How To Convert From EQ Ratio To AFR?

To convert EQ ratio to AFR, divide the stoichiometric AFR with the EQ ratio. It’s as simple as that and just a matter of a simple mathematical formula. But this is the commanded AFR. You can also find the AFR from the lambda by multiplying it to the stoichiometric AFR. 

As a tuning enthusiast, you may not come across the term equivalence ratio EQ ratio that often compared to say lambda units. But if you do end up seeing EQ ratio, you might be wondering how it relates to air fuel ratio AFR. Don’t worry.

We are going to show you how to convert from EQ Ratio to AFR?

But that’s just not it. Like everything else in the world, nothing is perfect. So isn’t fuel combustion physics. There is ethanol and consider that in your calculation too. How? Let’s find that out.

Some Common Terms

Following are some of the common terms that you need to know before getting into the details of EQ Ratio and AFR

AFR

Air-fuel ratio is the ratio of the mass of air present to the fuel present in the combustion process. 

Lambda Unit

Lambda unit is the ratio of the oxygen present in the combustion chamber to the oxygen required for perfect combustion with the fuel. It means if the lambda unit is 1, the mixture has the exact ratio of oxygen to burn the fuel. Greater than 1 means more oxygen or lean mixture and less than 1 means less oxygen or rich mixture.

Stoichiometric Mixture

Stoichiometric mixture is the balanced mixture of fuel and oxidizer without any excess in the mixture. 

EQ Ratio

Equivalence ratio is the ratio of the stoichiometric air-fuel ratio to that of the target air-fuel ratio in a mixture. 

How to Convert EQ Ratio to AFR?

In order to convert from EQ ratio to AFR you have to follow a mathematical formula. It is expressed as the following: 

Regular Gasoline

Commanded  AFR = Stoichiometric AFR / EQ Ratio

Mathematically it is shown in the following manner:

EQ Ratio  = Stoichiometric AFR / Commanded AFR

The value of Stoichiometric AFR is 14.7

Now, let’s assume the EQ Ratio is 1.45

So, the Commanded AFR = 14.70/1.45

=10.14

But let’s say you have a lambda unit that is 1. How do you go on to convert it to the AFR? 

Okay, you can relate the EQ ratio to Lambda unit in the following way:

Lambda = 1/EQ

EQ = 1/Lambda

So, Lambda = Commanded AFR / Stoichiometric AFR

So, if your lambda value is 1.20

Commanded AFR = Lambda * Stoichiometric AFR

                   = 1.20 * 14.70

                   = 17.64

But you can only get this value when ethanol is not being mixed with your fuel. So, the ratio in real life is definitely not going to be like this.

So how different will it be? 

E10

Let’s go through the example discussed but this time with E10 fuel. E10 fuel contains about 10% ethanol and the remaining 90% as gasoline. This mixture is sometimes addressed as gasohol. 

Now for E10 fuel, the stoichiometric AFR is 14.08

So, let’s go back to the EQ ratio calculator. 

Commanded AFR = Stoichiometric AFR / EQ Ratio

Similarly, EQ Ratio = Stoichiometric AFR / Commanded AFR

Now, let’s assume the EQ Ratio is 1.45

So, the Commanded AFR = 14.08/1.45

=9.7103

Therefore, you can see that for the presence of Ethanol, the ratio of commanded AFR drops by 4.24%. So, the effect of Ethanol can not be overlooked at all. 

E15

This is a situation where the combination is 15% ethanol and 85% gasoline. Typically, this is the highest range your car’s manufacturer will recommend you to have unless it is a flex fuel car. 

For E15 fuel, the stoichiometric AFR is 13.79

Now, the calculation looks the following

Commanded AFR = Stoichiometric AFR / EQ Ratio

Similarly, EQ Ratio = Stoichiometric AFR / Commanded AFR

Now, let’s assume the EQ Ratio is 1.45

So, the commanded AFR = 13.79/1.45

=9.5103

So, for E15, the commanded AFR decreases by 6.21% from no ethanol condition and by around 2% from E10 fuel. 

E85

This is not recommended to use unless your car is a flex fuel one such as Ford F-150, Chevy Silverado, GMC Yukon, or Chevy Tahoe. It contains about 85% ethanol and only 15% gasoline. 

For E85 fuel, the stoichiometric AFR is 9.765

Now, the calculation for equivalence ratio to AFR is shown below

Commanded AFR = Stoichiometric AFR / EQ Ratio

Similarly, EQ Ratio = Stoichiometric AFR / Commanded AFR

Now, let’s assume the EQ Ratio is 1.45

So, the Commanded AFR = 9.765/1.45

=6.734

So, for E85, the Commanded AFR decreases by 33.59% and from E10 the drop in percentage is by 30.65%. 

Methanol

Even though not so common, methanol can also be used as fuel for different car engines. Methanol is used as a fuel in Monster trucks, USAC sprint cars, and a few other racing cars. 

For methanol fuel, the stoichiometric AFR is 6.50

Now, the calculation looks the following

Commanded AFR = Stoichiometric AFR / EQ Ratio

Similarly, EQ Ratio = Stoichiometric AFR / Commanded AFR

Now, let’s assume the EQ Ratio is 1.45

So, the Commanded AFR = 6.50/1.45

=4.483

So, for methanol, the Commanded AFR decreases by 55.789% and from E85 the drop in percentage is by 33.43%. 

Diesel

Till now, the discussion was mainly gasoline engine centric. But the air-fuel ratio for diesel engines will be slightly different. This is because of the lower stoichiometric ratio in diesel.

