Typically hydrogen direct injection is performed after
the intake valves are closed to avoid displacement of intake
air (reduced power) and backfiring. Early injection refers
to strategies with start of injection (SOI) shortly after
intake valve closing typically resulting in fairly homogeneous
mixtures at spark timing. The required injection
pressures for early injection are in the range of 0.5–2 MPa.
On the other hand, late injection against higher in-cylinder
pressure at the end of the compression stroke requires
higher injection pressures of up to 10 MPa and typically
results in rather stratified mixtures. Finally, multiple
injection strategies with pulses during the compression as
well as the combustion phase require injection pressures
between 10 and 30 MPa.