|02-10-2001, 07:48 AM||#1|
Join Date: Aug 2000
Location: Mission Viejo, CA, USA
Exhaust Emissions Explained!!
Love 'em or hate 'em we all make 'em.
When combustion occurs in an engine certain by-products are produced. Here are a few.
Oxides of nitrogen
There are some others too. Carbon monoxide is the one we are most interested in. Please no e-mails from physicists criticising:
Carbon Monoxide (CO) we measure as a percentage of total exhaust emissions to gauge the air fuel ratio. The higher the CO figure the more fuel is in the mixture and the lower the figure the more air is in the mix. Catalytic converters convert CO to CO2 by combining it with the oxygen not used in combustion.
If the CO percentage is too low the combustion burn temperature will rise and cause engine meltdown under certain conditions. If the CO is too high then power is lost very quickly. Broadly speaking 4% CO and below is too lean for safety on a turbo car and 8% and above is too rich and will lose power. An Impreza 22B we had on the rolling road ran over 10% CO (off the gauge). When we reduced it to 7% CO it increased power by 20 bhp.
With you so far. But if you rely on CO% to measure safe engine running and a catalytic converter converts CO to CO2 how do you know if an engine is running to lean?
Good question, clever dick! Now we introduce lambda sensors.
For a cat to work (light up) it has to see a correct and accurate mixture. It then triggers a chemical reaction and converts bad gases to nice clean friendly ones. So that the cat sees the correct mixtures a sensor measures the mixture as it comes out of the engine. If it is too weak the ECU adds a tad more fuel, if it is too rich then the ECU reduces fuel mixture. This is fine until we want to make some proper power. If we were to run the engine at full chat at the mixture that a cat requires to work it would go bang. Therefore when the loud pedal is at a certain position (usually mashed into the carpet) a command in the software says "lets put some proper fuel in and make some power". Then the cat gives up and stops working and then we can measure CO%. All cars are like this. It is a fact that cats only work when the engine is on a constant throttle opening or idle. When you are driving like you damn well should the cat is doing nothing. True of a Nissan Micra or a Nissan Skyline.
Other exhaust gases can be used to tell us different things like combustion chamber efficiency, air leaks and cam profiles.
Is that clear?
I understand all that but why is an emission test carried out at idle and not under any load conditions when true bad emissions would show up?
Search me. Ask a politician.
Why do Subarus run so rich?
Tough one that. It is the same on every powerful Japanese turbocharged car. They always map their cars with far too much fuel.
Surely they know what they are doing at Subaru?
One would hope so, but have you compared your fuel consumption with a 275bhp Cosworth 2 litre 16v turbo? These do not blow up and they will return 30mpg no problem. So who is right?
Another bit of info to pass on from my UK friends.
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