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Old 12th April 2014, 10:12   #16
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Default Re: Turbo lag v/s Boost threshold

Quote:
Originally Posted by Jeroen View Post
...does opening the waste gate earlier introduce more/less turbo lag or lowers/increase the boost threshold or both?

Neither.


The above answer is both correct and incorrect.

Why?
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Old 12th April 2014, 18:50   #17
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Default Re: Turbo lag v/s Boost threshold

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Originally Posted by gotzuk View Post
How will opening a waste gate earlier, say at 9 psi instead of 10psi lower/ increase boost threshold and/or turbo lag?
Yes, that is a different way of phrasing the same question

Quote:
Originally Posted by VeyronSuperSprt View Post
Neither.
The above answer is both correct and incorrect.
Lets start with the definitions:

Straight from Wikipedia, but good enough:

Quote:
The boost threshold of a turbocharger system is the lower bound of the region within which the compressor operates. Below a certain rate of flow, a compressor produces insignificant boost. This limits boost at a particular RPM, regardless of exhaust gas pressure.
Quote:
Turbocharger lag ("turbo lag") is the time required to change power output in response to a throttle change, noticed as a hesitation or slowed throttle response when accelerating as compared to a naturally aspirated engine. This is due to the time needed for the exhaust system and turbocharger to generate the required boost. Inertia, friction, and compressor load are the primary contributors to turbocharger lag.
Now I don't have all the answers either, but I was involved in a similar discussion sometime ago.

Here's my take:

Boost threshold is for a particular turbo a given, specific, value. I don't think opening or closing of the waste gate will make any difference.

With respect to turbo lag, there the waste gate does play a role. Obviously, in the design stage going for an external versus internal waste gate can have a profound impact on the turbo lag.

But clever operations of the waste gate can have an impact on turbo lag as well. Now you could argue that is a design aspect, but still.

Waste gate valves and actuators are engineered for a specified boost level and turbine inlet pressure. As the inlet pressure rises, the waste gate might start opening. This happens because turbine inlet pressure also increases as boost pressure rises. What designers do is to introduce a bigger, more powerful spring to keep the wast gate close until the respective boost is achieved. The flip side is you require more force (a bigger, heavier actuator) to overcome the powerful spring.

Even so, ensuring that the waste gate stays shut tightly until the turbocharger comes up to peak boost, will ensure it revs up more quickly.

To put it in context of the question; allowing the waste gate to open earlier will result in the turbo spinning up less quickly. you could argue whether you would still call this turbo lag, but against the above definition it really is. Because it is perceived by the driver as such.

Its been a very long time since I looked into the design of turbo's and even then it was marine diesel application. They are a lot bigger, but a lot less complicated then modern car turbo's.

I have been involved in a few cases where friends reported enhanced turbo lag on their cars. Obviously, what they felt is not necesserily due to additional turbo lag, but could be due to other factors as well. But a waste gate that isn't functioning properly could cause such symptoms. A sticky waste gate, or slow to close due to dirt can cause similar effects.

Jeroen







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Old 12th April 2014, 19:57   #18
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Default Re: Turbo lag v/s Boost threshold

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Originally Posted by Jeroen View Post

Here's my take ..
Here's mine - am trying to simplify it further: The question permitted varying answers dependent on the definition of "earlier" and here are 2 scenarios (for both maximum boost is assumed @ 4500 rpm and boost threshold @ 1500 rpm):

(a) If the wastegate opens at 4500 rpm under normal operation (maximum boost is achieved) and is mechanically / electronically induced to open "earlier" at 4000 rpm then the answer would be no increase in turbo lag or difference in boost threshold.

(b) In the same scenario, if the wastegate were to be mechanically / electronically induced to open "earlier" at 1000 rpm, (with boost threshold @1500 rpm) then there would be substantially higher turbo lag and an increase in boost threshold.

Note: the opening of the wastegate @ 1000 rpm is assumed regardless of maximum boost for the purpose of the discussion.
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Old 13th April 2014, 10:00   #19
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Default Re: Turbo lag v/s Boost threshold

Quote:
Originally Posted by Jeroen View Post
Yes, that is a different way of phrasing the same question
Haha. No. That was part answer/part question paraphrased as a question.

