**7. Conclusion**

16 Will-be-set-by-IN-TECH

Fig. 14. Predicted NOx emissions versus hydrogen content over the NEDC (Mariani et al.,

Regione Lombardia, Fiat Research Center, Sapio, CNR-Istituto Motori and Seconda Universitá degli studi di Napoli are involved in a project to test a passenger car fuelled by HCNG blends, varying the hydrogen content, in order to assess the impact of hydrogen addition to natural gas on combustion, exhaust emissions and fuel consumption, over different driving cycles,

Fig. 15. Urban bus tested with HCNG blends (Genovese et al., 2011).

Figure 16 (Prati, Costagliola, Torbati, Unich, Mariani, Morrone & Gerini, 2011).

2011).

Natural gas is employed as fuel since it is the cleanest fossil fuel with exhaust emissions from natural gas vehicles lower than those of gasoline-powered vehicles. Some of its drawbacks can be mitigated by enriching it with hydrogen to produce the so called hydrogen-natural gas blends.

The laminar flame speed of methane is lower than the gasoline one and the addition of hydrogen, which presents a laminar flame speed about eight times that of methane, significantly improves this main combustion property.

In the past years, many authors have proved both experimentally and numerically that the HCNG blends improve engine efficiency and reduce CO2 emissions because of the reduced C/H ratio and fuel consumption. NOx emissions are, instead, larger than NG because of the higher in-cylinder temperature attained, for a given equivalence ratio. Anyway, the use of lean AFR or the EGR definitely reduces NOx emissions and bring about an extra increase in engine efficiency. The good combustion patterns of HCNG blends help to keep low HC emissions.
