Investigation of Combustion Characteristics of a Heavy-Duty Diesel Engine Retrofitted to Natural Gas Spark Ignition Operation

Abstract

The conversion of existing diesel engines to natural-gas spark ignition operation by adding a gas injector in the intake manifold for fuel delivery and replacing the diesel fuel injector with a spark plug to initiate and control the combustion process can reduce U.S. dependence on petroleum imports and curtail engine-out emissions. As the conventional diesel combustion chamber (i.e., flat head and bowl-in-piston) creates high turbulence, the engine can operate leaner, which would increase its efficiency and reduce emissions. However, natural gas combustion in such retrofitted engines presents differences compared to that in conventional spark ignited engines. Subsequently, the main goal of this study was to investigate the characteristics of natural gas combustion inside a diesel-like, fast-burn combustion chamber using a unique array of experimental and numerical tools. The experimental platform consisted of a heavy-duty single-cylinder diesel engine converted to natural-gas spark ignition and operated at a low-speed, lean equivalence ratio, and medium-load condition. The engine can also operate in an optical configuration (i.e., the stock piston and cylinder block can be replaced with a see-through piston and an extended cylinder block), which was used to visualize flame behavior. The optical data indicated a thick and fast-propagated flame in the piston bowl but slower flame propagation inside the squish region. In addition, a 3D numerical model of the optical engine was built to better explain the geometry effects. The simulation results suggested that while the region around the spark plug location experienced a moderate turbulence that helped with the ignition process, the interaction of squish, piston motion, and intake swirl created a highly-turbulent environment that favored the fast burn inside the bowl and stabilized the combustion process. However, the squish region experienced a much lower turbulence, which, combined with the reduced temperature and pressure during the expansion stroke and its higher surface-to-volume ratio, reduced the burning velocity and the flame propagation, but also avoided knocking. Consequently, the bowl-in-piston geometry separated the leanburn natural gas combustion into two distinct events. To extend the optical findings, the metal engine configuration was used to investigate the effects of gas composition, spark timing, equivalence ratio, and engine speed on the two-stage combustion. The results suggested that operating conditions controlled the magnitude and phasing of the two combustion events. Moreover, 3D CFD simulations of the metal engine configuration showed that the squish region contained an important mixture fraction that would burn much slower and can increase the phasing separation between the two combustion events to a point that a second peak would appear in the heat release rate. Moreover, the rapidburn event in such an engine was much shorter compared to its traditional definition (i.e., the time in crank angle degrees between the 10% and 90% energy-releas...

Optical analysis of flame inception and propagation in a lean-burn natural-gas spark-ignition engine with a bowl-in-piston geometry

“…In addition, the FL images were showing many high-intensity pixels at MFB95 and significant activity at MFB98. As preliminary 3D CFD simulations of the lean-burn operation in this engine suggested that the burned region contained no unburned fuel, 34 the FL was most probably due to a combination of a late burn of the fuel trapped near the straight corners of the cylindrical bowl and/or of the reflections from the late burn inside the (non-visible) squish region. 34,37 Furthermore, this late burn inside the bowl probably helped maintain the temperature required for late oxidation.…”

“…As preliminary 3D CFD simulations of the lean-burn operation in this engine suggested that the burned region contained no unburned fuel, 34 the FL was most probably due to a combination of a late burn of the fuel trapped near the straight corners of the cylindrical bowl and/or of the reflections from the late burn inside the (non-visible) squish region. 34,37 Furthermore, this late burn inside the bowl probably helped maintain the temperature required for late oxidation. Consequently, in addition to the contribution from the combustion phenomena taking place inside the squish region, the combustion of the unburned mass trapped at the bottom of the bowl also contributed to the low slope of the AHRR toward the EOC, as shown in Figure 3(b).…”

“…37,38 In detail, when the flame front reached the entrance of the squish region and the bottom corner of the cylindrical bowl, the flame speed reduced due to the smaller local turbulence intensity, lower unburned gas temperature, and more heat transfer to the boundary (due to the high surface-to-volume ratio). 34 Furthermore, Figure 5(b) and (c) suggests a start of combustion (SOC) between −8 and −4 CAD ATDC. Therefore, the cycle-to-cycle variation can be partially explained by the different spark inception period.…”

“…However, the 3D CFD simulations predicted no trapped fuel at the bottom of the bowl for the metal engine. 34…”

“…The goal of this study was to use flame luminosity (FL) and in-cylinder pressure measurements to investigate flame kernel growth and the subsequent flame propagation inside the bowl region. To ensure that observations of FL such as those in this article will also apply to production engines with similar bowl-in-piston geometry, preliminary 3D CFD simulations 34 of both metal (i.e. production) and optical engine configurations investigated engine operation under similar operating conditions.…”