Quote:
Originally Posted by VeyronSuperSprt View Post
Here's mine - am trying to simplify it further: The question permitted varying answers dependent on the definition of "earlier" and here are 2 scenarios (for both maximum boost is assumed @ 4500 rpm and boost threshold @ 1500 rpm):

(a) If the wastegate opens at 4500 rpm under normal operation (maximum boost is achieved) and is mechanically / electronically induced to open "earlier" at 4000 rpm then the answer would be no increase in turbo lag or difference in boost threshold.

(b) In the same scenario, if the wastegate were to be mechanically / electronically induced to open "earlier" at 1000 rpm, (with boost threshold @1500 rpm) then there would be substantially higher turbo lag and an increase in boost threshold.

Note: the opening of the wastegate @ 1000 rpm is assumed regardless of maximum boost for the purpose of the discussion.
Let's just reiterate again what boost threshold and turbo lag is.

Boost Threshold: the point where the inlet manifold goes from vacuum/negative pressure to positive pressure.
Lag: The time taken to achieve max boost from the point boost threshold is achieved.

I find the idea of waste gate actuation linked to rpm wrong, even though it is just for discussion.
This discussion about lag cannot be complete without taking into account target boost and at what rpm it is achieved and the fact that target boost is achieved at different rpm in different gears and engine loads.

If you are opening the waste gate at 3500 rpm, then when do you shut it?

Let's re look at the above example with wastegate actuation linked to boost pressure.
For sake of discussion we will presume boost to be 12psi @4500 rpm, 11psi @4000 rpm and 8 psi @3000rpm

If the wastegate opens at 12psi under normal operation (maximum boost is achieved) and is mechanically / electronically induced to open "earlier" at 11 psi then the answer would be no difference in boost threshold but lag time would be decreased since max boost (11psi) is being achieved 500 rpm earlier.

Now, if we make the waste gate to open when it sees 8 psi, then the system achieves max boost at only 3000 rpm, reducing lag time even further.

My 2 cents, FWIW.

Cheers
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Old 14th April 2014, 08:22   #20
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Default Re: Turbo lag v/s Boost threshold

I think the difference in definition makes for a somewhat different outcome.

I don't think there is a standardization body that deals with the exact definition for boost threshold and or turbo lag, so anything goes and does.

I think you're definition is more technical, especially on turbo lag whereas mine is more as how it is perceived the driver. So for me, boost threshold, or the turbo spinning up to threshold could just as well be part of the turbo lag. I put my foot down and until something starts happening it's all lag. That would even include any mechanical play in linkage etc.

You are correct in that lowering the opening pressure would reduce lag time. My thoughts are more around how a driver would perceive it. He/she might feel it as more lag as the engine doesn't pick up as quickly or as aggressively as before.

When diagnosing problems on a car, sometimes it can be very enlightening how drivers perceive something and what the underlying problem could be.

Just because a car is slow to pick up doesn't mean you need to replace the turbo and or ECU as I've seen suggested by some members. In fact, those are probably the last components on a car to fail.

Sticky or slow closing waste gates, might give this impression that the engine is suffering from turbo lag. Consider this scenario; accelerating fast, the waste gate is opening and at that moment you come of the throttle only to press on it again say 3-4 seconds later. The waste gate is sticky/slow to close. Technically, as per your post, lag is less, but the driver will experience a longer lag time, as the waste gate closes slowly.

Jeroen
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Old 14th April 2014, 20:31   #21
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Default Re: Turbo lag v/s Boost threshold

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Originally Posted by Jeroen View Post
Well, you can't just switch a supercharger on/off without it having being designed so. I certainly can't switch it of in my Jaguar. It would require at the minimum a clutch so you can disconnect the supercharger from whatever drivers it, e.g. pulleys, belts etc.
Jeroen
IIRC, most (if not all) the interwar Mercedes which were supercharged were clutched. The Stutz too, IIRC.
Anecdotically, that is how the Bentley Boys knew if the Mercedes's were being hard pressed.

