History of a Dimension
INTRODUCTION
THE
20 hp ROLLS-ROYCE ENGINE
Immediately
after the 1914-18 war Rolls-Royce recommenced manufacture of the Silver Ghost
car but realized that a smaller model was required to meet the prevailing
economic conditions. The first essential was a smaller engine and in 1919 a
design was put in hand by Mr Royce at West Wittering of an engine to give about
50 hp. By then, except for a short trial with eight cylinders,
Rolls-Royce had
settled down to six cylinder in-line engines. The new smaller engine followed
this lead. The first decision taken was to have a bore of 3 inches and a stroke
of 4 inches. The Royal Automobile Club rating of this engine was about half
that of the Silver Ghost-namely 20.6. From then on the drawing board settled the
general layout, on these basic dimensions, of an engine known as I.G.1.
A
cylinder bore centre distance of 4.150 inches was considered necessary to
provide adequate intermediate bearings and of 4.650 inches to provide a centre
bearing. It was on the smaller of these two dimensions that the future
development of the engine depended, and 4.150 inches proved an exceedingly good
choice.
The seven bearing crankshaft followed the rather niggardly practice of the early twenties, the main bearings being 2.000 inches diameter and the big ends 1.500 inches diameter. Fig. 1 shows this crankshaft.
The valve gear was of the most advanced design,
having two overhead camshafts. When I joined the company 11 years later,
the first engine was driving a chassis
bump test rig and one of my first jobs was to adjust the tappets. Far would it
have been from me to criticize, but the job took a whole day.
Certain other important dimensions were a connecting rod length of 8 inches,
a
gudgeon pin diameter of 0.750 inch, an inlet and exhaust valve throat diameter
each of 1.250 inches.
From all of the above basic dimensions the design shown in Fig. 2 emerged. The integral block and cylinder head were bolted to an aluminium crankcase. An all-speed throttle governor was present. A very advanced damper (much better than its successor) was fitted to the front of the crankshaft. The first engine was completed in 1920. Its power curve is shown in Fig. 3, a peak power of 53 hp having been achieved for a weight of 650 lb (including exhaust manifold, dynamo, magneto, starter and carburettor). In those days a good torque at low speeds was very necessary, 84 m.e.p. being obtained at 500 rev/min. Throughout this history low speed m.e.p. has been of paramount importance.
Production of this smaller car was needed for the autumn of 1922. Its price had to be about half that of the Silver Ghost. Changes were made to the engine and although a simpler push rod version was chosen, most of the basic dimensions remained unaltered, the connecting rod being shortened to 7.95 inches. Roller tappets were used. The crankshaft friction damper was separated from the crankshaft pinion spring drive. Figs 4 and 5 show this engine, the latter portraying the five gear train in the wheelcase.
During the next 37 years,
without increasing the cylinder centres, the output
increased from 53 bhp at 3000 rev/min to 215 bhp at 4200 rev/min and 4-inch
diameter pistons had been squeezed into the same centre distance.
No automotive engine ever gives enough power. For a given capacity more power has to be obtained by an increase in b.m.e.p. or an increase in rev/min. A six cylinder in-line engine has a unique way of resisting higher rev/min. Every crankshaft and flywheel assembly has a natural frequency of torsional vibration, the node being near the flywheel, but only an in-line six has a disturbing torque due to the motion of its pistons which is not only almost a pure sine wave, but one of considerable magnitude (Fig. 6). The exciting torque oscillating three times per crankshaft revolution reaches resonance when the rev/min is one-third of the natural torsional frequency. The inertia torque varies as the weight of the pistons, inversely as the length of the connecting rod and as the square of both the speed and the stroke. Its peak value in 1922 was about 170 lb.ft at 3300 rev/min.
The critical speed of the first production Rolls-Royce 20 hp engine was 3300
rev/min and equivalent to about 76 mile/h in top gear on the first series of
cars. Luck was with us for a time. A little below this critical speed the
distributor ceased to function and the cam wheel came adrift. Even this happened
very rarely as the flywheel, at 3100 rev/min, had a resonance of its own, whose
thunderous noise dissuaded most drivers from seeking an Elysium further on.
Little was known in the early twenties about the necessary test bed life of an
engine, but it was thought that a car should be capable of 10 000 miles of full
throttle driving on straight French roads. In actual fact the white metal
bearings (direct in the case of the connecting rod) lasted for about 9000 miles
of this treatment. Until the advent of motorways this engine had a reputation
for lasting for ever on English roads.
During the next seven years very few changes were made to the engine,
the most
important being a five ring piston and an oval web crankshaft. The latter
was the first in a long series of changes aimed at raising the critical speed.
