buletinul institutului politehnic din iaşi - Universitatea Tehnică ...

Bul. Inst. Polit. Iaşi, t. LVIII (LXII), f. 4, 2012 123
5.4 NOx Emissions
Fig. 5 shows the variations of NOx emissions with excess air ratio at
different hydrogen volume fractions. NOx emissions depend on temperature and
oxygen concentration in the cylinder. NOx emissions increases with the increase
of hydrogen addition fraction. Although the hydrogen-enriched engine produces
more NOx emissions when the excess air ratio is around stoichiometric
conditions, NOx emissions for all hydrogen enrichment levels drop to
acceptable value when the air fuel ratio is around 2.
Fig. 5 – NOx emisions versus air fuel ratio.
5.5. HC Emissions
HC emissions are mainly caused by the unburnt hydrocarbons in IC
engines. Fig. 6 displays the profile of HC emissions at 3000 rpm. As it is shown
in Fig. 6, although the λ increases, the decreased HC emissions go with the
increase of hydrogen addition level, reflecting the capability of hydrogen
enrichment on improving engine combustion. The explanation is that the
increased oxygen concentration would help the fuel be completely burnt, and
thus benefit reducing HC emissions at lean conditions (Rankin, 2008). The high
flame speed of hydrogen also benefits the hydrogen–gasoline fuel mixture to be
fully burnt and results in less HC emissions. Moreover, the short quenching
distance of hydrogen permits the flame of the hydrogen–gasoline mixture to
propagate much closer to the crevices such as the gaps between piston and
cylinder wall than that of the pure gasoline, and thereby reduces HC emissions
(Kahraman et al., 2009). To conclude, HC emissions at lean burn limit are
gradually reduced with the increase of hydrogen addition fraction for the
HHGE.