FIAT A.S.8 ENGINE.

jerryw said:

The Fiat A.S.8 engine was designed and developed in Italy during the early years of WW2 but was not intended for combat use. Its function was to capture the outright air-speed record installed in a specially built aircraft known as the CS15.
The A.S.8 was a V-16 design with individual cylinders and 45 deg. between the banks. Bore and stroke were 140 X 140mm giving a swept volume of 34.5 litres.
As with the 24 cylinder A.S.6 seaplane engine, the A.S.8 was designed to turn counter-rotating propellers.
Info on the internal arrangements of this engine is hard to find. Did this engine have two, side-by-side crankshafts each driving a propeller or was it a conventional V-16; if so, then the reduction gears at the front must have been of unusual design?
Any information welcomed.

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The Red Baron said:

Salve, sono Italiano non parlo Inglese,
per caso mi sono imbattuto sul Vostro Forum e vedo che cercate info sul Fiat AS8

vi allego la scheda tecnica tratto dal libro "Aereonautica Italiana museo storico catalogo motori"

Buona lettura

Saluti

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AS8 engine technical specifications
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Type: special engine for record plane, 16V at 45 degrees with supercharger and reduction gear for coaxial counter-rotating propellers

Cooling: liquid

Bore: 140 mm

Stroke: 140 mm

Displacement: 34.482 dm3

Mixture distribution system: four over-head valves each cylinder two overhead camshafts for each row

Compression ratio: 6.5

Fuel supply: depression carburetors

Compressor: centrifugal one stage

Reduction gear: with double output for coaxial counter-rotating propellers rate 1:1.666

Ignition: double with four magnetos

Max power: 2250 CV* approx. at sea level, 3200 RPM

Dry weight: 790 kg

Specific weight: 0.35 kg/CV

Power/displacement rate: 65.25 CV/liter

Year of manufacture: 1939-40

Quantity: unknown (few items)

Aircraft of destination: Fiat-CMASA CS15


* note CV = Cavallo Vapore (Horsepower); 1 CV = 0.99 Hp approx.

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During spring of 1939 germans - in few days and by means of two different aircraft -
broke the all-time speed record held by italian MC72 racer powered by Fiat AS6 engine.
So Fiat, in agreement with italian government, decided to build a new machine in order
to reconquer the losted world record: eng. Manlio Stiavelli was charged with aircraft
development and prof. Antonio Fessia with new engine.

The new engine was a V16, an unusual layout (even if engine of that kind were built
since 1918 but with considerably less power) for the intended high power output: this
choice was due mainly to the special duty required for the engine but also for making full
use of the great know-how Fiat had gained in those years.

The engine was built with the minimal cross-section, even less than that of AS6.
The V16 configuration had already been took in account in 1929 by eng. Zerbi during his
preliminary study on a 2300 CV engine, as requested by ministry, but was rejected at
that time because of the predicted unreliability of the needed eight-crank shaft for the
given cylinders displacement (45-50 liters); and more, the excessive size of cylinders would
have leaded to greater cross-section of the engine and unsatisfacfory speed range.
In 1939 most things were changed but the eight-crank shaft still was a trouble in a
powerful engine (more than 2000 CV for the new engine against 1400-1500 CV for the
AS6)(see figure at page 116 bottom: layout scheme for AS8 engine's crankshaft).

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The AS8 engine was greatly derived from AS6, so it had almost the same cylinders size
(bore for AS8's cylinders was only 2 millimeters more than AS6's), and consequently
pistons, rods, valves etc, and the fuel system was very similar to the latter; on
the other hand the pressure supplied from supercharger system was slightly less than in
AS6.
In order to minimize torsional vibrations in the long crankshaft all actuators of auxiliary
devices (fuel system, magnetos, pumps, etc.) were placed near the power train output.
The torque reaction was minimized adopting counter-rotating propellers, but, in a
different way from AS6, there was only one crankshaft and it drove the two propellers by
means of a special reduction gear.
At the beginning of 1940 the engine was already on the test-bench, showing great
power of endurance and reliability, which was unusual thing for such powerful device at
that time. The engine ran safe and easy for tens of hours at high speed without failures
and, though it was rated for a max power of about 2000 CV, it supplied up to 2500 CV
for periods. At 3200 RPM, 2250 CV the mean effective pressure was 18.35 kg/cm2
against a value of 16.8 for the AS6 at 3100 CV.
In conclusion AS8 was an outstanding engine, the best ever built in Italy in all times, the
only one that could beat the AS6.
Unfortunately, at the outbreak of the war development of the new record plane was
stopped and so the fine tuning of AS8 was never completed and it hadn't the chance to
show its great value in flight.

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