Performance investigation of a novel Franchot engine design

Abstract

Summary The double‐acting Franchot engine is inferior to the double‐acting Siemens engine under configurations limited by the Siemens engine. In this contribution, the performance of a novel Franchot engine design without the Siemens engine limitations is investigated with a new mathematical definition of the regenerator end temperatures, and the initial statement is challenged. The main advantages of the Franchot engine compared with the Siemens engine are the free control of the phase angle and the thermal separation of the cylinders. Here, the performance of a cylinder‐heated/cooled air‐filled Franchot engine is investigated at medium temperature under variations of engine speed, phase angle, geometry, dead volume, and gas density. A second‐order thermodynamic model with nonconstant, polytropic heat transfer is developed and implemented in Matlab/Simulink for this investigation. The nonconstant heat transfer is crucial to accurately model the behaviour of the direct cylinder heating and cooling. The results show that the phase angle and air charge density have the largest effect on the engine performance. An increase of the phase angle from 90o to 150o at a speed of 1000 RPM led to an increased output power of 58 W compared with a maximum power less than 20 W for a phase angle of 90o. The efficiency at a phase angle of 150o is approximately 25% which is slightly lower than the ideal Curzon and Ahlborn efficiency of 29.3%. This discrepancy can be explained by the nonconstant, polytropic heat transfer. In addition to the increase in engine power, the operation at higher phase angles reduces the pressure difference across the power piston by a factor larger than 4 which leads to a significant reduction in gas leakage across the power pistons. This shows that at higher phase angles, the 2 main disadvantages compared with the Siemens engine are at least reduced and arguably completely removed. Thus, the Franchot engine has the potential to be superior to the Siemens engine if freed from the operational restrictions of the Siemens engine.

Parametric Study of an Air Charged Franchot Engine with Novel Hot and Cold Isothermalizers

“…Figure 2 clearly shows the differences between the isothermal and adiabatic processes on the cycle work. Daoud and Friedrich [7] reported a novel Franchot engine design in which the heat transfer area was extended to the whole cylinder wall area, which allows the use of high temperatures. In addition, they increased the phase angle to allow faster speeds and hence increased the heat transfer.…”

“…However, for a double acting Franchot engine (see, for example, Figure 1 in [7]) the connecting rods will be inside the swept volume of one half of the engine and will thus influence the engine behavior. An increase in the connecting rod diameter will reduce the hydraulic diameter, which increases the heat transfer per volume, thus acting like an internal fin although it has unheated and uncooled surfaces (see Figure 4).…”

“…The study is based on a second order model derived in a previous contribution for the Franchot engine [7]. The model assumes an ideal Franchot engine with polytrophic expansion and compression processes and ideal regenerator.…”

“…The optimisation was performed manually until the maximum power is reached for the speed range 100-500 rpm. Each single speed, phase angle and connecting rod diameter were changed manually with a step size of 50 It is shown that adding fins is superior to the phase angle control method [7] in terms of power and efficiency. The engine with fins works closer to the Curzon efficiency at the maximum power than the phase angle method.…”

“…It is shown that adding fins is superior to the phase angle control method [7] in terms of power and efficiency. The engine with fins works closer to the Curzon efficiency at the maximum power than the phase angle method.…”