Pressure vs flow 1

[Mark911]

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?

 

[yiannakis]

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)

 

[Mark911]

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.

 

[Warpspeed]

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

 

[globi5]

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?

 

[Warpspeed]

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.

 

[iolair]

>>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!

 

[globi5]

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?

 

[iolair]

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

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

 

[globi5]

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)?

 

[Warpspeed]

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.

 

[iolair]

>>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.

 

[Mark911]

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.

 

[iolair]

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!

 

[Warpspeed]

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.

 

[iolair]

Carbon Seals Available

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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!

 

[Warpspeed]

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.

 

[iolair]

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

 

[Warpspeed]

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.

 

[iolair]

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

 

[Warpspeed]

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.