newer engine air/fuel ratio ?

[Truckedup]

Do I understand correctly that newer gas engine fuel air ratios stay around 14.5 rather than get leaner during part throttle as they they used to? I was told this is due to proper functioning of three way catalytic convertors...?

[mk e]

As I understand it they maintain the a/f ratio for emissions and get fuel economy with egr.

[rebelrouser]

The three way converter needs some oxygen left in the exhaust stream to get rid of NOX emissions. It is kind of complicated, but if I remember right, yes the computer some times changes the mixture to make sure the catalyst has enough free oxygen to function. I would not say its only at part throttle.




https://www.arb.ca.gov/research/seminar ... mooney.pdf

[MadBill]

Do I understand correctly that newer gas engine fuel air ratios stay around 14.5 rather than get leaner during part throttle as they they used to? I was told this is due to proper functioning of three way catalytic convertors...?

They've been doing this for many years to support reduction (the opposite of oxidation) of oxides of nitrogen, which can't occur with leaner than stoichiometric mixtures. The AFR 'dithers' rapidly in a narrow range around stoichiometry, allowing both oxidation and reduction reactions to simultaneously transpire. An example I know of is the late eighties/early nineties Buick 3800 engine which after some seconds under steady highway conditions would go into 'lean cruise mode' , ~ 16.5:1 AFR. The EPA then deemed it an "emissions defeat device" and the cars lost several highway MPG running at stoichiometry.

[gruntguru]

The three way converter needs some oxygen left in the exhaust stream to get rid of NOX emissions. It is kind of complicated, but if I remember right, yes the computer some times changes the mixture to make sure the catalyst has enough free oxygen to function. I would not say its only at part throttle.
https://www.arb.ca.gov/research/seminar ... mooney.pdf

Its the other way around. The 3 way cat can reduce NOX at stoichiometry or richer. The oxidising catalyst can reduce HC and CO at stoichiometry or leaner. The solution is to run at stoichiometry (or richer for NOX then add air before a 2 way to oxidise CO and HC).

[peejay]

As I understand it they maintain the a/f ratio for emissions and get fuel economy with egr.

EGR isn't used anymore, cam timing is played with instead. Alter exhaust gas residuals with exhaust timing and alter intake timing to change manifold vacuum (or lack thereof) to reduce pumping losses, and drive by wire because we're now at the point where engine power has little to do with throttle position, so throttle position as a primary input is no longer a valid means of control. It is now an effect, not a cause.

Air/fuel ratio is generally controlled "grossly" with the front O2 sensor, but the rear O2 sensor is used to trim that, because the whole point of feedback control is to keep the cat happy and the rear O2 watches what comes out of the cat. The rear O2 is no longer just a catalyst monitor, it's part of the fuel control and has been for at least ten years.

That is basically all there is to it: keep the cat happy. Fuel control is down to three things: Warm the cat up as quickly as possible, then keep it lit, and enrich as necessary under heavy load to keep it from melting. This is kind of the way it has been since 1975 but the controls and sensor tech has gotten a lot better in the past 40+ years.

This is how we are able to have, say, engines that run stoich up to near 20 pounds of boost (well, that and some serious CFD as regards to cooling flow in the engine), and engines that cruise and even idle at WOT in order to get maximum efficiency.

[Brian P]

Catalyst light-up is helped by using late (often after TDC) ignition timing during warmup (for the first several seconds until the catalyst starts working). Cold start enrichment is minimized. With good fine atomizing injectors and the right injection timing and late ignition timing (more time for fuel to vaporize), there's less need for it.

As soon as the O2 sensors start giving reliable signals, the engine goes into closed loop operation with air/fuel ratio regulated very close to stoichiometric. As mentioned above, that's necessary for the three way catalyst to both reduce nitrogen oxides and oxidize HC and CO. If the exhaust stream is lean, there will be excessive nitrogen oxide emissions, and if the exhaust stream is rich, there will be excessive CO and HC. To meet current emission standards, it is necessary to limit all of these emissions to a few parts per million after the catalyst starts working. That short period of open loop operation after a cold start has to be offset by much lower emissions afterwards.

Lots of engines nowadays operate in closed loop at stoich over almost the entire speed / load range. High RPM full load is not seen in the EPA test procedures, but low RPM full load is often seen ... modern powertrain calibrations get good fuel economy numbers by turning the engine as slowly as possible for what the driver is asking for, but to meet the emission limits, it has to run at stoichiometric under those conditions.

