Hydrogen
253
reduces the risk of sudden firing. The hydrogen molecules are smaller than
other gases and hence it can diffuse through many materials considered air
tight or impermeable to other gases. Leakage of liquid hydrogen evaporates
quickly as its boiling point is very low (–253ºC).
In the case of leakage of hydrogen, buoyancy and diffusion effects in the air
are often overshadowed by the presence of air currents from a slight ambient
wind and very slow vehicle motion. In general, these serve to disperse the
leaked hydrogen more quickly.
9.4.3.5 Flammability
Flammability limits are the range that the engine can operate over the wide
range of air–fuel mixtures. Hydrogen–air mixtures are flammable in the
broad range, –4–75% concentration, whereas gasoline flammability range is
less than 10. In the hydrogen–air mixture, 4% hydrogen can provide combustion mixtures whereas the stoichiometric air–fuel ratio is 29.5% (Norbeck et
al. 1996). Hydrogen provides stable operation even in dilute condition also.
In terms of equivalence ratio the flammability range for hydrogen is 0.1–7.1
and for gasoline is 0.7–4.0. Hydrogen engines can run on more lean mixtures
than gasoline engines. Lean fuel engine operation reduces the fuel consumption as well as combustion chamber temperature. However, lean operations
reduce the power output of the engine due to the reduction in volumetric
efficiency of the engine.
9.4.3.6 Quenching Distance
Combustion flames are typically extinguished from a certain distance from
the cylinder wall due to heat losses that is called quenching distance. The
quenching distance for hydrogen (0.64 mm) is lesser as compared to gasoline
(2 mm). So, hydrogen flames travel very close to the wall before they are
extinguished. The smaller quenching distance will increase the tendency of
backfire and pass nearly to the intake valve.
The ignition and flammability properties of hydrogen in comparison with
methane and gasoline is given in Table 9.3.
9.4.3.7 Air–Fuel Ratio
Air–fuel ratio for gasoline is nearly 15:1 whereas for hydrogen 34:1. As the
gaseous hydrogen fuel enters through the intake system, it occupies more
combustion chamber volume than liquid fuel and hence reduces the amount
of air entering into it (Figure 9.2). At stoichiometric conditions, hydrogen
displaces about 30% of the combustion chamber, compared to about 1–2%
for gasoline. This reduces the volumetric efficiency of the engine as well as
power developed. As hydrogen has a wide flammability range, the hydrogen
engine can work in the A/F range of 34:1–180:1.
253
reduces the risk of sudden firing. The hydrogen molecules are smaller than
other gases and hence it can diffuse through many materials considered air
tight or impermeable to other gases. Leakage of liquid hydrogen evaporates
quickly as its boiling point is very low (–253ºC).
In the case of leakage of hydrogen, buoyancy and diffusion effects in the air
are often overshadowed by the presence of air currents from a slight ambient
wind and very slow vehicle motion. In general, these serve to disperse the
leaked hydrogen more quickly.
9.4.3.5 Flammability
Flammability limits are the range that the engine can operate over the wide
range of air–fuel mixtures. Hydrogen–air mixtures are flammable in the
broad range, –4–75% concentration, whereas gasoline flammability range is
less than 10. In the hydrogen–air mixture, 4% hydrogen can provide combustion mixtures whereas the stoichiometric air–fuel ratio is 29.5% (Norbeck et
al. 1996). Hydrogen provides stable operation even in dilute condition also.
In terms of equivalence ratio the flammability range for hydrogen is 0.1–7.1
and for gasoline is 0.7–4.0. Hydrogen engines can run on more lean mixtures
than gasoline engines. Lean fuel engine operation reduces the fuel consumption as well as combustion chamber temperature. However, lean operations
reduce the power output of the engine due to the reduction in volumetric
efficiency of the engine.
9.4.3.6 Quenching Distance
Combustion flames are typically extinguished from a certain distance from
the cylinder wall due to heat losses that is called quenching distance. The
quenching distance for hydrogen (0.64 mm) is lesser as compared to gasoline
(2 mm). So, hydrogen flames travel very close to the wall before they are
extinguished. The smaller quenching distance will increase the tendency of
backfire and pass nearly to the intake valve.
The ignition and flammability properties of hydrogen in comparison with
methane and gasoline is given in Table 9.3.
9.4.3.7 Air–Fuel Ratio
Air–fuel ratio for gasoline is nearly 15:1 whereas for hydrogen 34:1. As the
gaseous hydrogen fuel enters through the intake system, it occupies more
combustion chamber volume than liquid fuel and hence reduces the amount
of air entering into it (Figure 9.2). At stoichiometric conditions, hydrogen
displaces about 30% of the combustion chamber, compared to about 1–2%
for gasoline. This reduces the volumetric efficiency of the engine as well as
power developed. As hydrogen has a wide flammability range, the hydrogen
engine can work in the A/F range of 34:1–180:1.