This one change provided a valuable 400 rev/min, raising the critical speed from
3300 to 3700 rev/min. One rather interesting comment on the records of the
Inspection Department reads, All standard 20 hp chassis despatched on and
after 1.8.23 have a Thackeray washer fitted to stop dynamo drive rattles.
Nearly all the remaining change notes refer to modifications tried in the
crankshaft spring drive and damper to overcome knocks, rattles and vibrations.
It seems opportune here to mention a piece of apparatus almost constantly in use when I joined the Rolls-Royce Company in 1930. This was the Summers indicator, used to measure the amplitude and frequency per revolution of torsional vibrations. Fig. 7 shows how this indicator works, and I shall always remember it as a piece of apparatus which never lied but often flew into pieces. Nowadays beautiful instruments can be obtained in America, but in 1930 we made our own and rather frequently had to renew them.
Coachwork did not grow lighter as the years proceeded. Better acceleration of the car was required. An unceasing quest for torque and power had begun.
20/25
hp ENGINE (3.68 litre)
I
have already said that six cylinder engines are dogged by a critical speed which
falls almost certainly within the desired range of useful speed.
It
seems appropriate to digress and expand on this one feature so perpetually in
the minds of designers and development engineers. In the early twenties the
critical speed was an accepted nuisance to be borne bravely, the half speed
period (six per rev) due to gas torque being rendered inaudible by the
crankshaft damper.
In
1928, Major B. C. Carter, then working at Farnborough, published his formula for
calculating crankshaft stiffness and critical speeds. Major Carter had been
working on this subject since the 1914-18 war, when failures in the Arab engine
had to be overcome. It is of such interest that I quote it in full:
It
will be seen immediately that the critical speed can be raised most
significantly: (1)
by reducing the flywheel inertia; (2)
by reducing the inertia of the crank damper fixed hub; (3)
by increasing w,
the width of the crank webs and, to a lesser extent, by increasing the
diameter of journals and crank-pins, the last adding quickly to the distributed
inertia.
Between
1922 and 1959 the critical speed rose from 3300 to 5400 rev/min in spite of the
arrival of balance weights whose inertia was all on the debit side. No less than
38 different crankshafts appear on the lists of parts up to 1939!
One
might ask why apparently no attempt was made to damp the critical speed and run
through it. The disturbing force is large and of the worst possible shape.
The
accelerations connected with the critical speed are enormous. 3,
three times
per revolution at 5000 rev/min, corresponds to an acceleration of 125000
radians/sec2. The forces required to retain working parts slightly
out of adjustment were beyond the strength of the materials and their attachment
to the crankshaft.
Apart from
vibration, things seemed in favour of more power. In 1930, when the compression
ratio rose from 4.6to 5.25, the valves did not suffer and the bearings gave less trouble. Metallurgy had more than kept pace with requirements.
Now that engine speeds of 3500 rev/min and over could safely be used on the road the flywheel vibration at 3100 rev/min had to be eliminated. A flywheel vibration was known to coincide with the roughness felt by the passengers because at 3100 rev/min the turning marks on its periphery became blurred. With the aid of a micrometer, measurements showed that the flywheel departed from its normal plane of movement by 0.020 inch, when 3100 rev/min was reached. With the gearbox removed optical methods gave a picture of the vibration shown in Fig. 8. Improvements could not be assessed without difficulty and outstanding was the effect of a counterweight on No.12 crankshaft web. A weight opposite the pin equal to the whole of the web, half the crank pin and big end, and one quarter of the reciprocating weight, removed all trace of the vibration. Unfortunately 11 similar weights, one on each other web, reduced the speed of the torsional vibration excessively and a compromise had to be sought.
The
effect of balance weights could be detected on No. 12 web and also on webs 6 and
7, where they reduced the centre bearing load and hence increased the life of
the most highly loaded bearing. The compromise resulted in what became known as
R's eight weight scheme, invented by Sir (then Mr) Henry Royce. It is
shown in Fig. 9 and appeared in 1932. A yet stiffer shaft offset the greater
inertia. One hundred per cent local balance was not attainable, but the flywheel
itself responded to a change in shape. The flange connecting the heavy rim to
the crankshaft, hitherto very stiff, was reduced to a diaphragm 0.100 inch
thick. The rear end of the crankshaft could now oscillate in a swash manner with
less disturbance at the flywheel rim.
Almost
identical troubles were being encountered at this time by the Packard Company in
America. To determine the best balance weight arrangement they mounted a
crankshaft vertically and applied forces to it at each crank pin equal to the
centrifugal forces of rotation (Fig. 10). They found that a single force 140
from Nos 3 and 4
crank pins pulled the crankshaft straight again. From this they devised a
four weight scheme on webs 1, 6, 7 and 12, the end ones being 20 from opposite
the pins and the middle ones 40 from opposite the pins. This arrangement with
overhanging weights was tried with success at Derby but was never standardized.
l speed often shaking them loose. Our designs are shown in Fig. 11.
