What is Lambda?
Lambda is a measure for the mass air to fuel ratio (AFR) present during combustion. When exactly enough fuel is combined with the available free oxygen, the mixture is chemically balanced and is called stoichiometric.
Lambda = 1 —stoichiometric mixture
Lambda < 1 —mixture is rich, excess fuel present
Lambda > 1 —mixture is lean, excess air present
The amount of air that is needed depends on the type of fuel used. In the case of gasoline/petrol, a stoichiometric mixture consists of an air to fuel ratio of 14.7 to 1. For different fuel, different ratios apply.
Narrowband Lambda is a measurement method where the AFR range is limited from 14:1 to 15.4:1. The sensor reading switches very sharply between the thresholds of lean and rich areas, providing a signal which indicates either a rich or a lean mixture but not to what degree.
This works well in controlling an engine for emissions, however, the limited range makes narrowband Lambda unsuitable for accurate tuning.
Wideband Lambda sensors are designed to give an exact reading of Lambda. This is particularly useful when the precise mixture needs to be known in order to tune the engine for optimum power. The measuring range can span from 0.7 to 32 Lambda for a 5 wire sensor type.
Wideband Lambda sensors use sophisticated controls, as the temperature change needs to be taken into account to be accurate.
There are two concepts for measuring wideband Lambda:
4 Wire Wideband Lambda Sensor
This technology takes advantage of the fact that the sensor's voltage output is based on not only the oxygen differential between the exhaust pipe and atmosphere, but also on the temperature of the sensor itself. Sensor impedance varies with temperature, so not only the sensor voltage, but also the sensor impedance needs to be measured. Systems which do not use at least four wires typically have errors in displayed Lambda as high as 8 percent!
5 Wire Wideband Lambda Sensor
This newer technology determines the air fuel ratio of an engine by measuring Lambda sensor voltage output and the current required to hold the sensor voltage output constant. This method offers increased speed and accuracy over the older 4 wire sensor technology.
The tuning objective dictates the target Lambda. Typical gasoline/petrol engines produce
- peak power at Lambda between 0.84 and 0.90
- best economy at Lambda equal to 1.05
- optimal emissions at Lambda slightly lower than 1
MoTeC ECUs allow for a Lambda goal table based on load and RPM.
Referencing the measured Lambda, the Quick Lambda function in the software adjusts the values in the fuel control table at the specified load and RPM site to achieve the goal Lambda.