This page is dedicated to how the Motronic functions by taking the raw Lambda input and converting this signal so that the computer can understand it. Some computer programming and flow charting will be discussed. 'Oxygen sensor' or 'O2 sensor' are other commonly used names for the Lambda sensor.
The Lambda is a minor system in the Motronic. It is an option, an add-on. As such, it is not a critical system. It is a smog device and in no way does it enhance an engines power. Just the opposite. All that it is doing is fine tuning the fuel to reach an ideal air/fuel ratio. This original value of fuel is read from maps just like it is without the Lambda system installed. It is only capable of making minor changes to the fuel level.
BOSCH,
the designer of the Motronic, defines the following:
VLean
= +0.45V
VReference = +0.475V
VRich = +0.50V
A simplified Lambda circuit for an early Motronic ML1.2 is shown below. Four series resistors, from +5 Volts to ground, make up the voltage divider section (left). This produces voltages of +0.45V, +0.475V, and +0.50V. Two comparators (LM139) use the +0.45V and 0.50V as threshold voltages. As the Lambda voltage(Vl) increases and passes though each of the threshold voltages, the output of one of the comparators will changes.
Strategically between VRich and VLean, is a midway point called VReference. This leaves a ±25mV wide window between lean and rich. If the Lambda sensor is not connected, the input to the comparators will remain at VReference. If the Lambda sensor is present, the comparator inputs will toggle back and forth between 0V and 1V.
This schematic has been simplified. The Lambda sensor connects to R801, and with C801, make up a low pass filter. Noise frequencies above 1Hz are filtered out. Through R808, this point is connected to VReference. R802, R803, R804, and R805 make up the voltage divider as previously described. Each comparator is wired to trip at a different threshold voltage. The comparator outputs go to the 1802 processor.
EF3 and EF4 are hardware interrupts. They set a flag that the processor routinely monitors. Once alerted to the flag, the processor investigates the cause. Opposite to what is commonly thought, the processor does not constantly check the Lambda's status. The Lambda circuitry tells the processor when there is a change in status and then the processor gets involved.
The outputs of the two comparators change as the inputs transition from rich to lean and back again. The following table shows the binary equivalent of the Lambda output voltage. This is how an analog input Vl is converted to a two bit binary number.
Name |
Lambda Output Voltage (Vl) |
Voltage | Output | Binary | ||
| EF3 | EF4 | EF3 | EF4 | |||
| Rich | > 0.50V | 0 | 0 | 0 | 0 | 00 |
| In-between | 0.45V to 0.50V | 5V | 0 | 1 | 0 | 10 |
| Lean | < 0.45V | 5V | 5V | 1 |
1 | 11 |
This is an ideal representation of the Lambda out voltage vs. time. This is a engine at
idle. At part throttle, the repetition rate is about double. TD
is dead time. During dead time, all fuel level adjusting activities stop. Dead time is
about double for an idling engine than at partial open throttle. It is less demanding at
idle.
This is a less ideal diagram but more realistic. The Vl
oscillations, shown here between the two threshold voltages, is from "cylinder
scatter". Differences in each of the four cylinders can cause the Lambda voltage to
jump about and even go in the reverse direction that the Motronic is trying to correct to.
Starting with a slightly lean condition, the output is 11binary. The fuel is increased while changes in the output is monitored. As the output changes to 10binary, we enter the 50mV neutral area and continue incrementing the fuel level. When the output changes too 00binary, the fuel incrementing stops. There is a wait period. After the wait period is passed, the fuel level is decreased incrementally from 00binary, passing through 10binary, and 11binary and again it stops for a wait period. This cycle continues endlessly.
The basis ML1.1 system, the early Euro 944, has no Lambda system. The air flow sensor and speed sensor are used to determine fuel flow from a map that is stored in system memory.
Next to it is the ML1.2 Lambda system flow chart, the early USA 944, which has a Lambda sensor (O2 sensor). The fuel level is a two level control process, map and Lambda control. The Lambda system also uses the air flow sensor and speed sensor to determine fuel flow from a map. The Lambda control makes positive or negative changes to the fuel level in order to fine tune the fuel output. The range of the Lambda control is quite limited which requires the fuel map to contain values very close to Lambda=1 (air/fuel ratio 14.7). Comparing the maps of a non-Lambda (Euro 944) and Lambda system, the Lambda maps will appear flat and unimpressive. The power is in the non-Lambda map.
The fuel map is always used with a Lambda system. One common but false saying is that the fuel map is no longer used when the car is equipped with an Lambda sensor. This is far from the truth. Where does the original fuel value come from? Is it a guess? No. Any change in load or engine rpm, the DME must go to the fuel map to get its initial value. Then the fine tuning begins. The Lambda sensor subroutines take charge and the fuel is continuously toggled back and forth. If the Lambda sensor is disconnected, the Motronic gets it fuel levels from the map only. It never just "goes rich to protect the engine" which is another famous and incorrect saying. When the Lambda is disconnected, it uses the fuel map.
For simplicity sake, the contribution of temperature has been omitted from the discussion. Fuel atomizes poorly at low temperatures requiring some mixture enrichment.
