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.

The Franchot engine is a double acting Stirling engine with only two cylinders and freely controllable phase angle. The hot and cold cylinders of the Franchot engine can be directly heated and cooled and thus act as heaters/coolers. However, the cylinders are necessarily long and thin to increase the heat transfer area and hence the power. The long strokes result in long cranks and connecting rods which lead to large and unwieldy engines. In this contribution, the directly heated and cooled multi-cylinder Franchot engine is dynamically studied with a novel balanced compounding mechanism. Thus, the balanced compound Franchot engine would be more compact, cheaper and more efficient due to the removal of the rotational parts. The new mechanism includes a linkage between two connecting rods in a conventional Franchot engine for which, four pistons (an expansion, compression and two guiding pistons) move as one reciprocator. The influence of different engine parameters, such as number of cylinders, temperature, dead volume and reciprocator mass, on the new configuration is investigated. The possible phase angles for each number of cylinders are given. The balanced compound Franchot engine changes the order of piston motion so that the largest of these phase angles is obtained. The theoretical analysis shows that increasing the number of cylinders, dead volume and reciprocating mass reduces the frequency and increases the stroke; increasing the cylinder diameter increases the frequency and decreases the stroke; increasing the load decreases the stroke and slightly decreases the frequency; and increasing the temperature increases both the frequency and the stroke. Thus, different engine parameters can be used to maximise the power generation without the piston hitting the cylinder head. The dynamic load, which is a function of the speed, does not prevent the balanced compound Franchot engine from self-starting while static friction can prevent the engine from self-starting, especially if the pistons are around the mid-stroke point. The most promising configuration is the three-phase engine which has the lowest number of cylinders, preferable phase angle and phase shift of 120 o and potential for electricity generation and heat pumping.

The global cooling demand is one of the fastest growing energy demands and is putting a strain on the electricity infrastructure. Solar-powered cooling could provide most of the cooling demand due to the coincidence of the cooling demand and the solar irradiance. In particular, the solar-powered Stirling-cycle cooler has low maintenance requirement, high theoretical efficiency, and use of environmentally friendly gases. However, Stirling-cycle coolers are expensive due to high driving temperatures, complex heat exchangers, and expensive solar tracking so that they have so far only been successful at high-temperature difference applications. This study introduces a novel directly coupled solar Stirling cooler for which the hot engine cylinders are deployed inside evacuated tube collectors. The machine uses air as working fluid, and its driving mechanism is based on the free-piston, balanced compound technology that was patented by Finkelstein. A second-order mathematical model is used to investigate the performance of the machine for different cylinder arrangements, gas leakage rates, chilling temperatures, and solar irradiance. In addition, the regenerators are optimised to maximise the cold production. It is shown that mechanical frictions can be reduced to 20% by selecting an appropriate cylinder arrangement. The solar cooler achieves a maximum cold production rate of 367.5 W/m 2 without using external heat exchangers at load temperature of 7 C, which is comparable with photovoltaic powered coolers.In addition, the machine is relatively simple, has safe and quiet operation, uses ambient air as working gas, and is able to produce a wide range of chilling including sub-zero temperatures without changing the working gas. The direct thermal coupling of the Stirling cooler to evacuated tube collectors significantly reduces the complexity of the machine and removes intermediate heat transfer steps which reduce the performance. Thus, the suggested cooling technology has great potential for solar refrigeration, especially for low power and near ambient cooling.

The Franchot engine is a double acting Stirling engine that has a freely controllable phase angle and no shuttle and axial conduction losses but is inferior to the Siemens and free piston Stirling engines in terms of its ability to self-start. In addition, the Franchot engine is not widely used with the reliable slider crank mechanism due to vibrations. Here, the multi-cylinder Franchot engine is thermodynamically and mechanically studied with the simple slider crank mechanism with the aim of improving the self-start capability and to reduce the vibrations. Both instantaneous power and engine arrangements are used to judge the mechanical performance for different engine parameters and configurations. The optimal phase shifts and phase angles are derived and it is shown that both are governed by the number of cylinders. The theoretical analysis shows that by increasing the number of cylinders, different engine vibrations are reduced and the engine becomes self-starting. Hence, the Franchot engine can be superior to the Siemens engine, particularly due to the ability to remove the rocking couples for engines with more than two phases. Thus, the engine operation is stabilised and the simple slider crank mechanism can be used with the multi-cylinder Franchot engine.

Abstract:The Stirling engine is an external combustion engine that uses heat exchangers to enhance the addition and removal of energy. This makes the engine power-dense but expensive, less efficient and complicated. In this contribution, the Stirling engine based on the Franchot engine has novel cylindrical fins working as isothermalizers to improve heat transfer without the complications of heat exchangers. Enhancing the power density by isothermalizing work spaces is compared to the bare cylinder optimized by varying the phase angle. The theoretical analysis shows that both the adiabatic and isothermal fins increase the power and efficiency, achieving the Curzon and Ahlborn efficiency at the maximum power point. In comparison to the phase angle method, the finned engine resulted in much lower gas mass flow rate, which leads to a reduction in the regenerator pumping and enthalpy losses. Thus, the Stirling engine has the potential to be simple, cheap, efficient and power-dense, and thus can be used effectively for different applications.