From: Peter Hill on 12 Apr 2006 19:30
On Wed, 12 Apr 2006 07:36:17 +0100, Austin Shackles
>I've been thinking about liquid phase injection...
>The injection system on the sierra (K-jet) runs, IIRC, at a nominal 5.5 bar
>(or something around that, anyway). This is not far different to the
>pressure that you get in a propane tank, provided the temperature is
>is there any reason why I can't feed liquid LPG from the tank to the fuel
>distributor inlet on the K-jet system?
>OK, it'd presumably need adjusting to get the fuelling right - but since the
>fuel contains less energy but is at a higher pressure, it might not need
>that much adjustment.
>Actually that was just an example, what I was wondering about really was
>more modern injection systems - for example, if I buy a 2-litre ford minibus
>which I have my eye on, I imagine it has the injection version of the DOHC
>engine as fitted to later sierras. I don't think it's new enough to be a
>duratec, even if there is a 2-litre duratec.
>But the same applies - you already have liquid injection and a metering
>system for it. dunno what pressure it runs at, mind you, but to be
>effective an injection system has got to be fairly high pressure. [aside:
>the book says the lucas hotwire in the later rangies is 2.5 bar]
>However, there ought to be scope for regulating the input pressure, and in
>fact, if you get the input pressure right, it should be possible to vary the
>pressure by just enough to compensate for the different specific energy of
>the fuel, and not have to adjust the metering at all. e.g. if the fuel has
>10% less specific energy, then upping the pressure by 10% should deliver
>more fuel. Get the ratio right and it should run on LPG or petrol with no
>OK, tell me why it won't work...
Petrol system is pumped and regulated to a constant pressure. Flow
rate is proportional to square root of pressure drop across injector
metering orifice. At constant regulated fuel pressure, hold injector
open x secs you get y cc's fuel, hold injector open 2x secs you get 2y
cc's fuel. Double the pressure and those flows are now 1.4y cc's and
2.8y cc's, you have to reduce the "on" time to about 7/10 of what it
was. LPG tank pressure depends on temperature your pressure changes a
over a wide range. At -42?C it's 0 (gauge), at 0?C it's 4.5bar and at
45?C (possible hot day temp in car boot when in full sun - I've seen
dashboard temps of 55?C in UK) it's 17.6bar. Pump and regulate LPG
pressure in same way as petrol, to get a constant pressure on the
system and thus constant calibrated fuel flow at all tank pressures,
would require a pressure about 1 bar above maximum that you will see
in the tank - about 18.6bar. You would need microscopic holes in the
injectors to meter the flow at that pressure.
Or you just use tank pressure (with a boost pump for low tank temps
and pressure) and then have to have an ECU that can take fuel pressure
into account, reducing "on" time when fuel pressure is high and
increasing it when fuel pressure is low. Not a normal petrol
injection system and definitely not K-jet unless you are clever enough
to make the required add-on. At high tank pressure the injector on
time could be very short, too short to actually get any controlled
flow. Most injectors would fail to open with high pressure keeping
the pintel on it's seat, unless you wack 100v though a 12v coil. If
it does open the hard landing when it's switched off and slams shut
could damage the seat very quickly.
Any attempt to regulate fuel pressure to a pressure below tank
pressure will result in fuel being vaporised, it boils in the
regulator. This causes delivery of froth and accurate metering is not
possible. How many bubbles went in? How much ice will form on the
regulator? What damage will it do to surrounding parts? Will the
regulator work at low temps as the LPG sucks heat from it?
The pressure in LPG liquid side of system is high enough to trigger a
whole load of legal requirements that all parts be tested to a safe
working pressure much higher than any possible system pressure. Your
petrol injectors would burst.
If you want a constant pressure of 4.5bar you need a constant tank
temperature of 0?C. So you need a tank that's both refrigerated (and
can cope with days when it's 30?C in the shade) and heated (maybe a
few days a year in England but much more often in Scotland and
Europe). That's additional system bulk and a constant fuel burn
(assume an Electrolux gas fridge like camper vans use) if it's to be
ready to go instantly and you don't want to wait while the fuel is
conditioned to working temp.
