"LP" is actually a mixture of Propane and Butane. Butane has more BTU's than the Propane, but it also "freezes" (stays liquid) at 32 degrees F. The Propane vaporizes at something like -70 degrees F. so more of it is mixed into the gas blend in the winter when it's needed for getting the vaporization process started. Once the engine begins to run and the coolant warms up, the heat exchanger can start vaporizing the Butane to take advantage of the extra power available by turning it into a gas that can be mixed with air for the right power output.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!