Stu @ M Developments

iTrader: (12)

Dread the thought Rob... how bloody boring!

Mike Gurney

Right i'm online - where are we

anyway - the information i've been given (from a very well known source) is

"basically as a car comes on boost it is possible for the turbo to struggle to provide enough air that the engine is capable of consuming, the engine will kind of cough - this is evidence of surge

generally speaking - It happens on engines with a good Volumetric efficiency as they come on boost"

this is contraditory to Stu and Karls explanation

below is the compressor map from a 4x4 saff cossie



the surge line is on the left of the map where the flow rate is low ie the turbo is not delivering much air - ie its coming on boost

this supports the alternative explanation that i have been given - that surge occours when a car is coming on boost

something that could be done to prove this is to find an istance of surge :-
1. note the boost pressure and the hp (air flow can be calculated from this point)
2. and then plot it on the compressor map itself

this should show us if the turbo is surging or not

so can somebody give me some details of the surge rpm / boost they were getting and back it up with a dyno chart.

turbo man

hmmmm... this is getting interesting, i'm gonna have to follow up on this with the big boys. bak when i got some info.

Rob M666SCC

Come on Stu, Mike's been waiting for a reply for 7 hours what ya doing sleeping on it

Sorry i'll get my coat

Anonymous

No,he is frantically looking on the net for a reply..ROFL

Stu @ M Developments

iTrader: (12)

Ok,
I can still see where your coming from here, believe me, i truly can, but i still dispute it as being correct. I wish you would answer my 2 earlier questions, as they WOULD help you to see my side.. however, given you either cant or dont want to, lets try and address this another way.

The engine is coming onto boost ok, so lets pick an engine speed ok? right, 3000rpm, lets use that figure.

At 3000rpm, your average YB power unit is flowing X amount of air, the X amount matters not the way im going to try to explain my thinking... (usual laymans way )

So with X amount of engine air consumption, and conversely for its output, engine waste air generation we have the turbo making some form of boost due to turbine speed.. agreed?

We agree they DO surge? Ok?

Ok, now if the turbo cant supply the engines demand for air at the time of surge, explain this for me.

why do turbos only surge when above 1bar absolute pressure??

Surely,
If the engine was consuming more than the turbo could supply, the turbo could NOT pressurise the inlet tract?

Surely before we hit the surge condition, our intake boost pressure would get lower and lower as the engine gobbled up all our turbo could give,and then began to try and take MORE as you suggest?

Another FACT** that may help:
To stop a turbo surging i apply more air pressure to the wastegate capsule, effectively opening the wastegate a little and slowing the turbo down. If i map less pressure to the gate and speed it up, it comes out of surge and stalls promptly so this is NOT an option.
Now if your scenario was true, surely id be doing the REVERSE of this?

Last point:
Why do the turbos surge/stall when you close the throttle on boost with no dumpvalve? Surely, with no throttle the engeine's demand for air is ZERO?


Good discussion though Mike I salute you for standing by your guns and arguing your point mate, and i truly can see where the confusion arrises, but believe myself to be right. If you bring me evidence to the contrary, i will both appologise AND thank you for the knowledge mate, cos i like to learn.. it keeps me alive.

Also,
Sorry if ive come across narcy on here lately, gotta lot of personal shit going on and ive been venting it in the wrong direction at times


**
*Please note i say FACT as i do this for a living, i havent phoned anyone, i havent looked it up, i know this as this is what ive chosen to fully understand to the extent people now pay me to do it. it wasnt meant aggressively or in any way defamatory*

Rob M666SCC

Well described Stu.....

keithG666

eh???

Stu @ M Developments

iTrader: (12)

Why thank you Rob

Billabong

Because anything less is a vacuum.... (at 'normal' altitudes)

.....because the pressurised air has no way of shifting, other than backwards through the inlet tract until it stalls the compressor as a reverse pressure wave tries to exit back out the way it came in - through compressor and back out the air intake (obviously doesn't happen with a d/v as that releases the pressure elsewhere....) Higher the boost pressure when this happens = the worse the scenario (maybe a factor in bowed IC's and blown off boost pipes?)

I may be wrong, but the only time an engines demand for air is zero, is when it is not running

Mike Gurney

good explanation Stu - i do acutally see your side of it belive it or not

quite enjoyed this discussion - i fookin love a techy chat - its not everybody's cup of tea

but i still don't buy it

so i'm gonna give it some more research and post my findings on here - not that i'm trying to prove a point - just cos i'm a stubborn bugger and like to see all sides of a conversation explored for myself

will post my findings on here

Mike Gurney

I've done a bit more research - an interesting article is here that discusses turbo sizing

http://web.ask.com/redir?bpg=http%3a...html&qte=0&o=0

its not an easy read - but it does go some way to explain that surge is where a turbo is struggling to provide enough air for a particular engine

(you can download an excel spreadsheet that shows surge lines)

I guess if you can alter the amount of air the engine is consuming ( an electronically part closed throttle should do the trick ) you can stop the turbo from surging

i'd be keen to do some calcs and see if i can plot a surging car onto its compressor map - Stu - next time you get a surging car in can you note the rpm etc i will have a go at working out the airflow etc

Oranoco

And the prize for the longest link goes to Mike

Mike Gurney

I've still got some info for you guys if you are interested to read it.

