Using Petroleum and Biomass-Derived Fuels in Duel-fuel Diesel Engines - Novel Combustion Concepts for Sustainable Energy Development - page 256

4.3 Combustion and Emission Characteristics of Dual-Fuel
Diesel Engines

Simulations were performed for one single-fuel case (100 % n-heptane), and four
dual-fuel cases, two with methane (90, and 97 % by volume), and the other two
with 95 and 98.5 % syngas (50 %CO-50 %H 2 ). As indicated in Table 3 , cases with
100 % n-heptane, 90 % methane, and 95 % syngas correspond to low-load con-
dition for which the total (fuel) energy into the cylinder is 390 J. The other two
cases with 97 % methane and 98.5 % syngas correspond to high-load condition
with total energy into the cylinder being 1,014 J. Also, for the two low-load cases,
the gaseous equivalence ratios are

ϕ g = 0.16 and 0.17, while for the two high-load

cases,

ϕ g = 0.58 and 0.60.
Figure 12 depicts the combustion characteristics for the 0, 90, and 97 % methane
cases in terms of the temporal pro les of pressure, HRR, n-heptane vapor mass, and

methane mass. For the first two cases corresponding to low-load condition, the

amount of energy input is kept fixed, whereas for the 90, and 97 % methane cases,

the amount of n-heptane injected is kept

fixed. Thus, with the increase in methane
mass, the indicated mean effective pressure (IMEP) increases from 1.88 to 7.36 bar,
as indicated in Table 3 . The increased load for the 97 % methane can also be seen
from the pressure and HRR plots in Fig. 12 . In addition, the HRR plots indicate that
the ignition delay is increased due to the addition of methane, as discussed earlier.
The combustion process in a diesel engine is generally characterized by a hybrid
combustion mode involving rich premixed combustion and diffusion combustion
(Dec 1997 ; Som and Aggarwal 2010 ). In contrast, depending upon operating
conditions, the heat release in a dual-fuel engine may also involve a lean com-
bustion mode with a propagating fl ame (Boehman and Le Corre 2008 ). The HRR

and methane mass pro les in Fig. 12 seem to support this hypothesis, especially for

the 97 % methane case. However, the methane mass pro le for the 90 % case seems

to indicate the extinction of the lean premixed fl flame due to ultra lean conditions at

low load. Various combustion modes and the flame propagation/extinction behavior

were analyzed by examining the evolution of some key species during combustion.
Figure 13 presents the CO, CO 2 ,andHO 2 mass fraction contours at different crank
angles for the single-fuel case. These plots are cut through at the peak

fl

value

ϕ

location. Black lines in each 3). The corresponding

plots for the two duel-fuel cases are shown in Figs. 14 and 15 . These plots can be
used to identify the different combustion zones. For instance, the high CO region
locates the rich premixed zone with

figure indicate contours (0.3

ϕ -

between 2.0 and 3.0, while the high CO 2
region corresponds to diffusion combustion zone with

ϕ

near 1. This is consistent
with previous studies (Azzoni et al. 1999 ; Aggarwal 2009 ) dealing with PPFs,
indicating that the most of CO is produced in the rich premixed zone, while the
most of CO 2 is produced in the diffusion combustion zone, where the temperature is
highest. Thus, the rich premixed zone is characterized by n-heptane decomposition
and partial oxidation, producing CO, H 2 , and intermediate hydrocarbon species.
These species are then consumed through oxidation reactions in the diffusion

ϕ