Exhaust temp

[paulj]

Hi to all
will unleaded increase manifold temprature ?
does anybody have an answer
please thanks Paul

[christopher storey]

The answer in theory is yes, as unleaded fuels burn at a higher temperature than leaded fuels did , and so there is more heat to dissipate . However, the effect on a large heat sink such as the head and manifolds combined will be very marginal

[paulj]

Hi Christopher
thanks for the reply
the question has stemmed from a paint problem on the bonnet,
just above the manifolds, where the paint developed a blister,
the paint shop seems to think it's to do with manifold temprature
I have my doubts.
but are asking me to fit some sort of heat deflector !
which i am not too keen on
so I just wanted a bit of conformation,that although the temp would be increased on unleaded, by what degree and thanks for the answer

regards Paul

[Heuer]

Paul

I have stainless steel manifolds which get very hot indeed but there is no sign of the bonnet paint being affected. If you are using the cast manifolds they will run cooler anyway. Sounds like a paint defect to me.

[ChrisC]

Glad you mentioned the Stainless manifolds not affecting the bonnet paint - I have been worrying about that :)

[PeterCrespin]

The answer in theory is yes, as unleaded fuels burn at a higher temperature than leaded fuels did

Got a source for that Christopher? I think it's a myth. Not that it changes the OP's answer, because he has a paint problem by the sound of it, not an engine problem.

But AFAIK all petrol fuels burn at basically the same temp. The octane rating is merely an indicator of the resistance to self-ignition at high temps/pressures - i.e. propensity to 'knock'.

Pete

[norman m. macleod]

Hi Pete,
Can't resist (but tried.....!) answering this one. some years of research (post-grad) in the various parameters applicable to combustion characteristics in the i.c. engine will guarantee my tuppence worth here...
Basically you are right, most petrols over the last 50 odd years burn, given a reasonable approximation to the correct stoichiometric (air/fuel ratio....) at pretty well similar temperatures. All other opinions are based on suburban myth, full stop. All the additives did (like tetraethyl lead....) was to increase the resistance of that mixture to commence burning prematurely, either by pre-ignition or by detonation (two quite separate conditions, by the way.....). You used the term self-ignition, I'm a bit pedantic on semantics, but that's a diesel......! I trust I have not strained your sense of humour..........
There are many of us mourn the passing of tetraethyl lead, and the Esso Goldens and the Discols that it permitted when I was a kid, and I could get away with a set of 10:1 pistons in my 998 Vincent.....whilst I appreciate the negative effects of lead on the human brain, I am cognisant of the carcinogenic effects (well documented....) of the recent additives attempting to get us back around the RON 100....Out of the frying pan into the ........?
Summary......? In my opinion he's got a paint problem.......... !!

P.S. Pete, don't worry about the under-engine shots, I located a new unused OEM 4.2 E-Type sump here in Oz...we shan't discuss the price, also found a baffle plate in good nick......Thanks
Regards to all,
Norman.

[PeterCrespin]

Hi Norman.

My dad had half a dozen patents in his name during his 25 years at Shell Stanlow refinery and he did a fair bit of work on knock research IIRC. Octel's plant was next door and I had a chemistry friend who did his industrial year there. Said you could see shiny globules of mercury in the runner channels for the lift doors in that plant!

I agree totally on your definition issue and it's not pedantic at all to point out the difference between various forms of errant combustion. It's just that I was somewhat in layman's mode (not that I know much else) when asking m'learned friend for his source. AKA 'being gentle with him' :-)

Christopher is obviously far from alone in thinking what he thought (hopefully past tense). There are all manner of OWTs doing the rounds about fuel, and oil for that matter. At least I now know we have a definitive source for this kind of thing and I shall cite you if it comes up again anywhere.

[christopher storey]

I hesitate to return to this debate, having been told I am wrong, ( and not having been concerned with this since my aviation days , so I am now more than 20 years out of date) but I think I must defend my thesis! The reason that modern fuels do release more heat than in the past is nothing to do with tetra ethyl lead, which in combustion terms - ignoring its lubrication effect on valves - was there purely to slow the progression of the flame front and prevent a shockwave developing , but stems from the fact that they are designed to be burned at leaner mixtures ( nearer to the stoichiometric ratio) than were past fuels . It is well recognised that flame temperature for any given fuel depends on the equivalence ratio between actual mixture strength and stoichiometric ratio , and that excess oxygen raises the achieved flame temperature . Thus for all practical purposes the flame temperature in current engines is markedly higher than was the case 30 years ago , and as I understand it this is one of the reasons why modern engines, though cleaner in CO terms, are sometimes dirtier in terms of the Nitrogen oxides , particularly NO2. Nitrogen in theory is inert in the combustion process, but in practice a small proportion is oxidised .

