Dale Amy
June 2, 2010

The Science of Propane
At normal room temperatures, propane is in a gaseous state, since its boiling point is a chilly minus 44 degrees (boiling point simply being the temp at which any given liquid changes "phase" to vapor). But much as automotive cooling systems are pressurized to raise the boiling point of their water/antifreeze mix, so is propane pressurized to keep it in a liquid state at typical ambient temperatures. Release it from that pressure and propane immediately vaporizes, expanding and cooling in the process. (Think of what happens when you open the valve to see what's left in your barbecue's propane tank.)

On older propane conversions for carbureted vehicles, that phase change to a gaseous state had to occur at the mouth of the carburetor, after which the mix of air and propane gas had to travel the length of the intake manifold runners and cylinder-head ports before reaching the combustion chamber. Without getting too buried in the science, these systems could be hampered by cold-start issues and had other inefficiencies.

Today's liquid propane injection systems, as employed by Roush, keep the propane in a liquid state right up until it exits the tip of the (specialized) fuel injector, immediately upstream of the intake valve. Like any form of fuel injection, this improves fuel efficiency but also helps make propane cold-start issues a thing of the past.

More propane tidbits:

  • Propane holds only about 86 percent of the energy per volume of gasoline, but this is partially compensated by its higher stoichiometric air/fuel ratio of 15.6:1 versus 14.7:1 for gasoline.
  • About 90 percent of U.S. propane is domestically produced; most of the balance comes from Canada.
  • Propane is a byproduct of both the processing of natural gas and petroleum refining.
  • Propane in gaseous form is heavier than air (about 1.5 times as dense) and therefore sinks and pools at the ground. However, liquid propane is significantly lighter than gasoline.
  • Propane has an octane rating of roughly 105 to 110, so will theoretically support higher compression ratios and more spark timing.
  • Propane burns cleaner than gasoline, meaning both fewer emissions-especially carbon monoxide-and cleaner engine internals.
  • Government incentives/tax credits help defray the cost of vehicular propane conversion.