The rationale behind the single sleeve valve design was two-fold: to provide optimum intake and exhaust gas flow in a two-row radial engine, improving its volumetric efficiency; and to allow higher compression ratios, thus improving its thermal efficiency. The arrangement of the cylinders in two-row radials made it very difficult to utilise four valves per cylinder, consequently all non-sleeve valve two- and four-row radials were limited to the less efficient two-valve configuration. Also, as combustion chambers of sleeve-valve engines are uncluttered by valves, especially the hot exhaust valves, being comparatively smooth they allow engines to work with lower octane number fuels using the same compression ratio. Conversely, the same octane number fuel may be utilised while employing a higher compression ratio, or supercharger pressure, thus attaining either higher economy, or power output. The downside was the difficulty in maintaining sufficient cylinder and sleeve lubrication. Bristol had introduced their first sleeve-valve designs in the 750 horsepower class Perseus and the 500 hp class Aquila that they intended to supply throughout the 1930s. Aircraft development in the era was so rapid that both engines quickly ended up at the low-power end of the military market and, in order to deliver larger engines, Bristol developed 14-cylinder versions of both. The Perseus evolved into the Hercules, and the Aquila into the Taurus. In 1937 Bristol acquired a Northrop Model 8A-1, the export version of the A-17attack bomber, and modified it as a testbed for the first Hercules engines. The first Hercules engines were available in 1939 as the 1,290 hp Hercules I, soon improved to 1,375 hp in the Hercules II. The major version was the Hercules VI which delivered 1,650 hp, and the late-war Hercules XVII produced 1,735 hp. In 1939 Bristol developed a modular, "unitized" engine installation for the Hercules, a so-called "power-egg", allowing the complete engine and cowling to be fitted to any suitable aircraft. The Hercules powered a number of aircraft, including Bristol's own Beaufighterheavy fighter design, although it was more commonly used on bombers. The Hercules also saw use in civilian designs, culminating in the 735 and 737 engines for such as the Handley Page Hastings C1 and C3 and Bristol Freighter. The design was also licensed for production in France by SNECMA. Except for the 2000 hp-range versions in the 1950s until there were lubricating oil improvements, it was considered to be one of the more reliable aircraft engines of the era. A total of over 57,400 Hercules engines were built.
