“I though the problem was always that gasoline only burns as a vapor,”
True. It burns as a flammable liquid with a low flash point (volatile). Heat generates the vapor.
” but under high compression it always liquefies. “
Yes and no. A can of gas is a liquid at ambient conditions. Gasoline vapor could be liquified if compressed and the hear of compression is removed from the compressed gas through a compressor after cooler (removing the hear of compression)
It’s impossible to liquefy the gasoline inside the cylinders of a running internal combustion engine. With cars’ cooling systems, the cylinders and pistons are running at high temperature.
hear ———> heat