Semester

Fall

Date of Graduation

2018

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Mechanical and Aerospace Engineering

Committee Chair

Cosmin E. Dumitrescu

Committee Co-Chair

Nigel N. Clark

Committee Member

Nigel N. Clark

Committee Member

Hailin Li

Committee Member

V'yacheslav Akkerman

Committee Member

Guodong Guo

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 lean-burn 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 rapid-burn 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-release fractions). A better solution was to use the inflection points in the apparent heat release to locate the fast burning stage. Specifically, the second inflection point of heat release rate can be regarded as the end of the fast inside-the-bowl burning. Furthermore, the operating conditions controlled the fuel fraction that burned in the squish region before the fuel inside the bowl completely burned, hence the phasing of the late combustion stage. This suggests that squish added to the combustion complexities of such retrofitted engines and that the combustion strategy should optimize the mass of fuel that burns inside the squish region. In addition, the results indicated that the coefficient of variation and standard deviation of peak cylinder pressure are better metrics to evaluate the cycle-to-cycle variations than variations in the indicated mean effective pressure because they were less affected by the combustion event inside the squish region. Overall, the reliable ignition, stable combustion, and the lack of knocking in this 13.3 compression-ratio diesel chamber showed promise for heavy-duty compression ignition engines converted to spark ignition natural gas operation under lean conditions, which would accelerate the introduction of heavy-duty natural gas vehicles in the U.S.A.

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