Quote:
Originally Posted by Jeroen View Post
Now, one for the turbo-nerds: does opening the waste gate earlier introduce more/less turbo lag or lowers/increase the boost threshold or both?
From the butt dyno/ seat of pants feel, will go with Gotzuk. Essentially because one is reaching xx% of full power earlier. Because the engine has been neutered!
If everything else remains the same (big cover backsides qualifier!), and one goes by measured values, rather than feel, lag will remain the same.

If one is really clever, I'm sure one can keep the lag, and neuter the engine!

Regards
Sutripta

PS. In a properly sized system, do we really need a wastegate? Any difference between petrols and diesels?
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Old 14th April 2014, 21:44   #22
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Default Re: Turbo lag v/s Boost threshold

[quote=Sutripta;3413180]
(big cover backsides qualifier!)/QUOTE]

The qualifier certainly helps - taking into account the completely different definitions on turbo lag.
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Old 15th April 2014, 07:14   #23
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Default Re: Turbo lag v/s Boost threshold

Quote:
Originally Posted by Sutripta View Post

From the butt dyno/ seat of pants feel, will go with Gotzuk. Essentially because one is reaching xx% of full power earlier. Because the engine has been neutered!
If everything else remains the same (big cover backsides qualifier!), and one goes by measured values, rather than feel, lag will remain the same.

If one is really clever, I'm sure one can keep the lag, and neuter the engine!

Regards
Sutripta

PS. In a properly sized system, do we really need a wastegate? Any difference between petrols and diesels?

That's because with his definitions you are thinking more as an engineer then how things are perceived as a driver, i.e. the different definitions.

But I'm fine with Gotzuk answer, because technically it is correct.

Not sure doubt the need for waste gates in a properly sized system. Design tends to be a compromise, not sure what/how to optimize this one. I need to ponder on that for a while. But essentially waste gates are deployed to reduce lag, it will allow turbo's to spin up quickly and then you bleed off excess boost.

I guess if you under dimension the turbo there would not be a need for a waste gate, but you would be left with a lot of lag.

In general all these sort of problems are less of an issue for diesels then for petrol burners.

Jeroen
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Old 15th April 2014, 07:49   #24
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Default Re: Turbo lag v/s Boost threshold

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Originally Posted by Jeroen View Post
In general all these sort of problems are less of an issue for diesels then for petrol burners.
Can you please provide more explanation on this.
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Old 15th April 2014, 11:12   #25
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Default Re: Turbo lag v/s Boost threshold

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Originally Posted by Bookkeeper View Post
Can you please provide more explanation on this.
Sure, it's a bit of a generic statement, so I'm sure some clever clog will come up with exceptions or different point of view.

Diesel engine tend to have a much smaller bandwith of speed/rpm than petrol engines. your typcial car diesel engine revs between 1000 - 4500 RPM.

That also means that the turbo needs to accomodate smaller speed variations then it's petrol version. It allows for improving turbo lag on a diesel compared to petrol.

Admitedly, on modern diesel the above does get offset to some extent by the fact that these modern diesel are extremely efficient. So the exhaust temperatures are getting low too. So whereas the changes of volume of air going through the diesel through the rev range is less then for a petrol version, (good for lag) the low temperatures also mean less energy for the turbo (bad).

Also, going beyond just the turbo, petrol engines need to be designed around very narrow compression band. Although the compression ratio is fixed obviouly, the end compression temperature and pressure also depend on the turbo boost pressure. In most petrol engine that means you need to introduce some sort of anti-knock detection. Diesel engines are virtually detonation / knock free so less complex from that point of view.

Sure there is more to consider, but these are just some thoughts.

I was just reading about Audi introducing an electric driven bi-turbo to their R8. If everybody start doing that we wont have to worry about boost threshold and turbo lag. Does introduce more electronics I dare say. All for it, because it would probably mean you can monitor and trouble shoot your electric bi-turbo through the OBD port.

