University of Sussex
Nsabwa Kigezi, Tom.pdf (5.82 MB)

Stability, control, and state estimation of free-piston engine generators

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posted on 2023-06-09, 17:23 authored by Tom Nsabwa Kigezi
This thesis investigates stability, control, and state estimation of free-piston engines (FPEs). Emphasis is placed on FPE electric power generators, currently targeted at potential application areas including electric vehicle range extension, efficient power sources in the field, and in combined heat and power systems. A general group of FPE configurations is considered; ranging from a single piston configuration through to an opposed piston configuration, which in each case may make use of either a bounce chamber, a mechanical spring, or second combustion chamber as the rebound device. To assist in verifying the theoretical results, one configuration type is physically modelled to create numerical simulation capability. The modelling includes representative descriptions of the combustion processes, the electrical machine, and the system dynamics. On stability, the thesis starts from first principles to newly develop a framework that relates key FPE physical parameters. Formal definitions of stability and instability are provided, and general technical statements on the stability of piston oscillations are proposed and verified. On control, the thesis newly applies model-based control theory to three control problems; namely, the control of compression ratio, engine start, and mitigation against abnormal combustion events such as misfire. Optimality and robustness are of key interest in addressing the control problems although other control approaches are investigated. On state estimation, the thesis newly develops a robust finite-time converging observer for FPE dynamics. The developed observer’s effectiveness is mathematically proven and verified for the estimation of in-cylinder pressure and piston speed. To partly verify the findings in experiment, the thesis describes the design and creation of a two-stroke gasoline FPE in hardware. The hardware rig is used to provide measurement data for model validation, observer-based state estimation, and also to discuss FPE stability. In model validation, a parameter identification scheme is proposed in form of a general optimisation problem. In state estimation, the mean observer error is found to be very small; 0.05 bar and 0.1 m/s for the estimation of in-cylinder pressure and piston speed respectively.


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  • doctoral

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  • eng


University of Sussex

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