Using Petroleum and Biomass-Derived Fuels in Duel-fuel Diesel Engines - Novel Combustion Concepts for Sustainable Energy Development - page 251

operation become less clear at low loads, and issues concerning reduced engine
power and higher UHC and CO emissions need to be addressed by optimizing other
parameters, i.e., injection timing, EGR, amount of liquid (pilot) fuel injected, and
gaseous fuel equivalence ratio (Chatlatanagulchai et al. 2010a , b ; Tomita et al. 2009 ).
Engine experiments using syngas-diesel combination (Boehman and Le Corre 2008 ;
Sahoo et al. 2012 ) indicated that the engine performance and emissions are strongly
in uenced by the syngas composition, depending upon the load and other conditions.

In general, increasing H 2 fraction in syngas was found to improve engine perfor-
mance, reduce CO and hydrocarbon emissions, but increase NO x emissions.
The literature review indicates extensive experimental research dealing with
dual-fuel (NG-diesel and syngas-diesel) CI engines, but relatively few computa-
tional studies on this topic (Singh et al. 2006 ; Shah et al. 2011 ). In particular, many
details of the two-stage ignition process, different combustion modes, and emission
characteristics in dual-fuel engines remain unexplored. Motivated by this consid-
eration, we performed a numerical investigation on the effect of gaseous fuels on
the ignition, combustion, and emissions in a dual-fuel diesel engine. The following
sections provide a brief description of the computational model, and some repre-
sentative results focusing on the two-stage ignition behavior, various combustion
models including lean premixed burning and extinction, and emissions.

fl

4.1 Computational Model

Simulations were performed using a 3D CFD software CONVERGE (Senecal et al.
2003 ), which incorporates state-of-the-art models for spray injection, breakup and
atomization, turbulence, droplet collision and coalescence, spray-wall interaction,
and vaporization. Details of these models are provided elsewhere (Senecal et al.
2007 ). The mathematical model is based on Eulerian-Lagrangian formulation for the
two-phase turbulent reacting field is described using the

Favre-averaged Navier-Stokes equations along with the RNG k-

fl ow. The gas-phase fl ow

turbulence model.
The spray is represented by a stochastic system of a discrete number of parcels, which
are tracked computationally using a Lagrangian scheme. The two phases are coupled
through the mass, momentum, and energy exchange terms, which are present in both
the liquid- and gas-phase equations. The injection process is simulated using a blob
injection model, which injects liquid droplet parcels with a diameter equal to an
effective nozzle diameter. The subsequent breakup process is simulated by using
models based on the Kelvin-Helmholtz (KH) and Rayleigh-Taylor (RT) instabilities
(Patterson and Reitz 1998 ). The droplet evaporation model is based on the Nusselt
number and Sherwood number correlations of Chiang et al. ( 1992 ). The CFD solver
uses an innovative modi

ε

ed cut-cell Cartesian method along with an adaptive mesh
resolution technique for grid generation (Richards et al. 2013 ).
Computations consider a 1.9L 4-cylinder GM light-duty diesel engine, which
has been extensively used for experimental studies at Argonne National Laboratory
(Ciatti and Subramanian 2011 ). The engine has a 7-hole common-rail injector in