There are several benefits of direct cylinder injection with spark-ignited, gasoline engines, but it's illegal in F1.
First, it improves mass flow of the intake charge, since the fuel mass fraction does not have to pass through the ports and by the intake valves. This small (<10%), but useful, increase in air mass flow helps with power.
Second, the reduction in charge temperature, due to the latent heat effect of the evaporating fuel, is more pronounced when the fuel is injected later in the compression cycle.
Third, a measure of charge stratification can be achieved with a carefully directed fuel spray. This will allow a leaner A/F ratio to be used without detonation, and thus a lower fuel consumption.
2008 FIA F1 engine reg's:
"5.7 Fuel systems
5.7.1 The pressure of the fuel supplied to the injectors may not exceed 100 bar. Sensors must be fitted which directly measure the pressure of the fuel supplied to the injectors, these signals must be supplied to the FIA data logger.
5.7.2 Only one fuel injector per cylinder is permitted which must inject directly into the side or the top of the inlet port."
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