Air fuel ratio on modern vehicles

[Truckedup]

On modern gasoline engine, let's say a few years old, what are typical A/F ratios? I have heard since the use of three way catalytic convertors, lean A/F is no longer the case and it stays pretty close to 14.7 except for full throttle...is this correct?

[MadBill]

Yes. Unless it's changed since I retired from the auto industry a decade back, to both oxidize unburned hydrocarbons and reduce oxides of nitrogen, most (all?) modern North American gasoline road vehicles at less than ~85% of W.O.T. 'dither' the AFR in a small range either side of stoichiometry, relying on the chemical storage capacity of the catalytic converters to 'smooth' the exiting gas composition. Even at W.O.T. some apps maintain stoich. for a period of perhaps 15 sec. to reduce overall emissions while still protecting components from overheating. US NOx standards preclude lean burn for DI gas engines (and give diesel mrfrgs. fits)

[peejay]

Practically everything nowadays has wideband front O2 sensors.

Some/most direct injected engines are running lean with a heavily stratified charge under light load. I never did get an answer about how they're keeping the cat happy with that, but my guess is that they are modeling catalyst temp and only go into lean running after the cat is hot enough, and most everything bolts the cat straight to the cylinder head or turbo anyway so it probably won't "go to sleep".

[Truckedup]

I found this about 02 sensors

?

Broadly speaking there are two different types of O2 sensor available, wideband and narrowband. Both sensors have their place in tuning an engine, narrowband sensors are designed to be used in conjunction with a catalytic converter.

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.

Unfortunately a narrowband O2 sensor is unable to determine exactly how rich or exactly how lean the engine is running making it useless for full power engine calibration (as under full power we need to run the engine much richer that 14.7:1 so we don’t melt pistons!).

A wideband O2 sensor on the other hand is designed to read a much broader spectrum of air to fuel ratios (The Haltech CAN wideband reads accurately from 10:1 – 20:1 air fuel ratio) which makes it the ideal tool for engine calibration.
Narrowband or Wideband?

So for engine calibration purposes, diagnostic purposes, datalogging and performance work a wideband O2 sensor is the correct tool for the job. For emissions related purposes used in conjunction with catalytic converters a narrowband O2 sensor is essential (normally with a wideband O2 sensor in place as well for the full power engine calibration).

[In-Tech]

Afr is a number highly confused. Lambda = 1

[Geoff2]

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...

[Truckedup]

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.

[80427]

Though I am far from an expert I will tell you what I know. I played with the tune in my 2002 K2500 with a 8.1 for about a month last year. I did a lot of work with cruising timing looking for mpg gains. Nothing really significant there but I also played with attempting to lean out the cruising a/f mixture. You could set it as lean as you wanted and the computer would correct it back to lambda or approx 14.7 to 1. My buddy who owns a performance shop and tunes 3-5 cars a week saw the same thing when messing with his daily driver. GM vehicles that are built in Australia actually have the ability to lean out the cruising mixture and actually have a table to tune it that some owners have activated. I assume the 14.7 is required for proper catalyst operation and to prevent NOx levels about what the converter could handle. There could also be something with excessive heat, but after reading a "lean of peak" in the aircraft world I would guess a proper tune would prevent excessive EGT.

I think any gain we are seeing is mpg currently is from transmissions keeping engines at peak efficiency and smaller engines or cylinder deactivation.

[ap72]

Quick point- I chased after running as lean as I could on a car I was tuning a while back. It ran fine but I got slightly better mileage when I went from "as lean as possible" to "as small of injector duty cycle as possible." I think the best was around a mid to high 15:1ish ratio for that car.

[Tuner]

Disregarding misfire, which may depend on several things, fuel ignitability, ignition energy, exhaust dilution, etc, a single cylinder engine efficiency improves the leaner it is up to the point where the work required for friction and intake, compression and exhaust pumping loss becomes greater than the work available from the power stroke.