The temperature is also affected in reality ( as opposed to in the adiabatic i.e. no heat loss situation) by the fact in petrol engines not only that mixture strength increases as speeds rise, which tends to cool the flame , but also by the fact that the effective CR rises from perhaps 4 : 1 at low speeds and throttle openings to say 9:1 at full throttle , which increases the combustion pressure and therefore temperature

Quite whether this higher temperature prevails in older engines is something I am not certain about , and nor do I know what the effect of ethanol incorporation is , but since in many jurisdictions leaner mixtures,( sometimes with air injection e.g. MGB ), are required than the engines were designed for , it seems to me that it is likely that there is a modest rise in temperature even in our engines unless we raise the mixture strength to a degree which these days is frowned upon

It is a very complex subject and I really don't know enough about it to speak authoritatively but FWIW I put it forward

Chris

[PeterCrespin]

I hesitate to return to this debate...but I think I must defend my thesis! The reason that modern fuels do release more heat than in the past is nothing to do with tetra ethyl lead, which in combustion terms - ignoring its lubrication effect on valves - was there purely to slow the progression of the flame front and prevent a shockwave developing , but stems from the fact that they are designed to be burned at leaner mixtures ( nearer to the stoichiometric ratio) than were past fuels . It is well recognised that flame temperature for any given fuel depends on the equivalence ratio between actual mixture strength and stoichiometric ratio , and that excess oxygen raises the achieved flame temperature . Thus for all practical purposes the flame temperature in current engines is markedly higher than was the case 30 years ago , and as I understand it this is one of the reasons why modern engines, though cleaner in CO terms, are sometimes dirtier in terms of the Nitrogen oxides , particularly NO2. Nitrogen in theory is inert in the combustion process, but in practice a small proportion is oxidised .

The temperature is also affected in reality ( as opposed to in the adiabatic i.e. no heat loss situation) by the fact in petrol engines not only that mixture strength increases as speeds rise, which tends to cool the flame , but also by the fact that the effective CR rises from perhaps 4 : 1 at low speeds and throttle openings to say 9:1 at full throttle , which increases the combustion pressure and therefore temperature

Quite whether this higher temperature prevails in older engines is something I am not certain about , and nor do I know what the effect of ethanol incorporation is , but since in many jurisdictions leaner mixtures,( sometimes with air injection e.g. MGB ), are required than the engines were designed for , it seems to me that it is likely that there is a modest rise in temperature even in our engines unless we raise the mixture strength to a degree which these days is frowned upon

It is a very complex subject and I really don't know enough about it to speak authoritatively but FWIW I put it forward

Chris

Nothing wrong with good-natured debate - I think that's what the forums are for, no? Defend away Don Quixote!

The above post is interesting but on a totally different subject. You're now talknig about the heat effects of varying fuel/air ratios and modern engine technologies but your initial answer was about modern fuels in an XK. How hot any given fuel burns at different fuel/air ratios is not at issue - or wasn't - not least because whatever criterion one applies ('lean' is hotter' or 'modern cars run leaner' etc.) applies to all fuel types and still wouldn't show any difference twixt unleaded/leaded.

I was merely querying a source for your suggestion that "...unleaded fuels burn at a higher temperature than leaded fuels did...and so there is more heat to dissipate. '

This is still not defensible, sorry, no offence.

It follows that your supplementary statement is also incorrect (again, no offence) since it is based on the above false premise:

'...the effect on a large heat sink such as the head and manifolds combined will be very marginal'

I suppose one could argue that 'non-existent' is the same as 'marginal to the Nth degree' but you wouldn't want to do that I'm sure... 8) More accurately, as there is effectively *no* significant change in heat output from today's unleaded compared to yesterday's leaded, there is no 'effect' to speak of, on any heat sink, large or small.

Meanwhile, FWIW, heat sink effects in car engines are essentially only concerned with peaks and troughs. They affect changes in temp over time, not the absolute temps reached when warmed up. Heat-in still equals heat-out and our gorgeous huge XK engines use thermostats in the same ranges as any other engine. So the large head/manifold heat sink affects warm up times and heat soak after switch off. but is irrelevant in a steady-state situation, where temps are essentially static and the waste heats of combustion and friction are carried to atmosphere by the cooling system.

Which, coming back to the original question, DOES actually mean that the bonnet paint would be hotter than ambient for longer after switch-off with an XK than, say, the B-series lump in an MG. But I think we all agreed in that issue was more likely to be paint-related than engine related.

