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 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) and 0.990 means there is 1% too little oxygen (1% rich) 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.
This Lambda calculation determines the relative balance of Oxygen to Combustibles in the exhaust gas by direct measurement of oxygen andcombustibles bearing gases. Lambda calculated in this way is independant of the degree of combustion - whether due to combustion inefficiencies, 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 independance to combustion efficiency, it does not matter whether pre or post CAT exhaust gases are used for the calculation.
excess air caused by exhaust system air leaks, or air injection will cause
the exhaust-gas-calculated Lambda to indicate lean values - above 1.000.
These air dilution conditions are usually indicated by high O2 readings in
the exhaust stream along with an overall reduction in the concentration of
the other gases. To improve the correlation of the EGA Lambda to the
Engine Management System mixture control point, air injection should be
disabled, and exhaust air leaks should be corrected.
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