Intercooler for Montego TD?

[marky]

Hi,

my first post, about doing my first mod to my first car!
Just bought a Montego TD estate 'countryman'. it's in good shape,
but i'm an engineer and so can't leave something alone. has anyone
any experience of fitting an intercooler to a TD enginer? A few companies make new ones for £250+, but I can pick up a used one off ebay for £50 or less, with scope for retrofitting say a Sierra cosworth intercooler onto the Perkins.

Any ideas, please comment. It's got to be worth doing!

mark

[MGTurbo]

There is only one way to approach this IMO. As i've recently got the same car, the performance difference at night time is huge, and if an intercooler was fitted, 100bhp+ would easily be acheived on the same boost setting with a large extra chunk of torque too.

However, fit a front mounted intercooler in the same style as the Montego/Maestro Petrol Turbo will not only mean you'll lose some cooling, but get some serious turbo-lag, not a good thing, as there's already lag present on the cars. A top-mounted intercooler in the same style as the XUD diesels in PSA cars would be the way to go, as long as some ducting could be added and a shield to prevent heat soak. It's something i'll look into one day...

Gareth

[e692wtt]

'Turbo Lag' on a Turbo Diesel?

[MGTurbo]

You obviously havent driven one then. On the Perkins theres a very flat period where the turbo does not spin fast enough to produce any boost and it feels slower than a 1.0 Nissan Micra. Only modern TD's with variable nozzle Turbo's can effectivly reduce this, the VW TDI being one, of them with the high pressure PD fuel system, its actually better than a petrol!

Gareth

[e692wtt]

That's not 'Turbo Lag'. Just low-down lethargy on a basic TD engine.

[MGTurbo]

Stop being pedantic. A NASP Perkin's is sharper low-down than my Monty TD although the slightly lower compression can account for that. I know what i'm talking about anyway's, i've been driving Turbocharged cars long enough.

Gareth

[e692wtt]

Pedantic?

[MGTurbo]

Yes! Your trying to tell me my Turbo Diesel car does not have turbo-lag, or what i'm describing to you is NOT Turbo lag... But it is.. Your were spliting at hairs to try and trip me up, its not going to work..

Gareth

[e692wtt]

Unless a Turbo Diesel engine has a throttle in its air supply, it cannot have 'Turbo Lag' like that experienced driving Turbo Petrol engines...

'Low Rev Lethargy' in both petrol and diesel Turbos 'feels' the same though, and is *not* 'Turbo Lag'.

Softer throttle pedal 'response' on a TD as the turbo 'cuts in' may well result from the fitting of a longer inlet tract (front-mounted Intercooler), but this is not 'Turbo Lag' either.

[Mat_C]

I've always considered 'Turbo Lag' to be mainly due to the combined mass of the turbine and impeller.

Interesting point about the lack of throttle on a diesel, but surely under WOT conditions the throttle can be neglected on a petrol. Seeing as spool up under WOT depends mainly on exhuast gasses, I can't see how a TD cannot have turbo lag. I've driven a TD with a boost gauge (Citroen XM ::hangs head in shame::) which seems to confirm this.

[e692wtt]

1. What's WOT?

2. Doesn't a Diesel, turbocharged or not, always run under WOT conditions, unless it has a throttle in its air intake, by definition?

3. Won't a given Turbo Diesel engine, therefore, begin to give a 'Turbo Kick' at X engine rpm regardless?

4. If one, for example, approaches a corner, in a Turbo Diesel-powered car, at Y mph and snicks into, for example, 3rd gear such that the engine revs are above X rpm (where boost 'kicks in') at the start of the corner, then when one presses the Accelerator Pedal at the exit of the corner (as per Roadcraft...) the 'kick' and acceleration will be instantaneous.

5. Is there a 'pause' on the Turbo Diesel-engined car, as the throttle is pressed from its closed position to partially- or fully-open, as on an early Turbo Petrol engine? *That* (on the Turbo Petrol car) is 'Turbo Lag' and Turbo Diesels just don't do it, by the nature of the engine running at WOT all the time.

6. On a Diesel-engined car, the throttle pedal controls the rate of fuel injection, not the rate of air intake (poor english alert...).

7. Lack of urge from 'initial take off' is a lack of inertia in the Turbo (due to the low rpm at idle, also known as 'Low Rev Lethargy'), I'll give you that, but once a Montego TD is rolling and the revs are over, I dunno, 2000rpm, there will be, by definition, no 'Turbo Lag' as the engine operates at WOT.

