When you are at wide open throttle, what function do the O2 sensors perform?

 

10% Ethanol replaced MTBE back in 2005 when they decided MTBE was polluting ground water. I have been running 10% Ethanol ever since then since in most states that is all you can get, unless you buy straight unleaded 87 octane gas in 1/2 gallon cans from Home Depot.

Bill

 

I glossed over some details and jargon in the first response. At wide open throttle you go into power enrichment mode and the O2 sensors are ignored because they don’t respond fast enough. But the input from O2 sensors during cruise is carried over into power enrichment via the fuel trims.

Suppose the default air to fuel is 14.7 for pure gas and you are running E10 which needs 14.1. During cruise, the O2 sensors have picked this up via the logic in my earlier posts, and the way the change actually appears inside the engine computer is a positive fuel trim, meaning that at any given operating point, the computer injects more fuel than the default table calls for. If the default air to fuel is 14.1 for E10 and you are running pure gas, the same thing happens in reverse, only the O2 sensors react in the opposite direction, and the engine computer is running on a negative fuel trim, injecting less fuel than the default table. So it doesn’t matter what the default air to fuel is. The fuel trim as driven by the O2 sensors brings it to the correct point for the fuel that the car is using.

Now “floor it”, and you go into power enrichment mode. This is built into the default tables. I don’t know exactly what the design numbers are, but suppose the default table is for pure gas, which would mean 14.7 air to fuel at cruise. In power enrichment, you want a richer mixture, so the default table may call for a richer, 12.7 air to fuel (again, I don’t know that exact number, I only know that it is richer). The O2 sensors don’t respond fast enough, so their input is ignored in power enrichment. But the fuel trim is not ignored, it is carried over from what it was during cruise. So in this example, suppose you are running E10. Your operation during cruise would have responded to the O2 sensors and picked up a positive fuel trim to shift air to fuel from the default table’s 14.7 down to 14.1. When you go into power enrichment by flooring it, that trim carries over, so instead of the computer going from 14.7 to 12.7 as it would on pure gas with zero fuel trim, it goes from 14.1 to 12.1, as influenced by the positive fuel trim which is carried over from cruise.

Overall, what the computer is doing is tracking another technical term that I haven’t gotten into, namely stoichiometry. I don‘t want to go into that whole discussion, as it gets lengthy. Other posts by both me and other people have done that, so you can search the term is you are curious. But after all is said and done, what stoichiometry means is that the engine computer uses the O2 sensors and resulting fuel trim to adjust things to have the right air to fuel for the fuel that is being used. Many other things besides ethanol can impact the proper air to fuel ratio. I mentioned one of them in an earlier post, namely, the liquid density of the fuel. That changes quite a bit during winter due to the butane that is added to boost vapor pressure to help cold starts in cold weather. Another thing that impacts the exact air to fuel ratio is the relative amounts of paraffins, naphthenes, olefins, and aromatics in any given batch of fuel. So what is really being targeted by the engine computer is not air to fuel ratio. It is getting the air to fuel ratio right, as influenced by stoichiometry, as measured by the O2 sensors, and as manipulated by the fuel trim.

 

I used to go 25 miles out of the way to get "real gas". (I live out a ways in the boonies) I use it exclusively in my Vettes, boat and all the 2 stroke toys and tools I own.

In the last couple years many of the stations in the area now have "Hi test" none E gas available.

Contrary to what some people say crap ethanol gas makes a performance and mileage difference. It was quite noticeable in my 07 Z06 and my boat lost 3 MPH off the top end. Knowledgeable boat owners will tell you that it takes 15 HP for each MPH increase over 50, so do the math.

I do not know what the effect is with my 15 Vette as I only put none e gas in it!

 

It depends on what type of engine management system a given engine has. Cars are mandated to have closed loop control with O2 sensors, and as noted in my earlier posts, all that happens in such systems when you shift from pure gas to E10 is that a bit more fuel is used to compensate for the lower energy content per gallon of fuel. Maximum power does not change. If someone claims to have seen a maximum power reduction due to E10 in a car built after the early 1990’s, either their engine control system is malfunctioning or they have an overly-active imagination.

I do not know with certainty whether boats have such engine management systems. Very few carburetors have active air to fuel ratio control, so if your boat’s engine is carbureted rather than fuel injected, it almost certainly doesn’t have such controls. Even if fuel injected, it may have only a default air to fuel table without the fuel trims driven by O2 sensors. If the engine does not have closed loop control with O2 sensors, then you will see a reduction in performance going from pure gas to E10. An engine without closed loop controls will run leaner and have less power on E10, so your observation of less speed/power on E10 would be perfectly reasonable. But as noted in the prior paragraph, it would not apply to cars, since all cars have all had such systems since the early 1990’s.

