GST with PSI
DSM Wiseman
- 2,728
- 1,503
- Jul 27, 2005
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San Diego,
California
I've got into the habit of helping others with tuning, and as a result, have received may questions via PM asking for various advice. One of the reoccurring questions that seems to get asked is about target air fuel ratios, and how they relate to what your wideband O2 ultimately sees.
When you're first introduced to them, these concepts can be confusing at first. I'm hoping to help others understand these concepts, as they are critical to understanding and targeting proper air fuel ratios while tuning.
I'm going to break this down in what I believe is a simple way for most people to understand. This is by no means an all inclusive guide to these principals, nor do I claim to know the in-depth operational principals and engineering behind these systems. However, for the average Joe tuning their own car, this explanation should satisfy the basic knowledge required to be successful.
Wideband O2 Sensors:
Let's start with the wideband O2. Nowadays, few people tune without one, and for good reason. A wideband O2 sensor is probably one of the best tools to allow the user to determine how their engine is operating. As the name suggests, a wideband O2 (WBO2) sensor simply reacts to changing oxygen levels in the exhaust. The nuts and bolts operation of a wideband O2 can get fairly complex, but for the sake of this discussion, I'm going to keep this at a very basic level. If you would like to understand the details regarding WBO2 operation, you can do some searching on Google, or reference the links below:
http://www.innovatemotorsports.com/resources/news3.php
http://tayloredge.com/reference/Science/oxygensensor3.pdf
https://www.turbobygarrett.com/turbobygarrett/airfuel_ratio_tuning_rich_vs_lean
Now, there are some features of a WBO2 that set it apart for the typically narrow band sensors, but again, for simplicity's sake, just know that most WBO2 sensors very accurately determine air fuel ratio (AFR) by measuring oxygen in the exhaust stream (and as a result, unburned fuel as well). Depending on the ratio of oxygen and fuel detected in the exhaust stream, the O2 sensor outputs a voltage reading, which is then interpreted by some type of gauge or human readable system output (for example, ECMlink).
Air Fuel Ratios:
OK, so your WBO2 will tell you your air fuel ratio (AFR). So, what is AFR, and why does it matter? As the name implies, AFR is simply the ratio of air to fuel. We'll divide AFRs into 3 states:
-Rich
-Stoichiometric
-Lean
Given these 3 states, we'll start by talking about stoichiometric, since it's key to understanding the other 2 states.
Stoichiometric:
Stoichiometric air-fuel ratio is where the major byproducts of combustion (emissions) are at their combined lowest point. This means for a given fuel, you have the correct amount of air and fuel to produce a chemically complete combustion event. Put simply, if you burn all the air and fuel mix without too much of either left over (which in turn reduces emissions such as CO, NOx, etc.), your oxygen sensor will read stoichiometric. This air fuel mix that is chemically balanced is always referred to as stoichiometric, or stoich for short.
Now, each fuel has its own stoichiometric air-fuel ratio. Here are some examples of fuels and typical stoich ratios for each:
C16: 14.8:1
Pump gas: 14.7:1
Diesel: 14.6:1
Q16: 13.6:1
E85: 9.8:1
Methanol: 6.4:1
Given the known stoich value of a specific fuel, we can go back to our 3 states (rich, stoichiometric, lean)
and determine which condition exists:
Anything over stoich = lean
Stoich = stoich (obviously)
Anything under stoich = rich
So, if we were tuning on pump gas (stoich is 14.7:1), a value of 9:0:1 would be rich. A value of 16.0:1 would be lean.
Lambda:
So, W T F is lambda? Lambda is another way of measuring air fuel ratio, and like stoichiometric, indicates AFR based on a standard reference point. So, let's break it down again into 3 states:
-Rich
-Lambda
-Lean
Now, here's where things can get confusing. The lambda value at the stoichiometric air fuel ratio for ANY fuel is always equal to a value of 1.00. The easiest way to understand this is to remember what the stoichiometric AFR actually is. Again, stoichiometric means for a given fuel, you have the correct amount of air and fuel to produce a chemically complete combustion event. So, no matter if you've got E85 or pump gas in the cylinder burning; if the ratio was such that all of the air and fuel were used up in the combustion process, your WBO2 will read stoich, or a value of 1.00 lambda. So once again, going back to our 3 states (rich, lambda, lean):
Anything over lambda = lean
Lambda (1.00) = stoich
Anything under lambda = rich
Lambda vs Stoichiometric:
I like to think of lambda and stoichiometric ratios as two scales which essentially mean the same thing. In a nutshell, they both indicate AFRs. However, lambda is more universal in the sense that it's a constant regardless of the fuel being used. Meaning, if you're using your WBO2 and determine .80 is the optimal target AFR to make power for your application, you will be targeting a .80 AFR regardless of the fuel being used.