8. Don't trust the Instruments in a Citroen XM - Peter Simpson of Car Mechanics mag had no end of trouble with the Instrument Panel in the mag's 'project XM Turbo Diesel', especially the Rev Counter, due to 'faulty earth wiring'.

9. WOT stands for 'Wide Open Throttle'. Or, 'no throttle at all' in the case of Maestro and Montego Diesels and Turbo Diesels.

10. How often does one run one's Petrol engine, Turbo or not, at WOT for any length of time without either scaring one's self or running out of road? Turbo diesels run like this all the time... No, don't answer that... but not very often???

[Mat_C]

When a petrol is WOT (wide open throttle) it effectively has a diesel intake system - so this is the easiest state to compare the two IMHO. i.e. At X rpm at WOT, the quantity of air 'consumed' in both cases is proportional to X.

The point at which the turbo 'kicks in' is variable. It will kick in earlier going up a hill for example.

When you drop it into 3rd, and you get instantaneous accelleration - the turbo is reasonably 'spun up' before you've dropped down, so when you do drop it and floor it, the reaction should be instantaneous.

On every TD I've driven, yes there is a pause. It is no-where near as dramatic as on a petrol, but it is there. Point taken about Citroen XM electrics - the boost gauge was about the only thing than worked :laugh:.

[MGTurbo]

You didn't need to list the basic working's of a diesel engine to try and make your point more valid. But you said it yourself in point 7.

Lack of urge from 'initial take off' is a lack of inertia in the Turbo (due to the low rpm at idle, also known as 'Low Rev Lethargy'), I'll give you that, but once a Montego TD is rolling and the revs are over, I dunno, 2000rpm, there will be, by definition, no 'Turbo Lag' as the engine operates at WOT.



THAT, is the turbo-lag i'm talking about! Regardless of a diesel operating at WOT (which i DID already know stood for Wide Open 'Throttle') And the same thing also applies to petrol turbo's as well. Once in the turbo range (be it anything above 3k in the petrol and approx 2k in the diesel) there is no turbo-lag when the 'throttle' is depressed.

Your case of diesel's having no throttle butterfly really does not work, as here, it makes no difference!

If the turbo is not spinning fast enough to create any boost and you want to overtake another car for example (which i would never do of course)in 5th gear, you would have to change down to get the revs within the power-band to get any movement out of it..IF the Turbo was a better design with variable-nozzle etc, this would not occur half as bad, as they start spooling from around 1100rpm! It's still simple, it's still turbo-lag, and i don't know why you had to question it.

Why you had to go into 10 point's of stuff i already knew about i don't know...

So back to the point of this thread, fitting an intercooler to the front of the car and using similar set-up to the petrol system on Maestro/Montego's WOULD increase turbo-lag over the standard set up, and this would be the problem in fitting this system, hence the other solution i came up with.

Gareth

[derek mclean]

Not much experience of turbos, but I have to agree with Gareth, in that if the turbo isn't spinning fast enough to create boost there will be a delay before it does get itself up to the requisite revs. to do so. That's called "Turbo lag". And it makes no odds whether the engine is a P or a D, or whether it has a throttle in the inlet tract. The turbo speed is a function of the exhaust gases that are passing through the exhaust turbine. A sudden demand for more power from a relatively slow engine (therefore turbo) speed will not elicit an immediate response, because it takes time for the turbo to "spool up" to the higher speed needed to generate positive pressure.

As for the original question, sorry, can't offer anything constructive, except to suggest looking at some other turbo installations, e.g. Mondeo.

Derek.

[e692wtt]

In a petrol Turbo engine, take your foot off the accelerator and the turbo's impeller slows down (reduced exhaust gas flow). When you reapply your foot to the accelerator the turbo's impeller speeds up (responding to increasing exhaust gas flow) and begins to provide boost again. The time taken for the turbo to begin giving boost is called 'Turbo Lag', the time for the turbo to respond to the increased throttle opening. Assuming that the turbo was giving 'boost' at the beginning...

This cannot happen in a diesel Turbo as the engine always passes a full flow of exhaust gas, limited by the engine speed (leading to 'Low Rev Lethargy' and slow initial 'spool up' from idle) and *not* by any restriction to the air flow to the engine in the first place (no throttle)... So no 'Turbo Lag' when the turbo is boosting - and 'Low Rev Lethargy' is not 'Turbo Lag' as engineers describe it (see first paragraph).

Yes, this 'Low Rev Apathy' is all-but absent on the latest Diesels, which is a vast improvement.

Why will fitting a front-mounted Intercooler increase 'Low Rev Apathy'? Please explain.

