53 replies to this topic

[Victor_RO]

As Mulsanne's Corner put it extremely well, "Audi has a habit of doing this following a Le Mans win, <<Oh by the way, we were using this cool new technology.>>" This time, it was variable-geometry/variable-vane turbochargers on the V10 TDI engine in the back of the R15+.

What are the detailed technical challenges of VGTs on a racing diesel? Also, Audi and Peugeot tend to use the same turbo manufacturers, so, obviously, there's a rumour/possibility that fitting these turbos pushed the Peugeot V12 to breaking point and caused the triple failure which shot their challenge down in the final hours of the race. Any opinions on that aspect?

[primer]

A slight hijack: does anyone know if VGT are used in Euro CV diesels? AFAIK Scania, Volvo, Mercedes DAF et al do not use VGT, just an 'ordinary' turbo?

It seems strange that they are so prevalent in cars, yet are not relaible enough for trucks and busses.

Edited by primer, 15 June 2010 - 15:21.

[J. Edlund]

As Mulsanne's Corner put it extremely well, "Audi has a habit of doing this following a Le Mans win, <<Oh by the way, we were using this cool new technology.>>" This time, it was variable-geometry/variable-vane turbochargers on the V10 TDI engine in the back of the R15+.

What are the detailed technical challenges of VGTs on a racing diesel? Also, Audi and Peugeot tend to use the same turbo manufacturers, so, obviously, there's a rumour/possibility that fitting these turbos pushed the Peugeot V12 to breaking point and caused the triple failure which shot their challenge down in the final hours of the race. Any opinions on that aspect?

To make the turbochargers survive the higher temperature would be the greatest challenge. Usually the variable vanes tend to become sluggish and stick at temperatures much above 800 degC.

They have been banned for Le Mans racing in the past I believe, so usually Garrett TR30R racing turbochargers have been used.

A slight hijack: does anyone know if VGT are used in Euro CV diesels? AFAIK Scania, Volvo, Mercedes DAF et al do not use VGT, just an 'ordinary' turbo?

It seems strange that they are so prevalent in cars, yet are not relaible enough for trucks and busses.

Variable geometry turbochargers are used on several commerial diesel engines, Scania for instance use them, although mostly to be able to control the exhaust pressure for EGR control.

[TDIMeister]

VGT turbochargers have been permitted in the ACO LMP1 rules since Audi's entry of the R10 in 2006. With this in mind, there was a lot of speculation whether the R10 was equipped with this or not, but having read a definitive paper by Audi's Ulrich Baretzky and closely seen and touched the car/engine at various venues like the 2006 Essen Motor Show, 2007 Performance Motorsport World Expo in Cologne and 2008 24H at Spa Francorchamps, I can be absolutely certain that the R10 did not. I can't speak for the Peugeot 908 HDI FAP though.

Aside from that, it is interesting speculation that either one or both Peugeot and Audi are running steel pistons instead of aluminum.

[TDIMeister]

There are some trucks that use VGTs. Apart from Cummins, Hino and International, I can't list other specific examples though.

[J. Edlund]

VGT turbochargers have been permitted in the ACO LMP1 rules since Audi's entry of the R10 in 2006. With this in mind, there was a lot of speculation whether the R10 was equipped with this or not, but having read a definitive paper by Audi's Ulrich Baretzky and closely seen and touched the car/engine at various venues like the 2006 Essen Motor Show, 2007 Performance Motorsport World Expo in Cologne and 2008 24H at Spa Francorchamps, I can be absolutely certain that the R10 did not. I can't speak for the Peugeot 908 HDI FAP though.

Aside from that, it is interesting speculation that either one or both Peugeot and Audi are running steel pistons instead of aluminum.

Both are using steel pistons these days.

Audis diesel V12's ran aluminum pistons from Mahle Motorsport, they are made from an 4032 based alloy with a higher content of alloying elements than regular 4032 alloy (Mahle call this alloy 142). The pistons are forged and a steel oil gallery is welded to a cast iron top ring carrier which is then electron beam welded to the piston. It is cooled by two spray jets, one directed at the oil gallery and the second one at the underside of the piston. The skirt is graphite coated.

Audis V10 diesel ran steel pistons from the beginning, infact it was what did the V10 possible. The larger diameter pistons of the V10 required steel due to the higher piston load caused by the diameter increase. Peugeot used steel pistons from 2008, though they were offered for both teams Audi decided to wait.