Except
in the original engine 1.G.1., from 1922 to 1932, the arrangement at the front
of the crankshaft was a small damped spring drive for the crank pinion and a
separate friction damper for the six per rev vibration, the hub of this damper
and the fan pulley being rigidly mounted on the crankshaft. In 1932 the low
inertia spring drive appeared. A comparison is made in Fig. 12. Now only the
hub of the main damper was fixed to the crankshaft, it having been agreed that a
figure for spring drive rate and damping could be found which took care of the
six per rev vibration and also cam wheel rattles. Another 250 rev/min had been
added to the critical speed.
Valve
spring design easily matched the increasing rev/min, but bearings needed a
better life and in 1933 the crankshaft was nitrided. It still ran in white metal
bearings. 30 000 miles of French testing at considerably higher speeds was now
the order of the day.
In
1934 a yet stiffer crankshaft was devised, camshaft accelerations were increased
and a cam balancer was incorporated to eliminate rattles of the five gears
at the front of the engine (Fig. 13).
Parallel
with the development of the Rolls-Royce engine, refinement having been of
paramount necessity, a different cylinder head, known as J1, had been on the
test beds. Having a higher compression ratio and a turbulent bath tub shape
(Fig. 14), more but rougher power was available if ever required. The
acquisition by Rolls-Royce of the Bentley Company could not have been better
timed. After a short experimental digression with a 2.36 litre supercharged
engine, the J1 engine was fitted to the first 3 litre Bentley made at Derby.
The engine differed from the Rolls-Royce 20/25 engine in cylinder head, camshaft,
compression ratio and carburettors (2 S.U.'s). The connecting rod
length of production engines was 7.950 inches, although the prototypes had been
inch longer at 8.250 inches. The critical speed had now reached 5000 rev/min
and a red mark on the rev counter limited the user to 4500 rev/min
uncomfortably close!
After
a year of production more power was obtained from the Bentley engine by
abandoning split skirt pistons in favour of the fluted Aerolite' design. A
small noise increase was accepted. 25/30
hp ENGINE (4.257 litre)
Cars
of the same basic design seem to get a little heavier each year and to preserve
acceleration axle ratios become lower. The safe rev/min could be reached in top
gear and more torque was once again required. The bore therefore was again
increased, this time from 3¼ to 3½ inches, the Rolls-Royce being known as the
25/30 and the Bentley as the 4 litre. White metal bearings no longer sufficed
and aluminium-tin bearings, invented in the Rolls-Royce laboratory, were fitted
to both main and big end bearings. This material needed more running clearance
than white metal and was responsible for an increase in noise and roughness.
An
interesting lubrication problem had come to light during recent development.
Although the Bentley engine would stand 160 hours full throttle at maximum revs
on the test bed, big end failures occurred quite soon in Continental motoring.
Believing that high-speed overrun might be the cause, an engine was run up light
on the test beds. The 160 hours had come down to a mere fifteen seconds.
Fortunately, two oil holes in each crankpin at 90 and 270 solved this
problem.
So
far this history has been mostly of designs which reached the production stage,
but four interesting experiments had also been done.
The first of these
related entirely to engine roughness. Instead of the separate iron block and
aluminium crankcase an integral cast iron crankcase was made. For a weight
increase of something over 1 cwt the engine was noticeably smoother both at full
throttle low speeds and during high speed on overrun. As an amusing sideline it
appeared to have a far better performance, the 8.250-inch connecting rods having
been fitted instead of the 7.950-inch ones! The fact that the engine went
together is a nice demonstration of where the compression volume normally
resided.
The
second experiment was a further attack on the critical speed. A four-bearing
crankshaft and suitable crankcase were made to suit the existing cylinder
centres. This was obviously not ideal but a rise of 400 rev/min was recorded, using the same journal and pin diameters.
Thirdly,
an overhead camshaft version of the engine was made. The open exhaust power from
3.68 litres proved to be 160. A cross-section of the engine is shown in Fig. 15.
This would have been a very worthwhile increase in power, but the development to
achieve sufficiently silent valve gear would have taken a long time.
Fourthly,
as a rival to the overhead camshaft engine a centrifugal blower running at about
six times engine speed was tried on both the test bed and in a car. Although
reasonably silent in operation the increase in m.e.p. was all at high rev/min
and rarely available to the driver. For most motoring a slightly higher m.e.p.
at low speeds is worth more than a big increase in power.