[derek mclean]

I take the point about the higher air flow in a diesel with no throttle compared to any engine with a nearly closed throttle, but the speed of the turbo will still be affected, albeit to a lesser extent, by the temperature of the exhaust gases, and this in turn is dependent on the load as well as the speed. O.K. the effect may be less prominent with a diesel on WOT, but if it is there at all then it is significant, and it is still turbo lag to me.

If engineers choose to call it "Low speed lethargy" instead of "Turbo lag", who am I to argue with engineers. I am just a mechanic. So maybe you (Rich) would be willing to explain it all to the rest of us in terms that we can understand.

As for the question about an intercooler increasing lag, I would offer the following theory, based on logic rather than knowledge. The longer the inlet tract, the longer it will take for the pressure generated by the turbo to increase, and to reach the cylinders.

I too have been accused of being a pedant, and it is not nice to be described in an unfriendly way. But I really would like to understand what you are saying, so please offer us all the engineers' theory.

Derek.

[MGTurbo]

Yes, please do explain the difference for me too. I run my own business and build and tune engine's for a living so i must be doing something wrong.

I'm having great difficulty understanding why a diesel with a turbo won't suffer lag because it doesnt have a throttle. To me, the facts are simple.

To generate boost, a higher volume of exhaust gas needs to be created. In the diesel, this is done by increasing the fuel/air ratio, and in turn, a higher volume of exhaust gas is created. This will then of course turn the turbine which turns the compressor which creates boost. And this take's time. It doesnt matter if its a petrol or diesel. There's still 'lag'. Or to use your technical term, 'low-rev apathy'.

Why will fitting a front-mounted Intercooler increase 'Low Rev Apathy'? Please explain.

Again, its simple. Your increasing the length of the induction system, by nearly 1.5 metres! Put together with the large bore hoses used on the Maestro/Montego set-up, and the intercooler matrix causing yet another restriction to the system, you WILL get increased lag over the standard set up! Why don't you understand that, as i've tried to put it as simply as i could, and didn't want to cause an arguement, but you seem to disagree with a concept thats very simple to grasp, even a small monkey could get it's head around it.

Gareth

[John S]

....and I thought turbo lag was simply due to the turbo not going 'on boost' within a certain rev band:eek: :laugh: Correct me if you like, but doesn't the turbo add an approx. extra of 25% more power or 20 bhp to a normally aspirated Perkins engine?
For anyone who's never had experience of a car (expecially under 50 bhp) I think they would find the acceleration almost startling in a TD maestro/montego. I find Isuzu engines (e.g. 2.5 TDi - Vauxhall pickup) use the turbo much more progressively through the rev band rather than coming on all in a rush e.g. on the Perkins 2.0TD prima.
I'm pretty used to my car now but its still fun to be behind a car doing 35 - you change down from fourth (economy gear, not good to go slower than 35 in 4th though!) into third to overtake then FLOOR IT and turbo boost right past 'em:cool: *cough*

[derek mclean]

As I thought, you can't explain it. So much for "engineer's speak". I think I'll stick to my opinion until somebody comes along with a coherent and logical alternative, and I suspect Gareth will agree with me.

I see nothing above that tells me anything useful.

I am still willing to change my mind if you can prove that you are not talking through bluster, but it looks as if we may have a long wait.

Derek.

[derek mclean]

Sorry, I see I have just spoken out of turn. Didn't look at the name and insulted the wrong person. Need to wait a little longer to get a sensible answer.

Derek.

[e692wtt]

Eh? I drive a Bedford Midi 2.2TD every week and my major gripe is the column gear shift... no 'Turbo Lag' on *this* installation.


Otherwise no comment, apart from "insulted the wrong person"???

Shoud report this to the moderator...


PS You can always edit a Post, as opposed to making a new Post.

Ghurka, me, not take Prisoners or New Posts.........

[Jonathan]

'The Moderator' is aware.

We have standards on this forum, rudeness doesn't meet them. I haven't a clue about the technicalities behind turbos but I'm sure you can dispute them politely.

[e692wtt]

"Confucious say 'woman with skirt up run faster than man with trousers down'..."

Me ghurka, take no prisoners...

Throw it wide open...


Anyway, anyone heard of "Death By Bongo"???

[Austin-Rover]

Originally posted by e692wtt
[B
Anyway, anyone heard of "Death By Bongo"??? [/B]

Nope. But it sounds like a fun way to go...

:laugh:

[derek mclean]

It looks as if we are destined to remain forever ignorant, Gareth.

Derek.

[MGTurbo]

I guess.... Meanwhile, my Monty is still suffering from low-rev apathy, apparantly them Bedford TD engine's are lag free and boost from idle, should get me one of them i reckon ;)

Gareth

[e692wtt]

???

