A new semifree-piston rotary generator concept is modelled dynamically and reduced to a single equation for piston stroke motion. This new concept comprises a toroidal-segment piston and cylinder, which orbit on separate generator disks, coupled by a pair of torsion springs to form a balanced mass-elastic system capable of spin. Conventional cyclic combustion takes place in the cylinder causing resonant motion of the disks. A two-part control strategy is proposed and tested by simulation to address the multi-objectives of maximum mechanical power transfer, minimum peak generator torque, and accurate piston top dead center (TDC) position control. A Part I strategy initially assumes that the combustion gas pressure is a function of time only. This produces torque control that follows a stroke velocity feedback law, which maximizes power transfer and implicitly minimizes generator torque, at the same time as power generation. When stroke-dependent gas pressure is introduced, however, the Part I strategy creates an unstable self-excited nonlinear system. The Part II strategy is designed to control piston TDC position and stabilize the response. This uses proportional control of gas pressure rise, assumed possible through fuel injection control and in-cylinder pressure sensing. An ideal-air-standard-dual-combustion two-stroke cycle is then adopted for nonstochastic simulation purposes, excluding the effect of delays and coupled system dynamics. A study is undertaken of a nominal 1.42 l, 200 mm orbit-radius, constant-pressure-scavenged diesel design with three different spring stiffness values. By focusing near the minimum compression ratio for diesel, to give a lower bound on the possible ideal output power, control gains are found that produce stable motion with piston TDC position errors of less than 1%. The power range is from 16 kW to 336 kW, depending mainly on spring stiffness. Since the concept can also store significant kinetic energy, it is potentially attractive as a range-extender for electric vehicles.

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