EdmontonSS

Combustion temps will be much cooler at 16:1 compared to 14.7:1 (for a gasoline engine)

All true, however in a steady state condition, an engine can be set richer for better NOX conversion, and leaner for better CO conversion (but, of course, still within the narrow operating band of the three-way catalyst) The air/fuel does not need to be "dithered" for the catalyst to work.

MadBill

In the years I worked on emissions calibrations (1981-92) and oversaw elements of natural gas calibrations (1998 -2003) we for sure needed to cycle rich/lean, and every engine I've scanned through that time and since still shows rapid crossover counts on the O2 sensor readings, signalling the same process is on-going.
The trouble codes set on OBDII-compliant vehicles (i.e, virtually everything in the US and Canada since ~2000) for converter malfunction are triggered by the downstream O2 sensors outputting fluctuating voltage ( = varying AFR) due to ineffective converters failing to store oxygen and thus passing the varying mixture by the downstream sensors. This is why you need simulators for them to produce the 'correct' signal if operating without cats.

EdmontonSS

The cycling is for diagnostics only. Does not need to be done for conversion... I was just clarifying that fact. In addtion, the Miratech systems I've been working on recently diagnose sensor health by some other means, as they do not dither the ratio. For example, older carburated vehicles had no means of "dithering" the air fuel ratio and the catalysts were effective.

MadBill

I don't know about off-road applications, but I can assure you that all the automotive systems I have encountered since the late eighties do indeed dither the AFR at all times once the engine is operating in closed loop. I have spent many hours watching GM Tech II diagnostic screens and a number of aftermarket scanners. The 'cross counts' of O2 sensor activity is a major indicator of emission system health.

From 1975 to ~1986, '2 way' (oxidation only) converters were used and steady lean air fuel ratios at idle and part throttle were necessary. Carburetors vanished at that time.

jimmyblue

iTrader: (11)

The nonlinear nature of the O2 sensor and its time-lag
dependence on heat make stabilizing a linear loop across
all regions of operation really iffy. You'd then also care a
whole lot more about system noise. So about everybody
likes to operate them in a switching mode, and make the
average work out to stoich. Then you have the problem
of -getting- it to switch enough to be meaningful, now.
That means dithering mixture on purpose, on a useful
timescale.

If you are consistently offset in mixture then your fancy
3-way catalyst is probably not going to have what it
needs to crack the nasties, especially since we do not
run a full time AIR pump.

Lean Cruise only is active at fairly low cylinder pressures
and RPM, and so lessens any chance of burning things up.
If you went and tried stupider things you could probably
find a way to hurt yourself. Thank goodness for the
caring hand of government around your neck, eh?

Stoichiometric is what it is, a simple definition of chemical
balance. It does not imply anything other than complete
(ideally, in steady state) reaction with no leftovers. But
here we are in a discontinuous, pulsed flow, every-other-
stroke firejug situation; compromises have to be made,
and "rules" bent to best advantage.

That curve isn't "mine", I just found it lying about some
years ago and put in pocket. It appears to be empirically
based and reasonable, is all I can say. But that's good
enough for me.

2000 cam'n SS

iTrader: (1)

hi everyone....i had a question about my car i have a two thousand ss and am having some problems with it....on some days my car will run like nothing i have ever riden in before...then most days throughout the week it runs it just runs like it has no power....i have looked at the air fuel ratio gauge the last time it ran good and it read that i was running rich during wot.....when it runs bad its running lean.....should it run better when its lean?????and what can i do to fix it to where it runs consistent

Draco

As EdmontonSS has been saying, leaning the mix past beyond 14.7:1 will reduce combustion temps. As you lean the mix, combustion temps AND flame speed decrease, reducing the end-gas auto-ignition known as detonation.

Highest combustion temps occur at a gasoline AFR of around 12.8-13.0:1. This is where maximum heat and flame speed is released by combustion and hence the greatest chance of detonating the end-gasses. As you go richer or leaner from this point of highest temps, temps decrease, and you could infer that things get 'safer'.

All else being equal, Going WOT at 13.0:1 will be more likely to detonate then WOT at 14.7:1, which in turn will be more likely to detonate then WOT at 16:1 or leaner.

There is much research into 'lean burn' engine operation to increase efficiency and reduce NOx emissions. Due to the low flame speed and temperature of lean mixes (and hence lack of power), turbocharging is used. If turbocharging a lean mix doesn't count as 'high load', I don't know what does . The problem is the difficulty in keeping the air fuel ratios stable at these lean mixtures (too close to the misfire limit), and also the increased ignition system power needed. Higher ignition requirements to light off the mix would also support the idea that detonation is less likely to occur in these lean mixes.


Reference:
Brinkman et al. defines the maximum octane requirement as that occurring at an equivalence ratio of 1.1 corresponding with the maximum brake mean effective pressure. Either side of this equivalence ratio, the octane requirement decreases.

