It's a heat exchanger that converts the liquid LP to a gas, using hot engine coolant. Simply releasing the pressure on the fuel in the tank does not allow enough of it to evaporate to run anything larger than a good-sized lawn mower engine. After doing maintenance on a fleet of LP-powered vehicles, it would be hard to run fast enough to give me a system like the one pictured. The fuel vapor passing through the same throttle body as the air displaces enough air to cause a 10% or greater power loss on the average LP-fueled engine. The only ones I've seen that produce acceptable power inject the liquid fuel below the throttle plates. The engine also needs to have a very high compression ratio to make use of the fuel efficiently, since LP gas has about a 110 octane rating.
Jerry
