Tank is in the back. The injector only works properly at elevated fuel pressure otherwise it would just drip out. Like a shower head with no pressure, it would just dribble out.

Valve lets in a mixture of air and fuel, not just fuel, and allows compression to happen. If that wasn’t the case you’d have to have a mixture injector that closes during compression and explosion. Without it, the explosion would expand in the intake runner instead of pushing down the piston.