I've been wondering about this for some time now...

[amwebby]

... and it is now bugging me.

Earlier this year the cat on the driver's side failed and I had it replaced.

Prior to that I had a constant problem with corrosion on the right side tailpipe. The end pipes are stainless steel so it was a maintenance issue, involving a vigorous cleaning with metal polish and I didn't think too much of it.

I was always intrigued by why it was only the driver's side. Then the cats were replaced and the problem disappeared so I put two and two together and worked out that the failing cat had somehow caused the rust.

What I don't know is why? Anyone able to shed light on this?

 

[Dieselman]

Because it doesn't heat the exhaust gas temp above the condensation point so you get loads of water being ejected...which happens to be acidic as well.

 

[jaymanek]

i was about to type "cue dieselman..."

 

[Dieselman]

i was about to type "cue dieselman..."

Which end..the sharp pointy one or the blunt one.?..

 

[Dieselman]

It's worth noting that a Cat equipped car emits greater Co2 and thus water content than a non Cat equipped car.
The only reason you don't see them chuffing like steam trains is because the cat is heating the vapour up so the vapour turns to steam, which is invisible.

 

[amwebby]

So that explains why I see more clouds of what looks like water vapour emitting from the exhaust of cat-equipped cars in colder weather?

 

[Dieselman]

Until the Cat raises the temperature of the gas, yes. If the Cat fails they emit clouds of vapour all the time.

A Cat equipped petrol car has to run rich all the time to feed the Cat some wasted fuel, otherwise it can't fire up.

 

[stats007]

That's only for the first minute or so until they're hot.

 

[Dieselman]

That's only for the first minute or so until they're hot.

What is.?

 

[stats007]

Running rich. The reason most cats fail is down to constant unburnt fuel passing through them.

 

[Adam230K]

nevermind

 

[amwebby]

Alright, I won't.

 

[Dieselman]

Running rich. The reason most cats fail is down to constant unburnt fuel passing through them.

All Cat equipped petrol engines run richer than required to pass some fuel for the Cat.
This is why a Cat equipped car loses some performance. The cat can cope with the volume of flow no problem.

Without fuel the Cat can't operate, if it could you would be getting soemthing for nothing which is impossible.

You are confusing standard settings and misfires.

 

[stats007]

I think your understanding of modern cat technology is a little flawed. Modern (petrol engined) cats do very little work - engines run lean when need be and the temperature gradient between inlet and outlet is quite small. There is no need to overfuel the cat - all it requires is sufficient oxygen for the oxidation cycle - overfuelling just pre-heats the cat to a working temp.

 

[Dieselman]

Only diesels use 2 way oxydising cats, petrols still use 3 way reduction cats which require fuel for the process to work.

http://en.wikipedia.org/wiki/Catalytic_converter

These three reactions occur most efficiently when the catalytic converter receives exhaust from an engine running slightly above the stoichiometric point.

A petrol engine produces maximum power when the mixture is slightly leaner than stoichiometric but cars with Cats have the mixture set to stoichiometric for the cat to work well. This reduces engine power.

 

[stats007]

I've not read that wiki entry - but a 3-way cat has a reduction, oxidation and control cycle. Stoichiometric is a theoretical point at which all fuel is burnt which goes against what you said earlier:

A Cat equipped petrol car has to run rich all the time to feed the Cat some wasted fuel, otherwise it can't fire up.

Maybe I'm being pedantic but running rich is above stoichiometric isn't it? .

A petrol engine produces maximum power when the mixture is slightly leaner than stoichiometric but cars with Cats have the mixture set to stoichiometric for the cat to work well. This reduces engine power.

Agreed - cats also do reduce flow slightly which is why 100 cell cats are available - or the DIYer just goes at them with a drill to take the core out when they're knackered!

 

[Dieselman]

Maybe I'm being pedantic but running rich is above stoichiometric isn't it? .

No. Running rich is past the point of peak power for general cruising, hence a Cat equipped car has less power. A stronger mixture is required for full load WOT driving though.
In addition the Lambda sensor switches the mixture stronger and leaner than stiochiometric to fuel the cat for the reduction side to work and then the oxy side to work.
Lean burn technology was banned in the EU when Cats were introduced because it didn't provide the wasted fuel that the Cat needs to run.

Some manufacturers now incorporate lean burn but only on very light load conditions.

Try removing the Cat from an exhaust and see how much power it liberates..virtually zero because the cat is correctly sized originally.

 

[Adam230K]

Alright, I won't.

I asked a question about catalysts... (aimed at DM) but then i went and read about how cats work on the internet and answered my own question .

But i've heard a few times on this forum that the cats can have the mesh inside them knocked out, and closed again, and still pass the MOT! i'm not sure how that works...

 

[stats007]

No. Running rich is past the point of peak power for general cruising, hence a Cat equipped car has less power. A stronger mixture is required for full load WOT driving though.
In addition the Lambda sensor switches the mixture stronger and leaner than stiochiometric to fuel the cat for the reduction side to work and then the oxy side to work.
Lean burn technology was banned in the EU when Cats were introduced because it didn't provide the wasted fuel that the Cat needs to run.

Some manufacturers now incorporate lean burn but only on very light load conditions.

Try removing the Cat from an exhaust and see how much power it liberates..virtually zero because the cat is correctly sized originally.

Rich IS above stoichiometric - the question was rhetorical .

Removing cats gives maybe a 3-5% increase - no different from fitting a different exhaust or air filter and the market for these is huge.

 

[Number_Cruncher]

Petrol engines give their maximum power when running slightly rich, and their maximum economy when running slightly weak when compared to stoichometric.

The value reported during MOT testing is the calculated lambda value, and this represents the excess air ratio. So, if lambda is greater than 1, the mixture is weak. In engine fuelling calculations, there's also a commonly used greek letter, phi, which denotes the excess fuel ratio. Lambda = 1 / phi

This value is not related to the output of the car's lambda sensor which is sometimes called an oxygen sensor. The car's lambda sensor cannot calculate a lmbda value, it switches near the stoichometric point, and simply tells the engine management system rich or weak. This means that the sensor is quite non-linear.

During warm up, the lambda sensor is too cold to switch, and the car relies on its open loop mapping (the open loop mapping is the starting point for all the fuelling calculations). As the exhaust heats up, the lambda sensor switches for the first time - this signal is detected by the engine management computer which then switches into closed loop mode.

When running in closed loop mode, the cat needs to be held close to stoichometric to allow both oxidation and reduction reactions to occur. As the lambda sensor is downstream of the combustion, the signal it reads is delayed. This delay, combined with the non-linear response of the sensor means that the control cannot be smooth, like idle speed control for example, but oscillates between rich and weak. This oscillation is about once per second at tickover, but, becomes faster at higher engine speed as the delay becomes shorter.

This non-linear response is also why the lambda sensor doesn't have full authority in closed loop mode. For example during a transient, like acceleration, the lambda sensor can only "say" weak, it can't give any indication about how weak the mixture is. So, there's no quick way for the ECU to reach the right fuelling point quickly using that approach. Instead, the open loop mappings plus stored adaptive corrections are used to get close to the operating point quickly, and then use the lambda sensor to prod the system back towards stoichometric.

Removing the cat may remove an exhaust obstruction, but as the lambda sensor is still holding the engine operating point too weak for power, and too rich for economy, it's not a great solution.