Swordfish Mk I, close up view of engine
The Bristol Pegasus was another important poppet valve engine used extensively during World War II, deriving its design largely from the Jupiter and Mercury engines. It was essentially a longer-stroke version of the Mercury, with bore and stroke dimensions of 5.75 inches and 7.50 inches, respectively. This resulted in a total displacement of 1753 cubic inches (28.7 liters), which, while substantial, provided a specific power output that was generally lower compared to other contemporary engines.
The Pegasus was typically rated at 1065 horsepower at 2600 rpm. It had a notably high piston speed of 3250 feet per minute, which was a distinctive feature among engines of its era. Mechanically, the Pegasus shared many design aspects with the Mercury, including the use of a two-piece crankcase, two-piece crankshaft, and a similar valve gear arrangement.
One notable difference in some versions of the Pegasus was the inclusion of two-speed supercharging, which allowed for better performance at varying altitudes by adjusting the boost pressure delivered by the supercharger. This feature was not standard across all Pegasus engines but did mark an advancement over the Mercury’s single-speed supercharging system. Despite its limitations in specific power output, the Pegasus proved to be a reliable and widely used engine in various aircraft throughout the war.