[J. Edlund]

A picture of a variable geometry turbocharger, taken from Scanias website. A Holset (Cummins) turbocharger I believe.



Higher resolution version: http://imagebank.sca...e/06719-001.jpg
As installed on an engine: http://imagebank.sca...e/07600-035.jpg

Edited by J. Edlund, 20 June 2010 - 21:14.

[TDIMeister]

@J.Edlund, do you have a source or paper that you could disclose or share which states the piston construction ? I would be interested to read up on it.


Cheers,

[OfficeLinebacker]

A picture of a variable geometry turbocharger, taken from Scanias website. A Holset (Cummins) turbocharger I believe.



Higher resolution version: http://imagebank.sca...e/06719-001.jpg
As installed on an engine: http://imagebank.sca...e/07600-035.jpg

forgive me for being dense...what's the part that actually "varies?"

[primer]

forgive me for being dense...what's the part that actually "varies?"

The vanes. But in that Scania image I cannot see how the vanes could move, that looks more like a variable volume turbocharger.

[dosco]

forgive me for being dense...what's the part that actually "varies?"

Usually it would be the inlet guide vanes, which I don't see illustrated.

I think it was in another thread that someone mentioned variable exhaust guide vanes ... the vanes are on the right side of the illustration, however it is difficult to say if they are variable.

[dosco]

The vanes. But in that Scania image I cannot see how the vanes could move, that looks more like a variable volume turbocharger.

What makes you think it is "variable volume?"

[primer]

What makes you think it is "variable volume?"

When I made that post, I could not see from the Scania's image how the vanes could be controlled. That, and the design of the shaft suggested that the the entire vane assembly could be moved on the axis to alter volume. I had confused the compressor for the turbine housing on the turbocharger.

This has kept me intrigued and after your post I googled and discovered this:

The Holset VGT uses a turbine stage where the swallowing capacity is automatically varied while the engine is running. This permits turbine power to be set, providing sufficient energy to drive the compressor at the desired boost level wherever the engine is operating. This is achieved by varying the area of a nozzle; a set of guide vanes that control the flow of exhaust gas through the turbine.

Conventional designs pivot the vanes to achieve different nozzle areas. The Holset VGT is extraordinary in that the vanes do not pivot but slide axially; this design ensures a high level of durability and reliability, essential for today's commercial diesel applications.

The sliding nozzle ring alters the aperture through which the exhaust gases flow onto the turbine wheel. This alteration in the geometry of the turbocharger increases the boost as the nozzle is closed down. Reducing the aperture increases exhaust manifold pressure and increases the turbocharger speed. As the nozzle ring opens up, the exhaust pressure reduces and the turbocharger boost decreases. In effect it creates an infinite number of fixed geometry turbochargers.

I had no idea this kind of VGT existed. I thought that controlling the angle of the vanes was the only way, and this also explains why I thought they were never used in commercial diesels (I recall reading somewhere that there are (or were) problems with long term durability of the movable vane mechanism itself).

[Tony Matthews]

When I made that post, I could not see from the Scania's image how the vanes could be controlled. That, and the design of the shaft suggested that the the entire vane assembly could be moved on the axis to alter volume. I had confused the compressor for the turbine housing on the turbocharger.

So, are we saying that the narrow disc, like a small water-wheel, to the right of the shaft and bearing assembly, is the exhaust turbine, and can move in or out on the shaft to expose more or less of the vanes? In the illustration I can't see that it can move out much further, but it could move in (left) I suppose.

[primer]

Thread needs turbo pron.

[dosco]

I had no idea this kind of VGT existed. I thought that controlling the angle of the vanes was the only way, and this also explains why I thought they were never used in commercial diesels (I recall reading somewhere that there are (or were) problems with long term durability of the movable vane mechanism itself).

Wikipedia talks about it: http://en.wikipedia....ry_turbocharger

Turns out the variable intake vanes that I am familiar with are not how turbos are normally managed (according to Wiki they are managed on the hot side).

I do recall seeing something about turbos with varibale intake (cold side) vanes, but I can't recall for the life of me where I originally saw it. Go figure.

[dosco]

So, are we saying that the narrow disc, like a small water-wheel, to the right of the shaft and bearing assembly, is the exhaust turbine, and can move in or out on the shaft to expose more or less of the vanes? In the illustration I can't see that it can move out much further, but it could move in (left) I suppose.