Jeroen
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Old 15th April 2014, 16:06   #26
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Default Re: Turbo lag v/s Boost threshold

Quote:
Originally Posted by Jeroen View Post
I was just reading about Audi introducing an electric driven bi-turbo to their R8. If everybody start doing that we wont have to worry about boost threshold and turbo lag. Does introduce more electronics I dare say. All for it, because it would probably mean you can monitor and trouble shoot your electric bi-turbo through the OBD port.
Jeroen
Thanks for your explanation.
The electronically controlled turbo on a car sounds interesting, will try and read more about it. I'm sure they'd need a pretty high end system to keep it running.

This kind of a setup would employ electric motors to spin the turbos, right? I'm guessing that the motor rpm will be in sync with the throttle application so as neutralize any lag.

I remember reading your earlier post about auxiliary turbos driven via electric motors and used in marine applications, how were the motors controlled? Were they controlled independently or one just had to flick a switch / throw a lever and the system would regulate itself?
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Old 15th April 2014, 20:28   #27
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Default Re: Turbo lag v/s Boost threshold

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Originally Posted by Bookkeeper View Post
This kind of a setup would employ electric motors to spin the turbos, right? I'm guessing that the motor rpm will be in sync with the throttle application so as neutralize any lag.

I remember reading your earlier post about auxiliary turbos driven via electric motors and used in marine applications, how were the motors controlled? Were they controlled independently or one just had to flick a switch / throw a lever and the system would regulate itself?

I haven't seen the details, but yes I assume the electric motors rpm would be closely in synch with the throttle application, position and rate of movement. The interesting thing of a (DC) direct current e-motors is they can develop huge torque in an instant, even from standstill. So very little if any lag.

Also, you should be able to adjust the boost pressure much more easily and maybe not need waste gates at all. You might have to introduce some extra sensors on the engine and a clever control box, but in theory this set up should give you the most optimum turbo performance you can imagine. So maybe that's the ultimate answer to Sutripta question about optimum sizing as well?

The marine application I'm familiar with, is just an extra turbo blower, driven by an e-motor and it was controlled through an on/off switch. I seem to recall we could also put it on auto. Not sure how it got triggered then, by rpm or inlet pressure.

What we did is, when we were expecting to be maneuvering at very low rpm for prolonged periods we would just switch it on. Bear in mind, this was on a 12.000 BHP, two stroke, Diesel. As I said, part of why the reason of adding a separate turbo on these low rpm's is that two strokes, especially the very large ones, have difficulties in purging out the exhaust fumes. There is no 'exhaust stroke' as on a four stroke. Getting rid of the exhaust and putting fresh air into the cylinder needs to be done in an instant, when the piston is around the lowest position more or less by differential pressure over the inlet and exhaust port and or valves. So at those very low rpm's, it needed a bit of help, hence the extra e-motor driven turbo.

Hope this helps

Jeroen
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Old 16th April 2014, 21:45   #28
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Default Ot

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Originally Posted by Jeroen View Post
Design tends to be a compromise,
We can take the compromise thing as a given, and not really dwell on it.
And I would go so far as to say not tends, is. (A very strong view in a world where we hedge by not using words like ever/ never/ always/ etc.)

(Have mentioned this compromise aspect a number of times. A search brought up this
http://www.team-bhp.com/forum/4x4-ve...ml#post1742114 (Mahindra Thar revealed at Autoexpo 2010))

Any thoughts on modeling this turbo lag? System response to a step change (in input)?

Regards
Sutripta
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Old 17th April 2014, 07:31   #29
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Default Re: Ot

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Originally Posted by Sutripta View Post
And I would go so far as to say not tends, is. (A very strong view in a world where we hedge by not using words like ever/ never/ always/ etc.)

Any thoughts on modeling this turbo lag? System response to a step change (in input)?
You know me, I weigh me words very carefully, never try to be direct, in order not to upset anyone!

Shouldn't be difficult to model. Especially on an electric driven turbo it is a real straightforward response, very precise, very predictable, only a few variables.
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Old 17th April 2014, 20:37   #30
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Default Re: Ot

Quote:
Originally Posted by Jeroen View Post
You know me, I weigh me words very carefully, never try to be direct, in order not to upset anyone!
??

Quote:
Especially on an electric driven turbo
Was thinking about the traditional setup.
Then model/ factor in a wastegate.

Regards
Sutripta
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