- Messages
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- Reaction score
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- Location
- Wrexham
- First Name
- Keith
- Elgrand
- E51
- Region
- Wales
Jeez @Lpgc you lost me at the end of the first sentence lol...There is ceramic in cylinders of every petrol engine - Ceramic insulates the centre spark plug electrode from the outer electrode. But not to worry about ceramics on spark plugs because they hardly ever break up or fail despite being inside the combustion chamber... Spark plug ceramics don't tend to fail unless the engine has been detonating or running too lean a mixture. Lean mixture can increase detonation and increases the potential for oxidation (because if we burn something with a lean mixture there is oxygen left over from the burn, where-as if we burn something with correct or rich mixture there is no oxygen left over after the burn).
We know that the ceramic matrix in Elgrand cats can and does break up.. but it isn't so much the 'heat of the burn' or 'heat of exhaust gas leaving the engine' that causes the cat matrix to break up. Spark plug ceramics look in better condition having run on some fuels that we expect to burn a bit hotter than they do running on other fuels that we expect to run a bit cooler.
There can definitely be a correlation between cat heat and cat failures. If we buy a new Elgrand cat and never fit it to a vehicle, never heat it up, it could sit on a shelf for hundreds of years and the ceramics inside wouldn't break up. If we found the remains of a catalytic converter in an Egyptian pyramid it's likely all the metal would have rusted away but the remaining ceramics would be in perfect condition.
The temperature of a cat isn't only effected by the temperature of the exhaust gas feeding it. Also the more of that gas (like when the engine is burning more fuel) the hotter it will get. And even more so, the more chemical reactions it has to do during cleaning up emissions the hotter it will get. If cats didn't produce heat themselves the hottest they could get would be to equal the temp of exhaust gas but cats get much hotter than heat of exhaust gas. If heat of exhaust gas were extremely hot but there wasn't much of it the cat might not get as hot as it would with a bit cooler exhaust gas but a lot more of it - I can hold my hand a distance over a small flame on a gas cooker but if I increase the size of the flame I would have to move my hand higher over the flame to not burn my hand, yet the small flame and the big flame are at the same temperature. If I ran the cooker on acetylene and oxygen instead of on natural gas and air I could expect to have to lift my hand further because the smaller acetylene flame could have overall more heat than the bigger natural gas flame. If instead of my hand I fitted a catalytic convertor over the cooker, the cat would also see the same incoming heat as my hand but the cat could get hotter than my hand because besides the heat coming from the cooker flame it would make it's own heat doing chemical reactions. When I ran the cooker on natural gas the hottest flame was a nice blue flame, if the cooker were a bit dodgy and gave a yellow flame it would be cooler. I could have my hand lower over a yellow flame than a blue flame but it isn't the same for a cat... A cat might get hotter over the yellow flame, because with the yellow flame it would make more heat inside the cat.
Nissan did a 'fuelling recall' which was supposed to address the cat problem. Looking at difference in readings from Elgrands that haven't had the recall compared to those that have had the recall the recall changed a few things but only slightly. Without the recall Elgrands continue to inject around 0.5ms of petrol on over-run and run a richer mixture than most engines on full load. After the recall Elgrands inject no fuel during over-run and fuelling enrichment at full engine load is leaned off a little (though is still rich). All cars go rich at full engine load so there's nothing unusual about going rich, or even a bit richer than we might expect. And in practice the fuelling recall seems to have done little to help the cat problem - I wouldn't expect it to either. To me the problem does seem to be a design issue on Elgrands.. design of the front cats, the position of them, and of course the 4 cat setup.
But what you really don't want if you have front cats still in place on Elgrands is overly rich mixture at high engine loads...
High engine loads means lots of exhaust gas flowing (bigger flame on the cooker, though this doesn't tell us the temperature of that flame).
Lean mixture means hotter exhaust gas, though that doesn't tell us the 'size of the flame' or the temperature that the cat will reach because with lean mixture the cat won't make as much heat inside itself, maybe not even with 'bigger flame'.
Rich mixture means cooler exhaust gas, though that doesn't tell us temperature that the cat will reach because again we don't know size of the flame, and with rich mixture the cat will do lots of exothermic chemical reactions which will vastly increase the temperature of the cat, exothermic chemical reactions can see the cat get hotter than it would with a lean mixture even though the temperature it was 'fed' at was cooler with the rich mixture than with the lean mixture.
The base temperature of exhaust gas doesn't directly correlate to the temperature that the cat will reach inside, that can depend more on the reactions the cat does or doesn't have to do inside (mixture, and type of fuel) and size of the flame (how much exhaust gas is flowing).
The worst situation is very rich overall at high engine loads (dirty emissions and 'big flame' so lots of dirty emissions to clean up) but with some cylinders running extremely rich mixture and some cylinders running a bit lean mixture, because then there's both way too much fuel plus oxygen feeding the cat and this will increase the temp inside the cat even more than a very rich mixture.
Wouldn't expect to find ceramic cats in a pyramid because they didn't have cats lol, but we do find Egyptian ceramics. Wouldn't expect to find metal matrix cats because they didn't have cats, and if they did they'd probably all have oxidated away by now ;-)
To sum up - Elgrand cats are shite, made even worse by having two pair in series/parallel. Matters made worse by the average age of our cars and the life expectancy of said cats.
Best solutions..
1. Don't buy an Elgrand
2. If you are stupid enough to buy one then get get some bloke called Simon from Yorkshire to convert you to LPG and rip all four cats out ASAP
Btw @Lpgc the Egyptians did have cats, little furry ones that they worshipped