Mm s h mm M hb h k bss bss

Aerodynamic friction

engine auxm 1aries

Transposing the line Tl-H' onto a new diagram, Fig. 9.5, the only correction now left to make is to include some allowance for the power consumed by the engine auxiliary equipment. This may comprise a lubricating-oil pump, an electric alternator, or generator for battery charging, lighting and control functions, a fluid compressor, cooling water pump, radiator fan, air preheater drive and miscellaneous other devices. The power consumption of the auxiliaries is most likely to increase as the speed or pressure level increases.

The net power cycle efficiency for the engine, driving its auxiliaries, is represented in Fig. 9.5 by the line 'J-J'. This then is perhaps characteristic of the shape of performance curve that might be measured by dynamometer brake testing of the engine.

operating characteristics of real engines Power ai id efficiency

Confirmation of the above was provided by Meijer (1959a) with the publication of the brake power and efficiency characteristics shown in Fig. 9.6. These were measured on a 30 kW (40 hp) single-cylinder rhombic drive Philips Stirling engine of the piston-displacer type with hydrogen as the working gas. The engine cylinder bore was 88 mm (3.46 in) with a power piston stroke of 60 mm (2.36 in). The pressure compression ratio Pmiix/Pmí» was 2.0 with a maximum related pressure of 13.7 MN/nr (20001b per sq in). The nominal temperatures for the engine were 700 °C (1290 °F) in the heater and 15 °C (60 °F) in the cooler.

FiC. 9.5. Power output and thermal efficiency as a function of engine speed and pressure •■;vel Effeejs of .engine^".» xiljjiry eoninment.

Fin. 9/». Biafcc power nml Ihcrmnl eflicicncy ol single-cylinder rhombic-drive Philips Stirling engine (after Meijcr 195«>a).

Fin. 9/». Biafcc power nml Ihcrmnl eflicicncy ol single-cylinder rhombic-drive Philips Stirling engine (after Meijcr 195«>a).

Torque/speed

Other data provided by Meijer included the engine torque curve as a function of speed reproduced in Fig. 9.7. The virtually flat torque/speed curve of the Stirling engine is particularly favourable for automotive applications. High torque at low speeds is desirable to achieve good acceleration. I he inherent high torque at low speeds of Stirling engines allows the use of a relatively simple transmission system for vehicular use and helps offset the increased capital cost compared with internal combustion engines.

Cyclic torque

In addition to the flat torque/speed characteristic discussed above, the Stirling engine has a particularly favourable cyclic torque characteristic. Ibis is the torque variation experienced, at the output shaft, during one revolution. The cylinder torque of a Stirling engine is much less variable than for an internal combustion engine of the same power.

Meijer (1970b) made the comparison shown in big. 9.8 for four-cylinder Stirling and spark-ignition engines of about 73 kW (100 hp). The reason for the virtually constant torque of the Stirling engine is that the range of the pressure variation in each cylinder is small, pnxuJpm{„ = 2 approximately, and there is a complete cycle in each cylinder every revolution. In the internal combustion engine the pressures may range from (1.08 to 5 MN/nr (12 to 725 lb per sq in) per cycle and, in a

Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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