Air fuel ratio on modern vehicles

[MadBill]

Us standards permitted that until the mid-nineties; after 'X' sec. of highway cruise (15?), the AFR for some vehicles would go to ~ 16.5:1. Upon further consideration, the EPA decided it was an 'emission defeat device' and was no longer legal. I had cars with build years straddling the change and my mileage (mostly highway driving) dropped ~ 8%..

[automotive breath]

It's an exciting time for engine development, personally I believe GDI changes everything.

A quote about GDI from the link below:
"The engine management system continually chooses among three combustion modes: ultra lean burn, stoichiometric, and full power output. Each mode is characterized by the air-fuel ratio. The stoichiometric air-fuel ratio for gasoline is 14.7:1 by weight (mass), but ultra lean mode can involve ratios as high as 65:1 (or even higher in some engines, for very limited periods). These mixtures are much leaner than in a conventional engine and reduce fuel consumption considerably."

https://en.m.wikipedia.org/wiki/Gasolin ... _injection

[Truckedup]

I found this about 02 sensors

?



In brief a catalytic converter works on a saturate/starve principal where the fuel injection system saturates the converter then starves the converter, i.e. it runs rich of 14.7:1 (saturate) then lean of 14.7:1 (starve) and as such a narrowband O2 sensor only reads rich of 14.7:1 or lean of 14.7:1.

It's yet another example of internet myths.

If an engine was using a carburettor then the AFR would be stable at close to a lambda of 1, the catalytic converter would work correctly.


The whole reason that the AFR wanders up and down about lambda 1 is due to electronic controls.
You've got a target AFR, a detector(o2 sensor) an error amp (difference between target and actual)...the error amp sends feedback to the computer to tell it to add or subtract fuel.

It's got zero do do with making the cat work correctly by making rich , lean, rich , lean.

It's nothing more than an explanation from someone who doesn't know how things actually work.

My quote was from this internet myth site...

http://www.haltech.com/

[David Redszus]

A oxygen sensor cannot measure air/fuel ratio.
It can only send a voltage based on the partial pressures of oxygen.

The target a/f ratio of 14.7 is a myth.
It is a convenient generalized target value for discussion purposes. The actual value,
even if the fuel and air mass are unchanged will depend on the composition of
the fuel. Which is why lambda is used for mapping and not a/f ratio.

For emissions and performance testing, a specific certification fuel is used. It does not resemble
the characteristics of most pump gas.

Most catalysts have an operational window of +- 1% from stoich, that is 0.99 to 1.01. Modern
ECU mapping is designed to maintain this value except for starting or warm-up conditions.
This value is often confounded by the presence of lubrication oils in the exhaust stream.

Closed loop mixture control is not often used (except at idle) due to variations produced by
mis-fires and cycle to cycle inconsistencies. It is used to set error codes and alarms.

In a street engine, the objective is to burn all the fuel.
In a race engine, the objective is to burn all the air.

[Leftcoaster]

In a street engine, the objective is to burn all the fuel.
In a race engine, the objective is to burn all the air.

Bullseye

[Tuner]

MadBill is 100% correct about the A/F 'dither' to feed the cat.

[user-23911]

Electronic controls don't and can't work without dither.

[user-23911]

MadBill is 100% correct about the A/F 'dither' to feed the cat.

So.......Catalytic converters don't work behind a carburettor because there's no "dither".
Really?

[mk e]

MadBill is 100% correct about the A/F 'dither' to feed the cat.

So.......Catalytic converters don't work behind a carburettor because there's no "dither".
Really?

Yes, plus a carb doesn't control the mixture adequately.

[digger]

the reality is a fuel injected engine in open loop or carb isnt going to cruise at constant AFR anyway it always varies a little so there will be some effectivity you'd think.

i must admit i always thought dither was an inherent outcome of the process, rather than the goal

[Truckedup]

MadBill is 100% correct about the A/F 'dither' to feed the cat.

So.......Catalytic converters don't work behind a carburettor because there's no "dither".
Really?

Yes, plus a carb doesn't control the mixture adequately.

So your saying a cat can't be used on a carbed engine? You mean like the 10's of millions of vehicles from 1975- the mid 80s or something else like specialized like a high HP engine?

[Tuner]

Carbs naturally 'dither' the exhaust stream to the cat because A/F distribution isn't perfect. Apparently we have a few here that have no experience with adjusting the PWM dwell (solenoid rich and lean limits) on an ECU controlled QJet.

[ijames]

MadBill is 100% correct about the A/F 'dither' to feed the cat.

So.......Catalytic converters don't work behind a carburettor because there's no "dither".
Really?

A two way cat will work behind a carb so long as the engine is usually a bit lean of stoich. That provides excess oxygen in the exhaust which the cat uses to oxidize CO and HC into carbon dioxide and water (CO2 and H2O). The first generation cats were two way cats, up until somewhere in the 80's. Some used air injection pumps to add fresh air to the exhaust so the engine wouldn't have to stay slightly lean since that improved driveabilty. To also deal with NOx a three way cat is used, and in this case the NOx must be reduced, not oxidized, so there must be no oxygen and some unburnt fuel in the exhaust to provide the proper chemical environment for the three way cat to convert NOx to nitrogen (N2). In order to provide both an oxidizing and a reducing environment the dither was introduced. While the engine is a bit lean the CO and HC are oxidized and the NOx is absorbed by the third cat section. Then when the engine dithers and goes a bit rich the excess CO and HC are absorbed in the first two cat sections and the stored NOx is reduced in the third section. I don't know if the NOx catalyst is mixed with the first two or in a separate section these days, but the HC and CO catalysts are intimately mixed and applied as a very thin coat to the honeycomb support. So, a two way cat doesn't need the dither but a three way cat definitely does.

[mk e]

Electronic controls don't and can't work without dither.

A proportional only system has error but not dither and can be used if your goal is to eliminate dither......but as others have explained the dither in mixture is there very much on purpose.

[mk e]

So your saying a cat can't be used on a carbed engine? You mean like the 10's of millions of vehicles from 1975- the mid 80s or something else like specialized like a high HP engine?

Not modern cats as others have explained and the carbs needed stuff like airpumps and electronic add-onset.

I'm not saying carbs aren't good, but a simple carb just can't re-jet itself when the weather changes like electronic systems effectively do