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From: Austin Shackles on 13 Apr 2006 03:39
On or around Thu, 13 Apr 2006 00:30:25 +0100, Peter Hill
<peter.usenet1(a)nospam.demon.co.uk> enlightened us thusly:
[snip good exposition on why it won't work]
OK, so how do you do liquid-phase injection? Did I gather rightly that LDV
fitted this to the Convoy, or was that simply a different kind of SGI?
the gist is that it's going to be complicated, and not worth it. If the
occasion arises, I'll probably go with SGI since that's reasonably easily
not sure I agree with your tank pressures, mind - one of the local garages
has a gauge on the pump and I've not noticed that much variation (it does
vary, naturally, but I don't recall even on hot days that it's gone as high
as 200 psi which is about 14 bar) - might just be that the tank temperature
doesn't vary that much, I suppose - certainly, I've never observed -42?C...
Austin Shackles. www.ddol-las.net my opinions are just that
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From: Stewart Hargrave on 13 Apr 2006 20:20
On Wed, 12 Apr 2006 22:36:02 +0100, Austin Shackles
>On or around Wed, 12 Apr 2006 19:59:54 +0100, Stewart Hargrave
><SpamOnlyToHere(a)MiserableOldGit.Me.uk> enlightened us thusly:
>>On Wed, 12 Apr 2006 07:36:17 +0100, Austin Shackles
>>>I've been thinking about liquid phase injection...
>>Thing is, because these are pressures that are regulated by a return
>>to the tank, the pressures are relative to tank pressure - ie the
>>absolute pressure (relative to atmosphere) is going to be up to 14
>some error there. absolute pressure in a vented tank is going to be more or
>less 1 bar...
Tank pressure on petrol is 1 bar, working pressure of K-Jet is around
Tank pressure of LPG is 7 bar +, so working pressure of K-Jet using
LPG is going to be tank pressure + working pressure = 12 bar + (not
sure where I got 14 from, though).
>>My other initial thought is, that as the injectors open simply when
>>the pressure behind them gets above 3.5 bar, then even when you have
>>turned the engine off, they are going to continue venting LPG. You'd
>>either need a shut-off solenoid on each injector, or find a way of
>>increasing the opening point to somewhere comfortably above 7 bar.
>You'd have shut-off solenoids, of course. But point taken; although how
>does the petrol system avoid this, or perhaps it doesn't?
When the petrol pump is shut off, the pressure drops below the opening
point almost instantly.
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From: Tom Woods on 14 Apr 2006 03:59
On Thu, 13 Apr 2006 08:39:34 +0100, Austin Shackles
>On or around Thu, 13 Apr 2006 00:30:25 +0100, Peter Hill
><peter.usenet1(a)nospam.demon.co.uk> enlightened us thusly:
>[snip good exposition on why it won't work]
>OK, so how do you do liquid-phase injection? Did I gather rightly that LDV
>fitted this to the Convoy, or was that simply a different kind of SGI?
I have been told that Vialle make a LPI kit
This thread (http://www.saabforum.nl/viewtopic.php?t=9102) is about it
(but not in english)
From the picture it just seems to be using 4 solenoids (above the
intake manifold), with 4 flexy pipes going from them to the manifold
where the petrol injectors go in - Sounds fairly simple!
The car in the picture has K-jet too.
I would like to know more about the vialle system! Does it use special
There must also be a special LPG ECU to control the injectors, but i
suppose that if you were determined you could use something like a
megasquirt kit to control them yourself.
Has anybody got and prices for vialle kit?
From: Peter on 15 Apr 2006 11:21
> is there any reason why I can't feed liquid LPG from the tank to the fuel
> distributor inlet on the K-jet system?
> OK, it'd presumably need adjusting to get the fuelling right - but since
> fuel contains less energy but is at a higher pressure, it might not need
> that much adjustment.
> Actually that was just an example, what I was wondering about really was
> more modern injection systems - for example, if I buy a 2-litre ford
> which I have my eye on, I imagine it has the injection version of the DOHC
> engine as fitted to later sierras. I don't think it's new enough to be a
> duratec, even if there is a 2-litre duratec.
Two big problems:
* delivery of liquid phase right to the injectors where it can get pretty
hot. I believe around 70C LPG will always turn into vapour phase regardless
of how much pressure you apply
* prevent injectors from freezing in cold engine
Plus all the stuff others have mentioned about metering & pressure