[H48HPE]

im no expert but i just fancied chipping in.

Ive driven turbo diesels at work and they definitly do have what i considered to be a lag, if im moving away with my foot hard down in any of the lower gears the engine accelerates up to a point and then the turbo definitely kicks in and it shifts it. this i had rightly or wrongly assumed was because at low speed the engine hadnt kicked up enough exhaust to spin the turbo.

i have always been told that the lag is the time taken from the power being applied to the turbo actually opperating. and this is very evident to me even on new stuff like the mondeo.

Im an engineer for the record (electrical) and its worth pointing out that the best engineers are those who listen to the lads on site/shop floor (to an extent), after all its them that are putting the job together and they know it inside out (only if they are the best lads on site though i supose!)

at the same time its give and take, they might come to me wanting technical advice, above all, manors are everything and theres no need for anyone to be insulting, in the end it causes upset and nowt gets done.

andy

[E_T_V]

Before I start driveling on spouting my 2p worth perhaps I can define a few basics so that people are using the same terms and not meaning different things. These aren't written in stone but are just my definitions. I’m not trying to teach people to suck eggs just allow the less experienced people a feel for what we are arguing about.

Lag - This is in reality the time it takes from hitting the "loud" pedal until anything exciting happens. What this means is physical terms is a combination of things but the biggest factor is the inertia of the turbo. These things spin at incredible speeds (150,000 rpm if I remember that lecture from Garett right) and take a while to get there. The bigger the turbo the longer it will take to get there (all other factors being equal)

Boost – This is the increased air pressure of air entering the cylinders caused by the action of the turbo. All turbos try to produce boost from idle. Some manage it effectively some don’t, but I’ll go on about that later.

Turbo – Made up of two parts an exhaust turbine that takes energy from the exhaust gasses and uses it to drive the other part of it, the compressor. This compresses air entering the cylinders (more air in effectively increases the capacity of the engine)


(stolen from this website as it puts it better than I ever could – it has a great explanation for people who are wondering what we are going on about)
http://auto.howstuffworks.com/turbo.htm


One way to decrease turbo lag is to reduce the inertia of the rotating parts, mainly by reducing their weight. This allows the turbine and compressor to accelerate quickly, and start providing boost earlier.

One sure way to reduce the inertia of the turbine and compressor is to make the turbocharger smaller. A small turbocharger will provide boost more quickly and at lower engine speeds, but may not be able to provide much boost at higher engine speeds when a really large volume of air is going into the engine. It is also in danger of spinning too quickly at higher engine speeds, when lots of exhaust is passing through the turbine.

A large turbocharger can provide lots of boost at high engine speeds, but may have bad turbo lag because of how long it takes to accelerate its heavier turbine and compressor.

(right back to my drivel)

Turbo chargers are only efficient over partial engine rev range (unless you go to the variable vane technology). So you can either have engines that boost from idle but the wastegate opens early so you don’t gain much top end power, or you have engines that have boost higher up the rev range to boost the engines power yet retain good economy during normal driving which is what most people go for. The exceptions are diesel truck engines, which have such a limited rev range, that they are almost always on boost. Diesels have more limited rev ranges than petrols so they are on boost for a larger percentage of their rev range without the wastegate being open.

Right now on to the contentious issue of whether adding an intercooler to a diesel will increase the lag. As usual I’m gonna sit on the fence as you all have valid points. But from my simplistic calculations a non turbo diesel gobbles up about 30litres of air per second at 2000rpm which means that an increased intake length will give a softer throttle response but only by a very small amount as it isn’t in the inlet tract for long due to the fact that diesels run full throttle all the time (the same reason as why they don’t need dump valves too) and also fuel is injected directly into the cylinders unlike petrols which allows the turbo to spool up faster. (I find this concept difficult to explain but I’ll try).
As soon as you hit the throttle in a diesel it can start to make lots of exhaust gases (due to the fact it runs full throttle all the time) and these cause the turbo to spool up. In a petrol you have to wait for the carb damper to open first which at low RPM’s can take a while and also as the turbo isn’t boosting yet it only draws in a small amount of fuel at first rather than the diesel which dumps a full load of it in. (that is why diesels smoke like hell when under full throttle before the turbo spools up to speed). This factor may be large or small, I’m not sure which but it will be there.

All in all if I’ve got the time and equipment I’ll be installing a front mounted intercooler to my TD where there is more space and better airflow.

[J199 HHG]

Well I might as well wade in with my opinions.

