Heinrici 2.5" Stirling Engine, Germany

Heinrici 2.5 Stirling Engine.
Heinrici 2.5" Stirling Engine
Type: Closed (Stirling) cycle hot air engine.
Size: Bore 2.5". Stroke 2.5". Flywheel diameters 10". Swept volume 200cc.
Layout: Inverted vertical, in-line piston and displacer (beta configuration).
Power: Fractional; a few watts at 500rpm (bends it's crank if allowed to over-rev.).
Working fluid: Air at atmospheric pressure.
Regenerator: Annular gap around long loose fitting thin walled steel displacer.
Heating: Now LPG, but originally probably kerosene -or may have been coal, or wood.
History: Probably manufactured between 1900 and 1910, it is said to have originally run a jewelers lathe or a dentists drill at Kumara. Owned by the Haines family lately of Temuka from 1918 until acquired by myself.

Hot air engines are external combustion engines and can be closed or open cycle, valved or valveless.
Heinrici hot air engines are of the valveless, closed cycle type, generally called Stirling cycle engines, after Robert Stirling, the Scottish Presbyterian minister who pioneered their development in the early 1800's. They operate by alternately heating and cooling a quantity of air, called the working fluid, contained in the engine's internal spaces. Heat is applied externally and passes through the cylinder wall, heating the working fluid, which is then expanded against a piston to do mechanical work. After heating and expanding, the working fluid is moved to a cool space where it cools and contracts before being returned to the hot space for the cycle to repeat.
It has a displacer (just a loose piston), below and in the same cylinder as the power piston to which it is connected via cranks and linkages so as to lead by 90degrees of crankshaft angle. The displacer space and the piston space are connected by the annular gap around the loose fitting displacer so that the working fluid moves between these spaces and changes volume by the appropriate ratio as the engine rotates. Because they have no valves and experience no sudden pressure changes, Stirling engines are noted for quietness and reliability.

The other main type of hot air engine, (generally called an Ericsson cycle engine), uses valves to control the movement of air internally and can be either closed or open cycle. Open cycle hot air engines admit a fresh quantity of air into the heating space every stroke. Confusingly, Ericsson also made valveless closed cycle engines- Stirling engines- most commonly the Ericsson Rider style of which there are some examples in NZ.

Heinricis use air at atmospheric pressure for their working fluid, but for higher specific output (power for size) and better efficiency, modern Stirling cycle engines use pressurised gas- air, nitrogen, helium or hydrogen.

Most Heinricis in New Zealand were imported by a firm of Christchurch glaziers (Bradley Bro's Ltd) in the late 19th and early 20th centuries. They re-badged them to the Bradley name. Made in a range of sizes, Heinricis were a very successful design and made to a high standard of engineering for the time. Their most popular engine was this model - 2.5" bore and 10" flywheels. Their smaller engines had insufficient power for most practical applications, while bigger sizes had poor power outputs for their size relative to steam and internal combustion engines of the era. Fired by town gas, kerosene, coal or wood, Heinricis were popular when quietness and simplicity were more important attributes than power output. They were used to run water pumps, jeweler's lathes, dentist's drills, phonographs, sewing machines, domestic fans, and even shop window displays.

Because the power available from simple unpressurised types of Stirling engine such as this Heinrici is minimal, friction is a main factor limiting output. Therefore they tend to run better (but with more knocks and rattles) as they wear- until displacer rod and piston clearances allow excessive blow-by. This example has had many hundreds of hours use and runs very well. It is in original condition and has not been extensively restored.
Worldwide, there are many Stirling engine collectors and enthusiast groups- and some commercial manufacturers (mainly toys, models and alternative energy). Whisper Tech Ltd, founded by Dr Don Clucas firstly in Ashburton then in Christchurch (with support from Orion), has developed (since 1987) a sub-1kw 4 cylinder. Stirling engine combined heat and power (CHP) unit. Now based in Spain, they supply versions for home (AC) and marine (DC) applications and are world leaders in this field.

Peter Lynn, 105 Alford Forest Rd., Ashburton, January '12.

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