Widebands measure lambda (λ), not AFR.

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Widebands measure lambda (λ), not AFR.

Post by cgrey8 » Fri Apr 12, 2013 12:00 pm

Despite the fact that Wideband (WB) sensors help tuners adjust AFR of a tune, O2 sensors and Wideband O2 sensors do NOT measure the AFR of combustion. They measure lambda (λ), which is the ratio of oxygen vs fuel that is left over in the exhaust burn. This value is then translated to AFR assuming a particular fuel was used (e.g. gasoline, E10, E85, propane, etc) during the combustion. That's the short explanation. Now onto the specifics...

As stated, HEGOs and WB sensors are not "AFR" sensors, they are lambda sensors. They sense the amount of oxygen in the exhaust and report the condition in units of lambda. Lambda measurements indicate whether there was more fuel than oxygen or more oxygen than fuel during the burn. But in order for a wideband to do even that, it must assume that the entire makeup of the exhaust was involved in the combustion...this becomes important later. A lambda burn of 1.000 indicates the burn was stoichiometrically perfect (aka stoich) for whatever fuel was being burned. In the case of 100% gasoline, stoich is accomplished when the AFR of combustion is 14.64:1. Said another way, the intake air charge was made up of 14.64 parts air to 1 part gasoline as it entered the combustion chamber. For those that live in places where E10 ethanol-blended gasoline is sold, stoich AFR is closer to 14.08. For those burning summer blend E85, a stoich burn is attained ~9.76 AFR. To report the exhaust condition in terms of AFRs, what WB gauges and controllers do is ASSUME a fuel being burned and multiply the measured lambda value by that fuel's stoic AFR value. The default for most WBs is to multiply the measured lambda by 14.64 (the stoich AFR for gasoline). So regardless of whether you are burning 100% gasoline, E10, E85, propane, or wood, the WB, in this case, would report the exhaust condition assuming the fuel was gasoline. The WB controller can't "detect" what fuel was burned, thus why it has to be told. So if you are burning E85 in your engine, but your WB is still configured to report the exhaust measurements assuming gasoline, then despite the actual AFR going into the engine, the WB will report the AFRs with a stoich burn indicating 14.64. The same is true if you have an AFR gauge.

So what are your options when you are burning an alternative fuel?
  • Forget the fact that you are burning an alternative fuel and continue tuning as though your fuel is gasoline aiming for the same AFRs you would if it was gasoline.
  • Tell the WB what fuel you are burning so it reports the correct AFR for the fuel you use, but this requires that you get familiar and comfortable with a new AFR range. In the case of Innovate WB controllers, you use LM-Programmer to do that. If I remember right, there's a drop-down list of common fuels the Innovate WB controllers can assume or you can configure for your own custom AFR if you blend your own fuel (i.e. E40).
  • When its available, the best option is to view your exhaust burn condition in terms of lambda.
Setting the WB to report lambda instead of AFR lets you monitor purely in the terms of what is actually being measured by the WB sensor regardless of what fuel you are burning. This way, you don't have to remember what AFRs are correct for your fuel at stoich and WOT. It doesn't matter whether you are burning gasoline, E85, or some custom-mixed blend in between; at cruising, you want to see that Closed Loop floating lambda around 1.000. While at WOT on a naturally aspirated engine, you want lambdas in the .85 range for gasoline & common gasoline blends like E10. Stronger concentrations of ethanol might require a lower lambda. Boosted engines might take lambda down to .78-.80 range at WOT for gasoline & its common blends. Unfortunately, you don't see a lot of WB gauges setup to display lambda. 99% of them all are setup to display AFRs and they ASSUME gasoline. I suspect the digital gauges can be configured.

To get the most accurate account of the exhaust condition, monitor the WB through software that can communicate directly to the WB controller via a serial/USB connection as opposed to voltage translation.

All the above is a mostly theory...perfect world talk. Remember that assumption from above:
Lambda measurements tells you whether there was more fuel than oxygen or more oxygen than fuel during the burn and by how much. But in order for it to do even that, it must assume that the entire makeup of the exhaust was involved in the combustion.
Consider what happens if you have an exhaust leak which can allow unburned air into the exhaust. Or the more typical case of a heavy overlap cam that allows unburned intake air to escape out the exhaust valve at low RPMs. How does the wideband know this is happening? In short, it doesn't. All it knows is that there is a surplus of oxygen in the air and so it reports that condition which translates to the user as a leaner-than-actual condition. In the case of an exhaust leak, fix the leak to solve the problem. But in the case of a heavy overlap cam, the options are not quite that straight forward since the problem is just physics working against you. If this was only happening to the wideband, it wouldn't be a big deal. However a narrowband O2 sensor is going to behave the same way...reporting to the EEC that a lean condition exists when the actual combustion mix is most likely quite rich. If this happens, the EEC will respond by trying to enrich the mix until the O2 sensor is happy. This over-enriches the mix being burned in the engine. At idle, this can cause rich-idle instability where the engine either runs rough or RPM-hunts...not to mention possible black smoke and strong fuel smell coming from the exhaust. Generally heavy overlap cams only suffer this at idle and other light load conditions. RPM/Loads while cruising, accelerating and WOT are generally unaffected.

So what can you do about it? Well the obvious thing would be to put a milder cam in that doesn't have as much overlap. If this will be a daily driven vehicle, then you might be better off with a milder cam so you can run lower idle RPMs, get better driveability, and likely burn less fuel. But for those that are not concerned about fuel economy, emissions, or driveability, there are still options available. In these cases, either
  • Increase the idle RPM high enough that less intake air is making it out of the exhaust and the EEC can maintain idle a little better
  • Force Open Loop while at idle and manually set commanded idle AFR to values that idle the engine smoothly.

Each strategy is different in how to force Open Loop Fueling at Closed Throttle idle. You'll need research how this is done you your strategy using assuming it is possible with your def file.

So what's the difference between a Wideband and a narrowband O2 sensors that most cars today come with? To be so similar, they are actually quite different. So lets start with what an O2 sensor is.

O2 sensors are passive sensors that don't require much special to function...just heat and an exhaust environment to be exposed to. Most O2 sensors today, called Heated Exhaust Gas Oxygen (HEGO) Sensors, have a heater to help them get up to temperature quicker so the EEC doesn't have to wait as long to go Closed Loop. With the exception of the heater in HEGOs, narrowband O2 sensors don't need a power source to do what they do. An O2 sensor produces voltage when it is above its operating temperature (>600°F) and the sensor tip is exposed to an oxygen deprived environment. This is why an O2 sensor can be tested using nothing but a voltage meter and a propane torch. Refer to this link for more details on how to do just that:
Diagnostic Solutions: Rediscovering Oxygen Sensors

Wideband O2 sensors are fundamentally different than narrow band O2 sensors in both construction and in feedback method. For a complete explanation of exactly how WB sensors work, check out this webpage:
How 5-Wire (Wideband) Sensors Work

Anything more I could say about WBs do what they do would just be repeating what's covered in this link.

For tech/spec info on WB sensors, refer to these links:
Planar wide band O2 / Lambda sensor
Bosch Lambda Sensor LSU 4.2

Last bumped by cgrey8 on Fri Apr 12, 2013 12:00 pm.
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