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Understanding AFR: AFR vs Lambda

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AFR vs Lambda

04.22

00:00 We have just discussed AFR, but this is not the only way to represent the fuel/air mixture in the engine.
00:07 As tuners, the the two most common units we will use to describe air fuel ratio are AFR and lambda and its important to understand the difference.
00:18 The fundamental difference is that AFR describes the ratio between the mass of oxygen and fuel, while lambda on the other hand describes the ratio between the the actual AFR and the stoichiometric AFR.
00:32 In plain english this means that lambda describes how much richer or leaner the air fuel ratio is compared to stoichiometric as a percentage.
00:42 In units of lambda, a number of 1.0 means that the engine is running at a stoichiometric AFR.
00:50 Numbers larger than 1 represent a lean mixture or one with an excess of oxygen, while numbers less than 1 represent a rich mixture or one with an excess of fuel.
01:01 It might sound like a subtle difference, but the fact that lambda gives us a value relative to stoichiometric is very important, particularly if you are dealing with fuels that have different stoichiometric air fuel ratios.
01:16 First of all, let’s look at an example so you can see how the different units work.
01:21 Let’s say for example we have normal pump fuel with a stoichiometric air fuel ratio of 14.7:1, and we are running the engine with a measured air fuel ratio of 12.5:1.
01:35 If we were displaying the air fuel ratio in units of lambda instead, we would get a value of 0.85 which is simply 12.5:1/14.7:1.
01:47 If on the other hand we were measuring a lambda value of 1.05 and we want to see what this is in units of AFR, we can just multiply the lambda value by the stoichiometric value.
02:01 in this case the answer is 15.4:1. One of the reasons I prefer lambda is because it is much easier for me to look at the lambda value and figure out at a glance how rich or lean I am.
02:14 For example if I’m running at 0.85 lambda, I can subtract 0.85 from 1.00 which gives me 0.15, which means I’m running 15% richer than stoichiometric.
02:28 In reality though you don’t need a calculator and I know that 0.90 for example is 10% richer or 1.03 is 3% leaner than stoichiometric.
02:41 This can speed up your tuning by allowing you to make fast, accurate percentage adjustments to your fuel table to correct errors without the need for a calculator.
02:52 The same process can be applied if you chose to tune in units of AFR, but unless you are great with maths, the percentages aren’t so obvious.
03:03 Another advantage if you are tuning on fuels with different stoichiometric air fuel ratios is that with the lambda scale, often our target lambda doesn’t change much.
03:14 For example if we have a turbocharged engine and switch from pump fuel to E85, we could safely target a lambda value of 0.80 at 1 bar of boost on either fuel.
03:28 If we were tuning in units of AFR though our target would change significantly.
03:33 On pump fuel our target AFR would be 0.80 multiplied by 14.7 which gives us a target of 11.8:1.
03:44 E85 has a stoichiometric ratio of 9.8:1 though so targeting the same lambda value of 0.80 would result in a target mixture of 0.8 multiplied by 9.8 which is 7.8:1.
04:01 Failing to understand how the stoichiometric ratio of a particular fuel can effect your target AFR can be potentially disastrous.