Bristol Hercules engines
Derived from the Perseus design, the Hercules engine featured 14 Perseus cylinders arranged in two rows. It became the most pivotal Bristol engine of World War II, with around 65,000 units produced. The Hercules engine, with its 14 cylinders of Perseus size, had a total displacement of 2360 cubic inches (38.7 liters), delivering an initial power output of 1375 horsepower at 2400 RPM during its first type test in 1936. As development progressed, Bristol managed to nearly double this power output.
A notable challenge with the sleeve valve technology of the 1930s was the need for precise fitting of each sleeve to its corresponding cylinder barrel. This problem was not prioritized during the Great Depression due to limited production capacities. However, as the prospect of war became clearer towards the late 1930s, achieving interchangeability between cylinders and sleeves became crucial. Innovations in manufacturing and materials, such as an accidental discovery involving a dull grinding wheel for final sleeve diameter grinding, helped address this issue.
Similar to the Perseus engine, the Hercules utilized spur gears driven by the crankshaft, which were enclosed in the nose case. The sleeve drive cranks for the rear cylinders extended shafts that ran between the front cylinders, creating a highly intricate setup reminiscent of a mechanical Swiss watch.
The propeller reduction gearing used was the Farman bevel epicyclic system. The supercharger was typically a single-stage, two-speed unit with automatic boost control.
The crankcase was a three-piece aluminum forging split vertically along the cylinder centerlines, fastened together with seven studs. The crankshaft was supported by three large roller bearings and featured dynamic counterweights in the crank cheeks. Master rod bearings were either fully floating or fixed and were notably made of white metal instead of the more common copper-lead or silver alloys. The two-speed supercharger drive was managed by multiplate clutches with oil operation and compression springs to absorb torsional shock. Initially, Hercules engines used single-speed supercharging.
Fueling was managed by a Claude Hobson downdraft carburetor with automatic mixture and boost control. In 1942, this was replaced by a Hobson-RAE injection unit as the benefits of single-point Bendix-type injection became evident.
A key feature of Hercules and subsequent Bristol engines was the remote gearbox for driving accessories like hydraulic and vacuum pumps, and generators. A jack shaft at the rear of the engine drove the Rotol accessory gearbox, facilitating easier maintenance, fewer component swaps during engine changes, and reducing the overall engine length.
Insights gained from captured German aircraft led to improvements such as fan cooling and individual ejector exhaust stacks in later engine designs. This resulted in a revised exhaust system with a swept-back configuration and ejection through the cowl flaps, replacing the earlier system that routed exhaust to the front into a collector ring.
By the start of World War II, Hercules production was robust, delivering 1375 horsepower with reliable performance. By the end of the war, the Hercules HE.20 SM had been tested at 2500 horsepower, utilizing ADI and injection carburetion.