.....that would easily generate 40HP more at the same pressure ratio.......plus, it would make it much easier to explain. :)

Very true and a good point.   Flow is definitely increased when less products of combustion are trapped in the cylinder, when reversion is reduced, and when a more favorable intake delta p exists, all due to lower backpressure.  So the caveat should be that it's not the larger compressor that causes the increased flow, it's the larger turbine AR and the resulting benefits.  I would suspect that a larger AR on the original (smaller) compressor would do the same up to a point.  And of course, this extra flow comes at the price of increased spool up rpm.  

RE: Pressure vs flow

And it would probably be even better still if there was no exhaust turbine at all, as with a mechanically driven centrifugal compressor.

RE: Pressure vs flow

Not quite on topic. But isn't the reason the throttle plates were placed in front of the turbo inlets to make sure that the turbine and compressor wheel kept on spinning at high speed when the driver went off the throttle?

RE: Pressure vs flow

That is my understanding of it too. The compressor sees vacuum and will not violently surge, the compressor also produces less mechanical load under suddenly closed throttle, and the turbo will not decelerate as fast under suddenly closed throttle.

The engine itself will not decelerate fast under closed throttle either, especially with the clutch disengaged for gear changing. It has to use up all the stored high pressure air in the pipework and intercooler first. When you get back on the throttle the pipework has to fill again, that does not help response either. These days with EFI, nobody does it that way anymore. Throttle on the plenum or individual throttles work far better, and are what you will generally see everywhere. If it worked well everyone would be doing it.

It probably is on topic, after all the original post was all about airflow response to sudden throttle change.

RE: Pressure vs flow

>>That is my understanding of it too. The compressor sees vacuum and will not violently surge, the compressor also produces less mechanical load under suddenly closed throttle, and the turbo will not decelerate as fast under suddenly closed throttle.<<

Ah ha, progress.........and under part throttle acceleration, the compressor/turbine will spool up rather quickly because the compressor is running in a partial vacuum as opposed to a system that is under part throttle acceleration and the compressor is compressing a full load of air and the output is dead heading against a partially opened throttle plate or perhaps a partially opened BOV, thus it would be running at a slower speed.

Assuming identical engines and volumes after the compressor outlet, and with the above scenario, nail both throttles at the same time and the race would be on....one has to fill up the volume between the compressor outlet and the operating cylinder and the other would have to fill only beyond the throttle plate and cylinder.

Same scenario at high speed with on/off/on throttle application.

Problem being, prior to 1988 and the RA168E Honda, ball bearing turbos were not generally available and invariably the system with the throttle before the compressor inlet could pressurize the larger volume slightly faster that it took to completely pressurize the smaller volume because it's compressor was running slower and it took time to spool it up.

For racing applications (simple on/off throttle), modern ball bearing turbos don't slow down much and spool up rather quickly if they do. Bottom line.....you can put the damn throttle anywhere you want!

>>The engine itself will not decelerate fast under closed throttle either<<

You are generalizing, that is certainly not true of my Z, if you closed the throttle, it instantly shut the fuel off at the ports and had the drivability of a normally aspirated engine....very docile!

RE: Pressure vs flow

I'm not a throttle expert, but one could ask though why did BMW have an individual throttle for each intake on all its M5s?

RE: Pressure vs flow

Probably the current most technically advanced turbocharged racing engine in the world....the unbeatable Audi R8.

Note the plenum throttles.....not individual port throttles.



RE: Pressure vs flow

Again I'm just asking questions: Could it be that a turbocharged engine requires a sealed throttle body since the plenum is obviously under pressure (having less throttle bodies is safer and avoids leakage). And/or could it be because a turbocharged engine deals with some delay anyway (so why care about adding more throttle bodies). And/or could it be because a single throttle body is more of an airflow restriction than several throttle bodies and is therefore more noticable on a naturally aspirated engine (a naturally aspirated engine cannot simply increase boost)?

RE: Pressure vs flow

Agree with you globi5, the closer you can get the throttles to the intake valves the better the response. The more remote, the more sluggish throttle response. That is true for any engine regardless, and the reasons are obvious.

At wide open throttle, the throttle should have damned near zero restriction to flow anyway, so it really does not matter where it is located from the flat out power perspective when wide open. But for transient response, locating it as close to the intake valves as possible is always best. It also makes the engine more cam friendly, when there is a lot of valve overlap, there will be less exhaust reversion at high manifold vacuum if the throttles are very close to the inlet valves.

