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Hispano-Suiza V8 Aero Engine

History
When World War I began, production lines of the Hispano-Suiza company were stopped to allow production of war material. Marc Birkigt began work on an aero engine based on his successful V8 automobile engine. The resulting engine, called the Hispano-Suiza 8A (or HS-8A), made its first appearance in February 1915 and was a promising design.

The first Hispano-Suiza ‘Hisso’ 8A kept the standard configuration of Birkigt's existing design: eight cylinders in 90° Vee configuration, a displacement of 11.76 litres (717.8 cu in) and a power output of 140 hp at 1,900 rpm. In spite of the similarities with the original design, the engine had been substantially refined. The cylinder blocks were made of a solid piece of steel. The engine block and the cylinder heads were made of aluminium and were coated in vitreous enamel to reduce leakage. All parts subject to wear were made redundant: spark plugs, valve springs, magnetos, etc. All moving parts were fitted in a case/oil pan to protect them.

Although engine reliability was a great concern in these opening stages of aviation, the HS-8A also provided good power-to-weight ratio and development potential. The engine and its accessories weighed 185 kg (408 lb), making it 40% as heavy as a rotary engine of equivalent power. The design also promised far more growth potential than rotary engines which (in spite of being the most common type then in use with airplanes) were getting close to the limits of the type. Rotary engines of increased power generally had increased weight, which in turn increased the already important gyroscopic torque generated by the engine's rotation. A further increase in torque was considered not acceptable, and the power-to-weight ratio of the new rotary engines under development did not appeal to aircraft designers.

The new engine was presented to the French Ministère de la Guerre (Ministry of War) in February 1915, and tested for 15 hours at full power. This was standard procedure for a new engine design to be admitted in military service. However, because of lobbying by competing engine manufacturers, the HS-8A was ordered to undergo a bench test that no French-made engine had yet achieved successfully: a 50-hour run at full speed. The HS-8A was therefore sent back to Chalais-Meudon on July 21, 1915 and tested for 50 hours, succeeding against all expectations.

In such a context, it is hardly surprising that French officials ordered production of the HS-8A to be started as soon as possible and emitted a requirement for a new single-seat high-performance fighter aircraft using the new engine. The SPAD VII was the result of this requirement and would allow the Allies to regain air superiority over the Germans.

The HS-8Aa, which entered production in July 1915, was the production variant of the basic HS-8A type, with increased power output: 150 hp (110 kW) at 2,000 rpm. Early HS-8A engines were plagued with various problems which required further work. The 150-hp HS-8Aa was the standard powerplant for early-production SPAD VIIs. The demand for the Hispano-Suiza engine was such that other manufacturers began producing it under license, in France, Great-Britain (Wolseley in Birmingham), Italy (Nagliati in Florence and Itala/SCAT in Turin) and Russia. Total production of the HS-8Aa amounted to some 6,000 engines.

The HS-8Ab was yet another evolution of the HS-8A. By increasing the compression ratio from 4.7 to 5.3, Birkigt was able to increase the power output to 180 hp (132 kW) at 2,100 rpm. The HS-8Ab began replacing the 8Aa on SPAD VIIs in early 1917.

Hispano-Suiza 8be

The HS-8Ab had been plagued by engine failures due to the high compression ratio: piston heads were sometimes pierced and seized the engine. To further increase the power output of the engine, Birkigt chose to increase engine speed. Two versions of the HS-8B were made. The first, the HS-8Ba, kept the 4.7 compression ratio of the HS-8Aa but delivered 200 hp (147 kW) at 2,300 rpm. The second version, the HS-8Bb, had a compression ratio of 5.3. A reduction gear (3:4 ratio) was fitted to compensate for the increased engine speed. However the reduction gear system was fragile, and often broke down, sometimes with spectacular results ending up with the entire propeller, driveshaft and driven gear parting company from the airframe. Progressive refinement of the engine brought the available power to 235 hp by the end of 1917.

The HS-8B was used to power the earliest versions of the S.E.5a, all examples of the SPAD S.XII and SPAD S.XIII, front-line versions of the Sopwith Dolphin and several other Allied aircraft types, with its gear reduction easily identifiable in vintage WW I photos, from its use of a clockwise rotation propeller.

The HS-8F was basically an enlarged version of the HS-8B for use on bombers. The displacement was increased to 18.5 litres (1,127.3 cu in), with a power of 300 hp (220 kW) at 2,100 rpm (equal to 750 ft lbs torque). The engine now weighed 256 kg (564 lb) and was not suitable for aerobatics, hence its use on bombers and reconnaissance aircraft. Engine speed being inferior to that of the HS-8B, the reduction gear was dispensed of, hereby increasing engine reliability.

Summary

Sometimes referred to as the "Rolls-Royce Merlin of World War I", the Hispano-Suiza HS-8 engine was one of the most important powerplants to be produced during the First World War. A total of 49,800 engines were built by Hispano-Suiza and French manufacturers. Derivatives of the engine were also used abroad to power numerous aircraft types and the engine can be considered as the ancestor of another successful engine by the same designer, the Hispano-Suiza HS-12 which served in World War Two.

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