I
have already mentioned that the first Bentley made in Derby was originally
designed to have a supercharged smaller engine. This engine, known as Peregrine,
had a bore and stroke of 2.725 x 4.125 inches, having been scaled linearly in
the ratio of 0.8 of a larger engine of 4.404 litres capacity, known as J3. The
latter had a bore and stroke of 3.400 x 5.000 inches set in cylinder centres of
4.375 inches.
The timing gears were at the back, lengthening the crankshaft by an eighth
bearing. The interesting point about the engine, J3, was that it showed how not
to increase the power and torque from an engine of 3 x 4 inches. Its
critical speed, in spite of 2-inch diameter journals and 2⅛-inch diameter
pins, was so much lower that even the benefit of a higher axle ratio to give the
same torque at the rear wheels was more than offset. After a few experiments
with centrifugally loaded crank dampers in an attempt to give not too much
damping for the six per rev period and enough to silence the one at three per
rev, the engine was abandoned and every effort was re-concentrated on 4.150-inch
centres. ROLLS-ROYCE
WRAITH (4.257 litres)
When
the 25/30 Rolls-Royce car was replaced in 1937 by the Wraith, the 3½-inch bore
engine underwent some changes in design in the interest of simplification.
The
five gear drive at the front end became a three gear drive, the water pump was
moved to the front and the distributor to the other side of the engine. Belts
were still used only for the fan drive. A yet stiffer crankshaft, having
2.500-inch diameter journals and 2.000-inch diameter pins, was fitted.
I
have not so far mentioned induction systems. In a push rod engine, space has to
be found in the cylinder head for push rods, their clearance due to non-linear
motion,
To
have all the ports and push rods on one side of the engine was not the best
arrangement for power. Engines used in the Bentley cars had the induction system
on the non-push-rod side, deriving an immediate gain in breathing. Bentley
drivers could be expected to use their gearboxes and high m.e.p. at 500 rev/min
was therefore less important. A further departure from Rolls-Royce practice,
allowable in the case of the Bentley, was to install S.U. carburettors.
By
choosing the best size of connecting passage between the two halves of the
induction pipe, ram effect increased the m.e.p. over quite a large range of
middle speeds.
Before
going on to the first radical change in the engines tested by good fortune
during the 1939-45 war, I would like to refer to some of the niceties of design
which take into account how an engine is used. The engines described in this
history have all been fitted at the front of orthodox cars having rear wheel
drive. Apart from all the modern devices such as power steering pumps,
refrigeration compressors, and even independent front suspension, which have
made things more difficult, the engine has always had to share the bonnet
compartment with the steering column, the pedals and the exhaust manifold. The
latter three are rather uncompromising companions.
In
1922 the driver was by far the most important person in a car and Rolls-Royce
therefore put the hot exhaust system on the near or left-hand side. The
induction pipe seems to have shared this side for hot-spot reasons. At the
top of the steering wheel were controls for the hand throttle, the ignition
advance and the mixture. Their messages reached the underbonnet compartment by
concentric tubes passing through the steering mechanism. A Rolls-Royce job of
the subsequent levers, rods and ball-ends could be made only if the carburettor
and ignition distributor were on the steering side. These two considerations
accounted for the five-gear drive in the wheel case. AFTER THE 1939-45 WARB range of engines
In
addition to this new six-cylinder engine it had been decided to make a four
cylinder and a straight eight, either leaving off the end cylinders or adding a
further one at each end still 4.150 inches away (Fig. 18). The war prevented any
immediate sale of these engines, but an immense mileage was piled up in vehicles
run as staff cars or converted to lorries. During the war the quality of plain
bearings and poppet valves went forward enormously, all of which could be
incorporated in the new iron engines. By 1946 French testing of cars had taken a
new turn, at least 100 000 miles being required to make it necessary to look
inside the engine.
Wars usually make one
change one's policy and in a direction to counter monetary inflation. A possible
successor to the Phantom III car, using the straight eight, was never made in
large numbers but was limited to 12 for royal persons and rulers of states,
and
production began on a Rolls-Royce Silver Wraith and Bentley Mk. VI, both using a
3½-inch bore six cylinder iron engine of 6.4:1 compression ratio. The
difference between the two engines now amounted only to camshaft and carburettor.
Both engines had a four-port induction
system, the Rolls-Royce engine breathing through a dual-choke down-draught
Stromberg carburettor and the Bentley through two 1-inch S.U. carburettors.
A
spring drive, exactly as in the pre-war engines, was added to deal with cam gear
rattles. The cylinder head was of aluminium to save weight, flat tappets of cast
iron ran on a carburized camshaft and exhaust valves were again made of KE 965.
The top 2-inches of the cylinder bores were chromium plated. Water space
between each cylinder bore provided a good foundry location for the cylinder
barrels. A by-pass oil filter kept the oil looking remarkably clean and proved
to be a wolf in sheeps clothing.