In my opinion a diesel doesn't suffer from turbo lag. In my experience with my Monty TD (160K's worth), if the revs were below 2000 RPM, you were without turbo. Raise the revs above, and the turbo comes on and you're away. So if you are getting bogged down in 2nd gear going up a hill, dip the clutch, rev it up, and ride the clutch a bit to maintain high revs, and you're away.

Therefore I concluded that the turbo was ineffective below 2000RPM regardless of fuelling. And the reason for this is that the engine is breathing fully all the time, so the turbo must be such that the volume of exhaust gas (regardless of fuelling) is insufficient to spin the turbo.

Above 2000rpm, the volume of exhaust is sufficient to spin the turbo, regardless of fuelling, so there is no 'lag' as the turbo is already spinning when you stand on the throttle.

Now in reality there will be a small crossover around 2000 where the turbo isn't going and the increase in fuelling will cause it to spin up. But it's very small, and not really noticeable in my opinion.

Now in a petrol, you shut off the exhaust flow completely (almost) when lifting off the throttle, so you can effectively shut down the turbo even at high engine speeds, and when you reopen the throttle, you have to wait for it to spin up. Hence the turbo lag is noticeable at all engine speeds.

So petrols have lag, diesels have regions where they are off-boost, and cannot be made to get on boost regardless of fuelling. This is not lag, as if you keep a constant engine speed, you will _never_ get any significant boost. With turbo lag, at constant speed, you will start to get boost.

-clear as mud eh!

[E_T_V]

Makes sense to me, that was the point I was trying to get across with turbos being designed to operate in certain rev ranges. Diesels have lots of low down grunt so car designers make the turbo effective at higher revs e.g. above 2000rpm. A noticable excepetion is truck engines which are on boost most of the time as they red line at maybe 3000rpm.

[derek mclean]

I've only just discovered this thread has a second page, and have just read the terrific semi-technical descriptions above. This is what I was after, and it certainly makes the distinction much clearer for me. And for you too, I hope, Gareth.

A properly described and argued case is always a joy to see or hear, and I am grateful to you guys for offering it. I suspect those of us that are neither mechanical engineers, thermodynamicists nor intelligent turbo drivers will probably carry on calling it turbo-lag, but I now understand the reasons for not doing so, even if I may fall into the trap of saying it wrongly, because it is easier, and because others will probably not understand the correct term. A bit like "video", "alloy", "material", etc. which have all degenerated over the years for one reason or another.

I apologise to all concerned for my use of the word "insult" at the end of page 1. I was panicking after having realised my mistake with the previous message, and meant no offence to anybody. It has taken me several days to catch up with subsequent messages, due to my unfamiliarity with the ways of these forums, i.e. I couldn't find the later posts.

I too was an electrical engineer in an earlier life, hence my desire to understand things, and to argue a case through to a logical conclusion. The fact that I turned a hobby (fixing my own cars) into a career (fixing other people's cars) that doesn't have me designing things doesn't stop me from having an enquiring mind.

Perhaps I could just add one small item to the turbo argument. At any given speed there will be a more or less fixed mass of air passing through the engine, but when more fuel is added the exhaust gases will be hotter, and moving faster, so the turbo will start to speed up even before the revs have risen. But I accept that this may be a relatively insignificant part of the overall effect. I would appreciate thoughts on this.

Any "rudeness" on my part was unintentional.

Derek.

[Mat_C]

Quite a bit to wade through here - so i'll stick to just a couple of points.

>>One way to decrease turbo lag is to reduce the inertia of the rotating parts, mainly by reducing their weight. This allows the turbine and compressor to accelerate quickly, and start providing boost earlier.<<

Agree with above totally (and all of ETV's post). So to have a zero-lag turbo instalation, the center bearing must be totally frictionless and the turbines must have zero mass. Obviously not going to happen!

>> Now in a petrol, you shut off the exhaust flow completely (almost) when lifting off the throttle, so you can effectively shut down the turbo even at high engine speeds, and when you reopen the throttle, you have to wait for it to spin up. Hence the turbo lag is noticeable at all engine speeds.<<

Why don't deisels do the same?

>>So petrols have lag, diesels have regions where they are off-boost, and cannot be made to get on boost regardless of fuelling<<

Which doesn't necessarily mean that the turbo will be boosting above 2000rpm with little fuelling?

>>With turbo lag, at constant speed, you will start to get boost.<<

Now I'm confused!



:)

[J199 HHG]

>> Now in a petrol, you shut off the exhaust flow completely (almost) when lifting off the throttle, so you can effectively shut down the turbo even at high engine speeds, and when you reopen the throttle, you have to wait for it to spin up. Hence the turbo lag is noticeable at all engine speeds.<<

>Why don't deisels do the same?

