Hydrogen
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will start easily. Expansion ratio is the ratio of the volume at which a gas
or liquid is stored to the volume of the gas or liquid at atmospheric pressure and temperatures. Hydrogen’s expansion ratio is 1:848; that is, gaseous
hydrogen occupies a volume of 848 times more than at a liquid state. When
hydrogen is stored as a high-pressure gas at 250 bar and atmospheric temperature, its expansion ratio to atmosphere is 1:240. This necessitates that a
large volume of hydrogen is required to be carried for adequate running of
a vehicle.
9.4.2 Chemical Properties
Hydrogen atoms are chemically very reactive. When a small amount of
ignition energy in the form of spark is provided to the hydrogen–air mixtures, the molecules react with air in the atmosphere very actively and
release significant amounts of heat and water vapor. At room temperatures
these reactions are very slow, but is accelerated by catalysts such as platinum and spark. Very high temperatures (>5000 K) are needed to dissociate
hydrogen molecules into atomic hydrogen completely. Hydrogen is considered an energy carrier to store and transmit energy from primary energy
sources.
9.4.3 Fuel Properties
Hydrogen is a suitable gaseous fuel for SI and CI. The fuel properties of
hydrogen are given in Table 9.2 (Saxena et al. 2008). The self ignition temperature of hydrogen is very high and best suited for SI engines. This temperature plays an important role in storage pressure of hydrogen because as
pressure of the hydrogen inside the cylinder increases, temperature will also
increase. The higher self ignition temperature of hydrogen allows the use
of larger compression ratios without causing premature ignition. It is well
known that as the compression ratio of the engine increases its thermal efficiency will also increase. Hydrogen can be ignited at its low ignition energy
of 0.02 mJ as compared to 0.24 mJ for gasoline and 0.28 mJ for methane at
stoichiometric.
9.4.3.1 Minimum Ignition Energy
Minimum energy required for ignition is the order of magnitude less than
that required for gasoline. The minimum ignition energy is a function of
equivalence ratio. At the equivalence ratio nearer to 1, minimum ignition
energy for hydrogen–air mixtures is very low. As very little energy is required for the combustion of hydrogen, any hydrogen–air mixture can be
ignited due to wide limits of flammability of hydrogen. The hot spots in
the combustion chamber may cause premature ignition in the combustion
chamber and flash back also.
251
will start easily. Expansion ratio is the ratio of the volume at which a gas
or liquid is stored to the volume of the gas or liquid at atmospheric pressure and temperatures. Hydrogen’s expansion ratio is 1:848; that is, gaseous
hydrogen occupies a volume of 848 times more than at a liquid state. When
hydrogen is stored as a high-pressure gas at 250 bar and atmospheric temperature, its expansion ratio to atmosphere is 1:240. This necessitates that a
large volume of hydrogen is required to be carried for adequate running of
a vehicle.
9.4.2 Chemical Properties
Hydrogen atoms are chemically very reactive. When a small amount of
ignition energy in the form of spark is provided to the hydrogen–air mixtures, the molecules react with air in the atmosphere very actively and
release significant amounts of heat and water vapor. At room temperatures
these reactions are very slow, but is accelerated by catalysts such as platinum and spark. Very high temperatures (>5000 K) are needed to dissociate
hydrogen molecules into atomic hydrogen completely. Hydrogen is considered an energy carrier to store and transmit energy from primary energy
sources.
9.4.3 Fuel Properties
Hydrogen is a suitable gaseous fuel for SI and CI. The fuel properties of
hydrogen are given in Table 9.2 (Saxena et al. 2008). The self ignition temperature of hydrogen is very high and best suited for SI engines. This temperature plays an important role in storage pressure of hydrogen because as
pressure of the hydrogen inside the cylinder increases, temperature will also
increase. The higher self ignition temperature of hydrogen allows the use
of larger compression ratios without causing premature ignition. It is well
known that as the compression ratio of the engine increases its thermal efficiency will also increase. Hydrogen can be ignited at its low ignition energy
of 0.02 mJ as compared to 0.24 mJ for gasoline and 0.28 mJ for methane at
stoichiometric.
9.4.3.1 Minimum Ignition Energy
Minimum energy required for ignition is the order of magnitude less than
that required for gasoline. The minimum ignition energy is a function of
equivalence ratio. At the equivalence ratio nearer to 1, minimum ignition
energy for hydrogen–air mixtures is very low. As very little energy is required for the combustion of hydrogen, any hydrogen–air mixture can be
ignited due to wide limits of flammability of hydrogen. The hot spots in
the combustion chamber may cause premature ignition in the combustion
chamber and flash back also.