Hydrogen has been largely proposed as a possible fuel for
internal combustion engines. The main advantage of burning
hydrogen is the absence of carbon-based tailpipe emissions.
Hydrogen’s wide flammability also offers the advantage of
very lean combustion and higher engine efficiency than
conventional carbon-based fuels. In order to avoid abnormal
combustion modes like pre-ignition and backfiring, as well as
air displacement from hydrogen’s large injected volume per
cycle, direct injection of hydrogen after intake valve closure is
the preferred mixture preparation method for hydrogen
engines. The current work focused on computational studies of
hydrogen injection and mixture formation for direct-injection
spark-ignition engines. Hydrogen conditions at the injector’s
nozzle exit are typically sonic. Initially the characteristics of
under-expanded sonic hydrogen jets were investigated in a
quiescent environment using both Reynolds-Averaged NavierStokes
(RANS) and Large-Eddy Simulation (LES) techniques.
Various injection conditions were studied, including a
reference case from the literature. Different nozzle geometries
were investigated, including a straight nozzle with fixed cross
section and a stepped nozzle design. LES captured details of
the expansion shocks better than RANS and demonstrated
several aspects of hydrogen’s injection and mixing. Incylinder
simulations were also performed with a side 6-hole
injector using 70 and 100 bar injection pressure. Injection
timing was set to just after inlet valve closure with duration of
6 μs and 8 μs, leading to global air-to-fuel equivalence ratios 
typically in the region of 0.2–0.4. The engine intake air
pressure was set to 1.5 bar absolute to mimic boosted
operation. It was observed that hydrogen jet wall impingement
was always prominent. Comparison with non-fuelled engine
conditions demonstrated the degree of momentum exchange
between in-cylinder hydrogen injection and air motion. LES
highlighted details of hydrogen’s spatial distribution
throughout the injection duration and up to ignition timing.
Higher peak velocities were predicted by LES, especially on
the tumble plane. With the employed injection strategy, the
areas closer to the cylinder wall were richer in fuel than the
centre of the chamber close to the end of compression