Nobody that I am aware of places the throttle body in front of the turbo these days. There are pictures of formula one engines with carburettors around too, but nobody uses them anymore in formula one. All ancient obsolete history. Even those insane drag race guys that would sell their mothers for an extra .001 second  faster ET are not putting throttles in front of the turbo. Believe me, if it worked it would be common practice.

The truth is, that at part throttle cruise, there will be insufficient exhaust mass flow to have any sane size of turbo fully spooled up to full working boost pressure no matter where the throttle is located.

To do so would require a turbine and exhaust housing so small it would simply become far too restrictive at full power. Turbine tip speed has far more to do with exhaust velocity in the scroll than aerodynamic load drag in the compressor. The speed lines on the compressor flow map should tell you that.

Turbos all have a rather limited flow range and require a finite time to spool up. You pay your money and fit either a small turbo, or a large turbo, and suffer the results of the compromise.
 

RE: Pressure vs flow

>>Nobody that I am aware of places the throttle body in front of the turbo these days. There are pictures of formula one engines with carburettors around too, but nobody uses them anymore in formula one.<<

Let's see....maybe I can help. Look for the key words "prior to 1988" and "ball bearings".

>>The truth is, that at part throttle cruise, there will be insufficient exhaust mass flow to have any sane size of turbo fully spooled up to full working boost pressure no matter where the throttle is located.<<

Ok....now let's change a few words around.

The truth is, that at part throttle cruise, with the throttle in front of the compressor inlet, there will be more than sufficient exhaust mass flow to spool up a turbo of any sane size and produce zero working boost pressure.......until the throttle is sufficiently opened.......key words = open/sufficient.

>>Agree with you globi5, the closer you can get the throttles to the intake valves the better the response. The more remote, the more sluggish throttle response. That is true for any engine regardless, and the reasons are obvious. At wide open throttle, the throttle should have damned near zero restriction to flow anyway, so it really does not matter where it is located from the flat out power perspective when wide open. But for transient response, locating it as close to the intake valves as possible is always best.<<

Jeeze, after 6 years of such sluggish transient response, you would think that the Audi R8 engineers would be able to grasp such a simple concept and immediately implement a design change. Oh well, I guess they were too busy sluggishly posting 5 of 6 victories at Le Mans and 6 straight ALMS championships since 2000.

RE: Pressure vs flow

(OP)
I don't have an opinion on pre or post turbo TB location from a performance standpoint but I sure wouldn't want to be the driver with a pre TB if the boost blows an IC or any of the other many pipes, hoses or connections between the turbo and plenum!  Instant air leak and a stuck throttle like condition.   

RE: Pressure vs flow

Mark,

Interesting point......and I agree! It's probably always a good idea to have a kill switch handy.

BTW, I was reading an article on BMW's Valvetronic control system....engine air flow/speed is controlled exclusively by varying intake valve lift. Very cool!

RE: Pressure vs flow

iolar, you are aware of course that the Garrett ball bearing turbochargers only use a labyrinth non contact oil seal behind the compressor wheel. They rely on the fact that boost pressure leakage back behind the compressor wheel will always be higher than crankcase pressure to keep oil out of the compressor cover.

With a front mounted throttle, under closed throttle overdriven engine deceleration, the whole compressor will see massive engine vacuum. Oil will pour into the compressor. Ball bearing turbochargers are well known to be unsuitable for carburettor suck through applications for that very reason.  If you had ever actually tried this front throttle idea out yourself (with a ball bearing turbo), you would quickly have discovered this for yourself the hard way.

RE: Pressure vs flow

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Look, it’s just not a big deal to adapt a 360 dynamic carbon seal for any ball bearing Garrett GT.

But that's not the point....what will it take for you to understand the significance of "prior to 1988" when the throttles before the compressor inlets were popular and for good reason......ball bearing turbos in 1988 and the Honda 1.5 litre V6 twin turbo RA168E engine pretty much made the need irrelevant....I don't know how many times I have to restate that!

I posted the photo of the Audi R8 with the throttles on the plenum.......what does that tell you? Does that mean I am no longer using throttles before the compressor inlet......could be?

Their are still certain advantages to a compressor inlet mounted throttle, mostly in cruise spool up and eliminating the unnecessary work of compressing air all day long when its not needed. It can be controlled by manifold pressure in cruise mode or simply switched to the full open position for performance driving. You don’t have to deal with it and you also don’t have to deal with variable geometry induction systems……I’ll deal with it!