If you consider how the lambda scale somewhat standardizes tuning, you wonder why lambda isn't used more often. Well, the answer is traditionally, most WBO2 sensor displays are calibrated to readout on a standard pump gas scale. Meaning, regardless of the fuel being used, at stoich/lambda, the gauge displays 14.7. Now, remember, stoich and lambda both just mean we got a chemically complete combustion event. Your wideband O2 sensor in the exhaust pipe doesn't care what fuel is being burned, it only cares how much oxygen is left over.
So, let's say you decided to swap from pump gas to E85 one weekend. Let's also say your idle is dialed in great, and you're hitting a target AFR of 9.8:1 on E85. Since the O2 sensor simply reads the amount of oxygen in the exhaust stream, it's going to see a nice clean burn at 9.8:1 (stoich of E85), thereby producing a voltage that corresponds to stoich/lambda. The problem now comes when you're sitting in the drivers seat looking at your AFR gauge which is receiving that voltage reading. Remember, we have a gauge that's calibrated to read AFR on a pump gas scale. That means at stoich/lambda, the gauge will ALWAYS say 14.7:1, regardless of fuel. And, it also means your car that's now idling on E85 at 9.8:1 will actually read 14.7:1 on your gauge, since BOTH values are equal to stoich/lambda...So, now you know why you get guys using pump gas AFRs for different fuels, and why the lambda scale makes more sense when comparing fuels.
The one important thing to keep in mind when referencing lambda is differences between various WBO2 setups. For example, if you're using an Innovate MTX-L, the lambda scale and gauge readout will always be the same regardless of the fuel used. However, if you switch to say, an AEM UEGO, the lambda scale and gauge readout will probably be slightly different than that of the Innovate MTX-L, as each manufacturer calibrates how their WBO2s interpret the voltages and data being received from the oxygen sensor, controllers, and other system hardware differently.
Bottom line, if your gauge is calibrated to read lambda, stoich/lambda is always 1.00, and any value over or under is either lean or rich (respectively) regardless of the fuel used. There are formulas to figure out gauge conversions, lambda and various other things. I'm not going to cover them here, but they are easily found on the interwebs via a quick search of the function you're trying to perform. There are also some good examples here: http://www.dsmtuners.com/threads/what-is-lambda.228595/#post-50379068
Wrapping Up:
Again, I've tried to simplify the principals here to make sense to the vast majority of readers. Some of these principals can be explained much more in depth, but I've tried to present the fundamentals in order to appeal to the average dude, such as myself.
For you visual learners, here's a great video that breaks these principals down as I've tried to outline above:
When you're first introduced to them, these concepts can be confusing at first. I'm hoping to help others understand these concepts, as they are critical to understanding and targeting proper air fuel ratios while tuning.
I'm going to break this down in what I believe is a simple way for most people to understand. This is by no means an all inclusive guide to these principals, nor do I claim to know the in-depth operational principals and engineering behind these systems. However, for the average Joe tuning their own car, this explanation should satisfy the basic knowledge required to be successful.
Wideband O2 Sensors:
Let's start with the wideband O2. Nowadays, few people tune without one, and for good reason. A wideband O2 sensor is probably one of the best tools to allow the user to determine how their engine is operating. As the name suggests, a wideband O2 (WBO2) sensor simply reacts to changing oxygen levels in the exhaust. The nuts and bolts operation of a wideband O2 can get fairly complex, but for the sake of this discussion, I'm going to keep this at a very basic level. If you would like to understand the details regarding WBO2 operation, you can do some searching on Google, or reference the links below:
http://www.innovatemotorsports.com/resources/news3.php
http://tayloredge.com/reference/Science/oxygensensor3.pdf
https://www.turbobygarrett.com/turbobygarrett/airfuel_ratio_tuning_rich_vs_lean
Now, there are some features of a WBO2 that set it apart for the typically narrow band sensors, but again, for simplicity's sake, just know that most WBO2 sensors very accurately determine air fuel ratio (AFR) by measuring oxygen in the exhaust stream (and as a result, unburned fuel as well). Depending on the ratio of oxygen and fuel detected in the exhaust stream, the O2 sensor outputs a voltage reading, which is then interpreted by some type of gauge or human readable system output (for example, ECMlink).
Air Fuel Ratios:
OK, so your WBO2 will tell you your air fuel ratio (AFR). So, what is AFR, and why does it matter? As the name implies, AFR is simply the ratio of air to fuel. We'll divide AFRs into 3 states:
-Rich
-Stoichiometric
-Lean
Given these 3 states, we'll start by talking about stoichiometric, since it's key to understanding the other 2 states.
