I may try to go pure propane eventually, but for now I want to use propane assist so to speak. I'm running megasquirtnspark extra so it would be easy to trigger the solenoids only when needed, and lean the gasoline maps to compensate so it doesn't get overly rich when the propane kicks in.

in order to ensure I will be getting liquid and not vapor when the solenoid fires I was thinking of setting up a fuel pump with return to circulate the propane and purge the vapors.  The question is what kind of pump do I need?

Fuel injectors must pulse width modulate to properly meter the fuel.  Nitrous injectors have too much internal mass to modulate in the order of millisecond response time.

LPG vapor pressure is high, on the order of 8.5 BAR at 70 Deg F, and 17 BAR at 125 F.  To keep LPG in liquid form at these temperatures on an engine, you must keep at least 3.5 BAR above vapor saturation pressure, or you will have a mixture of liquid and vapor propane.  Now, add in underhood temperatures of around 225 F, and you can reach propane vapor pressures in excess of 400 psig (27 BAR).  This is out of range for even the most advanced over the counter fuel injector.  I have personally witnessed a production gasoline fuel injector blow apart at 150 psig.  Not a pretty sight.

If you just want to use propane as an octane enhancer, remember that the liquid expands 270 times from vapor.  Even the tiniest amount will quickly exceed the air fuel handling capabilities of many engines.

The article you refer to is a long ago project that never made it into production.  The pumps discussed were indeed GM production fuel pumps, mounted inside the fuel tank (custom tanks) and plumbed in parallel.

Lastly, this type of project is NOT for the novice high pressure hobbiest (actually, nor is this forum).  There are just a handful of companies in the world that even have the capabilities and expertise of this technology.  Even mighty Ford tried it for one project run on a large Ford Excursion but pulled it almost as quickly.

Franz eng-tips, by professional engineers for professional engineers

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