So, let’s just agree to move on down the road!

RE: Pressure vs flow

Fair enough, I should have remembered Turbonetics!! But Garrett ball bearing turbos which are far more common still do not provide the spring loaded carbon compressor seal as an option (a very great pity).

I am still doubtful if there will be enough exhaust flow under small throttle cruise conditions to spool up any turbo to significant enough compressor Rpm, regardless of where the throttle is located, but if you can do it, good luck to you.

The Garrett VNT turbines may be another different way to do it, but they are not without problems of their own. I run A Garrett VNT on my everyday gasoline road car, and have some very mixed feelings about it. But the only real way to learn is to test things for myself. The key to using the VNT is what you use to control the vanes, and that is by no means a simple problem.

RE: Pressure vs flow

Warp....go ahead, please tell me all about your experiences with the VNT. I thought I once read about VNT and temperature related problems with SI gas engines.

John

RE: Pressure vs flow

At the immense peril of hijacking this thread, and with apologies to Mark911, I will attempt to keep my answer brief.

I have had none of the mechanical reliability problems that others have reported with the vane mechanism due to the higher EGT of gasoline engines. My VNT turbo system has been driven almost every day now for just over three years.

The the turbo response is fantastic, boost threshold exceptionally low, but top end power is lacking. The problem is that all the exhaust goes through the turbine all the time there being no wastegate. Unfortunately this turbine is fairly small and beyond a certain exhaust flow, the turbine back pressure just goes parabolic.

I had assumed a 150Kw turbo from a 4 Litre Nissan diesel would be about right for a 150Kw 1.6 litre gasoline engine.

Now the vanes stand on end (going almost radial), and still control the boost pressure o/k, but excessive turbine back pressure just kills the top end power. A larger exhaust turbine would be better, but then the response and boost threshold would not be quite so good. These VNT turbos are fairly difficult to come by in Australia, and there is not a wide choice available to try something different.

I had thought of using a wastegate arranged to open and bleed off excessive exhaust back pressure. As the vanes are still able to control boost it should work. I have heard others have had success with something along similar lines.

Controlling the vanes is another issue. Nothing suitable comes with the turbo, because what you get will be off some sort of diesel. So something homemade will be required. I am using a dual diaphragm system that uses both plenum vacuum and boost pressure to keep the vanes open at small throttle for good fuel economy. Under boost the vanes control the final boost.

I am happy with it as a very flexible everyday road car, and it behaves very well in traffic, but it is just not powerful. For anyone thinking of trying this, I would suggest a suitably sized Garrett ball bearing turbo would give results at least as good if not better with a lot less trouble. The fact that these variable vane turbos have not made it onto production (gasoline) engines does not surprise me. The whole VNT concept rather intrigued me, and being a bit adventurous I thought it worth a try.
 

RE: Pressure vs flow

Warp, Interesting....so I assume for a power system you would recommend standard ball bearing units for now until a selection of VNT's are more redily available complete with a vane control system suitable for an SI engine.....correct?

Thanks!

John

RE: Pressure vs flow

That has been my experience for a common SI road application, and I know others that have tried the VNT and eventually given up as well.

From what I can see, the VNT idea is much more successful with diesel engines that have quite different requirements. For gasoline engines a suitably sized state of the art ball bearing turbo would probably offer better overall performance.

Production engines definitely point in this direction, and I very much doubt if the hot rodders and racers can do any better than the professional engineers.

RE: Pressure vs flow

I believe Porsche still use KKK turbochargers not Garrett turbochargers? VNT is Garrets descriptive name for the variable vane turbine housing. I am guessing that KKK (and Porsche) are calling essentially the same thing a VGT.

High temperatures are blamed for causing VNT unreliability, but I am not sure exactly why. The vanes see no higher temperatures than the turbine wheel, and we never see wheel failures due to temperature. The vanes are quite likely made from a similar alloy as the turbine wheel, and are under far less mechanical stress.

To me it seems more likely that the vane actuator sliding ring mechanism just eventually blocks up with exhaust deposits and finally jams solid. That is going to be due more to what comes down the exhaust pipe rather than just temperature. The thick crust that can sometimes build up on the back of the exhaust valve on high mileage engines is the sort of thing that could likely cause the problem. It takes more than just temperature to do that.

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