Propane Motor Fuel Injection

[dn]

> hehe, that's the best thing about propane. In the Seattle area unleaded
> regular is about $1.05 to $1.15, Propane at the pump (road tax applied)

Up  here  in  the  Great White North, unleaded premium is $CDN2.40 per
gallon, (and Alberta has the cheapest gas in Canada!),  while  propane
at the  pump  is just over a dollar.  Mind you, our gallon is a little
bigger and our dollar is a little smaller...

> fuel it's only $.75 per gal, delivered now, pay later. I have a hose on my
> tank for filling... uh .. er.. my tractor, ya that's it, my tractor.
 
I like your attitude...

> I have been experimenting with trying to get the stuff to stay in a liquid
> state all the way to the injector, off and on, for quite a while now. I
> could never quite get the line pressure to be stable enough under varying
> throttle conditions. Part of the trouble was I was not able to get the right
> injectors. I am thinking now that I can just pump up the tank with nitrogen
> to boost the tank pressure to about 300-400psi or so, regulate it  through a
> liquid type high pressure regulator to get the line pressure stablized at
> about 150psi or so, go with the high pressure injectors you guy have
> mentioned here, sized as tiny as possible so that I can get fairly good
> metering at idle.

I  don't  think  pressurizing  the  tank  with  nitrogen  will get you
anywhere, the pressure will still decrease rapidly as the  propane  is
used up and the fuel volume in the tank decreases.  Also, the standard
filling  equipment  probably  won't  be  able to push fuel in at  that
pressure, (you wouldn't be able to fill your, uhh,  tractor  off  the
tank anymore)  and  the propane system tank, lines, valves, etc.   are
only rated at 250PSI test...

I  been  doin some thinkin on this over the past couple of days, and I
think we may be approaching the problem bass  ackwards...    With  the
standard EFI system, the fuel pressure is regulated in order  to  make
injector on-time  calculations  easier.   Since the injector is really
just an orifice, the  pressure  needs  to  be  constant  in  order  to
maintain a predictable flow across the orifice.  Most EFI systems have
the  fuel  pressure regulator tied into manifold pressure, so that the
fuel  pressure  remains  at  a  set  amount  above  manifold  pressure
regardless of throttle opening.  All this is done so that the computer
doesn't  need to dink with flow metering or additional calculations to
come up with injector pulse width.  

Now,  if  we  were to plumb the injector directly onto the liquid line
from the tank, and put a pressure transducer on the fuel line so's  we
would  know  the  fuel  pressure  at all times, we could theoretically
compensate the injector pulse width to give the right  fuel  flow  for
the instantaneous  line  pressure  that we have... voila - no need for
messy regulators, nitrogen, or any complicated mechanical stuff. Since
(last time I checked, anyway) propane follows the laws of physics wrt
flow  across  orifices,  it  should  be  relatively simple to have the
computer compensate for existing pressure.  Whaddya think? Am I out to
lunch, or did I stumble onto something here...

Still have the problem of where to find the proper  injector  for  the
system, though.    Stock injectors are all made to run at 20-50 PSI, I
think liquid propane would blow it up real good...  Might have to have
2 injectors, a tiny one for idle/cruise, and a big nasty one for  WOT.
The system I have in mind would be a TBI type system, I'm not sure you
could do this with a port injection system unless you had a really big
engine...

dn


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