With conventional manifold-mounted petrol injection, fuel is mixed with the air in the intake tract. It then travels past the valve before entering the combustion chamber. Physical contact with the manifold walls and valve stem causes interruption to the mixing. This results in some amount of fuel loss and the absorption of heat by the fuel-air mixture.
In a direct injection engine, the injector sits between the intake valves. Petrol is injected directly into the combustion chamber as the air enters. This not only allows a homogenous mixture that doesn’t suffer any contact loss, but also allows a sudden cooling of the fluid, thereby letting a little more air in. It also allows an ultra-high compression ratio, which makes a noticeable difference in efficiency.
Because of its location, the injector is subject to high pressure and high temperature, but must continue to do its work as a precision device. Poor fuel quality can cause problems to direct injectors, which is why some countries/markets do not offer models with direct injection engines.