The Brettschneider equation is the de-facto standard method used to calculate the normalized air/fuel balance (Lambda) for domestic and international I&M inspection programs. It is taken from a paper written by Dr. Johannes Brettschneider, at Robert Bosch in 1979 and published in "Bosch technische Berichte", Vol 6 (1979) N0. 4, Pgs 177-186. In the paper, Dr. Brettschneider established a method to calculate Lambda (Balance of Oxygen to Fuel) by comparing the ratio of oxygen molecules to carbon and hydrogen molecules in the exhaust.
The Lambda result is a dimensionless value that indicates the balance of air to fuel compared to the stoichometric point, where Lambda = 1.000. As an example, Lambda of 1.010 means there is 1% too much oxygen (1% "lean" of stoichometric, not 1% lean of where the engine wants to be) and 0.990 means there is 1% too little oxygen (1% rich of stoicometric, not 1% richer than where the engine wants to be) in the exhaust gas makeup.
For gasoline, Lambda x 14.71 is the air/fuel mass ratio – as for gasoline, at the perfect stoichometric point the A/F mass ratio is 14.71 (1.00 Lambda).
That does not mean that and engine actually wants anything like 14.7:1, it just means that that's what the exhaust gases contain to back out the AFR, given the amount of air and fuel delivered and burned to some efficiency rate. When tuning an existing engine, 14.7:1 is essentially, a value that is only useful in a chemistry class.
This Lambda calculation determines the relative balance of Oxygen to Combustibles in the exhaust gas by direct measurement of oxygen and combustibles bearing gases. Lambda calculated in this way is independent of the degree of combustion - whether due to combustion inefficiencies, ignition timing, misfire, or poor CAT action, and is a good witness of the effectiveness of the vehicle EMS fuel control (lambda) closed loop system.
Also, because of the independence to combustion efficiency, it does not matter whether pre or post CAT exhaust gases are used for the calculation.
Exhaust Air Leaks
Reprinted by Factory Pro Courtesy of Robert Schrader - Bridge Analyzers, Inc. Copyright © Bridge Analyzers, Inc.