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I'd rather suspect that depends on whether you think rough performance is indeed an advantage.
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See now, we're all pretending to be only slightly / mildly / purely academically intrigued now aren't we..........
Heh heh !
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HB,
i've got a 'friend' interested in the subject....:-)
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Quite a few motorcycles have CATs now, with some it is simply a matter of taking off the end can, slipping the cat out out and putting the can back on.
I suspect that if the millions of vehicles on our roads used 10% less fuel, the need to produce CATs (energy intensive) was negated and albeit a little more NOX and CO was emitted we would be better off.
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I wonder if it would be easy to do on a ......oh I don't know........Mondeo or something maybe ? Just idly wondering really......
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Popular cars those Mondeos aren't they Humph. Hey, don't you have one of those? ;)
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As a cat-equipped car burns very rich to leave enough unburned petrol to fuel the cat; de-catting will almost inevitably fail the MOT unless the ECU is re-programmed to run lean enough to meet the cat emission limits.
If it's running that lean, then there won't be much power advantage - unless the programmes are switchable.
There isn't a great deal of back-pressure caused by a cat of reasonable size - it does compromise the optimal exhaust design on many fronts though.
Banning cats and their associated benzine-ridden petrols would dramatically reduce the amount of fuel needed [and thus CO2] and improve air quality too. They were a failed technology only introduced through lies and corruption and have improved nothing.
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I see that great minds may think differently, but may come to similar conclusions Screwloose... Cats are Californian Carp... CaCC for short
:o}
Edited by Lud on 16/10/2008 at 18:07
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I agree Screwloose but to say an ECU controlled car with a CAT runs a lot leaner than an 80's car on carbs did however it is true to say that an engine could run leaner, nearer stoichimetric without a CAT.
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>>however it is true to say that an engine could run leaner, nearer stoichimetric without a CAT.
No it isn't.
Catalytic convertors need to do two jobs simultaneously - they need to oxidise CO and HC, while also reducing (removing the Ox from) NOx.
If the cat equipped car were run rich, there would not be enough oxygen available to oxidise the CO and HC.
The the car were to run lean, there would be plenty of oxygen available, and the reaction to remove it from the NOx would not proceed.
This is why cat equipped cars need to run for most of the time within a very narrow window of air / fuel ratios about stoichometric, and hence why we need a lambda sensor in the exhaust to keep continually nudging the fuelling back towards stoichometric.
Yes, extra fuel can be used when cold to allow reactions in the exhaust manifold to heat up the lambda sensor and cat earlier, and yes, fuel may be enriched at full power, but, predominantly, cat equipped cars run at or near lambda = 1
Of course, petrol engines could run far leaner than this, and if lean burn had not been strangled by the stupid import of American style emissions laws, who knows? the problem of NOx reduction might now have been solved. But, with no route to market for such engines, research on them has died off somewhat. In our current framework, I don't think it will ever happen.
80's engines on carbs did run quite rich at tickover, under acceleration, and at high engine speed, but, very very lean at part load cruise conditions - Ford's CVH was very much a foray into lean burn; if only Ford hadn't inflicted the Venomous Venturi carburettor onto it!
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No it isn't.
....
>>Of course, petrol engines could run far leaner than this, and if lean burn had not been strangled >>
Hang on NC, I think we are in agreement, my point was that contemporary ECU controlled CAT equiped engines run leaner than older carbed units though could run leaner still without a CAT.
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You said;
>>however it is true to say that an engine could run leaner, nearer stoichimetric without a CAT.
Your statement directly implies that a cat equipped engine runs richer than stoichometric. That isn't true.
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You said; >>however it is true to say that an engine could run leaner nearer stoichimetric without a CAT. Your statement directly implies that a cat equipped engine runs richer than stoichometric. That isn't true.
Perhaps I should have said "consistently nearer stoichimetric without a CAT."
My understanding is that a typical 3 way CAT equipped engine will run a/f ratios of below stoichimetric under acceleration though lean burn techinques enable consistently leaner running.
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>>Perhaps I should have said "consistently nearer stoichimetric without a CAT."
No, that would be even further from the truth, as cat equipped engines tend to run very close to lambda = 1.
