Technology Status of Hydrogen Road Vehicles

part of the inlet stroke. It is thus characterized as being at essentially ambient pressure, and at a temperature
from near-ambient down depending on hydrogen storage temperature.
At its simplest, the hydrogen can come from a single device that meters the quantity for each cylinder in turn.
This would be analogous to the single carburetor of simple gasoline engines, but has the great disadvantage
for hydrogen of making the whole inlet manifold subject to backfire. Somewhat more complicated is the timed
injection where each inlet port is supplied with low-pressure hydrogen, limiting the backfire-prone mixture
to that in the immediate vicinity of each cylinder. This line also preserves the advantages of multi-port
sequential fuel injection as in modern gasoline ICEs: less mixture variation, improved acceleration, better
volumetric efficiency, and elimination of the carburetor or similar device, which insert pressure barriers to the
mixture.
With low-pressure hydrogen injection typical of external mixture formation, the injectors are relatively simple,
and can have a metering function only if the hydrogen storage pressure is high enough (which is usually the
case). The air flow is unthrottled, but the incoming hydrogen occupies considerable space because of its low
density. Turbocharging the air can help, but the relative power loss of hydrogen compared to gasoline remains
as high as 40%. Simple mass considerations show this loss to be at least 15%.
As with gasoline the inlet valve can be closed about halfway through the compression stroke, compression can
proceed to about 10:1 ratio, and spark ignition can occur in the final part of the stroke depending on engine
speed. The expansion and exhaust strokes proceed generally as in a gasoline engine, but hydrogen and its
combustion products with air (essentially only H 2O and small amounts of NO x) can also lodge in cylinder
recesses and be present during the inlet stroke.
At low engine speed and load, hydrogen combustion with external mixture formation proceeds smoothly over
a wide range of lean mixtures. Thermal efficiencies are high, and the exhaust contains only trace quantities
of NO x (the low temperatures of lean mixtures block NO x formation), as well as inevitable traces of CO and
CO 2 typical of lubricating oil combustion. Unfortunately, this excellent result starts deteriorating toward midpowers
and speeds with the advent of rough combustion caused by pre-ignition and backfire. The NO x levels
can increase rapidly and power levels typical of gasoline engines cannot be achieved, not only because of
hydrogen’s low density, but also because of deteriorating combustion.
The phenomena at work in irregular combustion during the inlet stroke have no doubt been investigated by
most researchers, but the most complete reporting has been made by Musashi Institute of Technology.
Koyanagi et al. (1994) describe the definitive experiments on a single-cylinder engine supplied with hydrogen
from a relatively large reservoir with spark plug ignition at 231 BTDC and 10:1 or 12:1 compression ratio.
Temperatures were measured by thermocouples near the surface of the spark plug, object lenses followed the
luminescence of the flame, recorded either by photomultiplier or by high-speed video camera, and of course
crank angle and internal pressures were recorded. They found that:
! Backfiring occurred more easily and frequently with leaner mixtures and at the lower compression ratio,
i.e., at lower temperatures.
! The spark plug can be eliminated from consideration as the source of pre-ignition and backfire.
! Special pistons with enlarged radial clearance at top land clearly showed that the exhaust gases trapped
in this clearance volume were the source of pre-ignition and backfire.
Because little can be done to eliminate such residual pockets of hot exhaust gas, the Musashi researchers seem
to have abandoned external mixture formation, because most of their remaining reports deal only with internal
mixture formation. Others prefer to continue with the external mixture formation, and ascribe the backfire
to other causes such as spark plugs and hot exhaust valves, but they try to get around the problem by two lines
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