[vina]

Everyone, sorry for bumping the thread after almost a month of inactivity, but here's a question:

At very low rpm, when the turbo is not yet spooling, how does a diesel engine breath? do turbochargers have something like "bypass valve" that allows air to go around them when the engine is creating partial vaccum and turbo has not yet spooled up?

[dhanushs]

Originally Posted by vina

At very low rpm, when the turbo is not yet spooling, how does a diesel engine breath? do turbochargers have something like "bypass valve" that allows air to go around them when the engine is creating partial vaccum and turbo has not yet spooled up?

AFAIK, the turbo is spinning, but not generating enough boost in low rpm's. The engine being a 'sucker', should suck the required amount of air. And at idle, there is the IAC valve.

Here's two video's with turbo spinning at idle.

YouTube - ‪Turbo at Idle‬‏

YouTube - ‪K24 Turbo Idle‬‏

[mohan41]

I still dont understand one thing. The 1.3 MJD 90BHP engine supposedly has significant turbo lag (I havent driven one yet, the only diesel I have ever driven is Figo).

Attached is the torque/power graph of this engine, I searched for it recently I found it in this website: Fiat Grande Punto: all engines output and torque graphs - car magazine, auto reviews at www.autopressnews.com. I believe this is what Fiat has published, as I had seen it in some website of Fiat also earlier.

According to this, the torque and power both increase linearly (almost) from 1000 RPM onwards upto about 1700RPM when torque reaches maximum. If there is a turbo lag how is this possible? Shouldn't the torque and power go up only slightly till some RPM when turbo activates, and then go up very rapidly after that RPM. Is this output just a theoritical one, or some setup that uses a turbocharger that activates at 1000RPM, or a turbo charger that is always active?

[vina]

Originally Posted by mohan41

I still dont understand one thing. The 1.3 MJD 90BHP engine supposedly has significant turbo lag (I havent driven one yet, the only diesel I have ever driven is Figo).

Attached is the torque/power graph of this engine, I searched for it recently I found it in this website: Fiat Grande Punto: all engines output and torque graphs - car magazine, auto reviews at www.autopressnews.com. I believe this is what Fiat has published, as I had seen it in some website of Fiat also earlier.

According to this, the torque and power both increase linearly (almost) from 1000 RPM onwards upto about 1700RPM when torque reaches maximum. If there is a turbo lag how is this possible? Shouldn't the torque and power go up only slightly till some RPM when turbo activates, and then go up very rapidly after that RPM. Is this output just a theoritical one, or some setup that uses a turbocharger that activates at 1000RPM, or a turbo charger that is always active?

There are two components to turbo-lag - the spooling of the turbojet (via exhaust gas mass - depends on both the volume of gas flowing i.e. rpm, and the position of TPS sensor - i.e. pedal) AND time taken to build pressure in the manifold.


AFTER the turbo is getting enough exhaust, its own rpm (it is a turbine that rotates much faster than engine's shafts) increases - that takes time. Then once the turbine is rotating its compressor section send air to the manifold and increases pressure there - that also takes time.

So if somebody is starting from a light just turned green, he will see car giving some pickup and then VROOM the moment all of the above have happened - that is what most people call turbo lag.



The curves you have are drawn with fully depressed pedal and engine run at constant rpm - they have very little (but not nothing) to do with real applications

[mohan41]

Originally Posted by vina

...
The curves you have are drawn with fully depressed pedal and engine run at constant rpm - they have very little (but not nothing) to do with real applications

That makes sense. So if I understand this, they get the throttle into full open position but somehow restrict engine RPM to the value at which torque has to be measured (1000, 1500, 2000 ....) and go past turbo lag time and then measure the torque output. In reality, we cannot do this. The real world torque curve would then depend on the RPM at which the turbo fully kicks in (assuming there is a fixed RPM of turbo kick-in). If that RPM is more than the peak torque RPM then we will see a massive lag, is my understanding right?

This also means the published torque/power graph is not dependent on whether the turbo is a VGT or FGT, right? Whereas, the real world behavior would be different based on VGT or FGT.

How do different manufacturers using this same engine get differing behavior, still using VGT? SX4 it is claimed has less lag than Linea and Manza (according to reviews).

[vina]

Originally Posted by mohan41

That makes sense. So if I understand this, they get the throttle into full open position but somehow restrict engine RPM to the value at which torque has to be measured (1000, 1500, 2000 ....) and go past turbo lag time and then measure the torque output. In reality, we cannot do this.