The turbine doesn't move axially, the vanes do. If I understand the Wikipedia entry, the guide vanes are not a uniform shape (they are sort of "notched") and control of the turbo is realized by moving the vanes in and out of the hot gas flow. Since the vanes are not uniform in shape, varying how much vane is exposed controls the amount of vane area presented to the hot gas ... thus pressure in the turbine inlet can be manipulated and control of the turbine speed is accomplished.

Edited by dosco, 21 June 2010 - 17:37.

[Tony Matthews]

Thanks to primer posting the cross-sections all is clear! Just goes to show that trick computer-generated illustrations aren't as good at conveying information as a regular cave-drawing.

[primer]

But this design looks so challenging to manufacture! In particular, what happens around the circled (sort of) areas?

You cannot have a vaccum and a closed seal behind the disc ("nozzle ring" in Holset speak), and it must be free to constantly move in and out as commanded. So how do they seal the exhaust gases from leaking through? Tolerances must be absurd, and the aerodynamic interaction between the edges of vanes and disc critical. This has more in common with aircraft engines than automobiles! How can they mass produce and sell these things?

Holset must have stolen engineers and IP from Rolls Royce Group.

[Tony Matthews]

Can't see a major problem.

[J. Edlund]

@J.Edlund, do you have a source or paper that you could disclose or share which states the piston construction ? I would be interested to read up on it.


Cheers,

I've got the information from several 'Race Engine Technology' articles, particulary the November 2008 issue where steel pistons are discussed with Mahle Motorsport. The design of the Audi diesel V12 is in the August 2007 issue but this was at the time when Audi ran aluminum pistons.

Basically Mahle Motorsport states that the piston is machined from solid in an undisclosed high strength steel alloy. The piston is pre-machined, heat treated, final machined and assembled. Compared to an aluminum piston it saves 10% weight and it has a shorter compression height, a shorter and smaller diameter piston pin, but the regular three piston rings. The piston is grafal coated and the rings run directly in the piston, there are no microwelding issues. It does not have a full skirt and the piston pin runs directly in the piston, hold in place by circlips. The top ring is PVD coated and made of steel with a barrel form working face, the second ring is iron or steel. The top ring is described as more then 2 mm deep. The piston works well together with a Nikasil liner; Audi uses a linerless Nikasil coated block. Peugeot uses cast iron liners, although it is not known if this is without a bore coating.

The piston pictured in the article could be described as a more compact and fully machined version of the Mahle Monotherm piston.
http://www.us.mahle....erm_221x174.jpg

forgive me for being dense...what's the part that actually "varies?"

Holset have an animation on their website: http://www.holset.co...2_5_1_5-VGT.php

[TDIMeister]

Thanks, I have a precious few copies of RET, and among the ones I'm missing are those you mentioned.

As for the subject of VGTs, the sliding ring method illustrated above is not common in automotive turbochargers. Rather, they use radially-moving guide vanes to change the angle of attack of the incoming exhaust gas against the turbine wheel in the same manner one can replicate with any number of means.

Hope the image below works:

[TDIMeister]

There are also variably-adjustable compressors (the diffuser vanes change angle, analogous to the animated image I posted above, but this time in the compressor and not in the turbine), but this has not yet found application in series passenger cars and trucks that I'm aware. There are also pre-swirl devices that mount upstream of the compressor inlet in development to "throttle" or give variable compressor operating points.

[OfficeLinebacker]

Thanks, I have a precious few copies of RET, and among the ones I'm missing are those you mentioned.

As for the subject of VGTs, the sliding ring method illustrated above is not common in automotive turbochargers. Rather, they use radially-moving guide vanes to change the angle of attack of the incoming exhaust gas against the turbine wheel in the same manner one can replicate with any number of means.

Hope the image below works:

Brilliant. Thank you for that perfect graphical explanation.

[mariner]

These pictures show the exhaust vane area changing, IIRC Toyota cheated in the Rally championship by secretly doing a similar thing on the compressor entry. It was a lovely peice of enginering but completely and knowing illegal from the beginning!

BTW whilst on high output/litre diesels I have recently read an article on injecting liquid LPG into a diesel for a considerable ncrease in power or economy plus potential NOX reduction.

I think the logic is that the LPG burns using the excess air in the combustion chamber per the diesel principle and whilst it has to be ignited by the diesel flame front it burns faster so " catches up".