This post prompted me to check something. I have a Chrysler van with the common 3.6 Pentastar + 6 speed automatic combination. It has about 60,000 km on it. I just went outside and wiped a finger on the inside of the tailpipe (the van hasn't run for several hours, so it's cold). The pipe feels clean inside and my finger picked up just the slightest bit of carbon... not even enough to call it black, more like light grey. There's not much rich operation happening!

[jred]

friend of mine works at the local Honda dealership he's one of their troubleshooters and usually gets the problems that the other tech's can't seem to correct,, he was telling me just the other day that Honda runs their engines at about 17 to 1 AFR on low load cruise..


he will be at my shop in the next few days and I will confirm this the next time I talk with him..

[peejay]

This post prompted me to check something. I have a Chrysler van with the common 3.6 Pentastar + 6 speed automatic combination. It has about 60,000 km on it. I just went outside and wiped a finger on the inside of the tailpipe (the van hasn't run for several hours, so it's cold). The pipe feels clean inside and my finger picked up just the slightest bit of carbon... not even enough to call it black, more like light grey. There's not much rich operation happening!

Had the exhaust manifolds off of the shop truck. It's a 4.8l 4wd Sierra that is frequently used for towing and hauling excessive loads in the bed. Engine had something like 280k miles on it (so, maybe 400k kms roughly?). Exhaust ports were clean except for a slight trace of gray.

Fuel control is really good nowadays! Interestingly power enrichment doesn't happen immediately when load is increased, like with carburetors or with early EFI. Catalyst temperatures are modeled, and as long as the model says that the cat won't be melting down, the engine stays running stoich (and therefore clean). If it's not hurting the cat, it's not hurting the exhaust valves either, so everything is fine. It takes three to seven seconds for modeled temps to climb up to where enrichment is actually needed. So the engine will given enrichment for climbing a long grade, as is appropriate, but not for a quick burst of power, when it's not really necessary.

And this is OLD tech, like two or three generations old.

[peejay]

friend of mine works at the local Honda dealership he's one of their troubleshooters and usually gets the problems that the other tech's can't seem to correct,, he was telling me just the other day that Honda runs their engines at about 17 to 1 AFR on low load cruise..


he will be at my shop in the next few days and I will confirm this the next time I talk with him..

Direct injected engines might run 40-50:1 overall. The fuel is concentrated near the sparkplug by piston shape, so the actual mixture that the plug sees is 12-14:1 so it lights off nicely, but that little kernel of combustible mix is surrounded by inert air that insulates the water jackets from absorbing the heat, so more of the energy is used to turn the crankshaft instead of heating the coolant. Rather like a Diesel, actually.

The future is a very interesting place to live!

[Circlotron]

Direct injected engines might run 40-50:1 overall. The fuel is concentrated near the sparkplug by piston shape, so the actual mixture that the plug sees is 12-14:1 so it lights off nicely, but that little kernel of combustible mix is surrounded by inert air that insulates the water jackets from absorbing the heat, so more of the energy is used to turn the crankshaft instead of heating the coolant.

The fact that the flame stops short of the cylinder walls under the above conditions presumably means that any oil film wont get burnt off the cylinder wall, further reducing emissions and maybe prolonging engine life too.

[vwchuck]

Yes the stupid emissions have been killing fuel economy for quite some time now.

[Truckedup]

Yes the stupid emissions have been killing fuel economy for quite some time now.

Examples please....Maybe the circa 1990 3 cylinder econoboxes with feeble acceleration?

[mk e]

As I understand it they maintain the a/f ratio for emissions and get fuel economy with egr.

EGR isn't used anymore, cam timing is played with instead. Alter exhaust gas residuals with exhaust timing and alter intake timing to change manifold vacuum (or lack thereof) to reduce pumping losses, and drive by wire because we're now at the point where engine power has little to do with throttle position, so throttle position as a primary input is no longer a valid means of control. It is now an effect, not a cause.

Yes, I sloppily lumped trapped exhaust in with egr but as you point out they are handled differently and probably shouldn't be lumped.

I think it was Bosch who first started treating the throttle as a requested torque sensor rather than a throttle...turbos, cam position, auto trans....lots of ways to get the acceleration the driver is requesting.

On my last turbo VW it was pretty clear emissions was baked in as well with massive throttle lag....foot to the floor but only gentle acceleration happens..... until you are about 1/2 way into the intersection, then lots of power and wheels spinning to the point that you can not turn without closing the throttle....not the kind of solution I expect from an OEM safety wise.

[dfarr67]

When ethanol content goes up, stoich goes down.