Semester

Fall

Date of Graduation

2025

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Mechanical and Aerospace Engineering

Committee Chair

Derek Johnson

Committee Member

Christopher Ulishney

Committee Member

Victor Mucino

Committee Member

Nigel Clark

Committee Member

Jaime Castro

Abstract

The primary focus of this research is the modeling and validation of crankcase methane emissions stemming from natural gas combustion engines commonly deployed in the natural gas compression industry. The models employed rely on equations predicting the flow rate and composition of losses, typically generated within reciprocating piston-cylinder systems. Since crankcase emissions originate in a piston–cylinder arrangement, multiple variables were considered in this study, including displaced volume, pressure, temperature, flow, and viscosity (with corrections applied to account for the thermal and compositional effects on the mixture of fluids). The initial data for this research was collected during three measurement campaigns at four natural gas well sites in northern West Virginia's Marcellus Shale region. Subsequently, a second phase of data collection was conducted in a custom-designed engine laboratory, testing a Caterpillar G3508J engine across a 224 kW to 430 kW load range. Parallel to this data collection, a simulation model based on a three-component blowby mixture methodology was developed and implemented in Simulink®. This model is capable of predicting crankcase methane emission rates for a variety of natural gas engines used in the compression industry. Engine crankcase methane emissions measured during field campaigns and in the laboratory (e.g., 366 g/hr at a 300-kW load point) were compared with values predicted by the model. In both cases, the measurements fell 100% within the model’s predicted range. This laboratory finding was highly significant, as this emission rate constituted 16.8% to 22% of the total engine hydrocarbon exhaust emissions. The model also confirmed that crankcase methane originates primarily from unburned fuel (containing ~96% methane) rather than post-combustion exhaust gas (containing ~0.15% methane). Finally, using a lean-burn natural gas engine inventory of 2,571 units, a national crankcase methane emissions estimate was calculated to be between 56.7 and 58.5 Gg/year. To contextualize this finding, this estimate was compared against the crankcase component of national 'top-down' combustion slip estimates. Based on literature suggesting crankcase emissions account for 3-20% of the total slip (estimated in one study at 328 Gg/year), the resulting plausible range for crankcase emissions is 9.8–65.6 Gg/year. The study's result aligns well with the upper end of this derived range.

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