The paper describes the CFD analysis, the arrangement and the
first experimental results of a single-cylinder engine that employs
an innovative low-pressure hydrogen direct-injection system,
characterized by low fuel rail pressure (12 bar) and consequent low
residual storage pressure. The injection is split in two steps: at
first hydrogen is metered and admitted into a small intermediate
chamber by an electroinjector (a conventional one usually employed
for CNG), next a mechanically actuated poppet valve, that allows
high volumetric flow rates, times hydrogen injection from the
intermediate chamber to the cylinder within a short time, despite
the high hydrogen volume due to the low injection pressure.
Injection must be properly timed to maintain pressure below 6 bar
(or little more) in the intermediate chamber and thus keep sonic
flow through the electroinjector, to maximize volumetric efficiency
and to avoid backfire in the intake pipe.
A preliminary CFD study based on code KIVA3v was carried out in
order to compare the behavior of different injection valve and
valve-seat geometries in controlling air-fuel mixing inside the
cylinder and to address the choice towards the most suitable ones,
i.e., those that allow adequate fuel distribution in the combustion
chamber at ignition time for every operating condition.
As well, all the modifications necessary to obtain the prototype
engine from a production one were based on KIVA3v predictions,
especially as regards the volume of the intermediate chamber and
diameter, maximum lift and opening duration of the injection
valve.
The prototype derives from the single-cylinder Aprilia-Rotax 650
cm₃ with five valves (three intake and two exhaust). The cylinder
head underwent deep changes to replace the central inlet valve and
pipe with the ones for hydrogen injection. Special attention was
paid to crankcase ventilation system to prevent the formation of
hydrogen pockets.
Experimental results show the effectiveness of the solution. The
engine proved to run correctly, without the typical drawbacks of
hydrogen engines (preignition, knocking, backfire, roughness) even
with stoichiometric or slightly rich mixture (for maximum power).
Maximum power was higher than for the engine fed with gasoline.
A new CFD study based on AVL_Fire code recently started taking
engine actual details into account with the aims to better predict
engine behavior and to address the improvement of the whole
system.