The
combustion chamber of an overhead-inlet side-exhaust valve engine can have a
large variety of shapes. As long combustion chambers were condemned in all text
books the exhaust valve was placed as near the cylinder bore as was considered
reasonable for a certainty of water space between the exhaust port and the
cylinder bore. Satisfied with the power of the early post-war engines,
small
changes were made to the first design of cylinder head in order to select the
one that caused the least gas torque roughness, the shape shown in Fig. 19 being
chosen.
Tappet
adjustment had ceased to be a maintenance worry. The exhaust tappet adjustment
low down on the side of the engine was far from accessible, but the clearance
never varied.
Engines
proved exceptionally reliable, although a few big-end failures, due to dirt
which had not been caught in the by-pass oil filter, pointed to the need for
something even better. Nitrided shafts and lead-bronze bearings last for ever if
fed with clean oil, but the combination is much less tolerant of dirt than the
pre-war materials. In customers hands the chromium part of the cylinder bore,
about 0.0015 inch thick, lasted some 40000 miles, after which bore wear shot up
to 0.001 to 0.002 inch per 1000 miles. Re-chromium plating a block was a costly
business, the whole engine having to be dismantled and all studs and gallery
pipes removed from their casting. Army combat vehicles A step into the unknown
The
compression volume had become larger, giving more scope in the shaping of the
combustion chamber. A smoother engine had appeared, giving more than the
expected increase in power. The reason was not immediately seen and several
years went by before advantage was taken of this knowledge. It was then 1950.
The first post-war cars were mounting in mileage, some having reached nearly 90
000 miles. Mysterious failures of cam wheels started to occur, and always in
France. Fabric had proved to have a finite life with rather a large scatter,
t he
end being accelerated by higher than normal oil temperatures, always an adjunct
of Routes Nationales.
Development
engineers were faced with the problem of how to assess the value of a change
where time and temperature, as well as load, played their part.
A rig in which
the cam wheel drove a flywheel through a Hookes joint running at an angle did
not, we thought, indicate more than one source of improvement.
A soft material
was preferred for its silence, but aluminium was finally chosen because its cold
failure strength could be quickly assessed and it would not be subject to
unpredictable time and temperature effects. The inertia of aluminium,
suitably
shaped, was less than the fabric. A smaller inclination to rattle offset the
extra cold pitch line clearance required to counteract the relative expansion
with the crankcase as the engine warmed up. Parallel
with motor car development further work on the War Office engines continued.
Full flow oil filtration had been a great success. Most troubles were due to an
installation fault difficult to avoid in advance in every one of the
multitudinous uses to which the engines were being put. As greater numbers of
engines were soon to be ordered a redesign solely for simplification of
machining was carried out, over 100 000 engines subsequently being made.
It was
decided that the cylinder heads should be of cast iron. A comparison with
identical heads in aluminium showed no compression ratio advantage for the
latter.
A
perpetual difficulty in the manufacture of an engine is removing all loose sand
from the casting and cleaning the cored or drilled oil ways. Examination of
bearings after running-in always showed some dirt trapped in the lead-bronze
shells. On the car engines the oil passage to the main bearings fortunately came
near to the surface of the casting in a most convenient place. A full flow
filter for bearing oil only was coupled up for all test bed running. Soon
afterwards the filter, quite small in size, remained a permanent feature of the
engine.
In
accordance with the current American practice, immediate post-war engines aimed
at a gentle tappet rotation caused by a reduction in cam width at full lift.
It
is true that most tappets did rotate but the direction of rotation was
frequently counter to that of the theory behind the shape of the cam. It seems
more likely that tappet rotation is caused by an epicyclic effect of the tappet
in its bore. The direction would depend on push-rod angularity and the vector
effect of several frictional forces. 3⅝-inch diameter bores
Without
any apparent effort engines become more reliable the longer a basic design stays
in use. Why not therefore increase the bore size a little further? It seemed
that a reasonable gasket could be made if the bores grew to 3⅝-inch diameter.
There would still be 0.4 inch of metal between adjacent pistons, 0.275 inch of
casting and twice 0.062 inch of liner. Gas torque roughness again decreased and
the gain in power was more than expected.
The introduction to the public of 3⅝-inch
bores (4.9 litres) coincided with an option to have an automatic transmission.
Early experimental samples of cars so arranged accentuated a roughness at 3300
rev/min which for a long time had been gently in the background. Suspicion soon
rested on the flywheel, which now had a rather flexible extra bearing in the
gearbox. It was soon discovered that 3300 was the natural frequency in bending
of the engine-clutch housing-gearbox unit. The exciting force was radial run out
of the flywheel, which can never be zero because of bearing clearance.
