9.5 Why are hydrogen-powered cars not available?
Description
This article is from the Gasoline FAQ, by Bruce Hamilton with numerous contributions by others.
9.5 Why are hydrogen-powered cars not available?
The Hindenburg.
The technology to operate IC engines on hydrogen has been investigated in
depth since before the turn of the century. One attraction was to
use the hydrogen in airships to fuel the engines instead of venting it.
Hydrogen has a very high flame speed ( 3.24 - 4.40 m/s ), wide flammability
limits ( 4.0 - 75 vol% ), low ignition energy ( 0.017 mJ ), high autoignition
temperature ( 520C ), and flame temperature of 2050 C. Hydrogen has a very
high specific energy ( 120.0 MJ/kg ), making it very desirable as a
transportation fuel. The problem has been to develop a storage system that
will pass all safety concerns, and yet still be light enough for automotive
use. Although hydrogen can be mixed with oxygen and combusted more
efficiently, most proposals use air [114,119,121-124].
Unfortunately the flame temperature is sufficiently high to dissociate
atmospheric nitrogen and form undesirable NOx emissions. The high flame
speeds mean that ignition timing is at TDC, except when running lean, when
the ignition timing is advanced 10 degrees. The high flame speed, coupled
with a very small quenching distance mean that the flame can sneak past
narrow inlet valve openings and cause backflash. This can be mitigated by
the induction of fine mist of water, which also has the benefit of
increasing thermal efficiency ( although the water lowers the combustion
temperature, the phase change creases voluminous gases that increase
pressure ), and reducing NOx [124]. An alternative technique is to use
direct cylinder induction, which injects hydrogen once the cylinder
has filled with an air charge, and because the volume required is so
large, modern engines have two inlet valves, one for hydrogen and one for
air [124]. The advantage of a wide range of mixture strengths and high
thermal efficiencies are matched by the disadvantages of pre-ignition and
knock unless weak mixtures, clean engines, and cool operation are used.
Interested readers are referred to the group sci.energy.hydrogen and the
" Hydrogen Energy" monograph in the Kirk Othmer Encyclopedia of Chemical
Technology [124], for recent information about this fuel.
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