I am not entirely convinced about the accuracy of the claims so has anybody heard of LPG injection as a diesel boost technique? The article implied that it was not uncommon in trucks as the benefits allowed for the high installation costs of seperate injectors and ECU.

[TDIMeister]

Considerable increase in power, yes, potentially. Economy is relative: on an energy basis you will use about the same to get a given engine brake output. If a larger share of it comes from propane, there is a running dollar cost saving since propane is cheaper than Diesel, weight for weight. This ignores the cost of the conversion and added hardware.

I don't agree with reduced NOx... there are two mechanisms at play: introducing propane will reduce the running air excess, sure. At full load, a modern diesel engine is running at about 20-40% air excess, but will never approach or go richer than stoichiometric on account of black smoke. With propane, you might bring the air excess down to about 10% before unacceptable smoke; this happens to be where NOx emissions actually peak. Secondly, the propane is introduced as a premixed mixture into the Diesel engine. The premixed phase of combustion will dominate; this is characterised by very high heat release rates, high rates of pressure rise and a distinct change in the tone of the running engine as a result. NOx increases with premixed combustion in the absence of substantial EGR.

[TDIMeister]

To amend my post re propane injection, you have to be careful not to increase the percentage of propane too much, as this will lead to dangerously high peak firing pressures. Pre-ignition can still occur with a rich-enough pre-mixture of propane under prevailing compression temperatures in a Diesel engine and the presence of any hot-spots...

[dosco]

But this design looks so challenging to manufacture! In particular, what happens around the circled (sort of) areas?

All this stuff is pretty straighforward WRT manufacturing.

You cannot have a vaccum and a closed seal behind the disc ("nozzle ring" in Holset speak), and it must be free to constantly move in and out as commanded. So how do they seal the exhaust gases from leaking through? Tolerances must be absurd, and the aerodynamic interaction between the edges of vanes and disc critical. This has more in common with aircraft engines than automobiles! How can they mass produce and sell these things?

Holset must have stolen engineers and IP from Rolls Royce Group.

Not sure how they seal the thing, I'd speculate it is some sort of carbon seal but perhaps someone on the board may actually know the answer.

The tolerances on this sort of equipment are tight but are easily realized. Garrett (which eventually evolved into AlliedSignal now Honeywell) cranks out larger versions of this sort of stuff routinely. Not rocket surgery by any means.

[OfficeLinebacker]

Not rocket surgery by any means.

LOL.

Edited by OfficeLinebacker, 22 June 2010 - 21:36.

[Lee Nicolle]

I've got the information from several 'Race Engine Technology' articles, particulary the November 2008 issue where steel pistons are discussed with Mahle Motorsport. The design of the Audi diesel V12 is in the August 2007 issue but this was at the time when Audi ran aluminum pistons.

Basically Mahle Motorsport states that the piston is machined from solid in an undisclosed high strength steel alloy. The piston is pre-machined, heat treated, final machined and assembled. Compared to an aluminum piston it saves 10% weight and it has a shorter compression height, a shorter and smaller diameter piston pin, but the regular three piston rings. The piston is grafal coated and the rings run directly in the piston, there are no microwelding issues. It does not have a full skirt and the piston pin runs directly in the piston, hold in place by circlips. The top ring is PVD coated and made of steel with a barrel form working face, the second ring is iron or steel. The top ring is described as more then 2 mm deep. The piston works well together with a Nikasil liner; Audi uses a linerless Nikasil coated block. Peugeot uses cast iron liners, although it is not known if this is without a bore coating.

The piston pictured in the article could be described as a more compact and fully machined version of the Mahle Monotherm piston.
http://www.us.mahle....erm_221x174.jpg



Holset have an animation on their website: http://www.holset.co...2_5_1_5-VGT.php

How do they control the expansion rate on the piston. I would have thought it was more prone to 'grow'under the extreme heat of a diesel than an alloy piston. I can understand how it would be stronger though probably not as durable. As Peugoet seemed to discover.
What sort of rpm are these engines working at ? 5500-6000rpm?

[J. Edlund]

These pictures show the exhaust vane area changing, IIRC Toyota cheated in the Rally championship by secretly doing a similar thing on the compressor entry. It was a lovely peice of enginering but completely and knowing illegal from the beginning!

