No, neither are stand alone, they are slave systems the same as all the others where the petrol ECU does fuelling, the LPG ECU just adds the fiddle factor. These are both the OBD type so instead of having someone drive around while you sit there in the passenger seat with the laptop creating the fuel map, it connects to the OBD socket and looks at the short term fuel trim adjustments being applied by the petrol ECU. From these it can see how far out its own initial map is and adjust it to get it to what it thinks is correct. Most installers agree that these make setting up easier, particularly the done in a day merchants, but most will also agree that it might get it close but never right and setting it up manually will always give a better result.
I may be teaching you to suck eggs, but I'll start from the beginning. To understand how the gas system works, you need a basic understanding of how the petrol injection system works so I’ll start there. Petrol is pumped from the tank to the engine where it goes through a pressure regulator. This keeps the fuel at a constant set pressure and returns excess fuel back to the tank. The fuel under pressure sits at the back of the fuel injectors which are nothing more complicated than electrically operated valves capable of being turned on and off very quickly.
The injectors are switched on and off, to open and close them, by the cars ECU (electronic control unit). The ECU takes signals from various sensors, engine temperature, intake air temperature, throttle position, air flow, engine revs and, by using an internally programmed ‘map’ knows how much fuel is needed under any given circumstances. The amount of fuel is altered by altering the amount of time that the injectors stay open for. At idle, it can be as short a time as 3mS (3 milliseconds, or 0.003 of a second), at full throttle under load, it can be as long as 15mS. As the fuel is at a set, known pressure, the length of time that the injectors are open for directly affects the quantity of fuel injected.
As a fail safe, the engine is fitted with a lambda sensor in the exhaust system. This measures the amount of oxygen in the exhaust gasses and can tell if the air/fuel ratio (the ‘mixture’) is correct. If it isn’t, the ECU will ‘fine tune’ the length of time the injectors are open for to keep the mixture correct and the engine running at maximum efficiency. This fine tuning is the short term fuel trim. If the ECU finds that the short term fuel trim is always being adjusted one way or another (maybe because the fuel pressure regulator has aged and the pressure is slightly different to what it used to be) it adjusts a long term fuel trim, it shifts the norm, so the short term trim is always wandering around it's mid point, sometimes adjusting a bit one way, other times a bit the other.
When running on LPG the pulses from the petrol ECU to the petrol injectors are intercepted and fed to the LPG ECU (which isn't really an ECU, more of a controller). As LPG has different characteristics to petrol, the pulse lengths will be wrong, they will be too short (something that is set by correctly choosing the LPG injector nozzle sizes and the gas pressure from the vaporiser). So, if at idle the petrol injectors need to be open for 3mS, the LPG injectors may need to be open for 3.8mS. So the LPG ECU map needs to be programmed to add 0.8mS. But at full throttle while going uphill towing a caravan, the petrol injectors may need to be open for 15mS but the LPG injectors may need 20mS, so at that point in the map, it adds 5mS. That is what is involved in programming the map, setting these fiddle factors for all points in the rev and load range. It is this that the two 'self programming' controllers you have seen do by connecting to the OBD port, looking at the short term fuel trims at different revs and loads and adjusting their own map to keep the trims in the centre. It's just a shame that they aren't very good at it......
Bear in mind that the LPGTech system is only for the ECU and wiring loom, you'll still need the vaporiser and injectors (as said, don't even bother thinking about trying to use 12 year old components). You'll also need things like a shut off solenoid and filters, pipework, etc. By the time you've finished you could have bought any of these
http://autogas-lpg.co.uk/150-6-cylinders or
http://www.lpgshop.co.uk/lpg-autogas-6- ... t=priceasc (just don't expect much in the way of after sales service....).