A slight
stiffening of the clutch housing raising its frequency only a few hundred
rev/min removed the sensation of roughness from the passengers. Although
difficult to explain, this phenomenon often occurs when trying to eliminate an
unpleasant vibration. The most likely explanation is that objectionable
roughnesses are due to two resonances coinciding; it is only necessary to alter
one of them. It is very probable that the flywheel vibrations which had caused
so much trouble in the early thirties were due to the flywheels own resonance
coinciding with the bending of the engine structure.
When one tackles a particular roughness one
usually becomes aware of some others of lesser magnitude. The issue is
frequently confused. In our case, at about 2900 engine rev/min,
elusive
vibration also occurred. This time it was of higher frequency and was measured
to be 8700 per minute, or three times per revolution. A brief investigation
showed that the starter motor and the induction system had natural frequencies
of about 8700 per minute with slight variation due to differences in casting
thickness.
This
might be a good moment to mention crankshaft balance. Every crankshaft up to
then had been machined all over and it seemed fair to assume that static
balancing on two knife edges was all that was required. Nitrided crankshafts are
always bowed, a maximum figure of 0.012 clock reading being acceptable.
A
crankshaft which is balanced when bowed either statically or dynamically will be
out of balance when it is straightened by crankcase bearings. A first
approximation to allow for the bow is to move the knife edges or dynamic
supports in from the end journals. If the bow is truly circular there is a
position for the knife edges which fortunately falls on journals 2 and 6, which
is very nearly right. For some years this simple procedure was adopted (Fig.
20).
Softer engine mountings and more critical
testing soon made even greater accuracy desirable. A dynamic balancer was
installed which included a centre bearing for the crankshaft. Although bow was
taken care of and great accuracy was available, the result was not good
enough. The flywheel was attached to the crankshaft during the balancing operation and
the fluid part was filled with oil; the effect of eccentric flywheel mounting
had been corrected but still some vibration remained. If road wheels and tyres
needed couple balance perhaps the flywheel needed it even more. This proved to
be the case. Each fluid flywheel assembly was balanced in two planes only a few
inches apart before being attached to the crankshaft. This series of operations
has been used ever since and seems to account for all the forces which can be
eliminated by external addition or subtraction.
Major Carter pointed out many years ago how
lucky it was that certain possible resonances were sufficiently outside the
usable speed range. The tensile pendulum of the piston stretching the connecting
rod has a frequency of about 100 000 per minute, but the torsional frequency
could be only two or three times the maximum speed of the engine. In the case of
a six-cylinder engine, excitation by the torsional oscillations of the
crankshaft might easily excite unwanted motion of the pistons and may in fact
have been responsible for broken piston ring stops when these were in vogue.
A
much more difficult calculation concerns the possible resonance of an individual
crankshaft throw restrained by its own stiffness and that of the crankcase. A
figure of the same order as that of the piston resonance seems likely.
Many years ago I was able to watch a
six-cylinder engine being motored over without its cylinder head. At high speed
all the pistons appeared to be stationary at top dead centre, but occasionally
to alter their height or to rotate a little; possibly this was due to proximity
to an unwanted resonance.
By 1955 more power and more torque were
required to engine a new body which had less drag and could be expected to go
faster. When the maximum speed of a car is raised either a higher axle ratio is
required or the engine must run faster, or, more likely, a compromise involving
both will be chosen. The short cylinder liners had for a few years been
accompanied in batches of engines by a longer liner in the lower part of the
bores. Porosity at the foundry had been an ever present problem. A change to a
full length liner seemed the obvious answer. A high-phosphorus iron and a
chromium-plated top ring were known to be an excellent combination for bore wear
and solved all running-in difficulties. Unfortunately it did not seem reasonable
to ask the foundry to do with less than 0.275 inch of casting between cylinder
bores. Without liners the cylinder size could have been increased from 3.750 to
3.875 inches, but the foundry scrap would have been excessive. The limit of bore
size at 3.750 inches had been reached while 4.150 inches separated the centres.
Incidentally a wet-liner engine cannot emulate this use of space nor, as far as
I know, can any other construction.
At
last the increase in engine output had to be achieved by skill rather than by
strength. A six-port cylinder head and slightly higher compression ratio (6.6:1)
met the immediate need. The six balance weights on the crankshaft were replaced
by four in the position invented by Mr L. H. Dawtrey in 1934. The weights
appeared on webs 2, 6, 7 and 11, and could now be forged integrally with the
crankshaft. The middle weights were exactly opposite the crankpins, the end ones
finding themselves in a rather inexplicable position except to provide static
balance (Fig. 21).
