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[VX1]

cheers paul ! quite a while since i had to use my brain like that ! been working as a mechanical fitter on steelworks for last 8 years !! just do cars at home on the side now !!

Glad to be of some help to you then MM. Still got the brain power to answer some of the questions of your past life  

yea i scare myself sometimes !! only hope it stays there for a good long time paul !!

Sorry mate was just modifying the post.  

[markey mark]

no worries fella ! know exactly what you mean though !!! drives me insane bud !

[Marks DTM Calib]

The modern lambda sensors can last the life of the car as can the cats and this is thanks to modern engiens and fuel injection setups.

Cats tend to be destroyed by contamination or over heating (miss fires result in unburnt fuel lighting on the precious metal noneycomb) but, the most common seems to be impact damage but, I guess thats thanks to all these bloody speed humps!

Ow yes, cats dont filter anything, they act as a catalyist to allow the various toxins and gases to chemicaly react and form less nasty gases etc.....hence why they are formed as a honeycomb in order to create the largest surface area possible for the gases to make contact with.

[markey mark]

The modern lambda sensors can last the life of the car as can the cats and this is thanks to modern engiens and fuel injection setups.

Cats tend to be destroyed by contamination or over heating (miss fires result in unburnt fuel lighting on the precious metal noneycomb) but, the most common seems to be impact damage but, I guess thats thanks to all these bloody speed humps!

Ow yes, cats dont filter anything, they act as a catalyist to allow the various toxins and gases to chemicaly react and form less nasty gases etc.....hence why they are formed as a honeycomb in order to create the largest surface area possible for the gases to make contact with.

what i said basically !!  

[Kevin Wood]

.. and as for Lambda sensors, Wikipedia has a good article.

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

Essentially the narrow band sensors used in most cars measure the balance of oxygen remaining in the exhaust gases. If the mixture going into the engine is rich (too much fuel) almost all the oxygen gets used up burning the fuel and you get very little left in the exhaust gas. A lean mixture results in almost all the fuel burning and some oxygen is left over, meaning a relatively high oxygen content in the exhaust.

The ideal fuel mixture, and the one where the balance of exhaust gases provide the right conditions for the catalytic converter to do its' job is somewhere between the two.

The voltage output of a narrow band lambda sensor underoes a rapid transition from about 0.2v to 0.8 v at this point, called the stoichiometric mixture.

During most running conditions (idling, cruising and gentle acceleration, with a hot engine) the engine management system runs in closed loop mode where it makes adjustments to the fuelling based on the lambda sensor output. The fuelling is constantly increased or decreased to keep the lambda sensor output "tipping" either side of stoichiometric. This is why people refer to Lambda sensors "switching". The constant corrections of the ECU cause the Lambda sensor output to cycle between 0.2 and 0.8 volts.

Put your foor flat on the floor and a stoichiometric mixture will give you poor performance and rather high combustion temperatures, possibly leading to knock, burnt out exhaust valves and melted pistons. For this reason, under heavy load the ECU will go open loop, ignore the Lambda sensor and use the inputs from the other sensors to determine from a map what fuelling to use to provide a considerably richer mixture. Stoichiometric mixture is a ratio of 14.7:1 air to fuel by mass. Under full load a normally aspirated engine will need to richen to probably around 12.8 - 13:1 for maximum power output. For a turbocharged or supercharged engine the mixture needs to be stinky rich - often 11 - 12:1). A narrowband Lambda sensor can't measure this rich so its' output is ignored in open loop mode.

A wideband Lambda sensor works over a much wider range of fuel mixtures and responds faster so an ECU can remain in closed loop operation over the whole range of engine operation. They are not widely used, but when they are, it's usually on turbocharged or supercharged engines where it's important to control the fuel mixture at a rich level under boost to protect the engine. They are also rather useful for mapping engines because they tell you exactly what's going on fuel-wise.