Ah, well, it's because they run on a fundamentally different cycle.

In a petrol, you have to fill the cylinder with a mixture of petrol / air (at 14.7:1 ratio air to petrol), with an amount of it commensurate with how much power you require. It gets lit in one place (the tip of the plug), and burns nicely across the cylinder, hopefully the flamefront passing through all the fuel on the way. If your fuel/air mixture is wrong, the flame may go out or burn too slowly or not produce the max volume of exhaust gasses. So this 14.7:1 is crucial and it has to me maintained. So in a petrol you have to control both petrol and air intake to maintain this ratio.

(note if the flame front hits the piston, it causes pinking, and
melts the piston - the piston should be pushed down by expanding exhaust gases only)

However, in a Diesel, you fill the cylinder with air, compress it, then squirt in some diesel. The compressing action heats the air, hopefully sufficiently hot so that when the diesel gets injected, it burns. Any excess (unburned) air doesn't matter, it will just get pumped out.
So you don't need to bother metering the air. Now at max throttle,
you are reaching the point where there is insufficient air in the cylinder to completely burn the diesel, and this gives black smoke.
So you should turn down your max fuelling screw to stop this happening as it's just wasting fuel, you're not getting more power for it.

So in a Diesel, you only have to control the amount of Diesel going in, as long as there is enough air, you're fine.

Further (!), the amount of air that is in the cylinder is determined by the boost pressure, so if you set up your max fuelling screw such that it puts enough diesel to completely burn off-boost, you won't get any gain from having a turbo. Hence the little pipe that runs from the inlet manifold to the injector pump to increase the max fuelling when on boost.

Some diesels did meter (throttle) the air intake - the Land Rover 2 1/4 diesel did so. This was simply to generate a vacuum for the brake servo. It has no deleterious effect provided there is sufficient air in the cylinder to burn all the injected diesel.

Petrols and diesels are more different than people realise, in fact, they are fundamentally different. I just can't think of the right terms.

>>So petrols have lag, diesels have regions where they are off-boost, and cannot be made to get on boost regardless of fuelling<<

> Which doesn't necessarily mean that the turbo will be boosting above 2000rpm with little fuelling?

Correct - you are forcing more air into the engine, but without fuel to go with it, all you are doing is passing more air into the exhaust. No harm done, but no gain either. Of course, passing lots of air into the exhaust keeps the turbo spinning for instant power when you do require it.

>>With turbo lag, at constant speed, you will start to get boost.<<

>Now I'm confused!

What I meant was, that in a petrol (when in the lag state, i.e. foot down waiting for turbo to come on line), you will start to get some boost, and a runaway effect takes place, and off you go. In a Diesel, in this state, i.e. off boost, with foot on the floor, you can stay there all day, because there will never be enough exhaust gases to get the turbo going enough to give a significant increase in power.

As for intercoolers, they do two things:
i) Cool the air (compressing air in a turbo heats it up, and it expands slightly), which increases its density, so you can get more in the cylinder.
ii) Cool the engine, reducing thermal stress on the pistons
So they are a good thing.

Is this helping? or am I muddying the waters further.

[J199 HHG]

Originally posted by J199 HHG
>> As for intercoolers, they do two things:
i) Cool the air (compressing air in a turbo heats it up, and it expands slightly), which increases its density, so you can get more in the cylinder.
ii) Cool the engine, reducing thermal stress on the pistons
So they are a good thing.


Sorry, forgot this point:
If you fit an intercooler, you will only get benefit from point 2 unless you increase your maximum fuelling screw to take advantage of the increased mass of air in the cylinder. If you don't do that, you'll reduce stress on the engine, but get no more power.

[Mat_C]

Well I'm not going to quote all that hehe! Understand everything with regards to not having to keep a stoichiometric (sp?) mixture in a diesel - but I can still see a rather sudden drop in gas flow when the accellerator is released - which should still result in a loss of turbine momentum, hence the need to spin it up again => turbo lag.

>> Correct - you are forcing more air into the engine, but without fuel to go with it, all you are doing is passing more air into the exhaust. No harm done, but no gain either. Of course, passing lots of air into the exhaust keeps the turbo spinning for instant power when you do require it.<<

So 'turbo lag' can be experienced above 2000rpm - its not a fixed point. So we can say for arguments sake that:-

Exhaust 'power' (EP)= RPM x Fuelling

>>In a Diesel, in this state, i.e. off boost, with foot on the floor, you can stay there all day, because there will never be enough exhaust gases to get the turbo going enough to give a significant increase in power.<<

So you have the fuelling, but not enough RPM to develop the required amount of EP to spool up the turbo, which still means you have turbo lag.

