Using Petroleum and Biomass-Derived Fuels in Duel-fuel Diesel Engines - Novel Combustion Concepts for Sustainable Energy Development - page 253

Fig. 9 Predicted and
measured ignition delays for
n-heptane/air, syngas/air, and
methane-air mixtures at high
pressures and ϕ =1

experiments, but were not duplicated in simulations (Dryer and Chaos 2008 ; Jain
et al. 2013 ). It is also noteworthy that the Chambers mechanism is able to reproduce
the experimentally observed two-stage ignition process and negative temperature
coef

cient (NTC) region.

4.2 Ignition Characteristics in Dual-Fuel Diesel Engines

In a dual-fuel engine, the ignition is initiated by the pilot diesel. Since ignition
represents a critical process with regard to engine performance and emissions, it is
important to examine the ignition of pilot fuel in the presence of a combustible
gaseous fuel-air mixture. In particular, the ignition chemistry of large hydrocarbons,
such as n-heptane, is known to be characterized by the NTC (negative temperature
coef cient) and two-stage ignition processes (Curran et al. 1998 ). Previous studies

have not analyzed the effect of gaseous fuels on these processes. Therefore, sim-
ulations were performed to examine these aspects by considering homogeneous
gaseous mixtures in a constant volume reactor and two-phase mixtures in a diesel
engine. Figure 10 depicts the

first and second stage ignition processes in diesel

engine in terms of the mass pro les of OH and QOOH with respect to crank angle

for the single- and dual-fuel cases, as listed in Table 3 . As indicated in the table, the
start of injection (SOI) is

after top dead center (ATDC), and injection duration
is 8.5 crank angle degree (CAD).
For n-heptane, the two-stage ignition process is well depicted in the

− 8 °

figure, with

the first-stage ignition occurring at − 4.8 ° ATDC and the second-stage at − 2.4 °

ATDC. The first-stage ignition is de ned by the value of integrated heat release