This
four-weight arrangement, which has proved most successful, is certainly not what
the mathematician would choose. The inherent hogging couple of a six throw
crankshaft would be countered by four weights on webs 1, 6, 7 and 12,
each 30
away from opposite its crankpin. By the Packard method, taking into
consideration differences in stiffness of the crankshaft in different planes,
one arrives at about 20 from opposite the end crankpins. The drop forger won
the day and Mr Dawtreys arrangement will live a long time.
I
said earlier on that the F head engine seemed very unwilling to accept a
higher compression ratio, without much effect other than an increase in engine
roughness. Each increase in bore size had given more than the expected result.
This could be due to some side effect of the increase in compression volume.
What exactly causes combustion roughness is still a mystery, but the F
head disliked a smaller throat area for the outgoing gases. The combustion space
throughout the years developed as shown in Fig. 22, the throat area being no
smaller, although the compression ratio had reached 8:1. An enormous inlet valve
of 2.150-inch diameter completed the picture, the engine giving 178 hp and 135
maximum m.e.p. when production of the motor car six-cylinder engine stopped in
1959, compared to 51 hp and 89 m.e.p. in 1922 and 132 hp and 122 m.e.p. in 1946
(Fig. 23).
The reliability at this final
power was always being improved because of the military parallel development,
mostly on the straight eight engine which had also been bored out to 3.750
inches. This engine was known as the B81 and one particular requirement was a
continuous run at maximum power of 168 hours. No stop of any kind could be
tolerated in spite of fuel containing 3.6 cc lead per gallon. Brightray exhaust
valves and inserted exhaust valve seats were required in addition to
considerable development of the pistons before a week's continuous running could
be achieved. On lead-free fuel the same combination of pieces would run for 700
hours, this, regrettably, being only of academic interest.
An
opportunity now arose to fit a B81 engine in a small track-laying vehicle for
the West German Army. At least 235 hp was specified and this entailed an
eight-port cylinder head and a very large dual downdraught carburettor, the
whole engine having to run under water. The engine now found itself in an
unusually hot compartment, the cooling system having three inter-coolers in
series with the radiator.
The Germans expect the worst to happen and, although the cooling system was pressurized to 10 lb/in2, insisted on
a test of 100 hours at maximum power with the coolant outlet at 1000C
unpressurized. Twenty-two hundred hours of development running on test beds were
needed before the Bundeswehr could be told that a feat never before performed in
the history of automotive engineering had been accomplished. The feat was
satisfactorily repeated at Aachen on three engines chosen at random in Germany. Before the demand that the engine should
run boiling but unpressurized, gasket failures and stretch of inlet valves were
occurring everywhere except at Crewe. Failures were reported from prototype
vehicles, and every attempt to run a type test in Germany with coolant at a mere
800C was unsuccessful. The knowledge that about forty type tests had
been successful at the factory was comforting but apparently not helpful. How
true it is that one designs engines to survive a gruelling on a particular test
bed without realizing the tens of years of improvement to test beds which has
unwittingly gone on all the time.
An entire vehicle was coupled to a
dynamometer, the cooling-fan noise being audible for miles, and failure now took
place under the noses of the development engineers. It is the lot of such people
usually to have to diagnose the cause of a failure from the tangled remains and
they consider themselves very lucky ever to see by accident the start of a
trouble. Pre-ignition seemed the most probable culprit; auto-ignition was
demonstrated by removing a plug lead after some fifty hours running of a new
engine or a new cylinder head. Thermocouples were daintily inserted as near as
possible to the surface of the combustion space near to the sparking plug. They
read almost uniformly in all eight cylinders at 1750C. But after 40
or so hours running they started to rise, and soon reached 3500C.
On removing a cylinder head a small trace of oil was noticed in the coolant.
Cleaning the water passages of the head with detergent brought the temperatures
back to normal. The heat exchangers in the coolant were rightly suspected.
Although a type test under vehicle conditions was not possible, victory was
short-lived.
Engines
still would not run on a test bed at Aachen, almost identical failures occurring
when the coolant was not contaminated. It was really this difference between
Aachen and Crewe that created the unusual demand that the engines must run
boiling. On the Aachen test beds steam was shown to form in the cylinder head
when the measured coolant outlet temperature was 800C. Fortunately
the test at 1000C had to be done with a vehicle installation and
development of a German test bed was side-stepped.
The
engine was mounted back to front in the vehicle, the radiator being alongside
and very little higher. A book could be written about the day and night work
which followed. Suffice it to say that by bleeding off one gallon a minute of
the coolant pump flow of thirty gallons a minute (all of which normally entered
the gallery beneath the exhaust valves) and sending this one gallon directly
into the cylinder head; by moving the engine outlet to the rear end;
by
obtaining reliable heat exchangers and, lastly, by altering the coolant system
so that the make-up water was fed to a Venturi just before the pump inlet,
putting an end to aeration on any gradient, 170 hours at maximum power could be
repeated at will. Double this figure was reached by stelliting the inlet valves, a change that was arrived at after pouring oil into half the carburettor
supplying four of the cylinders.
