The numerical study presented in this article is based on an automotive diesel
engine (2.8 L, 4-cylinder, turbocharged), considering a NG–H2 blend
with 30 vol% of H2, ignited by multiple diesel fuel injections. The
3D-CFD investigation aims at improving BTE, CO, and UHC emissions at low load,
by means of an optimization of the diesel fuel injection strategy and of the
in-cylinder turbulence (swirl ratio, SR). The operating condition is 3000 rpm –
BMEP = 2 bar, corresponding to about 25% of the maximum load of a gen-set
engine, able to deliver up to 83 kW at 3000 rpm (rated speed). The reference
diesel fuel injection strategy, adopted in all the previous numerical and
experimental studies, is a three-shot mode. The numerical optimization carried
out in this study consisted in finding the optimal number of injections per
cycle, as well as the best timing of each injection and the fuel mass split
among the injections. The analysis revealed that combustion can be improved by
increasing the local concentration of the more reactive fuel (diesel): in
detail, the best strategy is a two-shot mode, with SOI1 = −35°CA AFTDC and SOI2
= −20°CA AFTDC, injecting 70% of the total diesel fuel mass at the first shot.
As far as the SR is concerned, the best compromise between performance and
emissions was found for a relatively low SR = 1.4. The optimization permitted to
extract the full potential of the H2 enrichment in the DF
H2/NG–diesel combustion also at low loads: in comparison to the
DF NG case, combustion efficiency, and gross indicated thermal efficiency have
been improved by 45.7% and 61.0%, respectively; CO- and UHC-specific emissions
have been reduced by about 85.0%. Comparing CDC to the optimized DF 30 vol%
H2/NG–diesel case, soot emissions are completely canceled,
CO2-specific emissions have been reduced by approximately 42.0%,
NOx-specific emissions by 33.8%. However, further work has to be
done in order to reach comparable values of HC and CO, which are still higher
than in a standard diesel combustion.