In a multi-cylinder engine, a lean cylinder can actually not be misfiring but be so lean it doesn't make enough power to exceed its own friction and pumping loss. In an ideal situation where all cylinders have more nearly equal A/F, such as sequential EFI, it is amazing how lean (compared to “conventional wisdom”) an engine will run and run well, with good power at part-throttle were economy gains are most often found.

With EFI systems which have O2 feedback, either turn it off and tune the VE table (and timing table) to lean it or use an Innovate WBO2 with analog out and program it to lie to the ECU O2 input.

Anybody notice we have this same conversation every couple of years?

[Brian P]

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...

Modern engine designs have variable valve timing, don't need to run rich much of the time, and when an automatic transmission is involved, that transmission is a lot more efficient than it was in the old days.

I have two vehicles that are worth comparing to "the old days".

One is a Fiat 500 with a 1.4 litre gasoline engine and a 5 speed manual transmission. This is pretty much as close in size as you can possibly get to the 1978 Honda Civic with 1.5 litre CVCC (lean burn) gasoline engine and 5 speed manual transmission that I had years (decades) ago. The new car weighs more (airbags, crash structures, and the like). The new car has better aerodynamics. The old one had a lean-burn engine. The new one has Fiat's MultiAir system. Fuel consumption is very close to the same between the two! Of course, the new one has far better regulated emissions (CO, HC, NOx) - the old one didn't even have a catalyst. The new one has more power and it complies with today's safety standards. But in terms of fuel consumption ... very close to the same. I will just about guarantee that the modern engine runs stoichiometric almost all the time (as opposed to the Civic's lean-burn system).

The other one is a Chrysler van with their 3.6 Pentastar. This engine has variable timing on both the intake and exhaust sides and a 6 speed automatic and although it's shaped like a big box, they paid attention in the wind tunnel. Compare that to an old school van with no consideration at all for aero and with a 225 slant six and single barrel carb and 3 speed non-lockup automatic and it's a whole different story. The new one uses 11 - 12 L/100 km and has 280 horsepower. The old one would be lucky to see 15 L/100 km and had maybe 95 horsepower and it was smaller inside and with less load capacity ... The new one has better aero, a more efficient engine, a more efficient transmission.

I don't think it's so much that the new engine runs "lean", as that it almost never runs "rich".

[user-23911]

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.

[MadBill]

That ain't so, Joe!
I spent more than ten years working on/managing emission calibrations for one of the Big Three in the eighties and nineties and I can tell you we spent a lot of time dialing in the optimum 'dither' to balance the converters' ability* to both oxidize hydrocarbons and reduce NOx. I can't imagine it's any different today. *The 'downstream' O2 sensor(s) are just there to verify that the converter is adequately 'storing' pollutants until the next dither can neutralize them. When all is well, a constant λ 1.0 is registered. When the output λ fluctuation reaches a predetermined level, a check code for converter failure is set.

[RednGold86Z]

That ain't so, Joe!
I spent more than ten years working on/managing emission calibrations for one of the Big Three in the eighties and nineties and I can tell you we spent a lot of time dialing in the optimum 'dither' to balance the converters' ability* to both oxidize hydrocarbons and reduce NOx. I can't imagine it's any different today. *The 'downstream' O2 sensor(s) are just there to verify that the converter is adequately 'storing' pollutants until the next dither can neutralize them. When all is well, a constant λ 1.0 is registered. When the output λ fluctuation reaches a predetermined level, a check code for converter failure is set.

I've used the down O2 voltage as a target to adjust the dither/switch delay automatically (in our OEM code) - aim for ~600mV and emissions drop a lot. It works pretty well to get that last bit of emissions down. Without it, it's hard to keep CO below 0.1%, with it, 0.01% is normal, and NOx goes to 0, HC 1-10ppm.

You might first think that it's impossible with a switch type O2, but they are actually much smoother when placed after a cat. You can watch them switch from lean 300mV to rich 600mV over the course of a few seconds if you're not making big fuel changes. You have to use some tricky predictive reactions in the control loop to keep it good, but it works.

[Circlotron]

GM vehicles that are built in Australia actually have the ability to lean out the cruising mixture and actually have a table to tune it that some owners have activated.

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