If you preheat your fuel using heat from your exhaust pipes (keeping the fuel liquid since it is under pressure), then have the fuel immediately vaporize when it is injected/loses the pressure keeping it liquid. Would you get a boost in fuel economy? I could see netting less power/volumetric efficiency as the air heats up and expands. But what about fuel efficiency? Would you be able to gain a more complete burn at higher engine speeds? Or even extract more energy out of a given amount of fuel since the fuel would be closer to its ignition point.