N. D. Brinkman et al, "Exhaust emissions, fuel economy and driveability of vehicles fuelled with alcohol-gasoline blends" SAE 750120



There are many other research papers and university course notes that discuss knock limits compared to Air fuel ratios. Just do a google search for topics containing terms like "knock limit" "equivalence ratio" and "flame speed"

RednGold86Z

Leaner does reduce flame speed, and peak temperature. The problem is that knock needs time as well. That slower flame speed just gave the end-gas time to heat up to the auto-ignition point. That slow flame also just reduced the efficiency of the cycle (late peak pressure), and heated the piston more. The way to combat it would be to advance the timing. That's why you'll see lean-cruise add timing. GM gets busted on it in the US, because of the NOx formation associated with it (it "bypasses" an emissions device -- the cat).

The 3-way catalysts (as used on practically ALL cars sold nowadays) do indeed NEED a "dithering" or switching air fuel ratio going acrossed it. It's due to the catalysts OSC (Oxygen storage capacity), typically due to cerium washcoat. Basically, you let the A/F stay lean for a tad, where it stores Oxygen, then go rich, where it catalyzes (combusts) the HC and CO with the aid of palladium or platinum. The Rhodium takes care of the NOx. In order to be highly efficient at all three main emissions - HC, CO, NOx, it MUST SWITCH back and forth. Otherwise it will "break through" (not be efficient enough) on whichever emission it's biased against (HC and CO need O2 and NOx to complete the reaction, so no steady rich allowed, NO needs CO so no steady lean). The oxygen does not come from cam overlap on stock engines, because there isn't any. It's only an air fuel ratio adjustment. I typically aim for 2-5 cycles per second at load, with proper biasing for the cat/O2 sensor/load point combination (bias a tad rich at higher load, leaner at lower, as the drive cycle readouts indicate necessary). Much slower at idle (1 cycle per 2-3 seconds) with a lean bias (no NOx formed anyway, and provides much smoother idle control).

Higher flame speed (power), the production of CO releasing more energy (power), Catalyst "shut down" (de-activates, thus not overheating), Exhaust valve cooling, Piston cooling, and knock and pre-ignition resistance are why engines are run rich at WOT. Some vehicles now run stoich for full throttle for a long time (even trucks), but that's an emissions/economy thing. There are accurate catalyst temperature estimation models being used that will richen the mixture and advance the timing as necessary (shuts down cat, cools EGT). The chambers are designed to reduce knock tendency at stoich (fast burn -> low timing required = low time for end gas/knock/piston heating; quench and spark plug location optimized also), and knock sensors are used as back-up.

Draco

This is a good discussion but I'll respectfully disagree with what you wrote in the above quote. If you have an engine at a fixed RPM, and compare a mix with lower flame speed and temp, and a max power high flame temp/speed mix (say, 12.8:1), then the max power mix is going to be hotter, get to the end gasses sooner, heat the pistons up more while the cylinder pressures are highest, and promote auto-ignition. The leaner mix flame front will get to the end-gas later in the process when pressures are lower, and be cooler. It is simply not possible for the slower burning, cooler flame to heat the piston and end-gasses up more then the hot mix within a fixed amount of time.

Also towards the lean limit approacing misfire, Leaner mixes will actually increase the thermal efficiency of the engine. This is why a leaner mix of 15.5 or 16:1 which can still be ignited without misfire is good for fuel economy. You are right - more timing is used, but only to compensate for the slower flame speed to maintain driveability. And they can get away with more timing because the leaner mix has a lower flame speed, is cooler, and therefore less likely to detonate .

RednGold86Z

Not my words:
http://blizzard.rwic.und.edu/~nordli.../gasoline.html
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7.2 What is the effect of changing the air/fuel ratio?
Traditionally, the greatest tendency to knock was near 13.5:1 air/fuel ratio, but was very engine specific. Modern engines, with engine management systems, now have their maximum octane requirement near to 14.5:1. For a given engine using gasoline, the relationship between thermal efficiency, air/fuel ratio, and power is complex. Stoichiometric combustion ( Air/Fuel Ratio = 14.7:1 for a typical non-oxygenated gasoline ) is neither maximum power - which occurs around A/F 12-13:1 (Rich), nor maximum thermal efficiency - which occurs around A/F 16-18:1 (Lean). The air-fuel ratio is controlled at part throttle by a closed loop system using the oxygen sensor in the exhaust. Conventionally, enrichment for maximum power air/fuel ratio is used during full throttle operation to reduce knocking while providing better driveability [24]. If the mixture is weakened, the flame speed is reduced, consequently less heat is converted to mechanical energy, leaving heat in the cylinder walls and head, potentially inducing knock. It is possible to weaken the mixture sufficiently that the flame is still present when the inlet valve opens again, resulting in backfiring.