"Compressor surge is quite simply (as the map shows) the inability for a turbo to flow air in a stable manner at any given pressure ratio (boost pressure) for a given air flow. The surge is always to the left of the map where the air flow is lowest, so if you run into surge you either have to turn down the boost at that flow point, or increase airflow at that point (by porting, cam changes etc..) it shouldn't happen if a turbo has been sized correctly.

Compressor surge can vary in effect from undetectable to engine stopping noisy flow reversal and often ensued by compressor wheel damage!

This often explains the cause of apparently mysterious turbo failures on modified engines that are using large to very large turbochargers! "

Stu @ M Developments

iTrader: (12)

Is this new info Mike? Or just you telling us what ive already told you and you argued with?

foreigneRS

only read the last page of this tread, but it seems that this qote is what stu s saying, and what you have previously disputed mike.

the symptoms in that quote are indicative of the turbo supply MORE air than the engine is consuming, not less.

for example, the last part, increasing airflow by porting, camchanges etc... will make the engine capable of consuming more air, therefore bringing it more in line with what the compressor is supplying.

good discussion though.

Stu @ M Developments

iTrader: (12)

Even better if youve read it all i expect:

foreigneRS

at normal altitudes, and depending on the weather conditions, the air pressure will be around 1 bar absolute. it will be 0bar gauge pressure (so called as it is what you read on a gauge over and above, or less in the case of partial vacuum, absolute pressure)

the only way an engine can achieve a pressure of 1 bar absolute in the inlet plenum is if the volumetric efficiency is 100% and over. a normally aspirated engine, even at WOT can never acheive this, you can only get it on a forced induction engine. edited to inclde: ram air is also a form of forced induction

Stu @ M Developments

iTrader: (12)

Exactly...
So the turbo must at this point be producing more air than the engine can consume to push the plenum into a positive pressure situation.

If the turbo couldnt supply enough air, there couldnt be any boost?

Surely you agree with this Mike?

foreigneRS

i have now - what a thriller, i can barely stay sat down it's that exciting

Stu @ M Developments

iTrader: (12)

Wed forgotten all about it till Mike brought it BTTT again after a fortnight..

For the 2nd time

Still,
Gives my essay's some coverage, and it may earn enough views to join some of yours in the FAQ Forum too

starturbo

Just seen this and I know its months old but in very simplistic terms I seem to remember from my training as a jet engine tech years ago that stall is the incorrect relationship between the compressor or turbine rpm,speed of the incoming air and the angle of attack of the same.when this relationship breaks down it usually starts with one blade and has a knock on effect over the others.this results in engine surge (or turbo surge)wheras the whole unit cannot operate effectively.Ive had surges doin engine installation runs in aeroplanes and you can only get rid of them by shutting them down.you then have to check all the blades to make sure theyre all there and not bent/missing/nicked.If I am wrong I aint tellin you which airline I work for Must dig me books out and refresh my head.

Stu @ M Developments

iTrader: (12)

bttt

CosRush

Me too at Rolls Royce amongst other places, but dont forget Stall and surge are 2 different characistics of airflow though an engine/Turbo.

The stall as you rightly say is the effect of the airflow when the angle of incidence relative to the airflow is either too high or too low (in simple terms, Compressor is being used outside of its design range).

Surge on the other hand occurs when the engine demands a pressure rise from the compressor which is higher than the compressor can sustain, the subsequent breakdown in airflow causes the engine pressure to overcome the inlet pressure, pushing backwards towards the inlet.

Charlie Chalk

iTrader: (1)

Nice one Stu..

Ive only just tarted reading some Tech Essays


But needless to say I'll be reading ALOT more

Stu @ M Developments

iTrader: (12)

pee vee

Hi Stu.

A couple of questions if i can!

How can you recognise if your car is surging? chattering while
on the throttle? and fluctuating boost levels while throttle is held in one place?

having read what you said. and discounted most of the rest

im right in thinking turbos with larger Compressors
will surge far more easily/at Lower
boost pressures as they shift way more air?

Its becoming obvious (to me) that Surge is the reason PROPER head work and cam selection are crutial to Stopping this?
as well as a good mapper .

Sooooo many reasons why you shouldnt fook with the boost if you dont know what you are doing! and this is just the next one ive been shown!

These Tech articles have tought me that there is SO much more to a turbo/forced engine than i ever realised.

Stu @ M Developments

iTrader: (12)

Correct

pee vee

thanks stu

your essay IS clear!


So would the best set up be to have the Peak boost
of the car at the same point as the engine Reaches VE?

Or is it better to happen any time after its reached that point?

Stu @ M Developments

iTrader: (12)

Tricky one, as its down to the Turbochargers compressor and turbine maps, but yes, peak VE and Peak Boost will normally work well indeed.

James_RScos

Amazing read my head is hurting but i now understand.

GARETH T

a good blast from the past

SafeChav

Interesting read Stu, i always like to read yolur essays where possible they are very useful. I spend a lot of time in WHSmith recently

Now.........ive read and understood what you've said. Now i know there is this massive fued about dump valves, but as you've written when running high boost and the turbo surges, it places high loads on the shafts and bearings.

When would you say is a dump valve a necessity, if at all...? And what turbos would benefit the most from it