>>My understanding is that a typical 3 way CAT equipped engine will run a/f ratios of below stoichimetric under acceleration
Yes, there are some regimes of engine operation as described in my post above where temporary rich running is allowed. Overrun fuel cut-off being an example where temporary extreme weak runing is also temporarily OK.
However, for the cat to work, the time averaged lambda value must be very close to 1 - there's a narrow band of air/fuel ratios centred on lambda = 1 where the cat will work. This represents the vast majority of engine operation.
though lean burn techinques enable consistently leaner running.
That's definitely true.
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As a cat-equipped car burns very rich to leave enough unburned petrol to fuel the cat; de-catting will almost inevitably fail the MOT unless the ECU is re-programmed to run lean enough to meet the cat emission limits.
Drat and damn and blast it.
So in other words, when my MAF sensor went and the car was running too lean, all I had to do was remove the cat, then it would have been running roughly correctly *and* with better performance.
Now he tells me ;)
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A decat on "classic" Impreza Turbos is a common performance mod with an ECU remap too.
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The fitting of catalytic converters is not compulsory, the compliance with emission standards is (set by DOT and Euro land). The fitting of a cat is the easiest and surest way manufacturers have of ensuring that emission standards are met (over a wide range of conditions and time).
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So, does that mean that my friend with his catless cars is not actually breaking the law ? Providing they meet emissions regs that is.
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As far as I recall; the fitting of a cat is compulsory under EU dictat.
It's quite possible to build a cat-less engine to meet the current standards - just not allowed following pressure by the oil companies etc. The last thing they wanted was Ford's lean-burn engine....
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Screwloose is right. The legislators actually had the stupidity to legislate a solution rather than a problem i.e. "fit a cat" rather than "achieve these emission values...". Of course they're all highly trained and imaginative engineers so we're all ok with that aren't we? The following week I believe they were experts on salt content on food and the week after it was the maximum size of mesh in a fishing net. Alright I fibbed about the last one.
JH
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>>Screwloose is right. The legislators actually had the stupidity to legislate a solution rather than a problem i.e. "fit a cat" rather than "achieve these emission values..."
Yes, with the obvious result that petrol engine technology has barely budged, except in tedious and incremental ways, since the early 90's - while diesel's have changed dramatically.
Of course, there's more to the petrol/diesel engine development contrast than the dead hand of the law, but, it's a large factor.
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The Ford 'Lean Burn' programme was always particularly impressive to my mind given that the engineers were stuck with the rough old nails that Ford called its passenger car engine range at the time (Pinto and CVH), yet they still delivered something that showed a lot of promise. A 1960's designed engine governed by rudimentary 80's engine management technology, and running on a carburettor, that still managed the best part of 40 mpg and ultra low CO and HC emissions in a big family hatch.
I can't help but wonder where we would be today if this technology had been developed and applied to a purpose designed modern engine, and the processing power of a modern fuel system. Would we have 2.0 petrol Mondeos and the like doing 55 mpg in the real world?
I remember in the early days of cats, there were big power gains to be had "de-catting" cars, particularly the big engined stuff which used multiple cats. Word on the street at the time was that a Vauxhall Carlton GSI 24v would gain 10 bhp just through ditching the cats. Similarly the 24v Ford Scorpio Cosworth.
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>>Screwloose is right. The legislators actually had the stupidity to legislate a solution rather than a problem i.e. "fit a cat" rather than "achieve these emission values..."
Agreed!
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As far as I recall; the fitting of a cat is compulsory under EU dictat. It's quite possible to build a cat-less engine to meet the current standards - just not allowed following pressure by the oil companies etc. The last thing they wanted was Ford's lean-burn engine....
I don't think so. I've just read the Euro 4 directive and in several places it states "where fitted, catalytic converters......" meaning there are a host of measures surrounding cats (i.e. to protect from failure to safeguard emission levels etc), but only when the manufacturer has elected to use a cat to control emissions.
Lean burn engines have sky high Nox emissions due to very high combustion temperatures. VW (and others) have partly addressed this in their direct injection stratified charge (fsi) engines.
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The EU directive would have been issued around '90-91; I'm fairly sure that it required the fitment of cats from '93 - mainly to kill off the ultra-lean-burn Ford unit that had demonstrated 100mpg at 100mph in an Escort [and still met the CARB '96 regs with only a NOx cat.]
