Air fuel ratio on modern vehicles

[Belgian1979]

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.

In the EU, our gas is supposed to be mixed with 5% alcohol. The stoich ratio is actually more like 14.4 to 1 than 14.7 to 1.

That being said, I had a case involving claims from car owners against a company producing diesel fuel. One of the items being mixed in dieselfuel these days is plantoil of several sources. There are other items as well.

Now until that time the legal requirement was that the company had to mix 10% of biodiesel into the fuel over the entire year of production. What happened ? They bought biodiesel fuel when it was cheaper than regular oil and mixed it in proportions far exceeding the required % of biodiesel. On other times during the year the used almost exclusively dino oil. Then on other times the additives of the fuel were a lot cheaper and they started mixed them in larger quantities...until it went wrong of course. As long as the total % over 1 year was equal to the legal requirements there was not a problem (so to speak)

Recently rules changed : now the biodiesel has to be blended in always in quantities of 10%. However no such rules exist for the additives.

And gasoline is no better.

[MadBill]

Some of the more complicated carbs used in the early eighties, e.g. 'electronic' Quadrajets, had duty cycle solenoids that operated the primary metering rods.

[mk e]

Some of the more complicated carbs used in the early eighties, e.g. 'electronic' Quadrajets, had duty cycle solenoids that operated the primary metering rods.

I had one of those, an 84 buick. It had a check engine light its last 10 years but always ran fine

The tbi setups quickly replaced them, i think because they figured if you have to have the electronics why also have the expense of the carb?

[peejay]

I don't think we would be seeing the high miles per gallon that new cars are getting, if the A/F ratio stayed constantly at 14.7:1.

I remember in one of DVs books he was able to get combustion to occur with a very lean 28:1 ratio...

I hear people complain than new vehicles get poor fuel mileage compared to 15 years ago....Of course new vehicle generally have more power than older models.

Anecdotes aren't data but interesting nonetheless:

I own a 14 year old Volvo. A friend of mine has a now-three year old Focus. Both cars are roughly the same weight (2800 vs. 2900lb), although the Focus is significantly larger outside and roomier inside. Both cars have all aluminum DOHC 4-cylinder engines within 50cc of each other making 160hp at low RPM, although his is direct injected and mine requires a light pressure turbo.

I can expect 28-32mpg, with an absolute high of 37mpg on an extended cruise at 65mph with the cruise set and A/C off.
He, on the other hand, normally gets 35-37mpg, with an absolute high of 43mpg reported on a trip at 80mph cruise with the A/C set to "stun".

Slipperier bodies, reduced frictional losses (lower-friction bearings and seals everywhere), and more efficiency in the engine.

If people are seeing 'less economy" then they are comparing 3500-4000lb bloated cars to the 3000lb cars they replaced. A new Civic is leagues larger than the Accord used to be. A Honda Fit is similarly a lot larger and heavier than a Civic used to be. The late model Taurus is a wooly mammoth in comparison to what a Taurus was even 10 years ago. It's kinda like no matter how big your hard drive is, you'll find a way to make it full.

[peejay]

Sounds like it. Years back when I was interested in GM PCM hacking, there was a site detailing all of the little things they found in the 727/730 computers, one of the things was "Economy Mode" where the computer, when appropriate, would use the O2 to find where stoich was, and then lean out to a certain lambda. It would then periodically enrichen just long enough to find stoich again to recalibrate itself.

Then the EPA told them to knock it off since it was increasing emissions But the code is apparently still in the computers, just not activated.

In the back of my mind I sometimes wonder if this is why it was so hard to get certain cars to pass a sniffer test for NOx.

I wouldn't put the blame for the 3800's legendary fuel economy on such trickery. There's no magic to having a relatively big, low-revving engine that cruises in a fat part of the BSFC curve in a deceptively slippery chassis. For fuel economy you want something that is very good at making 20-40hp, and fat lazy engines tend to do that well. Especially since they all had EGR, which can be REALLY good for increasing fuel economy.

