The introduction of alternative fuels is crucial to limit
greenhouse gases. CNG is regarded as one of the most promising
clean fuels given its worldwide availability, its low price and its
intrinsic properties (high knocking resistance, low carbon
content...). One way to optimize dedicated natural gas engines is
to improve the CNG slow burning velocity compared to gasoline fuel
and allow lean burn combustion mode. Besides optimization of the
combustion chamber design, hydrogen addition to CNG is a promising
solution to boost the combustion thanks to its fast burning rate,
its wide flammability limits and its low quenching gap.
This paper presents an investigation of different
methane/hydrogen blends between 0% and 40 vol. % hydrogen ratio for
three different combustion modes: stoichiometric, lean-burn and
stoichiometric with EGR. The main objectives are to identify the
complex mechanisms involved in the combustion process and to define
the optimal hydrogen ratio for each combustion mode.
The study combines engine tests and 0D modeling. Tests were
carried out on a spark-ignited single-cylinder engine adapted to
CNG operation with 2 different compression ratios 9,5:1 and 11,5:1.
Computations allow studying separately the different phenomena
linked to the progressive addition of hydrogen in the fuel. Hence,
using 0D modeling, the effects due only to the combustion speed
evolution, as a function of hydrogen ratio in the fuel, were then
quantified using experimental results as comparison basis.
The engine test results reveal that the impact of hydrogen is
limited in stoichiometric conditions except for CO₂ savings. A
ratio of 10 to 20 vol. % of hydrogen seems to be optimal to reach
interesting HC emissions reduction without an excessive lowering of
the operation range. The results are more encouraging for lean-burn
operation as the lean limit of equivalence ratio is extended for
more than 0,125 with 40 vol. % of hydrogen. NOx emissions and
consumption were significantly reduced (-92% for NOx) while
maintaining constant HC and CO emissions compared to methane
operation. Hydrogen stabilizes and speeds up combustion especially
in very diluted mixtures. Furthermore, hydrogen increases EGR
tolerance for the same reasons as in lean-burn mode: in comparison,
identical very-low NOx levels were reached with smaller consumption
gains. Finally, the engine test results show that the highest
tested ratio of hydrogen (up to 40%) in methane for very diluted
mixtures with air or EGR drastically reduce NOx emissions without
penalty on engine performances.
Computations prove that the evolution in terms of thermodynamic
properties of the fuel is not the most important contribution to
the evolution of combustion. The effect of hydrogen addition
contributes mainly to increase the combustion speed. Moreover, lean
burn operation results demonstrate that the increase of combustion
speed due to hydrogen addition is enhanced. This can only be
explained by the increase of thermo-diffusivity which has an
opposite and larger effect than the slowdown of laminar flame
velocity conventionally described in the literature.