When a sensor dies it generally stops responding to the mixture or responds too slowly. The ECU will detect these conditions and light the EML so you don't need to change the sensors as a matter of course.

Kevin

[shyboy]

.. and as for Lambda sensors, Wikipedia has a good article.

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

Essentially the narrow band sensors used in most cars measure the balance of oxygen remaining in the exhaust gases. If the mixture going into the engine is rich (too much fuel) almost all the oxygen gets used up burning the fuel and you get very little left in the exhaust gas. A lean mixture results in almost all the fuel burning and some oxygen is left over, meaning a relatively high oxygen content in the exhaust.

The ideal fuel mixture, and the one where the balance of exhaust gases provide the right conditions for the catalytic converter to do its' job is somewhere between the two.

The voltage output of a narrow band lambda sensor underoes a rapid transition from about 0.2v to 0.8 v at this point, called the stoichiometric mixture.

During most running conditions (idling, cruising and gentle acceleration, with a hot engine) the engine management system runs in closed loop mode where it makes adjustments to the fuelling based on the lambda sensor output. The fuelling is constantly increased or decreased to keep the lambda sensor output "tipping" either side of stoichiometric. This is why people refer to Lambda sensors "switching". The constant corrections of the ECU cause the Lambda sensor output to cycle between 0.2 and 0.8 volts.

Put your foor flat on the floor and a stoichiometric mixture will give you poor performance and rather high combustion temperatures, possibly leading to knock, burnt out exhaust valves and melted pistons. For this reason, under heavy load the ECU will go open loop, ignore the Lambda sensor and use the inputs from the other sensors to determine from a map what fuelling to use to provide a considerably richer mixture. Stoichiometric mixture is a ratio of 14.7:1 air to fuel by mass. Under full load a normally aspirated engine will need to richen to probably around 12.8 - 13:1 for maximum power output. For a turbocharged or supercharged engine the mixture needs to be stinky rich - often 11 - 12:1). A narrowband Lambda sensor can't measure this rich so its' output is ignored in open loop mode.

A wideband Lambda sensor works over a much wider range of fuel mixtures and responds faster so an ECU can remain in closed loop operation over the whole range of engine operation. They are not widely used, but when they are, it's usually on turbocharged or supercharged engines where it's important to control the fuel mixture at a rich level under boost to protect the engine. They are also rather useful for mapping engines because they tell you exactly what's going on fuel-wise.

When a sensor dies it generally stops responding to the mixture or responds too slowly. The ECU will detect these conditions and light the EML so you don't need to change the sensors as a matter of course.
Kevin

Hi Kevin,
My EML isn't lit at any time. Does that mean that the initial burn off of oil on starting my 2.6l, due I reckon to failing valve stem seals, will not necessarily be damaging my cats? In other words, does burnt oil contamination cause the same effect as unburnt fuel? I'm hoping to delay taking the heads off for a little longer, but don't want to risk destroying the cats.
Cheers,
Bill.

[Kevin Wood]

The life of both the cats and the Lambda sensors is reduced with excessive oil consumption. If this only happens at startup it probably all burns off by the time the cats and Lambda sensors start working. Difficult to say how much is too much. I have seen 2  figures quoted. For Lambda sensors 1 quart per 1,000 miles (what is a quart? about a litre IIRC) oh, for a metric world!

For catalytic converters 0.0015 lb/bhphr is quoted. So, thumb in the air time - assuming an average speed of 60MPH at an engine output of 40BHP I make that very roughly 1/2 a litre per 1,000 miles.  :-/

EDIT: The cats and the lambda sensors are totally separate too. The cats can fail and the Lambda sensors will be none the wiser (except where post-cat sensors are fitted). Only time you'll know cats have a problem is at MOT or if they self-destruct in a spectacular way. A Lambda sensor failure should light the EML though.

Kevin

[shyboy]

1 quart = 1.1365l. I was just going to post that I've got a bit of time probably, and then checked MOT expiry. 8/3/08.
Going to get my answer sooner than I thought.