Agreed with Gilbert.
Can add that an OBD connection is by no means a substitute for accurate mapping (like Gilbert described). Accurate mapping is very important, OBD connection is not.
When we calibrate an LPG system we set up a 'slave map, which the LPG system uses to interpret the (would be) petrol injector pulses to arrive at a pulse length for the LPG injectors. Get this 100% correct and fuel trims on both petrol and LPG will be identical (one school of though), or zero (another school of thought), or somewhere between the petrol trim figure and zero (another school if thought). Depending on vehicle spec and what I note when setting fuel systems up, I usually (not always) opt for the latter as best compromise. Every aspect is a compromise.
Get the above slightly wrong and fuel trims may be different on different fuels. Fuel trims are the way in which the petrol ECU learns to give precisely the correct amount of fuel to your specific engine, particularly when you first move the throttle or engine revs change while driving. Aiming for an 'ideal fuel trim of zero (as most, including AEB wit OBD connection, usually do, can cause fueling issues on some installs).
If you get the LPG map wrong, then fuel trims will need to be re-learned every time you switch fuels. If fuel trims are too far out when you switch fuels, then the engine warning light will come on with all associated consequences (no need to go into those here).
An OBD connection allows the LPg system to read fuel trims and if found incorrect to gradually steer LPG fueling to bring fuel trims to correct (in terms of AEB this means to zero, some systems allow the programmer to enter a preset here). The operative (and problematic) word here is 'gradually'. The LPG system adjusting fueling of it's own accord has to be gradual, else the closed loop functionality of the petrol system would not be reflected - and this would bring many problems of it's own. But the slow nature of the LPG system fuel trim steering facility also means the system still has to be mapped at least, maybe, 70% properly, rising to maybe 90% at full loads. With a correct LPG map, the fueling will be almost exactly correct immediately when you change throttle position and/or rpm changes. The engine has many fuel trims (each covering a certain load range within a certain rpm range) but the LPG system does not know what these ranges are... This is why the LPG system cannot have it's own identical fuel trim system, and this is the reason for issues behind installs that rely too much on OBD connection. The LPG system effectively only has one fuel trim, the petrol system has very many.
The consequences for having an incorrect map with and trying to compensate by use of an OBD connection is.. Well, say you have fuel trims initially of -5% at idle, the LPG system will then grqadually subtract 5% from the fueling to bring fuel trims to zero, so now the LPG system fueling adaption is at -5%... Then you move your foot on the throttle (say to set off driving in drive gear), and now the trims might have been +5% (without the OBD connection, because the map is wrong) - so fuel trims would still only have reached a maximum of 5% out, but because of the OBD connection and LPG system (singular) fuel trim is still at -5% and adapting slowly, the would be +5% petrol fuel trim now would need to become +10% for the fueling to be correct.. So the OBD connection has actually made things worse and for longer! But, that extra 5% incorrect fueling will be accounted for when the LPG system slowly steers fueling to correct the issue... Maybe fast enough to prevent the engine warning light coming on, but not fast enough to prevent the incorrect fueling issue that went on for longer. By connecting to OBD, the installer has prevented the driver/owner of being aware of the fueling issue, has prevented any major problem, but has fudged the job by not calibrating the system correctly. The LPG system can only slowly steer adjustment to fueling from the -5% to the +5%, during which time the engine will be running rich at first lean when you touched the throttle. Without that OBD connection, the furthest fueling would have been out would have been 5%, but now (at some points) they are 10% incorrect. Meanwhile, if the map had been correct in the first place, the LPG system would not have compensated at all, the petrol system would not have needed to have done any further correction, so there would have been no lag in the fueling correction... More correct fueling more of the time, without the OBD connection but with accurate mapping.
Too often used by dodgy installers who don't map systems properly, but a poor substitute for accurate mapping. Not very often a good idea to connect an OBD connection - much better to get the mapping correct. Having said that, it is a good idea to use a system with OBD connection on a Jag engine'd Rangerover! That's more due to the fact the Jag engine'd models will require a fuel return. You cannot map such engines on petrol pulse time alone, you do need a scanner that reads live data (and the ability to see petrol pressure helps too).
A simplified explanation but hopefully getting the point across.
lamb190 wrote:
how do you program the map when you first install the system. on my landirenzo ecu im sure it gave me manufacture makes and models to choose from... was this a base map program?
Even if you could find a file labelled 'AEB calibration for 4.4 Jag engined RangeRover', without experience, on your model of vehicle, maybe without an OBD live data reader, you do it very badly mate. Then you pay some 'pro' to calibrate it for you, only to find they don't know their stuff... Or, you could have bought a kit from a pro who does know his stuff and offers free calibration to kit buyers, saved money when all said and done, been issued with a free safety cert...
BMW engine'd 4.4 Rangerover, no probs for a DIYer. Jag engine'd Rangerover not nearly as simple. Maybe learn how to change the map reference points and become familiar with OBD live fuel trim data, lambda ratios, before attempting to calibrate. Would have been good to have had aptitude and plenty experience before getting around to the V8 Jag engine'd vehicle.
There are systems now which claim to be able to adapt for 'poor quality fuel' etc... As if 'oh no there was some steam mixed in with the LPG on that last tank'.
What they do is adjust the multiplier in real time, based on a one time reading of petrol injection duration versus manifold pressure (and sometimes rpm). Most of these make a real mess of fueling because they don't take into account any of the other factors such as various temp corrections . No more can they accurately calculate multiplier, or nozzle size, or working pressure, than a washing machine can bring clothes in the laundry basket downstairs, sort colours, load itself and select correct wash temp. But some of these ECU facilities do a better job of it than some installers.
Depending on how much money you're prepared to waste and issues you're prepared to put up with, maybe see you later. 200 miles from Bristol, lots of people come much further!
Simon
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