I was researching liquid propane injection a couple of weeks ago. My Brother and I were thinking about it for high compression engine use.


During the search I found a couple of links that had some good information on liquid propane injection with some of the answers coming from engineers that are involved in the industry.

Links:
http://www.eng-tips.com/viewthread.cfm?qid=129931&page=1
http://www.eng-tips.com/viewthread.cfm?qid=215940&page=16

One of the issues to overcome is the need for very high pressure to keep the propane liquid at high under hood temperatures, especially during restarting a hot engine. One of the engineers mentions pressures of up to 450psi to keep the propane liquid at high temps.

Another issue for those of us in the US is the propane is too dirty for liquid injection. Leaves nasty tar, propylene, and sulfur deposits in the system.

Edit:
A couple more links:

Brief synopsis of propane as an engine fuel-
http://www.eng-tips.com/faqs.cfm?fid=1139

Propane vapor pressure chart-
http://www.engineeringtoolbox.com/propane-vapor-pressure-d_1020.html

Note how high the vapor pressure is before it even gets to 50deg C.