 

Never a problem with my automobiles, but I once had an early model Ducati Multistrada which developed a warped, leaking gas tank . . . twice. It was fixed under warranty, but the dealer said Ducati blamed it on ethanol gas. I have no non-ethanol gas available near me so I got rid of it for a Suzuki with a steel gas tank which still runs perfectly.

So you may be throwing some money away in your cars, but I'd keep the ethanol out of your Italian motorcycle.

 

I used ti get hung up on the ethanol thing, but now I am more concerned about detergent additives. I try now to fuel up with the Top-Tier gasolines.

 

You will be just fine with the e10 in it. never any issues. Automotive engineers know this is the fuel available and have adapted the vehicles to work with it. On the other hand I do use non ethanol fuel in my bike during the winter months when I don't ride it more than a couple times a month.

 

How do the O2 sensors and the computer determine what is rich or lean?

 

 

There is supposed to be a very small, but not totally zero amount of residual oxygen left in the exhaust gas. If the O2 sensors sense that oxygen goes lower than that (or all the way to zero), it is a rich condition, meaning too much fuel. If the oxygen goes higher, it is a lean condition, meaning too much air. Perfect one to one stoichiometry would mean oxygen is zero, but has just barely arrived at zero. But you can’t control to that point because you wouldn’t know if it really is just barely zero, or if there is excess unburned fuel meaning super-rich. So the set point is very slightly above zero. That way it can tell both whether it is lean (which is easy, because leaner and leaner means more and more residual oxygen), or whether it is rich (which is more difficult, because once it gets to zero, it can’t tell whether it is just barely to zero or whether it has gone way too far into serious over-richness).

 

You need to go to pure-gas.org and look at the map of each sate and see how many states have ethanol free gas for sale. For example, in New York and Vermont, there is a chain of convenience stores with over 200 of them sell ethanol free premium.

Sure, some of them don't have ethanol fee gas, but the vast majority of the states do sell ethanol free regular and/or premium. Some have a few stations, like California or Maryland, and some have a lot of stations like Oklahoma. Locally(Springfield, MO), I can buy ethanol free 91 and 93 octane gas without any ethanol at 23 different locations within a ten miles radius of my home, a mix of them non Top Tier(11) and Top Tier(12).

 

The ECM software programming defines what A/F ratios are maintained at various rpms and other conditions. The O2 sensors measure the A/F ratio and supply that info back to the ECM.

One of the things tuners sometimes do is to alter the OEM A/F mixture tables in the ECM software, which is one method of deriving extra HP, but too lean can grenade the engine. That's one of the primary reasons one has to be very careful w/ an aftermarket tune. OEM programming generally errs toward the rich side to protect the engine.

 

That’s mostly right, but if I get real picky, which may be necessary in this situation to avoid further misunderstandings, the O2 sensors do not measure air to fuel ratio. They measure residual oxygen in exhaust gas, which tells you where the air/fuel mixture is in a rich/lean sense. True, the computer varies air to fuel ratio according to its map of air to fuel desired for various power and RPM settings. Also true, it intentionally sets things richer at high power setting to protect the engine from detonation, and leaner at cruise power settings to maximize mileage. As it does that, it expects to see less residual oxygen at higher power settings (ie, richer) and more residual oxygen at cruise power settings (ie, leaner). But the relationship between air to fuel ratio and degree of rich/lean depends on fuel composition, and the computer does not know that in advance. So what the O2 sensors do is close that last linkage by determining whether the engine is at its expected degree of rich/lean. If it is leaner than the computer thought it would be (ie, more residual O2 than the computer expected), it adds positive fuel trim, shifting the air to fuel ratio lower. If it is richer than the computer expected, it does the reverse.

 

I was under the impression that the base line was preprogrammed into the computer and then the O2 sensor referred to that preset number to determine if the mixture was either rich or lean. I was also under the impression that that preprogrammed baseline could be changed by the the factory in their factory tune, or changed by a tuner that went into the computer and changed that baseline number, or the computer could be tricked into a different baseline by installing an aftermarket air breather that enlarged the ID of the tubing where the MAF is located, thus changing the sampling ratio of the MAF and then the computer thinks the baseline has been changed and thus the car runs leaner vs using the stock air breather that matches the sampling ratio designed into the system by GM.

By changing the preprogrammed A/F mixture ratio in the computer, the lean or rich as detected by the O2 sensors can actually cause the car to run leaner or richer than what the factory set as a baseline.

That's why I said that GM changed the baseline number from what was used in the C6's to what is used in the C7. Jim Hall has a lot of knowledge about designing air breathers to trick the computer to cause the car to run leaner than what GM designed. In order to design his air breathers he needed to know what baseline A/FM was preprogramed into the computer by GM and Jim has stated that GM set their baseline A/FM based on E0 gasoline for the C6's and different baseline A/FM for E10 for the C7.