BTW whilst on high output/litre diesels I have recently read an article on injecting liquid LPG into a diesel for a considerable ncrease in power or economy plus potential NOX reduction.

I think the logic is that the LPG burns using the excess air in the combustion chamber per the diesel principle and whilst it has to be ignited by the diesel flame front it burns faster so " catches up".

I am not entirely convinced about the accuracy of the claims so has anybody heard of LPG injection as a diesel boost technique? The article implied that it was not uncommon in trucks as the benefits allowed for the high installation costs of seperate injectors and ECU.

Toyotas cheat was a restrictor with a bypass. When the engine was under load the restrictor opened this bypass and as a result the airflow to the engine increased as it didn't had to flow through the restrictor. This restrictor appeared to be an ordinary single piece restrictor while it actually was made of several pieces and spring loaded in such a way it could open.

How do they control the expansion rate on the piston. I would have thought it was more prone to 'grow'under the extreme heat of a diesel than an alloy piston. I can understand how it would be stronger though probably not as durable. As Peugoet seemed to discover.
What sort of rpm are these engines working at ? 5500-6000rpm?

A steel piston grows less due to steels lower thermal expansion, and it is also much more durable. With an aluminum high performance piston you're very close at what aluminum alloys can handle. Most aluminum alloys rapidly lose strength (and also stiffness) beyond 150-250 degC and when subjected to this kind of temperature the alloys also lose strength and hardness over time due to aging. Steel also got much better fatigue properties. For a good steel alloy 200-400 degC (about what a diesel engine piston is subjected to) have no significant effect on the strength and stiffness of the alloy, unlike with aluminum. Since steel doesn't conduct heat aswell as aluminum, assuming identical piston design the steel piston should run a bit hotter, but since steel needs less wall thickness that will probably have a positive effect on the piston cooling.

Among commercial diesels, the highest output versions use steel pistons while lower output versions can get away with cheaper aluminum pistons.

Maximum engine speed is around 5000 rpm for the Le Mans diesels. There is no reason to use extreme revs.

Some diesel engine piston temperatures can be found on page four: http://www.kspg-ag.d...kw_diesel_e.pdf

[SteveCanyon]

Toyotas cheat was a restrictor with a bypass. When the engine was under load the restrictor opened this bypass and as a result the airflow to the engine increased as it didn't had to flow through the restrictor. This restrictor appeared to be an ordinary single piece restrictor while it actually was made of several pieces and spring loaded in such a way it could open.

FWIW the device was held in the open position (to bypass the restrictor) all the time, not just under load. When it was removed from the inlet pipe the extra holes closed as the clips were undone, the internal springs doing that job. The only give-away was the little 'click' as it closed up.

Edited by SteveCanyon, 23 June 2010 - 00:52.

[Tony Matthews]

FWIW the device was held in the open position (to bypass the restrictor) all the time, not just under load. When it was removed from the inlet pipe the extra holes closed as the clips were undone, the internal springs doing that job. The only give-away was the little 'click' as it closed up.

Peter Wright wrote an article for Racecar Engineering magazine about the Toyota turbo scam, and I did an illustration. That was early in our relationship, and I was expected to make a neater version of his diagram rather than produce my own technical illustration - as a result I can't remember the details, and I don't have the magazine or a copy of the drawing. However, my memory is that the bypass mechanism only opened under load. You may well be right, Steve, but a back-issue of RE would help!

[jeremy durward]

Peter Wright wrote an article for Racecar Engineering magazine about the Toyota turbo scam, and I did an illustration. That was early in our relationship, and I was expected to make a neater version of his diagram rather than produce my own technical illustration - as a result I can't remember the details, and I don't have the magazine or a copy of the drawing. However, my memory is that the bypass mechanism only opened under load. You may well be right, Steve, but a back-issue of RE would help!

I remeber the article and the drawing for that one. it would have been on of the first race car engineering magazines my dad bought for me when i was young. i'm reasonably sure it was activated by putting the assembly together... although it was a few years back now. I still have the magazine at home, i'll see if i can dig it out

[SteveCanyon]

Been having a poke through my hard drive & Google to try to find a copy of that image .... no luck.
When searching with Google for images with the tags Toyota Celica WRX Turbo Inlet Cheat, I found this!

[Tony Matthews]

Russia's answer to the Porsche 917/10...

