Go back up the page and read all of Frank Raso's posts. He seems to have the best handle on the issue.
You do not get a mpg increase going to propane. The only reason to go to propane is that the fuel costs so little that its miles per dollar comes out ahead.

Propane prices vary a lot. My parents had propane for heating house and water. They'd stock up when the price was low.

The problem as I see it with dual fuel systems is that an engine optimized for propane is different than an engine optimized for gasoline.

One thing that you all have missed is the decrease in air volumetric efficiency while on fully vaporized propane. Gasoline engines run best when the fuel enters the combustion chamber in the form of tiny droplets. The ratio between vaporized gas and liquid gas is about 1000:1. So to get the maximum amount of air into the engine you have to make the fuel take up less space. Propane is vaporized before or in the mixer and as the air/fuel mixture reaches the cylinder the fuel takes up a much larger percentage of the total, limiting air and thus limiting power.
BTW, the term "air volumetric efficiency" is something that I just made up. I don't know if there's a real term for this.

Here's my wonder....Why do we have to vaporize it before it enters the chamber? IMHO it might prove to be better if injected into the ports directly as a liquid. As it flashes to vapor (-156F) it'd greatly drop the temperature in the intake ports and increase the air volumetric efficiency. Even just a little LPG injected into the intake manifold near the start of the runners would supercool the air, increasing power.

I seem to remember that LPG stays liquid at 125psi at room temp.

As we now have Diesel engines in trucks running fuel rail pressures of 26,000psi, and the gasoline fuel injectors on cars have proven to be nearly bulletproof, the technology is there to give it a try. Watching Gale Banks' turbocharging video last night, he mentioned that using methanol eliminated the need for an intercooler.

How about using the liquid propane in an eductor setup to drag more air into the cylinder?


I think that we're ignoring the best uses for propane because it's easiest to just use what has been available for the last 50 years.

R.

Well, I goofed again. I did a search after I had posted rather than before. Google "liquid propane injection" and you get thousands of hits. Oh, well.