Measuring the air-fuel ratio with a wideband sensor and CUMPAN
The air-fuel ratio – expressed as a lambda or AFR value – can be measured and displayed on the engine. Three components are needed for this: a wideband sensor, a lambda controller and a display. The CUMPAN Cockpit Assistant outputs a wide range of readings in combination and can also serve as the display for the lambda or AFR values.
Fundamentals
The air-fuel ratio is decisive for every combustion engine – for smooth, healthy running, for making the most of the available power and, not least, for exhaust emissions.
Complete combustion of a fuel requires a minimum amount of oxygen. Combustion is called stoichiometric (complete) when all fuel molecules react fully with the oxygen in the air, with neither a lack of oxygen nor any unburnt fuel left over. In practice, the amount of air supplied often deviates from the stoichiometric requirement. The key figure describing this deviation is the air ratio lambda (λ).
Lambda = air quantity / stoichiometric air quantity → λ = L / Lst.
In English-speaking countries the AFR (Air-Fuel Ratio) is commonly used. As the name says, it is defined by the ratio of air to fuel. For petrol fuels the stoichiometric mixture (λ = 1) is 14.7 : 1 – 1 kg of fuel needs exactly 14.7 kg of air for complete combustion. The relationship is: λ = AFR / AFRstoich. (example: at the stoichiometric mixture AFR = 14.7, so λ = 14.7 / 14.7 = 1).
Because the composition of common fuels varies and many modern vehicles can run on different fuels, it makes more sense to speak of λ values rather than AFR.
An air ratio of λ > 1 indicates a lean mixture, λ < 1 a rich one. With petrol fuels an ignitable mixture lies between 0.7 < λ < 1.3. The operating point for a three-way catalytic converter is λ = 1, where oxidation and reduction run in parallel. The maximum power of a naturally aspirated petrol engine, by contrast, is reached with a fuel-rich mixture at λ = 0.88.
Wideband sensor
Vehicle manufacturers do not usually fit a wideband sensor to production engines. These are optimised for exhaust after-treatment at λ = 1, where the cheaper narrowband sensor is used.
The best-known wideband sensors come from Bosch and are designated "LSU 4.2", "LSU 4.9", "LSU ADV" and "LSU 5.x". As a rule they are not interchangeable. The established "LSU 4.9" is the most widely used – all the figures below refer to this sensor.
As the name suggests, wideband sensors have a broad measuring range from λ 0.65 up to pure air (λ → ∞), and they do so with high accuracy: ± 1 % at λ = 0.8, ± 0.7 % at λ = 1 and ± 5 % at λ = 1.7. An integrated sensor heater quickly brings the sensor up to operating temperature. Service life is around 100,000 km.
The narrowband sensor (also called a binary sensor) works on a different principle: its reading jumps at λ = 1. That is well suited to controlling exhaust gas to λ = 1, but values above or below cannot be measured meaningfully or used as a control variable. So a narrowband sensor cannot control to maximum power (λ = 0.88).
Lambda controller
The wideband sensor's signal has to be processed by a dedicated evaluation circuit (LSU-IC) and converted into a usable form (0–5 V). This is the job of the lambda controller.
A wide variety of devices is available on the market. In principle any controller with a linear characteristic and an analogue 0–5 V output can be used. For this the CUMPAN offers a two-point calibration based on the data sheet of the respective controller manufacturer. Devices from "ProSport" and "14Point7", for example, have proven reliable.
As a matched complete set we offer the Lambda and AFR controller set Spartan 3 with Bosch LSU 4.9 in our shop: a factory-calibrated 14Point7 Spartan 3 Lite v2, the established Bosch LSU 4.9 wideband sensor and the matching stainless-steel weld-in bung. The controller outputs its signal via a linear 0–5 V analogue output and can therefore be connected directly to the CUMPAN – no free-air calibration of the sensor is required.
A particularly versatile solution for some KTM models is the "Fuel Guard" from "MCE Performance":
"The MCE Fuel Guard is an extension to our ECU tuning for KTM/Husqvarna/GasGas motorcycles. The Fuel Guard consists of a Bosch LSU 4.9 wideband sensor and the control unit which, combined with our ECU tuning, makes it possible to run a full closed loop to a lambda value of our choice. This means the ECU no longer calculates the injection quantity statically from the maps alone, but uses the wideband lambda sensor to reach our target lambda value on its own. The ECU trims the fuel quantity continuously and fully automatically to deliver optimum power and rideability at all times."
In addition, the Fuel Guard outputs the lambda signal to the CUMPAN, so the lambda values – the basis of the control – can be shown live while riding. A malfunction or a fault on the vehicle can thus be detected early.
Display
A meaningful assessment of engine running – poorly used power bands, faults in mixture preparation – requires a display of the lambda value. To this end, external instruments translate the lambda controller's analogue 0–5 V signal into the matching lambda value and show it.
The CUMPAN Cockpit Assistant offers exactly this and integrates the lambda display into a freely configurable overview of various engine data such as RPM, oil and coolant temperature, oil pressure or a gear indicator.
The signal is connected via one of the analogue 0–5 V inputs. In the menu under [INPUT] / [LAMBDA] [AFR] you set a two-point calibration: 0.0 V as the lower and 5.0 V as the upper calibration value. The corresponding values come from the data sheet of the lambda/AFR controller used; for the most common controllers they are already preset. The details are described in the CUMPAN user manual in the LAMBDA/AFR section.
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