Hydrogen (knocking) before spark plug sparks the air-fuel mixture.

Hydrogen has an extensive combustible range
compare to other fuel such as gasoline, diesel and natural gas 1. Therefore,
highly lean or rich mixture of hydrogen-air can be burn. Greater fuel economy
achieved if hydrogen internal combustion engine run at lean operation 2.  The quantity of pollutant such as nitrogen
oxide lowered in exhaust due to lower combustion temperature. However, amount
of hydrogen injected into the engine should have the perfect limit when the engine
runs in the lean condition. Engine power output decrease significantly caused
by depletion in the volumetric heating value of the air/fuel mixture when lean
condition 3.Hydrogen has extremely low ignition
energy.  Combustion energy for hydrogen
is about one order magnitude smaller than needed for petroleum fuel. The
advantages of having least ignition energy to allow hydrogen engine to burn
lean mixture and secure quick ignition 3. However, there is a drawback of
having lower ignition energy. There is huge possibility of hydrogen-air mixture
can be ignited by different source rather than spark plug such as hot pots 4.Quenching distance of hydrogen is
shorter compared to petroleum fuel. Consequently, hydrogen fires move nearer to
the cylinder wall than gasoline before hydrogen quench.

Quenching the flame will be
difficult if the quenching distance is extremely smaller. This may raise the
tendency to backfire because the hydrogen-air blend easily reaches a nearly
closed intake valve than an isooctane-air flame. 5.Hydrogen
has a relatively great auto-ignition temperature.  This has a significant impact on a
hydrogen-air mixture is compressed. The compression ratio of an engine is
determined by the autoignition temperature of the fuel.The temperature rise is
shown by the equation: From the above equation, we know that compression ratio relies
on T2 which is temperature during compression.  This T2 is restricted by
auto-ignition temperature of the fuel to limit air-fuel mixture to ignited
automatically (knocking) before spark plug sparks the air-fuel mixture. An
engine can reach great thermal efficiency with high compression ratio when the
auto-ignition temperature fo of fuel is high 6.Hydrogen internal combustion engine able to achieve similarity
to the thermodynamically ideal engine cycle because of the high flame velocity.
Nevertheless, the flame speed reduces significantly when the engine operates in
the lean condition 7. Hydrogen capable of diffuse in air rapidly than
hydrocarbon fuel such as iso-octane and natural gas. This is due to high
diffusivity of hydrogen. This enables a uniform mixture of the air-hydrogen
mixture. Hydrogen will not pollute the environment if any leakage happens.
However, there are problems in hydrogen internal combustion engine due to very
low density. Hydrogen storage required a very large volume of sufficient
driving range and diminish power output due to low energy density. 8.

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