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Date of Award


Document Type

Campus Access Dissertation

Degree Name

Doctor of Philosophy in Mechanical Engineering-Engineering Mechanics (PhD)

Administrative Home Department

Department of Mechanical Engineering-Engineering Mechanics

Advisor 1

Jeffrey D. Naber

Advisor 2

Mahdi Shahbakhti

Committee Member 1

Brian J. Eggart

Committee Member 2

Gordon McTaggart-Cowan


The conventional internal combustion engine will continue to exist for a long time. Likewise, demand for higher output efficiencies, higher specific power output, increased reliability, and lower emissions will continue to grow. There is also a growing requirement to run on various gaseous fuels and natural gas, whether for environmental, economic, or resource conservation reasons.

This dissertation investigates a 6.7L diesel engine converted to run stoichiometric diesel micro-pilot / natural gas premix combustion with a maximum diesel contribution target of 5% of the total fuel energy with a three-way catalyst aftertreatment. The research centers on investigating the dominant factors and their impact on the critical barriers of this technology, including the positive and negative impact on combustion stability at low loads, the most influential factors and their impact on maximizing thermal efficiency at medium loads, the controlling parameters at preventing combustion knock at high-loads, and the ability of the three-way catalyst to minimize emissions.

A diesel-like efficiency of 41% brake thermal efficiency was achieved with a high load output of 23 bar brake mean effective pressure when operating in the micro-pilot mode. This operating condition reduced up to 25% brake-specific CO2 emissions compared to diesel-only. Low loads can be achieved by delaying combustion phasing, reducing the injection pressure, adding exhaust gas to the intake, and increasing the total diesel pilot quantity. Maintaining stable ignition of the diesel pilot becomes a challenge at low loads, as the intake pressure is reduced; the chamber pressure at diesel injection decreases, and the presence of a near-stoichiometric mixture of NG will act to inhibit the diesel ignition. As such, maintaining the stoichiometric combustion resulted in a minimum load output of 5 bar BMEP. The pilot injection pressure reduction improved combustion stability at lower loads. While lean operation enabled further load reduction, it precludes using a three-way catalyst to control NOx emissions. At medium loads, a design of experiments investigation revealed that, when the equivalence ratio is constrained at stoichiometric, exhaust gas recirculation and pilot injection timing are the most influential factors in controlling combustion and performance metrics. In contrast, intake air temperature and pilot injection pressure showed the least sensitivity. While it was possible to achieve 25 bar BMEP for high loads, such operation was limited by pre-ignition. Exhaust gas recirculation and pilot injection timing can mitigate abnormal combustion effectively. At a steady-state, near stoichiometric condition, it was observed that the catalyst operates efficiently, consistent with a three-way catalyst operation with very low NOx and unburned methane emissions.

Overall, this dissertation demonstrates that diesel-like performance can be achieved with the stoichiometric micro-pilot concept and provides an understanding of the primary controlling factors and their limitations.