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82 Victor Pantilie et al.<br />
many researchers have studied the effect of using hydrogen as a fuel (pure or<br />
mixed with another fuel) on pollutant emissions and engine performance (Das et<br />
al., 2000), (Al-Baghdadi & Al-Janabi, 2000), (Yamin et al., 2000), (Maher et<br />
al., 2003).<br />
The combustion of hydrogen does not create pollutant emissions such as<br />
CO2, CO, HC or PM because hydrogen does not contain any carbon. From the<br />
oil combustion, some fractions of unburned hydrocarbons are present inside the<br />
engine combustion chamber but these are negligible (Pantile, 2010). The massspecific<br />
lower heating value of hydrogen is almost three times as high as that of<br />
gasoline thus the brake specific fuel consumption is reduced ant the thermal<br />
efficiency of the engine is increased. Hydrogen has antiknock quality due to the<br />
self-ignition temperature of the hydrogen/air mixture which is greater than that<br />
of gasoline. A combustible mixture is produced by a small amount of hydrogen<br />
mixed with air, which can be burned in a conventional spark ignition engine at<br />
an equivalence ratio below the lean flammability limit of gasoline/air mixture.<br />
The resulting ultra-lean combustion produces low flame temperature and leads<br />
directly to lower heat transfer to the walls, higher engine efficiency and lower<br />
NOx emissions in the exhaust (Al-Baghdadi & Al-Janabi, 2003). The laminar<br />
flame speed of a hydrogen-air mixture at stoichiometric conditions is almost 6<br />
times higher than that of gasoline. At lean dosage conditions (λ=2) the laminar<br />
flame speed of hydrogen is approximately equivalent to that of a stoichiometric<br />
gasoline-air mixture (Verhelst & Wallner, 2009). The hydrogen flammability<br />
limits are wider compared to gasoline. This means that the load of the spark<br />
ignition engine can be controlled by the excess air-fuel ratio – the load engine<br />
qualitative adjustment. Furthermore, the compression ratio of the engine can be<br />
increased due to the higher octane number. The main disadvantage comes from<br />
the low density of hydrogen which influences the storage of hydrogen and the<br />
power output which is lower compared to liquid fuels (when comparing engines<br />
with external mixture formation), (Pantile et al., 2011).<br />
The U.S. Department of Energy and Department of Transportation have<br />
taken initiatives to shift towards a hydrogen-based transportation system. The<br />
aim of these researches is to develop and commercialize hydrogen fuel cell<br />
vehicles in an economic manner. However, hydrogen can be used as a fuel for<br />
transportation in internal combustion engines rather than in fuel cells at least for<br />
some decades (Romm, 2006), (Sukumaran & Kong, 2010).<br />
The lack of efficient and economical hydrogen infrastructure represents<br />
one of the major issues in exploiting hydrogen energy. Internal combustion<br />
engines can be considered as a transition technology for achieving economical<br />
hydrogen infrastructure (Safaria et al., 2009). Developing hydrogen programs<br />
are conducted by several automotive companies especially by BMW Group and<br />
Ford Motor Company. BMW has introduced its hydrogen engine in the 7 series;<br />
6.0 L V12 bi-fuel (gasoline – hydrogen) engine which operates with liquid<br />
hydrogen stored on board (Kiesgen et al., 2006). Also, Ford has optimized