So, this is something that's troubled me for some time.
As I understand it, O2 sensors measure the residual O2 in the exhaust flow. It's in the name - makes obvious enough sense to me....
If you're running lean, there's 'unused' O2 present in the exhaust because you've not burned enough fuel to consume all the oxygen.
The sensor can see that there's O2, and reports it as such.
If you running stoichiometric, then all of the O2 is consumed, leaving none in the exhaust for the O2 sensor to measure and report.
How does it know if you're rich? By which I mean how can the O2 sensor distinguish between your having consumed all the O2 because you've burned precisely the right amount of fuel, and having consumed all the O2 (and having some fuel left over) because the engine's over-fuelling?
I've never been able to get the definitive answer on this one....
But isn't the definition of stoichiometry that it is using precisely the right amount of fuel and oxygen to ensure that there's nothing left over of either? Or, is it that what we in the motor trade call stoichiometric isn't really what a chemist or physicist would call stoichiometric?
Combustion isn't complete when the gas leaves the cylinder, if it was the catalyst wouldn't change exhaust gas composition.
With a good working cat almost all the available oxygen should used up before the exit of the cat.
Way back they used to set the mixture of racing and aircraft engines by the colour of the flames exhaust --- yellow rich mixture, blue lean.
" We're trying to plug a hole in the universe, what are you doing ?. "
(Walter Bishop Fringe TV show)
Now this is where I get (more) confused. I thought this was why you get the rich - lean - rich - lean thing going on to provide O2 and fuel to the cat (deliberately dithering either side of the stoichiometric ratio a little bit). Have I misunderstood this bit? Perhaps I have in that I think that cars with an air-fuel sensor try to hold it at exactly stoichiometric (confused confused confused....)
Confusing especially when wide band sensors keep such tight control of the mixture.
Combustion takes time, modern engines are designed to have turbulence in the combustion chamber to maximise the mixing of fuel, oxygen and flame to speed up combustion and ensure as much combustion as possible takes place before the exhast valve opens. A lot of gas leaving the combustion chamber is carbon monoxide which is itself a fuel.
" We're trying to plug a hole in the universe, what are you doing ?. "
(Walter Bishop Fringe TV show)
Hey Paul. O2's can be confusing... Not the first discussion here I'd start with the narrow band O2. Don't know if you're a premium member on YouTube? I can post some videos if you want.
In case you're not here are 2 previews of SD Premium:
My understanding but open to correction.
Stoich is at that.5 switch point. A narrow o2 cannot hold or tell the ecu how to hold that switch point as it can't really see how rich or lean it is. Just that it's one or the other. I think of it as a toggle switch. It can't hold a happy medium.
That's why we now have wide band. They read how rich and lean the exhaust is and can hold a happy mixture.
A bit of thinking overnight, this is my very rough and ready take on it.
As practical guys we look at Stoichiometry as delivering the correct air to fuel ratio but it is really about achieving the correct Oxygen to fuel ratio.
The at the end of compression the cylinder contains fuel and air Oxygen and Nitrogen. The Oxygen and fuel should be in the correct ratio for stoichiometric combustion
As the Nitrogen in air is relatively unreactive it just a bystander in the combustion process, for the most part it goes in through the intake and out the tail pipe unchanged it just dilutes the Oxygen fuel mixture in the cylinder and in turn the exhaust gasses.
Although combustion is never 100% complete the fuel and Oxygen that is actually burned will be combine at the stoichiometric ratio.
The gases leaving the cylinder are a mixture of combustion products and Nitrogen, unburned Oxygen and unburned & partialy burned fuel (CO)
The combustion products are for the most part water and CO2 which are together with the Nitrogen are relatively unreactive.
The gas the upstream lambda sensor is exposed to comprises fuel and Oxygen mixture diluted by the water, nitrogen and CO2 which are for our purposes unburnable and inert. The upstream Lambda sensor can't see the rest of the gases in the exhaust it can only somehow detect the ratio of unburned Oxygen to unburned fuel.
My take on this is although the exhaust gas is 99% unreactive gases the remaing oxygen to unburned fuel ratio at the upstream Lambda sensor is going to be in the same ballpark as the Oxygen to fuel ratio at the start of combustion.
" We're trying to plug a hole in the universe, what are you doing ?. "
(Walter Bishop Fringe TV show)
Paul Marchant wrote: I'm not a premium member (finite amount of money and all that as cars are a hobby) - It'd be great if I could watch that video...
Cheers,
Paul
It's now unlocked. I do not claim to be the authority on this and I'm not even sure that my delivery was good on this one. But this video will at least let you know where I stand on this subject. Which, by the way, I stopped caring about. Is it a fuel sensor? Is it an oxygen sensor? It's both is my current standing....I think
That's made things a lot clearer - thanks for allowing me to see the video.
I think it's the bit where you say to consider it as a sort of catalytic converter.
My take is that the sensor works by watching the amount / speed of O2 transferred from the inner chamber (atmospheric) through to the outer (exhaust) and that a rich condition should be considered to have a negative amount of O2 in the exhaust (in that the remaining unburnt fuel instantly 'consumes' the oxygen), giving rise to a greater transfer of O2 ions though the wall of the sensor than simply having no O2 in the exhaust.
I think, in my mind, that it is still an oxygen sensor (rather than a fuel sensor), but that it needs to be considered in light of the 'negative' amounts of oxygen that a rich exhaust mixture provides.
Cheers,
Paul
Last edit: 7 years 5 months ago by Paul Marchant. Reason: Thought a bit more about it...
Topic Author
I'd start with the narrow band O2. Don't know if you're a premium member on YouTube? I can post some videos if you want.