Yours cordially

[norman m. macleod]

Hi all,
I wish to advise the Forum that I, too, like Christopher, "hesitate to return to this debate....!" Not for the same reasons, though, since I believe we are going off at a tangent on an enormously complex subject, the efficient burning of that witches' brew of (varying) hydrocarbons we call petrol. It occurs to me that there are thousands of XK engines out there in E-Types running on unleaded these days, and there are not too many reports of the problem that Paulj has with his paint on the bonnet, and I am sure a fair proportion of those have original paint, many having been re-painted with more modern paints. He mentions discussions with "his paintshop...." so I assume he has had it re-done recently, and I am pretty sure therein lies the problem..............however, I am no expert on modern paints.......
Thanks Pete, for summarizing a few points (pretty well correctly...) that I could have explained in a few thousand words (and have done in a previous life......!!) and thanks to both yourself and Christopher for entering into the Forum debate (isn't that what it's there for....?) In parting, however, I have to take issue with Christopher's comment that tetraethyl lead was not, repeat not, there just to, quote "purely slow the progression of the flame front" unquote, but to allow the use of higher compression ratios (which is related....) The work done under the PV curve of the I.C. cycle is directly related to the compression ratio, and this dictates the limitation of the use of this otherwise very useful cleaning fluid we call petrol in this cycle. It is also the fundamental reason that the diesel engine has overtaken the petrol engine these days, in thermal efficiency, with it's ultra-high compression ratios. Rudolph Diesel pointed the way to the world in the late 1890's, although I am not entirely convinced that he was aware of the implications at the time......! Second minor issue is that, quote "mixture strength varies with speed....."unquote. Why should it? Not for any reason that I am aware of...Any well-programmed electronic fuel management system, or for that matter an appropriately well-chosen and profiled SU HD8 needle might approximate to, would deny that assertion.. There are certainly uneven mixture areas in the induction tract flow at higher revolutions, due to centripetal throw-out on bends, etc., of the low-volatility fractions, but this all gets back to the difficulty of properly and evenly atomising this dreadful brew we call petrol, a problem which has been with the human race for the past 80 years or more.............Better inlet tract and cylinder head design have assisted, but the holy grail is still out there somewhere..
Hi there Paul, get back to your paintshop......!!
Best regards to all in the Forum, from the most isolated capital city on the planet, I hope you all noticed we got 101 E-Types on the cover of the E-Type magazine for the 50th Anniversary, I had reported it through David (Heuer) as 85 (my mistake.....) not too bad for a distant colonial outpost.....(oops...do we use that term anymore.........?)
Norman
Perth, Western Australia.

[paulj]

Hi
well seemed a simple question at the time!
not being a fuel specialist, I just pondered the following question raised by the paint shop.
On returning the car for inspection the manager felt the bonnet and commented on the temprature, which I had to agree seemed extremely hot
to the point where 2-3 seconds would have caused a burn!
so the car was left, and with the paint pulled away it showed oval rings of slight rust marks best descirbed like the rings of a tree section.
my conclusions were.
1, there was some moisture trapped under the paint and with the heat applied pushed the moisture out causing the first ring ,therafter the moisture would ingress and lift the paint another few mm, apply heat and the cycle began again ,
there was no apparent problem with the steel,the paint is solvent base 2 pack laquer finished.
2.the manifold temprature and solvent based paint.
This was not, I felt the problem, but with the paint shops concerns I felt it might be worth more consideration .especially as they had agreed to re-paint it at no cost, given all elements, now we have a gleaming bonett again.
I have had the manifolds coated,and fabricated two stainless heat shields also re-tuned the carbs, I can hardly go back if it re-occurs having made no attempt to take the paint shops concern on the manifold temprature.
thanks to all,
Paulj

[PeterCrespin]

Smart move by both parties Paul and the question was totally legit.

Although obviously the huge majority of E-types have zero paint problem like yours (and therefore the paint is fit for purpose) you might as well try some kind of heat shield in your case if you don't mind doing that. I bet in six months time you'll be fed up of it and everything will be fine.

Was your bonnet in primer for a while? Primer tends to absorb moisture invisibly that can cause microblistering later under the top coat. It's best not to leave cars in primer too long. Not saying that was your issue, just a general point.

[paulj]

Hi
no, car was tgaken back to bare metal,primed and top coated all in the same week
the heat shields actually look great, maybe not for purist but some peace of mind for me,
conclusion
this was a paint problem, the underside of the bonnet was unaffected
but painting doesn't come cheap
so I just wanted to make every effort to aviod a reoccurence
many thanks to one and all for the interest
paulj