[TDIMeister]

I have seen results of simulation work done of the PSA 5.5L V12 going to 6500 RPM. However, this does not mean the engines actually run to those speeds in race trim. Peak power was actually developed closer to 4000 and drops off after that. The Audi paper also had the R10's power peaking at 4000 RPM...

Sources: internal documents; Baretzky, U. et al. The V 12 TDI for the 24h of Le Mans - Victory of an Idea. 28. Internationales Wiener Motorensymposium 2007

Edited by TDIMeister, 23 June 2010 - 14:58.

[TDIMeister]

The Ricardo-Judd V10 Diesel Le Mans engine, based superficially on the VAG V10 TDI, also had peak power at of 468 kW @ 5500RPM from 4.6L, but the combustion development was investigated in CFD simulation up to 8000 RPM... Their argument for higher peak power RPM was that achieving competitive power at lower engine speeds required BMEPs that necessitated a heavy engine construction.

Source: Cornwall, Morrison and Sapsford. V10 Diesel Engine for Le Mans. MTZ worldwide 7-8/2005 Volume 66.

Edited by TDIMeister, 23 June 2010 - 14:59.

[murpia]

The Ricardo-Judd V10 Diesel Le Mans engine, based superficially on the VAG V10 TDI, also had peak power at of 468 kW @ 5500RPM from 4.6L, but the combustion development was investigated in CFD simulation up to 8000 RPM... Their argument for higher peak power RPM was that achieving competitive power at lower engine speeds required BMEPs that necessitated a heavy engine construction.

Source: Cornwall, Morrison and Sapsford. V10 Diesel Engine for Le Mans. MTZ worldwide 7-8/2005 Volume 66.

I thought the Ricardo-Judd was based on the Judd GV5 block (hence the lightweight / low BMEP / higher rev concept):

http://www.mulsannes...rdojuddv10.html

The VAG V10 TDI was developed by Perkins (now Caterpillar)

http://www.perkins.c...l...7&id=284098

I may be wrong...

Regards, Ian

[Victor_RO]

The VAG V10 TDI was developed by Perkins (now Caterpillar)

http://www.perkins.c...l...7&id=284098

I may be wrong...

Regards, Ian

The one that ran in 2004, yeah, surely. This VAG V10 TDI engine is a full Baretzky creation.

[TDIMeister]

I thought the Ricardo-Judd was based on the Judd GV5 block (hence the lightweight / low BMEP / higher rev concept):

http://www.mulsannes...rdojuddv10.html

The VAG V10 TDI was developed by Perkins (now Caterpillar)

http://www.perkins.c...l...7&id=284098

I may be wrong...

Regards, Ian

You're quite right, sorry. The Ricardo-Judd was claimed at 135 kg vs. 367 for the VW V10 TDI, both dry figures. The VW mill had been so heavily discussed and benchmarked in the cited article that until I read it more closely (for what must've been the 5th time), I somehow concluded that the designs were superficially similar.


Cheers

Edited by TDIMeister, 24 June 2010 - 11:52.

[gruntguru]

What are the detailed technical challenges of VGTs on a racing diesel? Also, Audi and Peugeot tend to use the same turbo manufacturers, so, obviously, there's a rumour/possibility that fitting these turbos pushed the Peugeot V12 to breaking point and caused the triple failure which shot their challenge down in the final hours of the race. Any opinions on that aspect?

It would be incorrect to ascribe any engine failures to VGT turbos (although one advantage of VGT is the availabiliy of boost at lower revs and this would reduce engine reliability if applied to excess.) Generally speaking a VGT system will see lower exhaust backpressure on the engine for a given boost, which should benefit reliability. Advantages of VGT in a racing Diesel application include:
- Lower exhaust BP than a wastegated system (WG open). Thermal efficiency will also be higher in this region (more power, lower fuel consumption)
- Boost available at lower revs
- Reduced lag

[J. Edlund]

It would be incorrect to ascribe any engine failures to VGT turbos (although one advantage of VGT is the availabiliy of boost at lower revs and this would reduce engine reliability if applied to excess.) Generally speaking a VGT system will see lower exhaust backpressure on the engine for a given boost, which should benefit reliability. Advantages of VGT in a racing Diesel application include:
- Lower exhaust BP than a wastegated system (WG open). Thermal efficiency will also be higher in this region (more power, lower fuel consumption)
- Boost available at lower revs
- Reduced lag

More boost at lower revs would probably not be a problem for a diesel. At low revs, the smoke limit tends to be the problem; a result of having too little boost. These engines can probably also control the boost both based on turbocharger speed and manifold pressure.

