The Hispano-Suiza 12Y was a French aircraft engine in the pre-WWII era. Developed from
the earlier, and somewhat smaller, 12X, the 12Y became the primary 1,000 hp (750 kW)
class engine and was used in a number of famous aircraft, including the Morane-Saulnier
M.S.406, Dewoitine D.520. The design was also widely used in other countries, and formed
the basis for licensed production of a number of designs, most notably the Soviet Klimov
VK-105 series.
The 12Y was a fairly traditional in construction, a 36-litre water-cooled V-12 with the
two cast aluminium cylinder banks set at 60 degrees to each other. A unique feature was
that the cylinder heads were not removable, instead the entire block could be quickly
removed from the engine. This made it somewhat famous for being entirely leak-proof, a
design that was considered by other designers and almost became a part of the
Rolls-Royce Merlin. The major design change from the earlier 12X was to use a
master-articulated cylinder rod system, instead of the fork-and-blade type. A single
overhead camshaft drove the valves, which were filled with liquid sodium for cooling.
Only a single intake and exhaust valve were used, unlike most designs of the era which
had moved to four valves per cylinder. A single-stage, single-speed supercharger was
standard, although the art of designing a useful intake was not as well developed as in
other countries, and altitude performance was always lacking.
The first 12Y test articles were constructed in 1932, and almost instantly the entire
French aviation industry started designing around it. At the time the engine developed
only 760 hp (570 kW), but it was clear it had potential to the 1,000 hp (750 kW) class.
An early modification led to the 12Ycrs which used a hollow propeller shaft to allow a
20mm cannon to fire through the propeller spinner (what the Germans called a motorkanone).
All later designs shared this feature. The 12Ydrs was the next major series, with a basic
rating of 836 hp (623 kW) at sea level with a compression ratio of 5.8:1, while the 12Y-21
offered 867 hp (647 kW) from a 7:1 ratio.
In 1936 the connecting rod design was changed slightly to create the 12Y-31, but the low
compression ratio was retained and the power was increased to only 850 hp (630 kW).
Nevertheless this became one of the most used engine designs of the pre-war era, used in
almost all fighter designs and prototypes.
A real effort to improve the performance of the engine in 1938 resulted in the 12Y-45,
which used the Polish-designed Szydlowski-Planiol S-39-H3 supercharger in place of the
indifferent French models. This allowed the compression ratio to be raised to that of the
-21's 7:1, boosting power to 900 hp (670 kW), although requiring the use of 100 octane
fuel. Combined with the fully-adjustable Ratier propeller, this allowed the D.520 to
perform as well as contemporary designs from Germany and England. Another improvement in
supercharging led to the 12Y-49, whose performance improved from 850 hp (630 kW) at sea
level to 920 hp (690 kW) at just over 10,000 ft (3,000 m). This improvement in power with
altitude was a common feature of many early engines, the result of the supercharger
"robbing" power at low altitudes to provide boost that then had to be dumped to avoid
overboosting the engine.
The final major version was the 1,084 hp (808 kW) 12Y-51, which had just started into
production when France fell in 1940. The -51 was the first version that came close to the
performance limits of the engine, although the supercharging still meant that it was
unable to compete with designs from England and Germany above 15,000 ft (5,000 m).
In the mid-1930s, Russian engineer Vladimir Klimov was sent to France to obtain a license
for local production of the 12Y. A series of design changes were added to cope with cold
weather operation, and the engine entered production in 1935 as the M-100 with about
750 hp (560 kW). However a series of continual upgrades increased the allowable rpm from
the 12Y's fairly low 2,400 to 2,700, thereby increasing power to 1,100 hp (820 kW). The
resulting design, the M-105, became one of the major Soviet engine designs during the war.
In 1941 designers were allowed to attach their own names to their designs, and the engine
became the VK-105.
The 12Y design was further modified to create the somewhat more modern 12Z, but production
had not yet started when France fell.
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Specifications
For 12Ydrs:
Layout: 12-cylinder two-row vee
Bore by stroke: 150 by 170 mm
Displacement: 36 l (2,197 in³)
Compression ratio: 5.8:1
Power: 890 hp (664 kW) at 2400 rpm at sea level, 930 hp (700 kW) at 2,950 ft (900 m)
Weight: 470 kg (1036 lb)
