In the late 1920's Montague Napier designed his E96 'H' Type 24-cylinder compression ignition (diesel) engine with a 5.75" bore x 5.5" stroke. After his death the design was resurrected and, as a direct result of the considerable experience gained with their successful 1,000 B.H.P. H-type Dagger engine, D. Napier & Son Ltd commenced design work on their design E101 engine. This was a horizontal 'H' water-cooled 24-cylinder C.I. engine. Two, six and twenty-four cylinder versions of this engine were built and it was named the Napier Sabre. Development work was slow and the design and was a long way behind two 6-cylinder opposed piston engines designed by Hugo Junkers, the Jumo 204 & 205 engines. The British Air Ministry persuaded DNS to abandon their E101 project in favour of building the Jumo 204 engine under licence. This became the Napier Culverin.
The Air Ministry however had a requirement for a new high-powered petrol aero engine and in 1935 DNS revived the E101 and redesigned it as the E107. The basic features of the Sabre engine, namely twenty-four cylinders, liquid cooling, sleeve valves, and compact design remained throughout the Series.
Within two years the first petrol Sabre engine was completed and successfully run. By 1941 the 2,090 B.H.P. Series II engine had overcome most of its initial teething troubles, and was chosen to power the Hawker Typhoon, then Britain's leading fighter aircraft. Subsequent developments in design provided a progressive increase in power output so that in 1945 the Series VA, which incorporated a boost corrected servo-controlled, ignition unit and single-lever cockpit control, gave a maximum of 2,600 B.H.P. The Series VII (E121 design) included water / ethanol injection brought the maximum output to 3,055 B.H.P.
Noteworthy features marking the successive development stages of development include, the strengthening of certain components to enable them to stand up to greater loads, the increased capacity of-the two-speed supercharger, the introduction of the Hobson-R. A.E. injector and single-lever cockpit control, and finally, the inclusion of water/methanol metering equipment.
Development in power output of 'Sabre engines
The "prototype" Sabre engine ran for the first time on 23rd November 1937, and completed its initial acceptance tests by 17th January 1938. By March the power delivered had already risen from its acceptance test figure of 1,350 B.H.P. to 2,050 B.H.P. Gradually the Series I (E114) engine was evolved type tested in June 1940 at a maximum power rating of 2,060 B.H.P.
In early 1939 tests were undertaken with a special, ground-boosted version of the Sabre to be used in a Heston aircraft for an attack on the world speed record. An air blast was applied to the air intake on the test bed to simulate a forward speed of 400 M.P.H. and under these conditions a maximum power reading of 2,560 B.H.P. was achieved. This was obtained at 1,000 C.R.P.M. and 9.2 psi boost pressure, which represented a 5 minute limit at full throttle. Unfortunately, this aircraft, through no fault of the engine, crashed during its initial flight trials and, owing to the war, further tests were abandoned.
The Sabre Series I engine was fitted with a supercharger with an 11 inch diameter impeller and a clutch which was capable of gear changing at maximum cruising speed only (designated Mark I clutch). After around 60 Series I engines had been produced the design was changed to include:
(i) a supercharger with a 1,2 in. dia. impeller
(ii) a supercharger clutch capable of gear changing at maximum
climbing speed, (Mark III)
(iii) larger carburettor chokes.
This standard of engine was designated Series II. The Sabre Series I and II engines were rated at the same take-off and combat boost pressure but because of the larger supercharger impeller the maximum power altitude of the Series II was raised by 1,500 feet in "M" gear and 2000 feet in "S" gear.
Horizontal 'H' Water-Cooled Engine
Design E101 1932 'H' diesel prototype
24-cylinder 5" bore x 4.3/4" stroke
Design E107 1937 Sabre I
24-cylinder 5" bore x 4.3/4" stroke 2,055 bhp