This could get very long indeed:laugh:

[J199 HHG]

Well I'm not going to quote all that hehe! Understand everything
>with regards to not having to keep a stoichiometric (sp?) mixture in
>a diesel - but I can still see a rather sudden drop in gas flow when
>the accellerator is released - which should still result in a loss
>of turbine momentum, hence the need to spin it up again => turbo lag.

I think there's a second set of factors here - a Diesel will have a smaller turbo than a petrol, which gives it less inertia. Coupled with the fact that it will remain spinning at a far higher speed off-load because of the open air intake means that any lag is negligible. I certainly never noticed any.

>So 'turbo lag' can be experienced above 2000rpm - its not a fixed point. So we can say for arguments sake that:-

>Exhaust 'power' (EP)= RPM x Fuelling

>>In a Diesel, in this state, i.e. off boost, with foot on the floor, you can stay there all day, because there will never be enough exhaust gases to get the turbo going enough to give a significant increase in power.<<

>So you have the fuelling, but not enough RPM to develop the required >amount of EP to spool up the turbo, which still means you have turbo >lag.

Well, not really because lag implies that it will pick up at some point, i.e there is a 'lag' or delay between pressing the loud pedal and an increase in power delivery. In this case, the 'lag' is infinite, as the engine has insufficient RPM to cause the turbo to spin up, so you will never get the increase in power you are demanding.

I think the definition of lag is the crux of the matter:
'Turbo lag is the delay between increasing the fuelling and the engine delivering a significant increase in power'. Note that the engine has to at some point deliver an increase in power.

My observations were that:
<2000RPM - no power increase ever.
>2000RPM - instant power increase.

i.e. no lag in either circumstance.

>This could get very long indeed

No matter, if it is interesting / thought provoking then that can only be a good thing.

[E_T_V]

I'll try and keep this short this time!

When you lift off of the throttle in a diesel it still pumps shed loads of air into and out of the cylinder (ok no fuel to make the gases expand but shed loads still the same) as there is no throttle butterfly to shut off the flow so the turbine spins down slowly.

In a petrol when you lift off you effectivly shut off the inlet tract and your turbo generates lots of pressure as it has no where to go as the butterfly is closed (so you need a dump valve to prevent the turbine slowing excessively/stalling). However this compressed air is vented to the atmosphere (hence the turbo car Tchhhh noise when changing gear), and so it doesn't go through the engine. This gives the turbine less energy to keep spinning as less exhaust gasses are generated. Combined with the increased pressure to push against (regulated by the dump valve) as the inlet is closed and less gasses to power it, it slows more than a diesel's turbo does when you lift off for example to change gears.

[Mat_C]

>>No matter, if it is interesting / thought provoking then that can only be a good thing.<<

Agreed:)

Now, back to the brawl :laugh:

Driving a petrol turbo, I see the boost rapidly increase, after a pause, when you boot it.

Driving a diesel turbo (with a gauge - ****roen XM) I can see the boost increase very slowly, and notice the accelleration increase accordingly. You're almost at max. revs by the time it reaches full boost.

IMO, although they take very different forms, they are both turbo lag as the turbine is spinning up in both cases. Its just the rate that is different.

[Liam]

Some interesting reading here, I think Chris explained it in terms I can understand. Just one question I have. Just out of interest, what happens in a petrol engine when the throttle is rapidly closed at high revs, the piston moves down so its trying to pull in air quite quickly but can only pull in a small amount through the almost closed throttle, so what happens? Does this cause a vacume? If it did surely the engine revs would drop very rapidly? Does my question make any sense?

[J199 HHG]

Originally posted by Liam
Does this cause a vacume? If it did surely the engine revs would drop very rapidly? Does my question make any sense?

It does cause a tremendous vacuum - in my petrol LandRover, I can get to nearly 30 inches of mercury - a total vacuum is 32inches.

A petrol always has some vacuum in the inlet system, which nearly vanishes at full throttle (just a bit of vacuum caused by the air filter). As you ease off the throttle, the engine is sucking against the butterfly, and the vacuum increases.

This vacuum will suck oil past the valve stems and up the bores so you get blue smoke on gearchange on a tired engine. Notice that even a tired old Diesel doesn't have this effect - no vacuum.

This vacuum is used to drive mechanical economy guages. They are simply vacuum guages calibrated in MPG. A bit of a loose connection, but reasonably good under ideal circumstances, i.e. top gear.

