Effect of natural gas/hydrogen blends on spark ignition stoichiometric engine efficiency

Hydrogen (H2) added to natural gas (NG), improves the combustion process of the air-fuel mixture. This gives the potentiality to develop engines with better performance and lower environmental impact. In any case how hydrogen is produced represents a crucial aspect. In general, if H2 is produced utilizing fossil fuels and not renewable or nuclear sources, the environmental benefit of CO2 reduction could be reduced. In this paper two engines, a light duty (LD) and a heavy duty (HD), were tested in stoichiometric conditions. The engines were fuelled with NG and with two blends of NG with a 20% and a 40% by volume of H2, respectively named NG/H2 20% and NG/H2 40%. The light duty engine was tested at different load and speed, with spark advance set by the electronic control unit (ECU). The ECU actuated a retarded ignition, especially at low load. With the heavy duty engine, the tests were carried out only at high load. Spark advance was tuned to obtain burning gravity centre at the same angular position with NG and the two NG/ H2 blends. Hydrogen positive effect on combustion development has been observed, even if global efficiency was only marginally affected for both the light and the heavy duty engine. Moreover, only with the heavy duty engine and NG/H2 40% blend, knocking phenomena have been observed, requiring great attention in spark advance, boost pressure and EGR rate optimization. For best performance with high H2 content, a specific combustion system design would be necessary to fully exploit H2 characteristics.