Understanding ignition processes in spray-guided gasoline engines using high-speed imaging and the extended spark-ignition model SparkCIMM Part B: Importance of molecular fuel properties in early ﬂame front propagation Rainer N. Dahms a,⇑ , Michael C. Drake b , Todd D. Fansler b , T.-W. Kuo b , N. Peters c a Combustion Research Facility, Sandia National Laboratories, Livermore, CA 94551, USA b Propulsion Systems Research Laboratory, General Motors Global Research & Development, Warren, MI, USA c Institute for Combustion Technology, RWTH Aachen University, Germany article info Article history: Received 14 September 2010 Received in revised form 16 February 2011 Accepted 24 March 2011 Available online 27 April 2011 Keywords: Engine Direct-injection Imaging Ignition Stratiﬁed abstract Recent high-speed imaging of ignition processes in spray-guided gasoline engines has motivated the development of the physically-based spark channel ignition monitoring model SparkCIMM, which bridges the gap between a detailed spray and vaporization model and a model for fully developed turbu- lent combustion. Previously, both SparkCIMM and high-speed optical imaging data have shown that, in spray-guided engines, large variations in turbulence intensity, equivalence ratio, and enthalpy along the stretched and wrinkled spark plasma channel favor localized ignition spot formations in rich-mixture regions. In combination with strong local ﬂow velocity, multiple successful ignition events along the re-striking spark lead to early non-spherical turbulent ﬂame fronts. In this paper, SparkCIMM is enhanced by: (1) criteria to capture localized ﬂame extinction phenomena, (2) a formulation of early ﬂame kernel propagation based on the G-equation theory that includes effects of non-unity Lewis numbers, and (3) an extended equation to compute turbulent burning velocities of stretched ﬂames in stratiﬁed mixtures. Localized rich ignition along the spark leads to early ﬂames, whose propagation is, due to initially small turbulent Damköhler numbers, signiﬁcantly inﬂuenced by molecular fuel properties. The analysis reveals that non-unity Lewis number curvature effects, intensiﬁed by heavy dilution by exhaust gas recirculation, strongly affect the early ﬂame-kernel development in spray-guided gasoline engines. In particular, these effects signiﬁcantly bias the ﬂammability limit of ﬂame kernels towards rich-mixtures while inhibiting their propagation in lean regions. Favorable initial conditions for combustion are found in rich-mixture regions, albeit in the presence of substantial equiv- alence ratio ﬂuctuations and scalar dissipation rates. This paper demonstrates that the full complexity of the model equations developed here is required to reproduce the characteristic experimental features (spark channel stretching, multiple re-strikes, local- ized ﬂame kernel formation, and early turbulent ﬂame front corrugation) of spray-guided ignition phenomena. Published by Elsevier Inc. on behalf of The Combustion Institute. 1. Introduction Rising petroleum-derived fuel prices and stringent emission regulations have put a strong demand on improving internal com- bustion engine fuel efﬁciencies and lowering engine-out emissions in equal measure. Compared to conventional throttled, homoge- neous charge spark-ignition combustion, stratiﬁed charge combus- tion can increase fuel efﬁciency by reducing pumping and heat losses and by providing a thermodynamically more favorable ratio of speciﬁc heats. Spray-guided combustion systems (SG-SIDI) [1] are now in production in Europe and continue under active re- search and development. In these engines, the centrally located fuel injector sprays along the cylinder axis towards a closely spaced spark plug. This conﬁguration allows better combustion phasing, and lower engine-out smoke and unburnt hydrocarbon emissions compared to wall-guided engines. Overall, SG-SIDI en- gines have a wider stratiﬁed charge operating range and a signiﬁ- cant fuel economy advantage over WG systems. However, the closeness of the fuel spray and spark electrodes can cause high cyc- lic variability and unfavorable conditions for ignition [2]. As de- scribed by Dahms et al. [3,4], SG-SIDI engines exhibit dramatic stretching of the spark plasma channel (‘  10 mm) and large ﬂuc- tuations in equivalence ratio, velocity, turbulent kinetic energy, and enthalpy along the stretched spark channel. These conditions 0010-2180/$ - see front matter Published by Elsevier Inc. on behalf of The Combustion Institute. doi:10.1016/j.combustﬂame.2011.04.003 ⇑ Corresponding author. Fax: +1 925 294 2595. E-mail address: Rndahms@sandia.gov (R.N. Dahms). Combustion and Flame 158 (2011) 2245–2260 Contents lists available at ScienceDirect Combustion and Flame journal homepage: www.elsevier.com/locate/combustﬂame