burna wrote:
Megasquirt ECU (V3 Board should run LPG injectors directly)
You might be interested in this thread -
http://my90.co.uk/lpg/viewtopic.php?t=4203&highlight=megasquirt
Unfortunately it reads a bit odd - I made some comments which were accidentally deleted, but you'll get the (one-sided) gist.
Vapour LPG injection is subject to more variables than liquid injection - meaning the Megasquirt has to have more sensors and take them into consideration in the map.
Vapour temperature is crucial, as it is vapour the temp affects the fuel density greatly. In practice it varies a lot in use, so you cannot hope for the best or guess an average.
Every commercial vapour injection sstem incorporates a gas temp sensor (NOT the vaporiser water jacket sensor) and uses it to compensate fuelling.
Vapour pressure is also crucial, though more predictable. Early vapour systems didn't include a pressure sensor, and relied on the mechanical gas regulator to be set up and to stay accurate. All current systems do have sensors for that bit more accuracy in compensation. High variation in demand (e.g. performance engines) makes a sensor more likely to give real benefits.
Gas injectors, like petrol injectors, come in more than one form. Most are saturated drive, Keihin are peak-n-hold drive, BRC too I think. Getting any data on most gas injectors will not be easy. Generally they will be spec'd up to a certain bhp (per cylinder) - but beware that only applies when they are used at their rated operating pressure. Many injectors are 'sized' to the application by using different sized restrictors, a few are constructed in a range of sizes. Sizing is crucial, too big is as bad as too small as it ruins low-range performance.
Operating pressure varies between makers. Most run at around 1 to 1.5 bar, and the regulator is referenced to the manifold - i.e. the absolute gas pressure varies as MAP goes up and down. The MS would need to compensate on the differential pressure though, as that is what influences the actual fuel delivery. Most current gas systems use both MAP and fuel pressure inputs to calculate this, some older ones actually use a differential sensor.
Some differ e.g. Prins runs up to 2.5 bar and at that pressure doesn't even need to be referenced to MAP unless it's a turbo. Most gas injectors won't even open against that pressure. (Prins use Keihin)
Vapour injector performance (opening/closing times) is way different to petrol injectors ; vapour injectors are necessarily bigger hence slower, I do not know if MS tweaking can cope with this properly.
Given the costs of acquiring all the bits and the plain fact that brand X injectors may not work well with brand Z regulator, it's probably just as cost effective to buy a complete front end kit and use it in its intended mode as a slave to the Megasquirt.
Throw away the gas ECU if you must! but then you've got to get MS to deal with fuel temp issues etc. and invent a strategy for dealing with e.g. chilled regulator or gas tank empty; and even make your own arrangement for displaying tank level.
A better match for MS would be liquid propane injection. I'd say you've no chance of sourcing the specialist parts for that - propane tank with inboard fuel pump, LPI injectors. Do not be tempted to use petrol injectors, they will burst when you switch off (liquid propane has a huge coefficient of expansion). This stuff has to be real high pressure rated.
I have to say I've seen several people talk of using MS for LPG but never heard of an actual implementation, but I expect there must be some!!
Anyways, all immaterial if you can't do it legally?
good luck, Ross K
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