Of course some tuners remove the O2 sensor(generally when they install aftermarket headers), thus the computer adjusts the A/FM based on the preprogrammed baseline in the computer and info from the MAF. There is no feedback from the O2 sensors as to the amount of oxygen in the exhaust pipe, as they are not in the system.

I stated that there was slight change in performance when going back and forth between E0 and E10, and that's because GM installed different baseline A/FM into the computer in the C6 vs the C7.

I believe it's a proven fact that the LS(C6) and LT(C7) engines run on the rich side and by leaning out the A/FM the cars picks up horsepower.

 

Understood, all I was saying in simple way is that the O2 sensors do effectively "measure" the A/F ratio just by providing residual 02 data to the ECM. As you said, the ECM is programmed to add a specified amount of fuel at various rpms, etc., based upon the residual 02 data it receives.

 

Thanks in return. We are in agreement.

 

If you take the O2 sensors out, I would agree that the baseline matters because it must run from the baseline with the only possible adjustment being from power setting, not fuel composition. In that respect, it’s like the old carburetor days when you couldn’t adjust A/F for fuel composition. If the baseline is pure gas and you run E10, you will be lean, and if baseline is E10 and you run pure gas, you will be rich. But if the O2 sensors remain active, choice of baseline shouldn’t matter.

At the risk of seeming redundant with earlier posts, suppose you have active O2 sensors and your default A/F map is set up for pure gas. If you then run E10, the O2 sensors will sense that there is more residual oxygen in exhaust gas than it expects (too lean), and regardless of where you are in the operating envelope, the computer will subtract about 0.6 A/F from the value in the default table. If the default map is set up for E10 and you run pure gas, the O2 sensors will sense rich, and will add about 0.6 A/F to the values in the table. So in the end, it doesn’t make any difference whether the default A/F table is for pure gas or E10. The O2 sensors will sense proper rich/lean, and will use the fuel trims to bring you back to the proper stoichiometric point in the map from either starting position.

Remember, it’s stoichiometry that you really care about, not A/F, and the relationship between A/F and stoichiometry depends on fuel composition. You’re stuck with using an A/F table since air and fuel are what the computer can manipulate, so the question is, how do I make that A/F table into a stoichiometry table. When you take an A/F table and then add in fuel trims as driven by O2 sensors, you then have a stoichiometry table which is what you really want. You would need different A/F tables for different fuel compositions. You only need a single stoichiometry table, regardless of fuel composition.

 

Then when an owner takes his C6 or C7 to a tuner and he changes the baseline A/FM ratio in the computer and then runs it on his dyno and it picks up 30 HP and the owner then takes the car to the dragstrip and it runs .3 tenths quicker and 3 MPH faster they are just imaging those gains because the O2 sensors ignored the change in the baseline and automatically adjusted the A/F M back to the baseline that was in the computer before the tuner got hold the car?

 

I don’t have significant experience with tuners so I can’t comment with confidence on what they are manipulating. I don’t doubt that they make changes that increase power, because as you point out, they generate dyno power increases and drag time reductions. Obviously they could make mechanical changes like cam, compression ratio, etc, or they could alter the spark advance curve, but I don’t think you’re talking about those. If you limit the discussion only to changing how rich/lean the engine runs at any given power setting, then as far as I know, there are only two ways to do that. I can’t rule out the possibility that there are additional ways, it’s simply that I can’t think of any others.

One way would be to disconnect the closed loop O2 sensor controls, which would cause you to run with whatever the default A/F mapping is. In that case, the default A/F mapping would obviously matter. You could either change the mapping in the computer, or you could indirectly vary the mapping by messing with the MAF sensor as you noted in post #55. But for the MAF trick to work, the O2 sensor circuit would have to be disabled, otherwise, fuel trims would bring you back to the original mapping. Trouble is, if you go this route, you lose the ability to have the computer adjust for different fuel composition. While ethanol is the single biggest variable in that area, even if you stick to only “pure gas”, ideal A/F can vary from about 14.5 to 14.9 depending on the exact composition of the batch of gas you happened to get. I’d also guess that this approach would set trouble codes that would not pass inspection in quite a few states.

If you leave the closed loop O2 sensor controls in place, then to change how rich/lean you run at any given power setting, you’d have to change the mapping in the computer of what residual O2 level it expected at any given power setting. I’m sure that could be done, and perhaps the reason we are disagreeing is that we are not using words that are clear to each other. What you describe as changing the A/F baseline may be what I describe as changing the residual O2 mapping. Another variant on that idea would be to change the degree of power enrichment built into the A/F mapping. Since you go off closed loop control when in power enrichment, then you could make yourself richer or leaner at wide open throttle without doing anything to the O2 sensors or residual O2 mapping. But none of the changes in this paragraph would result in any differences between pure gas and E10. The fuel trims generated by the O2 sensor circuit while in cruise would insure that the degree of richness would change equally for both pure gas and E10.