Propane Motor Fuel Injection

[dn]

Greetings.

I've had a hankering to get into the EFI arena for some time now, the 
Circuit Cellar Ink article by Ed Lansinger really stirred up my interest
again.  The system I have in mind is really unusual, however, since I want
to use PROPANE as the fuel instead of gasoline.

Before you dismiss me as a crank, let me tell you about the advantages of 
propane as a motor fuel:

1. cheap - at least here in Canada, less than half the cost of gasoline
2. high octane rating - upwards of 125 combined octane rating - this allows
   high compression engines with all their advantages.
3. engine wear - since propane is a gaseous fuel, there is no washing down
   of the cylinder walls as with gasoline, so cylinder and ring wear is
   drastically reduced.  It is not unheard of to have propane powered engines
   with 500,000 or more miles without a rebuild.
4. clean - both from an emissions standpoint and from engine internals, there
   is much less carbon buildup with propane.
5. fuel mixture - again since propane is a gas, there are no problems with
   improper fuel distribution and fuel puddling in the manifold, nor 
   problems with fuel vaporization at low temperatures.
6. simple - since the fuel is already under pressure, the system needs no fuel
   pump.  Also, the carburetors are extremely simple devices. 

There are, of course, some disadvantages as well, most notably:

1. propane has a lower overall energy content than gasoline, so you have to
   use more to produce the same power level. (similar to alcohol)
2. the system requires a water jacketed vaporizer to change the fuel state 
   from liquid to gas, since the loss of heat by vaporizing the fuel would
   quickly freeze the carburetor solid without it.
3. availability is much lower than gasoline, you can't get it at every service
   station.
4. propane carburetion systems are for the most part antiques - the design
   of the carburetor and vaporizer hasn't changed for thirty years.  Generally
   the systems currently available are dismally inefficient and oversimplified
   to the point where it is difficult to impossible to tune the thing properly.
5. propane requires specific engine mods  -ie stainless steel exhaust valves 
   and significantly modified spark timing curves.  This has given propane
   fuel systems bad press in the past since most systems are designed for dual
   fuel applications - you can flip a switch to convert from propane to gasoline,
   which in most cases results in poor performance from one fuel or both,
   depending on which fuel the engine operating parameters are optimized for.
   At best, it is a compromise, which gives mediocre performance from both.

Some of the disadvantages listed above could be greatly reduced by using an
EFI system for the propane fuel system.  A direct TBI type system would
allow the tailoring of the fuel delivery to exact engine needs, the computer
could adjust spark timing to suit the different advance requirements of propane,
and the direct injection of liquid propane into the airstream would lower it's
temperature drastically, resulting in much improved intake fuel air density
and thus volumetric efficiency.

The system itself would be remarkably similar to a gasoline EFI system, but
one as-yet insurmountable problem I have encountered is the fuel injector 
itself.  Firstly, it needs to be able to handle a fuel pressure of about 150
PSI.  The other problem stems from the amount of fuel needed.  Propane expands   
something like 270 times from it's liquid state to it's gaseous state, so the
injector would have to be accurate with very small volumes of liquid fuel in
order to meter the correct amount for a given air fuel ratio, especially at 
idle.  Unfortuneatly you can't simply regulate the fuel pressure down to a
more manageable pressure, at least not by any means I am aware of, since
there is nowhere for the unused fuel to go.  You can't simply plumb it back
to the tank as with gasoline EFI systems, since the tank is at a higher 
pressure than the returning fuel.  Pumping it back would kinda defeat the 
purpose, since not having to have a pump in the first place is a bonus.
The volume problem is a bit less severe in that you could possibly use a
single direct port injector in the throttle body injection system, since
they are made to meter much lower volumes than a TBI injector.

I currently have a 78 Camaro with a 1970 vintage angle plug 350 engine running
happily on propane.  The system was relatively expensive, (2000$CDN), but has
paid for itself twice over in fuel savings alone.  The tank is by far the most
expensive part of the system, it alone was about $1200.  I run a relatively 
high 10.5 to 1 compression ratio, and the engine puts out quite well, although
I haven't dyno tested it.  I think I could improve on this greatly, however
with some decent fuel metering and a modified ignition system.  I plan on
using an HC11 as an ECM core, firstly doing a timing control system, and
gradually adding the EFI to it.  If anybody out there has any ideas on how
to solve the injector problem, LET ME KNOW!!!.  

regards,
darrell




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 | Darrell Norquay           |   QLOG   | Internet: darrell at dlogtech.cuc.ab.ca|
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