Diesel engines require slightly less amount of air due to the higher compression ratio.  

For diesel fuel, the stoichiometric AFR is 14.50

Now, the calculation looks the following

Commanded AFR = Stoichiometric AFR / EQ Ratio

Similarly, EQ Ratio = Stoichiometric AFR / Commanded AFR

Now, let’s assume the EQ Ratio is 1.45

So, the Commanded AFR = 14.50/1.45

=10

So, the commanded air-fuel ratio is slightly lower for Diesel than Gasoline engines without ethanol. 

EQ Ratio to AFR Conversion Chart

Following is the chart that shows the EQ ratio to AFR values for different types of fuels. It also includes the Lambda value.  

EQ Ratio	Regular Gasoline AFR	E10 AFR	E15 AFR	E85 AFR	Methanol Fuel AFR	Diesel AFR	Lambda Value
1.45	10.14	9.71	9.51	6.734	4.48	10	0.69
1.43	10.28	9.85	9.64	6.83	4.54	10.139	0.70
1.39	10.57	10.13	9.92	7.02	4.68	10.432	0.72
1.35	10.89	10.43	10.21	7.23	4.82	10.741	0.74
1.31	11.22	10.75	10.53	7.45	4.96	11.068	0.76
1.20	12.25	11.73	11.49	8.14	5.42	12.083	0.83
1.04	14.13	13.54	13.26	9.39	6.25	13.942	0.96
0.96	15.31	14.67	14.36	10.17	6.77	15.104	1.04
0.85	17.29	16.56	16.22	11.49	7.65	17.058	1.17
0.71	20.704	19.831	19.422	13.753	9.155	20.422	1.41

The table shows the comparison of the values. Of course, as ethanol content keeps increasing the air-fuel ratio of that mixture also keeps decreasing.

Then the diesel air-fuel ratio is almost similar to the gasoline AFR only because of the stoichiometric ratio being close.

Finally, the lambda value has been obtained by just dividing the EQ ratio by 1. So, as the EQ ratio keeps decreasing the lambda value keeps inc

Factors Affecting the EQ/AFR Ratio

There are several factors like ethanol or methanol percentage, O2 sensor etc. that affect the EQ/AFR ratio. We have discussed them below in detail. 

Ethanol

At this point, it should leave no doubt that ethanol influences the air-fuel ratio. Even though in typical engines, the ethanol percentage is not higher than 15%. As the ethanol content increases, for example in E85, the stoichiometric air-fuel ratio decreases. 

What is stoichiometric air-fuel ratio? It is the ratio between air and flammable gas during the combustion process. 

When ethanol is added, it comes along with extra oxygen in its molecule. Oxygen helps in combustion. So, these extra oxygen molecules make the combustion relatively easier. Therefore, less fuel is required for burning. This reduces the overall ratio.

However, it has its downsides too. Ethanol is hydrophilic and can cause corrosion to your engine components. But you can maintain the ethanol content through fuel composition sensors. 

Methanol

Methanol is also used as a fuel mixture just like ethanol. Its biggest advantage is low flammability and it does not produce any opaque smoke when burned. Hence, it is used in different race cars so that whenever there is any accident, it won’t be engulfed by opaque smoke. 

Methanol molecules have oxygen in them. Oxygen helps in the combustion process through oxidation reaction and more oxygen in a molecule means it will be quicker to have this oxidation reaction required for combustion.

What does that mean? It means, methanol will require less air for the combustion process. Less air means the air-fuel ratio is reduced since it is the ratio of the air to the fuel required for combustion. 

Temperature and Humidity

Temperature plays a vital role in maintaining the proper ratio. Typically, it requires more fuel to maintain the air-fuel ratio in colder conditions. 

Here is the thing, warmer air is less dense due to lower ratio of oxygen molecules in it. That’s why you need leaner condition to drive during warmer weather and more richer condition during colder weather.

How do you manage that? Through jetting down your carburetor. 

How to maintain the correct ratio?

To maintain the correct ratio you have to do the following

Oxygen Sensor

Oxygen sensors are crucial engine components. It helps monitor the oxygen ratio in the combustion chamber and sends that information to the engine control unit ECU and powertrain control module PCM. Based on that information, fuel trimming happens. 

Well, a functioning oxygen sensor is necessary to maintain the correct air-fuel ratio in the engine and to have proper combustion. You can fix this by replacing the oxygen sensor. 

Update ECU and PCM

ECU and PCM helps to keep track of the fuel content needed for the combustion process. 

Now, if the ECU or PCM is damaged or not working properly then it won’t be able to do the fuel trimming process accurately. Hence, the fuel will not be properly mixed with the air for the combustion process.

Fuel Composition Sensor

It helps to keep track of the fuel composition of your car. Particularly for flex fuel vehicles. 

A faulty sensor will not keep track of the right fuel composition and therefore lead to improper combustion process and ratio. You can fix it by either resetting the fuel composition sensor or just replacing it. 

You need to maintain the correct ratio to avoid problems like engine knocking, high fuel consumption, reduced engine performance, etc. 

Summary:

There are a few electrical components like O2 sensor, PCM, Fuel Composition sensor that play a crucial role in maintaining the correct air-fuel ratio. But if these components are damaged, it changes the air-fuel ratio either through low fuel injection or sending wrong air-fuel mixture data.

FAQs

Conclusion

Now, you know how to convert EQ ratio to AFR. All it needs is a simple mathematical formula. We hope that you find this guide helpful. Use this for the conversion process. But make sure to remember what factors affect this ratio and how to maintain it for a long time.