You are right - the instrument that keeps the rpm constant (and measure torque) is called dynamometer

Originally Posted by mohan41

The real world torque curve would then depend on the RPM at which the turbo fully kicks in (assuming there is a fixed RPM of turbo kick-in). If that RPM is more than the peak torque RPM then we will see a massive lag, is my understanding right?

Sort of.

what happens is this: somebody in Punto at red signal floors the throttle as signal turns green. Next thing he sees is very low power (or torque) since the rpm wasn't much, engine in not generating power. This means he accelerates slowly, and rpm increases slowly.

This also means that engine will be very efficient in converting all the heat generated into work - not much will be left in the exhaust to spool the input turbine of the turbo.

Around the time the rpm reaches 1500 or so, the flow rate and waste heat in the exhaust are enough that the turbo starts to pick up speed (again slowly still - peak torque rpm is still 1700-1800) By the time the rpm hits 1700/1800 (a second or two) the turbo has high rpm and has been compressing the air into the intake manifold.


Suddenly you can see very high sustained torque, and must take your foot off the pedal (or you crash into the biker in front )


This is the usual perception of turbo lag.

Originally Posted by mohan41

This also means the published torque/power graph is not dependent on whether the turbo is a VGT or FGT, right? Whereas, the real world behavior would be different based on VGT or FGT.

How do different manufacturers using this same engine get differing behavior, still using VGT? SX4 it is claimed has less lag than Linea and Manza (according to reviews).

VGT etc. tend to flatten the torque curve out - imagine two curves with peaks at 1200rpm and 2200 rpm and superimpose them - you'll get very flate response between the two extremes. But they do not eliminate turbo lag. Nothing can.

The engines with less turbolag (Figo, Logan/Micra) are the ones that have tuned the peak-torque rpm to lower numbers. So turbo spools very early and there is no perception of turbo lag (at very low speeds usually diesels generate way more torque than you reasonably need even without turbocharger - by the time turbo lag can hit you, you'll be changing gears anyway)


All this is based on my reading on the internet and experience on my Figo and friends' Swift and Indica - so some bits can be wrong.

[dhanushs]

Originally Posted by mohan41

According to this, the torque and power both increase linearly (almost) from 1000 RPM onwards upto about 1700RPM when torque reaches maximum. If there is a turbo lag how is this possible? Shouldn't the torque and power go up only slightly till some RPM when turbo activates, and then go up very rapidly after that RPM.

The area from idle rpm to 1700-1800 rpm is called turbo lag. Ideally, for a car with minimal turbolag, its engine should generate enough torque (pulling power) right from the idle rpm. In a logan 1.5 dci mill, there is very minimal turbo lag. The difference between torque @idle rpm and max torque is less in logan (minimal turbo lag) but huge in 1.3 mjd.


You cannot judge pulling power just from a graph. Again, even the numbers do little justice. Please take a look at - http://www.team-bhp.com/forum/techni...ake-sense.html

Originally Posted by vina

There are two components to turbo-lag - the spooling of the turbojet (via exhaust gas mass - depends on both the volume of gas flowing i.e. rpm, and the position of TPS sensor - i.e. pedal) AND time taken to build pressure in the manifold.

As I mentioned above, isnt the turbo always spooling.

Originally Posted by mohan41

This also means the published torque/power graph is not dependent on whether the turbo is a VGT or FGT, right? Whereas, the real world behavior would be different based on VGT or FGT.

Power and torque curve IS different for same engines with fixed geometric and variable geometric turbo.

Originally Posted by mohan41

How do different manufacturers using this same engine get differing behavior, still using VGT? SX4 it is claimed has less lag than Linea and Manza (according to reviews).

Originally Posted by vina

VGT etc. tend to flatten the torque curve out - imagine two curves with peaks at 1200rpm and 2200 rpm and superimpose them - you'll get very flate response between the two extremes. But they do not eliminate turbo lag. Nothing can.

VGT = Variable Geometry turbo.

A fixed geometric turbo can generate some amount of boost only at a given rpm. A bigger turbo generates good boost, but at higher rpms and a smaller turbo generates less boost, but a much lower rpms.

Using a VGT, manufacturers try to get the best out of both worlds. ie, low end driveability, and high rpm boost.

Originally Posted by vina

You are right - the instrument that keeps the rpm constant (and measure torque) is called dynamometer

rpm constant?