Stoichiometric:
Stoichiometric air-fuel ratio is where the major byproducts of combustion (emissions) are at their combined lowest point. This means for a given fuel, you have the correct amount of air and fuel to produce a chemically complete combustion event. Put simply, if you burn all the air and fuel mix without too much of either left over (which in turn reduces emissions such as CO, NOx, etc.), your oxygen sensor will read stoichiometric. This air fuel mix that is chemically balanced is always referred to as stoichiometric, or stoich for short.
Now, each fuel has its own stoichiometric air-fuel ratio. Here are some examples of fuels and typical stoich ratios for each:
C16: 14.8:1
Pump gas: 14.7:1
Diesel: 14.6:1
Q16: 13.6:1
E85: 9.8:1
Methanol: 6.4:1
Given the known stoich value of a specific fuel, we can go back to our 3 states (rich, stoichiometric, lean)
and determine which condition exists:
Anything over stoich = lean
Stoich = stoich (obviously)
Anything under stoich = rich
So, if we were tuning on pump gas (stoich is 14.7:1), a value of 9:0:1 would be rich. A value of 16.0:1 would be lean.
Lambda:
So, W T F is lambda? Lambda is another way of measuring air fuel ratio, and like stoichiometric, indicates AFR based on a standard reference point. So, let's break it down again into 3 states:
-Rich
-Lambda
-Lean
Now, here's where things can get confusing. The lambda value at the stoichiometric air fuel ratio for ANY fuel is always equal to a value of 1.00. The easiest way to understand this is to remember what the stoichiometric AFR actually is. Again, stoichiometric means for a given fuel, you have the correct amount of air and fuel to produce a chemically complete combustion event. So, no matter if you've got E85 or pump gas in the cylinder burning; if the ratio was such that all of the air and fuel were used up in the combustion process, your WBO2 will read stoich, or a value of 1.00 lambda. So once again, going back to our 3 states (rich, lambda, lean):
Anything over lambda = lean
Lambda (1.00) = stoich
Anything under lambda = rich
Lambda vs Stoichiometric:
I like to think of lambda and stoichiometric ratios as two scales which essentially mean the same thing. In a nutshell, they both indicate AFRs. However, lambda is more universal in the sense that it's a constant regardless of the fuel being used. Meaning, if you're using your WBO2 and determine .80 is the optimal target AFR to make power for your application, you will be targeting a .80 AFR regardless of the fuel being used.
If you consider how the lambda scale somewhat standardizes tuning, you wonder why lambda isn't used more often. Well, the answer is traditionally, most WBO2 sensor displays are calibrated to readout on a standard pump gas scale. Meaning, regardless of the fuel being used, at stoich/lambda, the gauge displays 14.7. Now, remember, stoich and lambda both just mean we got a chemically complete combustion event. Your wideband O2 sensor in the exhaust pipe doesn't care what fuel is being burned, it only cares how much oxygen is left over.
So, let's say you decided to swap from pump gas to E85 one weekend. Let's also say your idle is dialed in great, and you're hitting a target AFR of 9.8:1 on E85. Since the O2 sensor simply reads the amount of oxygen in the exhaust stream, it's going to see a nice clean burn at 9.8:1 (stoich of E85), thereby producing a voltage that corresponds to stoich/lambda. The problem now comes when you're sitting in the drivers seat looking at your AFR gauge which is receiving that voltage reading. Remember, we have a gauge that's calibrated to read AFR on a pump gas scale. That means at stoich/lambda, the gauge will ALWAYS say 14.7:1, regardless of fuel. And, it also means your car that's now idling on E85 at 9.8:1 will actually read 14.7:1 on your gauge, since BOTH values are equal to stoich/lambda...So, now you know why you get guys using pump gas AFRs for different fuels, and why the lambda scale makes more sense when comparing fuels.
The one important thing to keep in mind when referencing lambda is differences between various WBO2 setups. For example, if you're using an Innovate MTX-L, the lambda scale and gauge readout will always be the same regardless of the fuel used. However, if you switch to say, an AEM UEGO, the lambda scale and gauge readout will probably be slightly different than that of the Innovate MTX-L, as each manufacturer calibrates how their WBO2s interpret the voltages and data being received from the oxygen sensor, controllers, and other system hardware differently.
Bottom line, if your gauge is calibrated to read lambda, stoich/lambda is always 1.00, and any value over or under is either lean or rich (respectively) regardless of the fuel used. There are formulas to figure out gauge conversions, lambda and various other things. I'm not going to cover them here, but they are easily found on the interwebs via a quick search of the function you're trying to perform. There are also some good examples here: http://www.dsmtuners.com/threads/what-is-lambda.228595/#post-50379068
Wrapping Up:
Again, I've tried to simplify the principals here to make sense to the vast majority of readers. Some of these principals can be explained much more in depth, but I've tried to present the fundamentals in order to appeal to the average dude, such as myself.
For you visual learners, here's a great video that breaks these principals down as I've tried to outline above:
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