[Note, as well, the involvement of SU and Stromberg carburetor
development along with this engine]
Merlin
By 1932 it was becoming apparent to Rolls-Royce that their best-selling
engine, the 21.2 litre 745HP Kestrel was coming towards the end of its
development life.A decision was made by Sir Henry Royce to develop a new
engine using some of the experience of the Schneider Trophy winning 'R'
engine. It retained the V12 configuration and geared supercharger of its
predecessors, but was of 27 litres swept volume. It was anticipated
initially that
this engine would be able to reliably deliver around 1000HP. The engine
was known initially as PV12 (Private venture, development intially
entirely
funded by Rolls-Royce). When in October 1933 the Air Ministry agreed to
finance the development, it was named Merlin (Rolls-Royce piston engines
were named after birds of prey, jets after rivers).
Ironically, the early development of the engine was plagued with
problems, with gear train failures and persistent failures of the water
jackets (the
cooling mixture eventually became 30% glycol as antifreeze in water at
+18PSI pressure). Eventually in July 1934 the Merlin passed its type
testing and
was rated at 790HP at 12 000 feet at 2500 RPM. The Merlin B was tried
with a ramp head to the cylinder which had improved fuel mixing and
flame
propagation in Rolls auto engines, and in February 1935 delivered 950HP
at 11000 feet equivalent. Based on their experiences, Rolls-Royce
decided
to make the crankcase and cylinder blocks as 3 separate castings, with
bolt-on ramp heads to the cylinders. This engine was the Merlin C. By
this time
the promise of a low-profile aero engine of 1000HP had persuaded both RJ
Mitchell and Sydney Camm to base their designs around this new untried
engine. This engine still had problems, but after some modifications as
Merlins E and F, a Merlin E passed a civil 50 hour Type Test at 955HP
(maximum 1045HP). As an emergency measure it was decided to scale up the
parallel cylinder head as used in the Kestrel to the larger engine
(Merlin
G). This engine easily passed its Type Test a month before the F (now
released as Merlin I) and was then designated Merlin II. This engine
weighed
1335lb and was rated at a maximum power of 1030HP at 3000RPM at 16250
feet, and ran on 87 octane fuel. It is worthy of note that in 1937 an
attempt was made to break the World Landplane Speed Record, using a
highly modified Spitfire I and a specially strengthened Merlin II. This
engine
actually generated 2160HP, and showed the potential for development of
the engine. Most of the modifications developed for this engine
eventually
found their way into production Merlins. The Merlin III was adapted for
the use of a constant-speed propeller and a constant-speed unit.
A variant of this engine with a higher supercharger gearing (providing
up to 12.5lb boost) and a Coffman cartridge starter was termed the XII
and
marked the difference between the Spitfire I and II.
In 1935, after problems with supercharger gearing, Rolls-Royce decided
to take out a license for the Farman 2-speed drive. The advantage of the
2-speed supercharger was that it could be run at low speed, using little
energy, in the thick lower altitudes, while being available to enrich
the air supply
at altitude. There are supercharged engines providing zero extra boost
at sea level being flown today. The first of these engines with a
2-speed
supercharger was the Merlin X. This added significantly to the length of
the engine.
In 1939 a decision was made to focus on 100 octane fuel for aero
engines. This fuel permitted higher boost pressures and temperatures
without
detonation, and allowed the use of +12lb boost rather than the previous
limit of +6lb.
The next major development of the Merlin came from Sir Stanley Hooker.
It was realised in the development of the Merlin for the World Speed
attempt that the efficiency of the Merlin supercharger was relatively
poor. Hooker, a mathematician by trade, examined the supercharger from
first
principles, and markedly improved its efficiency. He also improved the
flow characteristics of the air inlet, which improved the output power
at altitude.
Although the original installation was elongated even further, it was
found that by turning the carburetor around the length was similar to
the original
installation. This new engine was the Merlin XX, and allowed power to be
maintained at much higher altitudes (1175HP at 20 000 feet compared to
1160HP at 13 500 feet for the Merlin II). The single-speed supercharger
Merlin 45 incorporated many of these modifications, and this engine,
fitted to
the Spitfire airframe, became the Mark V Spitfire.
Some of these engines were modified for low-altitude power, since most
of the air combat was taking place around 6 000 feet. In these, the
supercharger impellers were shortened, and the speed of the
constant-speed unit increased. This gave a maximum power height of
around 6 000 feet,
and increased speed by around 22 mph at this height. If coaxed to higher
altitudes, however, the engine suffered badly.
The development of high-altitude bombers required the development of an
engine with a higher full-throttle height. Rather than move to
turbochargers,
Hooker suggested adding two superchargers in series. Since a high
altitude supercharger of the right size had already been developed, the
output from
the Rolls-Royce Vulture supercharger was simply fed into the
supercharger of a Merlin 46. The only modification required was the
incorporation of a
cooling stage between the two supercharger stages to prevent fuel
detonation. The new engine, the Merlin 60, had a full-throttle height of
nearly 30 000
feet. A redesign changed the supercharger gearing and introduced a
2-piece cylinder block to produce the Merlin 61. This engine produced
spectacular
effects when fitted to a Spitfire. Although intended for the Mark VIII,
it was possible to fit it to the Mark V airframe, and this became the
Spitfire Mark
IX/XVI series. The extra cooling necessary became evident by the
enlarged radiator under the left wing.
As the specific power from the engines increased, the focus of much of
the design was strengthening. An empirical approach was to run an engine
at
high power until something broke, then strengthen or redesign that part
and carry on. The consequence of these developments was the Merlin 130
with
a low level power of 2030HP, and an elevation of the height at which
1000HP was available from 16 000 to 36 000 feet. In late 1944 a Merlin
was
run for 15 minutes at 2640HP! After the difficult beginnings the ability
of the Merlin to withstand abuse became a watchword. Few engines
tolerate full
power loads for any great period, but there are examples on record of
Lancaster pilots losing one of their Merlins shortly after takeoff, but
simply
continuing the mission with all the remaining throttles pushed to the
stops. The engines rarely failed.
The 500, 600, and 700 series Merlins were mostly post-War developments
for civilian transports. In these the focus was not on absolute power
but on
component life. In total the production run on the Merlin was 168 040.
Carburettor design
One of the great problems as discerned by pilots was the tendency for
the carburetted engine to cut out under negative 'g'. Luftwaffe pilots
learned to
escape by simply pushing the nose of their aircraft down into a dive, as
their fuel- injected engines did not cut out under these circumstances.
By 1941
Miss Tilly Shilling in Farnborough had developed a partial cure for the
problem. A diaphragm across the float chambers with a calibrated hole
allowed
negative 'g' manouvres, and was fitted as standard from March 1941.
Sustained zero 'g' manouvres were not sorted out until somewhat later.
In 1942
an anti-g version of the SU carburetor was fitted to single and
two-stage Merlins. 1943 saw the introduction of the Bendix-Stromburg
carburetor which
injected fuel at 5psi through a nozzle direct into the supercharger and
was fitted to the Merlins 66, 70, 76, 77, and 85. The final development
was the
SU injection carburetor which injected fuel into the supercharger using
a fuel pump driven as a fuction of crankshaft speed and engine
pressures, which
was fitted to the 100 series Merlins.
Developmental lines
Single Stage, single speed supercharger
Merlin I
Merlin II
Merlin 45/46
Single Stage, two speed supercharger
Merlin X
Merlin XX
Two Stage, two speed supercharger
Merlin 61/64
Merlin 66/67/76/85
Merlin 100 series
Merlin 130
Merlin 140
--
My other Triumph runs, but....
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