7.3 What is the effect of changing the ignition timing
The tendency to knock increases as spark advance is increased, eg 2 degrees BTDC requires 91 octane, whereas 14 degrees BTDC requires 96 octane. If you advance the spark, the flame front starts earlier, and the end gases start forming earlier in the cycle, providing more time for the autoigniting species to form before the piston reaches the optimum position for power delivery, as determined by the normal flame front propagation. It becomes a race between the flame front and decomposition of the increasingly-squashed end gases. High octane fuels produce end gases that take longer to autoignite, so the good flame front reaches and consumes them properly.
The ignition advance map is partly determined by the fuel the engine is intended to use. The timing of the spark is advanced sufficiently to ensure that the fuel/air mixture burns in such a way that maximum pressure of the burning charge is about 15-20 degree after TDC. Knock will occur before this point, usually in the late compression/early power stroke period. The engine management system uses ignition timing as one of the major variables that is adjusted if knock is detected. If very low octane fuels are used ( several octane numbers below the vehicle's requirement at optimal settings ), both performance and fuel economy will decrease.

7.6 What is the effect of engine speed?.
Faster engine speed means there is less time for the pre-flame reactions in the end gases to occur, thus reducing the tendency to knock. On engines with management systems, the ignition timing may be advanced with engine speed and load, to obtain optimum efficiency at incipient knock. In such cases, both high and low engines speeds may be critical.

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RednGold86Z

http://www.contactmagazine.com/Issue...ineBasics.html
http://www.streetrodstuff.com/Articl...ne/Detonation/
Two other good knock/pre-ignition articles, for those that are interested.

joecar

In practice, I don't see this, I see the opposite, why...?

EdmontonSS

The catalyst does not NEED to be dithered to work. It does have to work with "dithering" due solely to the nature of the control system, it is the nature of a closed loop to vary rich and lean. Three-way catalysts did not always have oxygen storage, and were not practical for automobiles until this feature was added via oxidizing metal components. Link follows:
http://www.che.utah.edu/graduateProg...t&lectureID=23

Quoted from the patent that article refers to:
"All of the contaminants can be substantially removed if the air-fuel ratio is closely controlled within a "window" having boundaries of about 14.4 and 14.6, although it may be feasible to operate in the range of 14.2 to 14.9 depending to a large extent upon such factors as fuel composition. It is feasible to control the variations in air-fuel ratio within hese average limits. For example, the fuel supply system can be controlled by an oxygen sensor located in the exhaust gases. The normal variations of such a control system provide a continuous swinging of air-fuel ratio about the desired value, near the stoichiometric value. The variations are small, however, and the average air-fuel ratio generally remains within the operating window, and short excursions outside the window may not unduly adversely affect the operations."
Complete patent available at: http://xrint.com/patents/us/4157316

A very nice graph of catalyst conversion vs. lambda along with the "window of operation" on the top right corner of the first page of this article:
http://www.vehicular.isy.liu.se/Publ...SE_02_PALE.pdf

"Three-Way Catalyst (TWC) Found in most modern systems, the TWC uses a mixture of platinum, palladium and rhodium to reduce all three major harmful emissions concurrently. The efficiency of this device is largely affected by AF, with stoichiometric conditions being the optimum working range. Although fluctuations from 14.7:1 for a finite duration are tolerable, the average AF ratio must be very near stoichiometric."
Found at: http://www.affordable-fuel-injection...s-engineer.htm

More reading at: http://www.naftc.wvu.edu/technical/i.../CNG/CNG1.html under "combustion stagegies"

Reading and colour charts at:
http://ect.jmcatalysts.com/applicati...ducts-nscr.htm

The last two refrences are for natural gas engines, however the combustion byproducts of such are the same as a gasoline engine. The catalyst would not "know" a difference. With a very tight AFR control, dithering and oxygen storage are not required. I do believe that the control systems today are "good enough" that we could control AFR to stoichiometric and have effectice TWC conversions without dithering. However the automotive engineers like the dithering to evalate the health of the oxygen sensor, and it doesn't hurt as long as the catalyst does have the "oxygen storage" capability built in. I'm not saying it's not a good idea, or that dithering is "incorrect" in anyway, just that it does NOT NEED to be done for complete conversion to take place.

I'm not sure about this, I believe there is probably some overlap on even the smallest of stock LSX cams (this being an LSX forum). There is overlap in both of the factory LS6 cams, because I remember reading about the changes in GMHTP magazine (they compared the cam overlap in the article). And this overlap occurs even with the LS6 cams huge lobe separation. In any case, cam overlap or not, the catalyst is simply completing incomplete combustion. As long as the components in the initial reaction are near stoiciometric, the end products will be equivilant, and the three-way catalyst can work.