The average efficiency of the turbocharger increase, but there is a drop in peak efficiency.

[jpf]

The average efficiency of the turbocharger increase, but there is a drop in peak efficiency.

That sounds like a good change -- I was interested to hear about the narrow powerband of the R10 in this interview:

http://jalopnik.com/...tory-at-200-mph

I guess I had thought of diesels as having smaller overall rev ranges, but a good wide spread of power within the range they had.

[pugfan]

That sounds like a good change -- I was interested to hear about the narrow powerband of the R10 in this interview:

http://jalopnik.com/...tory-at-200-mph

I guess I had thought of diesels as having smaller overall rev ranges, but a good wide spread of power within the range they had.

I've always thought of it like this: 3000 - 5000 for a diesel would be equivalent to 6000 - 10000 for a petrol. It doesn't seem particularly narrow for a racing engine.

[Victor_RO]

(although one advantage of VGT is the availabiliy of boost at lower revs and this would reduce engine reliability if applied to excess.)

I'm pretty sure that increased boost over longer periods on a racing engine not originally developed for that could be a potential cause of piston failures, and it seems from various sources that the Peugeot engines did succumb to broken/blown/whatever pistons.

[J. Edlund]

That sounds like a good change -- I was interested to hear about the narrow powerband of the R10 in this interview:

http://jalopnik.com/...tory-at-200-mph

I guess I had thought of diesels as having smaller overall rev ranges, but a good wide spread of power within the range they had.

Well, a range between 3000 and 5000 rpm is sort of like having a formula one engine with a power range of 11000-18000 rpm.

I'm pretty sure that increased boost over longer periods on a racing engine not originally developed for that could be a potential cause of piston failures, and it seems from various sources that the Peugeot engines did succumb to broken/blown/whatever pistons.

Diesel engines are different than gasoline engines in this area. Boost itself will not harm a diesel engine as the airmass/combustion is not connected to the fuelmass/combustion as in a gasoline engine, infact, more boost will generally decrease both combustion and exhaust temperatures. At low engines speeds they are also not limited by cylinder pressures in most cases. Instead the smoke limit (emissions) and exhaust temperature tend to be what limits the output. This is because at low speeds the turbochargers are not able to produce enough boost so the engine run rich (rich for diesel engine that is). The maximum boost pressure is then limited by the regulations so that should in itself be anything new. The question is how much fuel they inject into the engine, what kind of injection durations and alpha angles (start of injection in crankshaft degrees before TDC) they use; because that is going to have a significant effect on both cylinder pressures and the thermal loads the engine is subjected to. Unlike a gasoline engine, there is also no warning sign for excessive combustion pressures in the form of knocking.

[jpf]

I've always thought of it like this: 3000 - 5000 for a diesel would be equivalent to 6000 - 10000 for a petrol. It doesn't seem particularly narrow for a racing engine.

Yeah, I agree -- it all depends on the gearing of the final drive, right? That's part of why his comments surprised me.

[gruntguru]

Yeah, I agree -- it all depends on the gearing of the final drive, right? That's part of why his comments surprised me.

Yes. It is bad practice to think of the width of a power band purely in terms of RPM. A power band 2000 rpm wide is great if the max is 5000 but very narrow if the max is 20,000.

Better to think in terms of percentage ie 3,000 - 5,000 is a 40% power band (ie revs can drop 40% on gearchange and remain in the powerband). This would be equivalent to a 12,000 - 20,000 power band in terms of driveability.

J.E. - agree with all your comments. My point on higher boost at lower rpm is simply this - if more boost is available at low revs the engineer will increase fuel to take advantage, so thermal stress will increase, even if the same exhaust temperature limit is observed.

Edited by gruntguru, 30 June 2010 - 03:23.

[murpia]

Better to think in terms of percentage ie 3,000 - 5,000 is a 40% power band (ie revs can drop 40% on gearchange and remain in the powerband). This would be equivalent to a 12,000 - 20,000 power band in terms of driveability.

To me, 40% is quite wide for a racing engine, but the Le Mans diesels are subject to both a boost restriction AND an inlet restrictor.

Wide powerbands are characteristic of choking the restrictor lower down in the rev range (I think). And I understand this to be true for both NA and turbo, diesel & gasoline.

Regards, Ian