You use this vacuum for engine braking if your driving technique allows you to do this (modern thinking doesn't allow it).

This vacuum causes SU carburettors to run lean, hence the popping / farting you got on old mini's and morris minors on overrun / gearchange. They fitted a valve in the butterfly to limit the vacuum on later carbs.

Even a total vacuum only causes 15PSI of pressure trying to crush the engine. Nothing compared to the combustion pressures.

HTH,

Chris.

[J199 HHG]

Originally posted by Mat_C

Driving a diesel turbo (with a gauge - ****roen XM) I can see the boost increase very slowly, and notice the accelleration increase accordingly. You're almost at max. revs by the time it reaches full boost.

IMO, although they take very different forms, they are both turbo lag as the turbine is spinning up in both cases. Its just the rate that is different.

To be fair, I have never driven a turbo diesel with a boost guage.

But I still say you haven't got turbo lag in your Citroen. Turbo lag is the delay between opening the throttle and the turbine reaching it's terminal velocity for that engine speed / fuelling. It sounds like max boost isn't reached in your citroen until you reach max engine speed and fuelling, anything less and the turbo isn't going flat out. This implies that the wastegate is never used, (has it got one?), and a smaller turbo would reach max boost quicker.

With turbo lag, a fixed throttle position gives a wildly varying power output. i.e. Without lag, power is a function of fuel and engine speed. With a turbo, another variable is introduced, time. Power increases with time for a fixed engine speed and fuel level.
It's this variation with time that is lag.

If in your citroen, you can watch the boost climbing slowly for a fixed throttle setting, then you have lag. If the boost is proportional to throttle setting, then you have not got lag, you just have a turbo that is running at a speed determined by engine speed / fuelling.

For a fixed speed, does modulating the throttle setting (say 1 cycle per sec) cause the boost guage to follow roughly, or does it ignore you.?

:-) - must find out how these smiley thingys work.

[Liam]

Yes that all makes sense, thanks for the explaination.

[Mat_C]

Oh, it's not mine - i would never waste my money on an overcomplicated unreliable turd of a car :laugh:

As for the XM turbocharger - I know nothing about it - but needless to say it's probably way to complicated than it needs to be!

>>If in your citroen, you can watch the boost climbing slowly for a fixed throttle setting, then you have lag.<<

Which is exactly what happens.

Unfortuately (or is that fortunately) I can't see myself driving that car for a while - its not mine, so I can't say about pulsing the throttle.

Just to claify, my personal definition of turbo lag is due to the energy that is wasted due to acceleration of the turbine, hence the power increase (air compression) the turbo can produce lags behind the exhaust gas presented to it. It is only when the wastegate(WG) starts to open that the exhaust energy is transmitted straight thru (but now your loosing a bit thru the WG). Or when under constant speed and the turbo has caught up. The turbo lag is not over as soon as things start get going once you floor it, just that this is by far the most noticeable part.

[E_T_V]

yes engines with closed thottles cause a vacuum, indeed trucks use this phenomena to great effect with a valve in the exhaust (or is it inlet I'm not sure) which when closed makes the engine braking huge, and makes a funny noise too!

[G Force]

( indeed trucks use this phenomena to great effect with a valve in the exhaust (or is it inlet I'm not sure) which when closed makes the engine braking huge, and makes a funny noise too! )

Its in the exhaust hence exhaust brake. It uses the massive pressure build up in the exhaust and cylinders to slow the engine down, it's a bit like sticking your finger over the end of a bicycle pump and it stops you pushing the plunger down.

Regards Gary

[J199 HHG]

Originally posted by Mat_C
to complicated than it needs to be!

>>If in your citroen, you can watch the boost climbing slowly for a fixed throttle setting, then you have lag.<<

Which is exactly what happens.

Unfortuately (or is that fortunately) I can't see myself driving that car for a while - its not mine, so I can't say about pulsing the throttle.

Just to claify, my personal definition of turbo lag is due to the energy that is wasted due to acceleration of the turbine, hence the power increase (air compression) the turbo can produce lags behind the exhaust gas presented to it. It is only when the wastegate(WG) starts to open that the exhaust energy is transmitted straight thru (but now your loosing a bit thru the WG). Or when under constant speed and the turbo has caught up. The turbo lag is not over as soon as things start get going once you floor it, just that this is by far the most noticeable part.

Well this Citroen sounds like a different driving experience to the one I had in my Montego TD. It sounds like the turbo is perhaps too big for the engine. I do seem to recall that the Montego had a very small (smallest available) turbo at the time.