Technology Status of Hydrogen Road Vehicles

Appendix 2. Performance of Hydrogen-Fueled
Internal Combustion Engines for Road Vehicles
During the century or so that the ICE and automotive transport have been such dominant features of life, there
has been little difficulty in suitably matching fuels (usually gasoline or diesel oil, but also other hydrocarbons)
and the engine design as it evolved over the decades to meet higher performance and environmental objectives.
The end of this process is in sight, partly because the overall smog levels in many modern cities are becoming
intolerable–the increase in vehicle population far outstrips the diminishing improvements per vehicle--and
partly because the CO 2 and other molecules cannot be removed and are held responsible for much-discussed
climate change.
It is worthwhile, however, to review briefly the manner of combustion in four-stroke Otto (gasoline) and diesel
engines, which represent the bulk of ICEs in use, because the performance of hydrogen fuel in these engines
will frequently be compared to gasoline and diesel.
A2.1 Combustion Processes in a Traditional ICE
A2.1.1 Four-Stroke Gasoline ICE
On the inlet stroke, the air-fuel mixture is drawn into the cylinder from the inlet manifold/inlet ports, originally
from a carburetor but increasingly now from gasoline injection into the air at each inlet port; the inlet valves,
originally one, now increasingly two or even three, remain open at least throughout the stroke. In
turbocharged engines, the air is compressed for better power output and the gasoline pressure suitably adapted.
As the piston rises on the compression stroke it swirls the mixture and evaporates the gasoline; the inlet valve
is often still open up to 301 after bottom dead center (BDC) for increased charging; compression ratios reach
10:1 or higher requiring additives (lead now being phased out in favor of benzene) to prevent pre-ignition and
power loss.
The expansion stroke involves combustion and expansion of the gases, whose temperature can reach higher
than 1200 o C, but ignition from a spark plug occurs before top dead center (from about 51 to 501 BTDC) as
the engine speed increases to give enough time for proper combustion. The spark is automatically advanced
by electronics compared to earlier mechanical devices; only 1001 crank angle (or a few milliseconds at high
revs) are then available in the compression stroke (between 301 ABDC inlet valve closure to 501 BTDC spark)
for full vaporization of the gasoline (which burns satisfactorily only as a gas); hence the importance of swirl
and squish and the automatic spark advance for good burn.
On the exhaust stroke, the exhaust valves (originally one, now often two) are open and the cylinder contents
are sent to the tailpipe with prior pollution control (originally only acoustic, now with catalytic converters for
unleaded gasoline controlling CO, SO 2, and NO x down to low levels); the inlet valve usually opens just before
TDC to purge the cylinder as much as possible before the exhaust valve closes just thereafter.
A2.1.2 Four-Stroke Diesel ICE
The main difference is that the mixture of air and diesel fuel, which has a low self-ignition temperature (about
250 o C compared to as high as 470 o C for gasoline), is compressed to much higher levels (the compression ratio
can reach 18:1 or more) and the spark plug can be dispensed with. Furthermore, diesel oil burns well as a
liquid, obviously subdivided into very fine droplets to create a homogeneous mixture.
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