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I am thinking of buying a diesel car, and have noticed on some models they mention a catalytic converter. I thought that these where only fitted to petrol models or am I wrong?
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They are also fitted to diesels but their longevity is reduced - some Audi's, for example, may have failures on the early A4 TDI cats at low-ish mileage I think. (vague or what?)
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As Guy says they are found on diesels. I look after a lower mileage but older Laguna 2.2D and also a Discovery TDi at 80K miles, both with cats.
I don't doubt Guy's comments but both of these diesel cats are OK so far.
David
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Think they utilise a different chemical process to petrol cats. Like petrol cats they produce foul smells but not the H2s bad egg odour from petrol. Quite distinctive either from my Xantia or following a modern London Bus on my bike. Anyone care to describe?
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Bad egg smell is the result of high sulphur in the petrol. H2SO4 is produced when the cat is still cold but disappears as the cat reaches normal operating temp.
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I don't know about the longevity of diesel cats but exhausts on diesel cars last far longer than petrol. Just replaced two sections of the exhaust on the wifes Maestro TD after 9 years.
I think it must be the greasy soot which coats the inside which protects them.
Alvin
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Alvin
It is also because diesel fuel has more carbon compared to hydrogen in its chemical composition and therefore produces less water vapour. So the exhaust does not rust out from the inside.
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Diesels don't have a three way cat, due to the higher sulphur in diesel it is impossible so far to have one which deals with NOx. Get the sulphur of fuel down and that is possible.
Not sure where Guy gets his info on diesel cats being fragile though.
BTW Vectras have a cat too. So far no problemo despite flloods, rough roads incl some loose surface stuff.
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I think the exhaust smells a bit like the stuff in my home gas-pipes.
I believe that 'natural gas, has very little odour, so, for safety's sake, a 'pong' is added (I seem to recall the additive is Ethyl Mercaptan - one of the smelliest substances around. I managed to synthesise some in the lab at school, 40+ yrs ago - stank the building out, but I was never caught!!)
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Roger,
Your comments about "Ethyl Mercaptan - one of the smelliest substances around" took me back. In a previous life I worked for "The Gas" in the engineering section. Our cabinet of useful things, in the corner of a large open plan office, had a small quantity of Ethyl Mercaptan. It was treated like a deadly animal and kept in a small container within two others.
If there wasn't much else going on we might just pop the bottle out and take the top off for a few seconds, perhaps drip some on the carpet. It was a devil keeping a straight face as those in other areas looked around with comments like "Jesus Christ, what the hell is that?".
Happy days.
David
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David
It's nice when the old grey cells show they are still functioning. It amazes me sometimes what I remember from schooldays. (Yesterday is MUCH more difficult)
Having discovered, in the Guinness Book of records that e-m was the smelliest substance, I looked it up in the Encyc. Brit. (No Internet then!), found the formula C2H5SH, and a brief description of how it was made.
So, off to the lab, heat flowers of sulphur with red phosphorus in a crucible, making a few crystals of phosphorus pentasulphide, allow to cool & add a few drops of ethyl alcohol. Bingo!!!
Roger
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It's 40 years since I did chemistry, but isn't H2SO4 sulphuric acid and H2S hydrogen sulphide (bad eggs)?
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Brian
Yes, and it's H2S that is produced and causes the smell.
Regards
John
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I had the idea some time ago that James Dyson's cyclone system could be used to remove particulates from diesel exhaust (before it reached the cat), but I got the 'not invented here' letter from Dyson.
I'd be interested to know how cats cope, a) with sooty diesel exhausts and b) with the thermal shock that they will experience on 4-wheel drives that venture through deep water. I suspect that both events will require expensive replacement.
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It's not that difficult to remove the larger particulates. It is the small 'aerosol' particulates that are the challenge and these are too fine to be extracted by 'cycloning'.
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If it's a particle, it should be possible to separate it (I should have thought). Aren't these the bits that are peculiar to small high-speed diesels (e.g. car engines) that are carcinogenic?
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James, two very good points but I'm afraid the answer is 'no' to each.
The mixture of gases and very small particles is referred to as an 'aerosol' because that is what it is, i.e. particles so fine that they cannot be readily separated from the gas. If you were to spray red paint from an aerosol can into the spout of a Dyson cleaner, I can assure you that it wouldn't come out the other end separated into a clear gas and a lump of red polymer! Mr Dyson is very clever with words but I don't think even he would claim that!
Many methods have been tried for separating fine particulates without real success, including cycloning, microwaving, imparting an electrical charge, etc. Peugeot's system is almost certainly the best commercially-ready solution so far, although it is complicated and expensive. It includes a store of a special chemical which doses the fuel in the tank at specified intervals to lower the combustion point of the particulates that will be produced, plus a catalyst that will increase the temperature of those particulates to above this lowered combustion point. The catalyst is activated by a shot of fuel into the cylinder between the normal injection events and this is all controlled by sensors and the ECU. Others are working on much simpler systems, for example an auto-selective system wherein an electrical discharge zaps the particulate as it enters a chamber .
Finally, particulate emission is not exclusive to small high-speed diesels at all. I cannot recall the figures but petrol engines are prolific producers and I believe direct injection petrol engines are amongst the worst. As I said before, diesels get the blame because their smoke is often visible, but particulates large enough to be visible are the least harmful. It has not been established that particulates are carcinogous but a strong reason for getting rid of them is their effect on asthmatics.
Sorry to get a bit serious on this normally light-hearted forum but I wanted to clear up these common misunderstandings.
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Thank you for the comprehensive reply. I stand corrected, although I didn't mean to imply that only small diesels produced particles - as you say, the larger they are, the less harmful they seem to be (presumably because they are less likely to be inhaled) but any sort of smoke is pretty objectionable.
I must admit to a strong aversion to diesels, largely because of the noise and the smell (and the terror of spilt fuel on roundabouts when I'm on two wheels), but also because their alleged 'greenness' seems unfounded. I gather than garages run them hard to clear the soot before checking their emissions for MOT, which seems to defeat the object!
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This is where I got the idea that diesel cats don't last as long...
Author: john hill (---.ilford.mdip.bt.net)
Date: 15-01-02 20:13
I own a Audi A4 110 TDI, wghich I purchased new on 1/8/98. To date the catalytic converter has been replaced twice. The present one is now defective and needs replaced, (the car has been serviced by Audi from new and has covered 68,000 miles).
When the last one was fitted by the Audi dealer( Sept 99) I was informed that the part had a 3 year waranty. When thisone began to rattle I contacted the dealer and was told that there was no waranty on this part.
I have contacted Audi stating my disapointment at the life span of this expencive part (£240 + VAT). They informed that this was normal for a catalaytic converter as it was a wearable part.
Please can you confirm if this normal for this part to have such a short life?
If not, what would you think a reasonable time for a catalytic converter.
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Well I must be extremely thick because I cannot for the life of me see the logical link that this one post would lead one to conclude that diesel cats are more fragile/prone to failure.
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