Experimental investigation of pilot-fuel combustion in dual-fuel engines, Part 2: Understanding the underlying mechanisms by means of optical diagnostics

Abstract

The pilot-fuel auto-ignition and combustion under engine like conditions in compressed methane/air mixtures have been investigated. Experiments were performed in an optically accessible Rapid Compression-Expansion Machine featuring quiescent charge conditions and a single-hole coaxial diesel injector mounted on the cylinder periphery. In Part 1, the phenomenology of the pilot-fuel combustion has been studied based on the thermodynamic analysis. With the addition of methane, a strongly prolonged pilot-fuel combustion duration was observed, especially at increased EGR rates. The aim of Part 2 is to improve the understanding of the underlying processes governing the pilot-fuel burning and premixed flame initiation in dual-fuel engines. For this purpose, the thermodynamic analysis was supplemented by the application of optical diagnostics including high-speed CH 2 O-PLIF, Schlieren and OH* chemiluminescence, and corroborated with homogeneous reactor and laminar flame speed calculations. The investigations focused on determining the role of (a) ignition location and number of ignition kernels, (b) stratification of the ignition delay due to the chemical influence of methane and (c) the role of flame propagation during the pilot-fuel burning. In the initial phase, the combustion was found to propagate through an auto-igniting front. When combustion reaches the lean zones with high ignition delay spatial stratification, the premixed flame propagation becomes the dominant mechanism owing to its higher spreading rate. Both processes influence the pilotfuel combustion duration. With increasing methane concentration the simulations predict increasing stratification of the ignition delay in lean regions, and a moderately increased laminar flame speed in the pilot-fuel lean regions. Overall, this explains the observed trends of longer pilot-fuel burning in the dual-fuel cases and indicates an increasing role of flame-propagation in the dual-fuel combustion pilot-fuel burning.

Impact of a split injection strategy on mixing, ignition and combustion behavior in Premixed charge compression ignition combustion

“…3, the light sheet was slightly tilted to match the 5° injector inclination. Since the fuel spray is pushed towards the piston head during the compression stroke, Srna et al [31] found that the effective symmetry axis of the spray cone has an angle of 3.5°, which was adopted for the arrangement of the light sheet in the present experiments. In both setups, LIF signals were directed towards a LaVision HSS6 high speed camera equipped with a high speed image intensifier (HS-IRO, gate width 150 ns, gain 62) using the same dichroic mirror (4'', HR 45 300-450 nm).…”

Verification of diesel spray ignition phenomenon in dual-fuel diesel-piloted premixed natural gas engine

“…The phenomenology of DF combustion and its underlying processes have been described to some degree in previous optical studies. 12,13 In these studies, an optically accessible rapid-compression-expansion machine (RCEM) equipped with a single-hole coaxial diesel injector is utilized to investigate the DF combustion, as it transitions from ignition to premixed-flame combustion using thermodynamic analysis as well as OH* chemiluminescence imaging and planar laser-induced fluorescence (PLIF) imaging of CH 2 O. The optical diagnostics showed that CH 4 increases the ignition delay and prolongs the pilot-fuel combustion duration.…”

A multiparameter investigation of syngas/diesel dual-fuel engine performance and emissions with various syngas compositions

“…In dual-fuel mode, due to the pilot injection of reactive fuel (as diesel fuel), there is the added complexity of the poorly understood physicochemical interactions of the gaseous syngas combustion interactions with the liquid pilot fuel spray. Currently, natural gas or methane dual-fuel operation are well covered by the literature [17][18][19][20][21] but syngas less. Moreover, to correctly optimize dual-fuel syngas CI engines, the effects of its composition should be well understood.…”