Until every improvement was included
success was not achieved and only later was it discovered that at Aachen a
restriction in the test bed coolant system had halved the flow and that the
engine thermostat had been removed, thereby throwing to the winds a valuable 5
lb/in2 pressure with its higher boiling point. A little moral that
can be drawn from this story is always to put the recording thermometer at the
hottest part of a cylinder head, a practice which had been forgotten since 1937. A final attempt
for power
While
some strive to overcome troubles, others are looking to the future. What could
be reached using the now available 100 octane fuel? A six-cylinder head of 10:1
compression ratio produced 205 hp with two carburettors and 215 hp with petrol
injection. The eight-cylinder engine reached 275 hp with petrol injection and
268 hp with three S.U. carburettors on 80 octane (motor) fuel. These figures may
not be startling by racing or even sports standards, but their progress has
always been constrained by an ability to idle smoothly at 400 rev/min and a
desire for the maximum possible torque at 1000 rev/min; the latter remained
essential even with an automatic transmission. When it was first decided to fit
two separate carburettors on a Rolls-Royce car there was some concern about the
possibility of a reliable tick-over. The problem was as usual tackled by an
appreciation of the niceties of the mechanics involved. The carburettors would
get further apart as the engine warmed up and their throttle spindles would not
be exactly in line. An accurate relative adjustment was required between them.
This adjustable coupling would inevitably have either slack or flexibility or
both. An initial approach was to have slack in the two intervening couplings,
each carburettor having its own throttle stop. With separate throttle springs
this provides a consistent tick-over, but a poor and inconsistent behaviour at
very small throttle openings. A better arrangement, used for some years,
employed a single throttle stop on one of the carburettors, the spring and
operating lever being at the extreme other end of the spindle assembly. Both
universals now always remained wound up in one direction. Couplings with slack
became outmoded and a steel plate with endwise flexibility replaced them. The
best arrangement now was to have once again a stop on only one carburettor,
the
operating lever and the spring being as near to it as possible. Finally, an
increase in radius of the throttle stop from the rather usual 1 inch to about 3
inches made a Rolls-Royce job.
So
ended the development of engines having 4.150 cylinder centres, 4 inches
stroke and a gudgeon pin of only 0.750-inch diameter (Figs 24, 25, 26 and 27).
An
experiment in size and weight
About 1952 a chance remark in America
suggested that these aluminium engines would work just as well if the dry liners
were removed. An old War Office engine was extracted from the store,
the liners
were removed and 3⅝-inch pistons were fitted at their usual running
clearance. This engine survived a test-bed gruelling and a year or so in a car
and then, like its predecessors, was forgotten.
Also just after the war some engines had
been made of 2.875-inch bore and 3.125-inch stroke, again using dry liners in an
aluminium crankcase. Considerable knowledge existed of dry-liner engines and
only the opportunity to use it seemed lacking.
A projected special military vehicle,
which
unfortunately did not materialize, needed a lighter engine of less overall
height. It seemed plausible to suggest that a B61 of shorter stroke and of all-aluminium
construction would fulfil the requirements. In 1949 a stroke of 3.9 inches had
been tried as an exercise in smoothness, but one of 3.6 inches was necessary to
meet the installation limitations. This figure would also enable some current V8
parts to be used.
An experimental engine was made using 3-inch
diameter pistons running in dry liners and a 2.15-inch diameter inlet valve. The
side exhaust valve in an aluminium block had not in the past given trouble,
although some new thermal problems could be expected. The cylinder centres
remained unaltered at 4.150 inches.
The
design of the cylinder head showed how a compression ratio of 11:1 could be
obtained in spite of the shorter stroke and it was argued that a reduction in
stroke in the ratio of 5:4 could be countered by running the engine 5/4 times
faster. An experimental engine of 7.8:1 compression ratio made mostly from
existing pieces proved the truth of this prediction, 190 hp being obtained after
a small amount of work on the induction system. CONCLUSION
An
organization which makes entire engines does not make, design or develop many of
the constituent or attendant pieces. It considers itself fortunate when
extramural engineering keeps pace with internal requirement. The extramural
engineering is shared by other engine makers and only some outstanding and novel
departure from the orthodox would leave it behind or find it wanting. Throughout
the forty years of this history, fuels, lubricants and metallurgy have kept pace
with m.e.p.s and rev/min. This single sentence can be written because
thousands of people have spent millions of pounds making it so. Only Youngs
modulus has resisted change and one can merely dream of what one would do if it
were doubled.
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The Motorcars
Bentley Motors Crewe
Rolls-Royce Motor Car Engines