Originally Posted by vina

usually diesels generate way more torque than you reasonably need even without turbocharger - by the time turbo lag can hit you, you'll be changing gears anyway

Turbo lag is fun, but is a curse whist city overtaking.

EDIT: Removing wrong info (image) about 1.5 dci mill.

[dhanushs]

Here's the torque curve of the 1.5 dci mill. We can see that, there is about 110NM of torque available at 1100rpm ie, about ~68.75% of max torque is available way low down the rpm. Where as in 1.3mjd its just about 50% of max torque.

But then, again, numbers don't do justice. Real world performance of both engines are way different.

[vina]

Originally Posted by dhanushs

You cannot judge pulling power just from a graph. Again, even the numbers do little justice. Please take a look at - http://www.team-bhp.com/forum/techni...ake-sense.html


As I mentioned above, isnt the turbo always spooling.

What I wrote was not for your consumption. Anyway, since you have jumped in - turbo's job is to compress the air into the intake manifold. This is a mass transfer thing and will as a result always take some time. I think I wrote about this earlier.

Power and torque curve IS different for same engines with fixed geometric and variable geometric turbo.




VGT = Variable Geometry turbo.

A fixed geometric turbo can generate some amount of boost only at a given rpm. A bigger turbo generates good boost, but at higher rpms and a smaller turbo generates less boost, but a much lower rpms.

Using a VGT, manufacturers try to get the best out of both worlds. ie, low end driveability, and high rpm boost.

You are right, and my comment was the reason why - VGT has effectively two (multiple actually) turbine geometries - smaller geometry for low rpm, large geometry for high rpms.

the overall effect is similar to overlap of all these geometries



rpm constant?

Isn't that right? Dynamometer will restrict the rpm from going any higher than what you have dialed in and then measure how much torque it needs to keep the rpm from increasing. At least that's what I thought (and wikipedia description seems to agree)


Turbo lag is fun, but is a curse whist city overtaking.

EDIT: Removing wrong info (image) about 1.5 dci mill.

(above post) Is this a fair comparison - Isn't the 90bhp punto engine smaller.

By the way, how does this engine (and MJD used in TATA) achieve a perfectly flat torque curve in the middle? Do you know of any electronic controls?

[vina]

This post http://www.team-bhp.com/forum/test-d...ml#post2428817

The author claims that because the engine has more displacement (for lesser power) there is lesser turbo lag.

Now Figo also has very little turbo lag and the engine again is larger than the MJD and produces less power.


So what does displacement have to do with turbo lag - this is not even turbocharger displacement

[dhanushs]

Originally Posted by vina

So what does displacement have to do with turbo lag - this is not even turbocharger displacement

I don't think the points mentioned are from concrete sources.

" turbo lag can be even worked out well with proper turbo tuning and gear ratios like Fiat & Tata have done in their cars." The 1.3 mjd is famed for its turbo lag. Moreover, Fiat punto 1.3 mjd sucks on lower gears (bad gear ratios).

I think he meant, that larger engines have more torque @low rpms, and hence driveability. Since lack of driveability is somewhat = to turbo lag.

I personally think that the 1.5 dci engine in the logan is very much practical for a car of that that segment/price.

[SYSTEMIZED]

Hello members,
Anyone having experience with TATA ARIA`s turbo lag. I am planning to buy a used one for hilly trips (Leh expeditions) and vacations but my known experts says it has turbo lag @ 1000 rpm. I am driving Renault Duster RxZ 110PS and satisfied with its negative turbo lag. So how is it if compared with TATA`s ARIA?

[latentpotential]

I have a vehicle with the 1.3 MJD VGT 90Bhp.

Using a tuning chip on this engine can also help in smoothing out the power output and giving a more linear power delivery. With the settings I have today on the chip in use, turbo lag is no longer felt during drives, even as a passenger.

[SYSTEMIZED]

Originally Posted by latentpotential

I have a vehicle with the 1.3 MJD VGT 90Bhp.

Using a tuning chip on this engine can also help in smoothing out the power output and giving a more linear power delivery. With the settings I have today on the chip in use, turbo lag is no longer felt during drives, even as a passenger.

Hello latentpotential! Does tuning chip have any negative impact on engine?

[latentpotential]

Originally Posted by SYSTEMIZED

Hello latentpotential! Does tuning chip have any negative impact on engine?

There is no 100% answer to this question. A good chip, used within the boundaries of the engines final capability will not harm anything.

If warranty, yes, it does void warranty.

There are several other threads where this has been discussed, look up race dymanics, race chip, petes tuning etc.