Cruising AFR

[BlueZ31]

Hey all,

 

Anyone know the typical cruising AFR (45-60 MPH) for a N/A vg30e? I recently installed a LC-1 and Innovate AFR gauge, and found that my typical cruising AFR at those speeds fluctuates between 16.5-18.0! Under full throttle it is between 12.5-13.5, showing that the previous tailpipe AFR and my buddies temp-loan wideband readings of 14.7-16 at WOT were off (granted that mine is calibrated correctly, having followed the instructions). I have a ka24e 60mm TB, and a new TPS, could the adjustment or voltage measured at the TPS affect cruising AFR, being that the ECU uses this reading? Thanks for the help.

 

-Rob-

[BLOZ UP]

Is it automatic? The manuals do not have a pot in the TPS, it's just a switch. Even if it does it's usually just for tip-in adjustments.

 

What's your fuel pressure at?

[BlueZ31]

My fault,

 

Its a 5-speed, new walbro 255lph pump, 30-32psi at idle, 36psi the second the pedal is depressed, as per the FSM.

[randy 77zt]

A modern dohc 4 valve engine with a centered spark plug can run 15/1 or 16/1 a/f with no problems.I was riding in a 2008 Subaru STI with some kind of laptop tuning software connected and saw a 16.2/1 at steady 60mph.Alot of it has to do with combustiion chamber design.

[BLOZ UP]

I've run the VG at 19:1 to get great MPG. It just skyrockets the NOx emissions. I'm not sure what AFR the OE ECU wants but it should be in closed loop during cruise and I imagine emissions are it's top priority.

[rejracer]

There is nothing wrong with running an engine that lean so long as the components can handle the added temps. NOx is caused by high combustion temps. I think (but have never tried) water injection would lower the combustion temps, and thus lowering NOx.

[Leon]

I've run the VG at 19:1 to get great MPG. It just skyrockets the NOx emissions. I'm not sure what AFR the OE ECU wants but it should be in closed loop during cruise and I imagine emissions are it's top priority.

Actually, NOx emissions peak around 16.5:1. At 19:1, NOx levels likely drop down to where they were at ~14:1, or lower, as combustion temps keep dropping. If you can actually run your engine at a consistent 19:1, your NOx emmissions are fine. However, if you're having intermittent incomplete combustion (likely, especially with an older engine at 19:1AFR), then HC will skyrocket from these misfires.

 

 

There is nothing wrong with running an engine that lean so long as the components can handle the added temps. NOx is caused by high combustion temps. I think (but have never tried) water injection would lower the combustion temps, and thus lowering NOx.

This is yet another common fallacy, so bear with me on this one. Burned gas (combustion) temperatures are dropping as the mixture leans out, not increasing. Higher cylinder temperature means higher cylinder pressure, higher cylinder pressure means more torque. Otherwise, if temps were increasing as you went leaner, you'd get best power from an engine by running it lean. This is not the case.

 

The maximum combustion temperature point is slightly below stoichiometric (e.g. rich, about 13:1). This is because the dissociation of certain chemical compounds in the cylinder presents a little bit of extra oxygen, which can be taken advantage of by putting in a bit more fuel.

 

Formation of NOx depends on temperature and the amount of O2 present in the exhaust gasses. The reason NOx is a maximum when the engine is lean (~16:1) is because as the mixture is leaned out, rising O2 concentrations initially offset dropping combustion temps.

 

This is akin to the torque and power relationship. The torque curve has a peak (dictated by the engine's design) at a certain rpm. Power is an equation dependent on torque and rpm (generally speaking: Power = Torque X Speed). Look at engine ratings, the rpm where max power occurs is always higher than peak torque rpm. Why? Because the increase in rpm initially makes up for the loss of torque.

 

Same thing with NOx! Except now you can look at combustion temps as the "torque curve" (maximum dictated by AFR, at ~13:1) and O2 concentration as the "engine rpm" (keeps increasing). NOx is your "power curve". As combustion chamber temperature begins dropping, O2 concentration increases. The initial increase of O2 offsets the decrease in temperature and therefore increases NOx to a maximum when slightly lean. After that, NOx drops back down.

 

Reference: "Internal Combustion Engine Fundamentals" by Heywood

[rejracer]

Thanks Leon. So in summary High temps + slightly lean (excess of Nitrogen and Oxygen) = NOx.

 

There are 3 possible ways to fix the NOx issue:

1. Lower the compression ratio to lower combustion temps

2. Water injection could take the combustion temps down, eliminating 1 factor in the forming of NOx.

3. Modify the engine to consistently run at 19:1 or leaner which would lower combustion temps.

[Tony D]

Modern-Schmodern! The Chrysler "Lean Burn" emissions engines of the 70's were running 19:1 and higher with heavy catalyst work to scrub the leftovers...

 

I would only Parrot what Leon had to say otherwise...

 

In response to the 'fix the NOx' issue, that is if you are compliant with Emissions. Actually the graph was posted by Rayaap on the relationship between the exhaust gasses. The leanest burn with the highest NOx is very close to the lowest point for HC/CO... This is how you can lean out the SU's to pass to catalyzed standards save for NOx.

 

With modern EGR theories, cooling that EGR allows for MUCH heavier recycling of the gas to allow the engines to run in this AFR region for naturally low emissions (as opposed to running closer to Stoch--in essence WASTING FUEL in order to leave enough to support catalytic action combustion!) and then just pump EGR to lower the combustion temp and drop NOx to a catalyst capable level.

 

Water injection works to do this, but the effect is also do to some cracking of the steam vapor into H2 and CO.

 

"Fixing" NOx really has been the job of the catalyst as the tradeoffs are pretty complex interactions. If they do it pre-catalyst, it usually involves HEAVY EGR usage. And that in the past has heated everything up causing other problems. Now with those coolers pirated from a Diesel...you could probably recirc a WHOLE LOT MORE EGR and drop that NOx quite low without that problem.

 

Of course, you then have to amply modify your coolant system to handle the rejection of the EGR cooler running all the time.