The Leonardo 175 is a feedback controller cobbled up from an open loop mixture controller. It is much the same as the feedback systems that were introduced to improve the petrol carburettor.
This description is for an OMVL 90E reducer.
Firstly the liquid is evaporated and its pressure reduced to something like 60 PSI. The gas is then passed through the solenoid valve before having its pressure reduced to about 15 PSI.
The demand valve senses the difference between the atmosphere and that at the mixer which is just before the throttle valve. That pressure difference is proportional to the rate of air flow through the mixer. The very large diaphragm in the demand valve opens up a valve, letting more gas into the chamber in response to that air flow.
The gas output is then fed into the mixer and provides a nearly correct fuel/air mixture to the engine. This is an open loop system which of course is not perfect, just like the petrol carburettor.
Firstly when you take your foot off the accelerator the diaphragm slams shut and cuts off the gas supply, and the engine dies. This can be somewhat embarrassing when the brakes and steering are powered. So there is a small bleed screw to provide gas at tick over. The amount of gas passed at full throttle is adjusted by the spring holding the diaphragm valve open.
In this way the gas/air mixture is coarsely controlled when open loop.
Most of the gas is then passed directly to the mixer. The exact proportion is controlled by a simple gate valve in the by-pass pipe. Because the mixture is almost correct it is only necessary to adjust a proportion to obtain a better fuel/air mixture at all times this is done by the stepper motor control valve in the second pipe.
When the Lambda sensor output is equally high and low it means that the fuel/air mixture was equally rich and lean. This is the same for all feedback systems including petrol injection. A further refinement is to make the switch over from rich to lean and vice versa as slow as possible. That means that the fuel/air mixture was as near optimum as possible. That is what the petrol Engine Management computer does. It does this by drawing a contour map of the petrol demand required for all combinations of power, speed, temperature and a whole host of variables, this also controls the spark timing. The value at each position is “trimmed” in response to any correction needed to keep the Lambda sensor at its optimum. This requires a fairly complex computer with lots of none-volatile memory so that the settings are not lost even when the battery is disconnected.
The Leonardo ECU takes in the Lambda sensor output and produce a “map” with Throttle position on one axis and RPM on the other axis, the Stepper motor demand is then the height of the map. There is a facility to bring engine coolant temperature into the equation but it is rarely used. The ECU stores the value required at the stepper motor to obtain the required Lambda output. The range of output values is 0 (off) to 255 (wide open). However, the system operates best if the value is 25 (default value) plus or minus 5 at tick over and between 20 and 125 at other times. If it gets to 0 or 255 then the control has gone “open loop” which means that there is a fault condition or the setup is wrong.
In common with all negative feedback systems there is quite a lot of scope for errors to be corrected by the feedback. The error only manifests itself when saturation (0 or 255) is reached.
However, there are some problems with vehicles which are injected because those systems are most likely to be Lambda feedback systems with On Board Diagnostics (OBD2).
Because the LPG controller disconnects the petrol injectors the closed loop petrol Engine Management Controller is now running “open loop”. That causes OBD2 faults with Malfunction Indication, in that system. Specifically the trim values reach saturation. Either it manifests itself as an engine “too lean” or engine “too rich” fault code because no matter what the petrol EMC does the Lambda sensor output is now controlled by the LPG ECU (Leonardo).
My vehicle is a V10 where the petrol EMC manages each bank of 5 as a separate engine. I have had on occasion “Bank 1 too lean” simultaneously with “Bank2 too rich”. There is a way out and that is to fit another bit of electronics that fools the petrol EMC.
This may just be a switch that keeps the petrol EMC in Closed loop start-up mode or it may just reset the petrol EMC clearing the trim values. The pair of yellow wires are for this purpose. The most complicated one (AEB 424,425 or 426) reads the trim values and sends a simulated Lambda sensor signal to keep the trim values exactly as they were. This fools the petrol EMC into thinking it is still operating in closed loop mode, so that switching back to petrol is seam free.
AEB even make a dual one for my engine. Of course there must be different units for the different OBD types, VPW, ISO and CAM.