NOx cats are a pretty common fitment now - but I'm still waiting to see a direct injection engine that can last any reasonable distance - and it won't be the disastrous FSi..... [When will VW learn?]
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I've just looked up "stoichiometry" on Wiki because I was feeling thick.
I still feel thick.
:-(
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It is quite simple HB.
It is the ratio of air to fuel that provides the most complete burn and in therory extracts the most energy out of the fuel, stoichimetric is 14.7:1.
An analogy would be with a bunsen burner, light it and you get a candle light yellow flame, the gas is not burning very efficiently and therefore not very hot, adjust the air supply however and you can obtain a very clear roaring flame, the gas is burning efficiently and much hotter.
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To go just a little further than cheddar's post;
For best economy, the engine should run a bit weaker than stoichometric.
For best power, the engine should be run slightly richer than stoichometric.
So, left to their own devices, engine makers would like to push the engine into the lean region during light load running, cruise, for example, and allow the engine to run a bit rich when the driver demands some go.
The requirement to run very close to stoichometric to allow the cat to work prevents or limits these mixture variations, and is limiting the economy we get. So, cats are both expensive and polluting to make, and they reduce the efficiency of the engines they strangle.
Having said all of this, I'm glad that we are rid of lead in fuel - I think that those who defend its use may have been addled a bit by it!
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NC
All pollutants are bad; but if the trade-off for reducing the lead in fuel [from only 0.15 micrograms/ltr] is measurable levels of highly-carcinogenic benzine in urban air, then it wasn't much of a bargain.
The study linking thick kids to playground dust lead levels was laughable.
Then again; my addled memory isn't what it once was.....
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Heh heh... I'm with Screwloose on this one, addled and all...
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I think Screwloose has a good point, after all the removal of lead and the introduction of CATs tackled different polutants though were intrinsically linked by the fact that lead contaminates CATs. However lead still could have been legislated against without the mandatory introduction of CATs.
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To add a little to NC's post:
As NC says, for best economy the engine should run a bit weaker than stoichometric. and for best power the engine should be run slightly richer than stoichometric. Hence high specific output motorcycle engines usually run at ratios under stoichimetric, even at light throttle openings because running too lean on part throttle can effect the throttle response and hence the balance when cornering. Not such an issue with a car.
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Ah, now I understand. Thanks chaps. It was the bunsen burner analogy which unlocked it for me.
All I need to do now is learn how to pronounce stoichiometric and I'm sorted !
Stoki metric
Stochi metric
Stocky metric
Sticky metric
I'll get there..........I'll use that in the snug at the Fox and Rabbit as soon as the opportunity arises..........Can't wait to see their faces......
:-)
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All I need to do now is learn how to pronounce stoichiometric and I'm sorted !
tinyurl.com/5khq4d Click on the red loudspeaker icon. Best of luck!
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Stoi (as on Koi Carp) ... keyo ... metric.
Depends on whether the Fox & Rabbit is in Solihull, Sunderland ...
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Cheshire as it happens. OK got it thanks !
" Ah well y'see its all to do with your Hokey Cokey metrics pal...get them sorted and and you'll be away. Take it from me......"
Slurp.....
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...Slurp...
Now where did I put those car keys?
Steady:)
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>>For best economy, the engine should run a bit weaker than stoichometric.
For best power, the engine should be run slightly richer than stoichometric.
So, left to their own devices, engine makers would like to push the engine into the lean region during light load running, cruise, for example, and allow the engine to run a bit rich when the driver demands some go.<<
This sounds a bit like v-tech technology to me!
Billy
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This sounds a bit like v-tech technology to me!
We are talking air-fuel ratio not valve timing, a V-Tech engine may employ this approach though if it does so it is not because it is a V-Tech engine.
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It doesn't need V-tech; a 1930s SU carb was capable of doing that.
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Emissions are not a problem for cars made before august '92. I've de-catted a couple of cars that I have that were pre '92 as quite frankly I couldn't care less about the environment and would rather have the extra power. On my current car I simply took the cat off and smashed out the contents - easy!
Oh and by the way V-tech make kids toys, I think you mean VTEC ;-)
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