Heck, years back I had a Nissan that got 42mpg until the EGR hose cracked and fell off. Economy dropped to 35mpg. I noticed it straightaway and spliced it back together right there at the fuel station!

[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%..

This was the feature that gave early G.M. 3.8 FWD cars exceptional economy wasn't it??

Some people reported as high as 40 MPG highway on Buick, Olds and Pontiacs for a few years then it dropped off.

Thanks
Randy

Exactly. I had a number of such vehicles. Even the last one, a supercharged 3.8 L. Park Avenue would get 40 M.P.I.G. at a steady 60 MPH; the same as my 1.8 L. Vibe.

[1989TransAm]

" But the code is apparently still in the computers, just not activated."

Yep, the same on my 1989 Trans Am. Like you mine is activated.

[Brian P]

I hear people complain than new vehicles get poor fuel mileage compared to 15 years ago....Of course new vehicle generally have more power than older models.

Anecdotes aren't data but interesting nonetheless:

I own a 14 year old Volvo. A friend of mine has a now-three year old Focus. Both cars are roughly the same weight (2800 vs. 2900lb), although the Focus is significantly larger outside and roomier inside. Both cars have all aluminum DOHC 4-cylinder engines within 50cc of each other making 160hp at low RPM, although his is direct injected and mine requires a light pressure turbo.

I can expect 28-32mpg, with an absolute high of 37mpg on an extended cruise at 65mph with the cruise set and A/C off.
He, on the other hand, normally gets 35-37mpg, with an absolute high of 43mpg reported on a trip at 80mph cruise with the A/C set to "stun".

Slipperier bodies, reduced frictional losses (lower-friction bearings and seals everywhere), and more efficiency in the engine.

If people are seeing 'less economy" then they are comparing 3500-4000lb bloated cars to the 3000lb cars they replaced. A new Civic is leagues larger than the Accord used to be. A Honda Fit is similarly a lot larger and heavier than a Civic used to be. The late model Taurus is a wooly mammoth in comparison to what a Taurus was even 10 years ago. It's kinda like no matter how big your hard drive is, you'll find a way to make it full.

Yup. If you want a car comparable in weight to the original VW Golf/Rabbit or Honda Civic - both of which were sold in huge numbers in the 1970s - your choices are Mitsubishi Mirage, Nissan Micra (in Canada), Fiat 500, smart fortwo, Chevrolet Spark. The current Honda Fit is like the third/fourth-generation Civic and the current Civic is like the third/fourth-generation Accord. The current VW Golf is over 3000 lbs, nearly double the original. VW makes a car that's like the size and weight of the original Golf - it's called the up! and it's two size classes below the Golf now.

But ... The newer cars have much, much better aerodynamics and more efficient engines and more efficient transmissions with (usually) more speeds.

And then there's trucks. My '84 Toyota 2wd long box did fine with its 4 cylinder engine; the truck weighed 2700 lbs. Just try finding a pickup truck that doesn't weigh the better part of double that now! And my Toyota had a bed height low enough that it was simple for one person to load and unload a motorcycle. The new ones are sky high and shorter people can't even see over the edge of the bed, nevermind grab something that's in the truck on the other side of that edge.

The vehicle manufacturers have done well to hold fuel consumption more-or-less steady. If powertrain efficiency were still where it was in the 1970s, that overgrown pickup truck would be getting 9 mpg instead of 18.

[In-Tech]

" But the code is apparently still in the computers, just not activated."

Yep, the same on my 1989 Trans Am. Like you mine is activated.

Woah, you gotta 89 TA? .......

[AUTOMAN]

Feedback o2 definitely does saturate/starve rich/lean on purpose. This a balance on HC and NOX oxidation and reduction.

Lean kicks/Rich kicks and tapers control how long it is rich vs. lean.

I test cats.

cliff

[RW TECH]

I test cats.

cliff

I used to do oven aging and dyno aging (4-mode) years ago.