Propane EFI

[Greg Hermann]

>Biggest problem with propane EFI is that it's boiling point is all over the
>map - from not at -42~ to 175+ at 100f~.  This implies pumping to a high
>pressure, special unobtanium injectors ( gasoline ~50 to 75~ psi) and lots of
>high pressure gas in the engine compartment.  That's the bad news.

If you wanna get serious about figuring out exactly what is going on here,
there are charts of the thermo properties of propane in the ASHRAE manual.
Analogous to steam tables, use 'em the same way to figure pressure v. temp
v. enthalpy, boiling point at a given pressure, etc. Boiling point at 1
atm. (sea level) is about -38 F. Don't remember critical point T & P off
the top of the noggin.
>
>The good news is if you are a fangler, you can tell the tank company to suck
>off.   Seems like a blend of liquid propane and butane about 50/50 is a direct
>replacement for freon 12.   Yet did know that propane is an established and
>excellent refrigerant?

Only in refineries, where the fact that it burns so well is no different
than everything else in the joint!

Hummm  Duh - that means any small auto a/c pump and
>coils can be used effectively as a pressurizing system - using a stock tank
>without exotic in tank pump.  That ought to give you a bunch of dead
>presidents to play with.

All you need to get pressure out of the storage tank reliably is a little
bit of heat under it, and a bottom feed outlet line. All (prolly more)
pressure than you could ever want that way.
>
>Next, unless I am sadly missing the point, the only advantage of liquid
>propane over gaseous propane is the latent heat of vaporization.  Gaseous
>propane introduced such that it can fully blend results in a homogeneous
>mixture that's incredibly smooth and even burning.  This premixed gas is why
>when by weight is much less than gasoline yet delivers within 10% of the
>milage of gasoline.  Direct port injection will not carry much of the fuel
>into the chamber liquid -44F bp and forfeit the homogeneous mixture advantage.

But the problem, and the reason that propane engines are down on power is
that vaporized propane, with its lower (44 vs. about 114) atomic weight,
displaces about 2.5 times as much oxygen as vaporized gasoline. So--like
always--less oxygen, less power!
>
>One alternative for fangling is to salvage a conventional water heated
>vaporizer and submit it to fangle torture.   Remove the water jacketing.
>Machine the upper chamber off.  Plumb the high pressure let down valve at one
>end of an A/C coil and the low pressure end at the other.  Stick the A/C coil
>in the intake - either the air box NA or as an aftercooler turbo'd.  Since the
>only reason for the water to add enough heat to keep the vaporizer from
>freezing, if the coil is large enough (not hard) it does that ok.  BUT you now
>get FREE charge cooling without having to obtain unobtainium high pressure
>injectors.

Try running the calcs here. There's more than enough heat of vaporization
to cool the intake charge below the boiling point of the propane, IF the
manifold were not LONG PAST saturated with propane vapor. (WHICH IT COULD
NEVER BE!) (meaning no more would evaporate!) Dat's why you need heat.

But where else is there heat available? Compression stroke. So flash SOME
(20%) of the propane into the intake, and use the latent heat from that to
cool the air charge to the point where it is saturated with propane, plus
to chill the rest of the liquid propane, while it is still at high (300
psi) pressure to whatever that temp is. reduce the rest of the propane down
to a normal (4 bar??) fuel rail pressure with a regulator. Let whatever
propane flashes from this process vent out of the rails through a float
vent and go into the manifold too. Now, you have liquid propane at about 60
psi above the manifold pressure in the fuel rails, and at whatever its
boiling point is at that pressure. So inject it into the ports through
conventional injectors, which will atomize it decently, if NOT well, and
most of the rest of the latent heat will absorb heat during the compression
stroke, thus reducing compression work, cooling things down, and increasing
net output somewhat.

Big hitch here is--you do NOT want to reduce temp at the time of ignition
(TDC) to the point where a stoich amount of ANY fuel in the chamber would
have a higher (theoretical) partial pressure than that particular fuel's
saturation pressure at the TDC temp. and volume--Cuz it will never get
there! If you try to get past the saturated fuel vapor condition, it just
WILL NOT happen--part of the fuel will still be liquid when the spark goes
off, cuz there wuz never enough heat AVAILABLE to vaporize it.

In case you hadn't guessed already, this is why alky fueled engines are
always soggy and hard to get to light--the evaporation of the alky
OVERCOOLS the charge and compression stroke to such a point that: first it
is tough to get a saturated fuel vapor pressure condition at TDC which is a
rich enough mixture to be above the LFL, and second, the remaining fuel
droplets are trying to quench the flame front! I suspect that a run through
the numbers with propane might show the same sort of problems as with
alky--unless you add enough heat to vaporize most of it from the water
jacket, like has always been done.

And this little discussion has not even addressed what to do with the
propane vapor from the fuel rails during a hot soak!

Regards, Greg