Motec LTCN User Manual download pdf (Page 24)

24 Operation

Lambda and Air Fuel Ratio

Lambda gives a measure of the Air to Fuel Ratio (AFR) that is independent of

the type of fuel being used.

Lambda 1.00 Stoichiometric ratio: no excess fuel and no excess air

Lambda > 1.00 Lean: excess air

Lambda < 1.00 Rich: excess fuel

Stoichiometric Air Fuel Ratio for various fuels

Lambda Air Fuel Ratio

Petrol Alcohol LPG Diesel

1.0 14.7 6.4 15.5 14.5

Calculations with Lambda

Fuel Stoichiometric

AFR

Measured

value

Calculated value

Petrol 14.7 AFR = 14 Lambda = 14/14.7 = 0.95

Diesel 14.5 Lambda = 1.1 AFR = 1.1 x 14.5 = 16

See Appendix F – Lambda to Air Fuel Ratio Table for a quick reference table.

Engine Tuning with Lambda

When tuning the engine, the target Lambda (or AFR) is dependent on the

tuning objective. The following table can be used as a guideline.

Tuning objective Lambda

Maximum power 0.84 to 0.90

Economy 1.05

Emissions 1.00

Normally the engine is tuned for maximum power at full load and for emissions

or economy at light loads.

The exact requirements for a specific engine and fuel can only be found by

experimentation. On turbo engines extra fuel may be desirable to reduce

exhaust temperatures and help avoid knock. If the vehicle is fitted with a

catalytic converter, extra fuel may be required to ensure the catalyst does not

overheat when not operating at Lambda 1.00

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Motec LTCN User Manual Page 24