In this article, we present some ways in which the air-fuel ratio (lambda or AFR) can be measured and displayed on the engine. This requires a broadband sensor, a lambda controller and a display unit. In addition to its ability to output a large number of measured values in combination, the CUMPAN Cockpit Assistant can also be used as such a display device for the lambda or AFR values.
Basics
The air-fuel mixture is crucial for every combustion engine in order to ensure good and healthy engine running, to make optimum use of power delivery and, last but not least, for exhaust emissions.
A minimum amount of oxygen is required for complete combustion of a fuel. One speaks of stoichiometric (complete) combustion when all fuel molecules can react completely with the oxygen in the air without oxygen being missing or unburned fuel remaining. The amount of air supplied often deviates from the stoichiometric air requirement. The key figure for characterizing this deviation for any combustion is the air ratio lambda(λ).
Lambda = air volume / air volume stoichiometric
λ = L/Lst.
In English-speaking countries, it is customary to specify the AFR(Air-Fuel-Ratio). As the name suggests, this is defined by the ratio of air and fuel volume. For petrol fuels, the stoichiometric mixture (λ = 1) is 14.7 : 1. Exactly 14.7 kg of atmospheric oxygen are required for 1 kg of fuel to be fully converted .
The relationship between lambda and AFR is: λ= AFR / AFR stoichiometric (example: λ=1=14.7/14.7).
Since the composition of common fuels varies and many modern vehicles can handle different fuels, it makes more sense to speak of λ values instead of AFR.
An air ratio of λ > 1 is referred to as a lean mixture and λ < 1 as a rich mixture. An ignitable mixture for gasoline fuels is between 0.7 < λ < 1.3.
The operating point for a three-way catalyst is λ = 1 (oxidation and reduction processes run in parallel).
The point for the maximum engine output of a naturally aspirated gasoline engine is determined with excess fuel and λ = 0.88 is reached.
Broadband probe
The broadband sensor is not usually installed by vehicle manufacturers on standard engines. Optimized for exhaust gas aftertreatment ( λ = 1), the more cost-effective jump sensor is installed .
The best-known broadband probes are manufactured by BOSCH and bear the designations “LSU 4.2” “LSU 4.9” “LSU ADV” “LSU 5.x”. The probes are generally not interchangeable. The most commonly used is the established “LSU 4.9” and all the variables mentioned below refer to this probe.
As the name suggests, broadband probes have a wider measuring range of λ 0.65 – λinfinity and a very high measuring accuracy:at λ = 0.8 of +/- 1%, at λ = 1 of +/-0.7%, at λ = 1.7 of +/- 5%.
The broadband probe has an integrated probe heater to reach the required working temperature as quickly as possible. The service life of the probe is approx. 100,000 km.
The jump sensor (also known as a binary sensor) uses a technically different principle: the measured value jumps at lambda = 1. This behavior is very well suited for exhaust gas control to λ = 1. Values above or below this cannot be sensibly recorded with this probe or even used as a control variable. It is therefore not possible to regulate to the maximum power(λ = 0.88) with a step probe.
Lambda controller
The signal from the broadband sensor must be processed by a special evaluation circuit (LSU-IC). The signal is converted into an evaluable form (0-5V). This is done in the Lambda controller.
A wide range of different devices are available on the market. In principle, all devices with a linear characteristic curve and an analog 0-5V output can be used. The CUMPAN offers the option of performing a two-point calibration in accordance with the controller manufacturer’s data sheet. Devices from the manufacturers “ProSport” and “14point7”, for example, were used successfully.
The manufacturer“MCE Performance” offers a special and particularly versatile solution for some KTM models with its “Fuel Guard” product:
“The MCE Fuel Guard is an extension to our ECU programming for KTM/Husqvarna/GasGas Motorcycles. The Fuel Guard consists of a Bosch LSU 4.9 broadband sensor and the control unit which, in combination with our ECU programming, makes it possible to create a “full closed loop” to our desired lambda value. This means that the control unit no longer just calculates the injection quantity statically on the basis of the characteristic maps, but instead uses a broadband lambda sensor to automatically approach our desired lambda value. The ECU adjusts the fuel quantity permanently and fully automatically to achieve optimum performance and drivability at all times.”
In addition, the Fuel Guard outputs the lambda signal to the CUMPAN so that the lambda values – the basis of the control system – can be displayed live while driving. A malfunction or defect in the vehicle can therefore be detected at an early stage.
Display device
A display of the lambda value is required so that an exact assessment of the engine running (poorly utilized power ranges, errors in the mixture preparation) is possible.
External display instruments can be used for this purpose, which translate and display the analog 0-5V signal of the lambda controller into the corresponding lambda value.
The CUMPAN Cockpit Assistant offers precisely this option and integrates the lambda display into a meaningful, freely configurable overall view of various engine data, such as RPM, oil and coolant temperature, oil pressure or a Gear indicator.
