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A Mechanica torse on a Diet

25th January 1957
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Page 42, 25th January 1957 — A Mechanica torse on a Diet
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Scammell Scarab 6-tanner with Perkins P.4 Oil Engine Shows 25 Per Cent. Improvement in Fuel Consumption on Local Deliveries

By John F. Moon,


ALTHOUGH it has frequently been found that by converting petrol-engined vehicle to run with an oil engine loss of performance has accompanied the fuel economy gained, this is not the case with the Scammell Scarab 6-ton mechanical horse.

Tests recently carried out show that the performance of the oilengined vehicle is almost identical with that of the petrol-engined version in respect of maximum speed and acceleration, whilst the braking performance is, in fact, slightly

better. The fuel-consumption rate has been improved by over 73 per cent. when running non-stop, and a.125-per cent, gain is shown under simulated delivery conditions.

The Scammell Scarab 6-ton prime mover was first announced as an oiler at the Commercial Motor Show last September. The engine chosen was a Perkins P.4 four-cytindered unit rated to develop 55 b.h.p. gross at 2,400 r.p.m., with a gross torque output of 134 lb.-ft, at 1,350 r.p.m.

The Scarnmell 2-litre fourcylindered petrol engine normally fitted develops 45 b.h.p. at 3,200 r.p.m., whilst the maximum torque output (net) is 93 lb.-ft. at 1,800 r.p.m.

As the engines have generally similar dimensions, the conversion was reasonably simple. Indeed, the only major mechanical alteration to the specification has been the use of a higher-ratio rear axle (75 to 1), as opposed to the standard petrolengine ratio of 11.23 to I.

Other modifications concern the position of the 14-gal. fuel tank. This is now located on the left of the chassis frame to reduce the length of the fuel lines, The vacuum reservoir of the braking system is on the right of the frame where, incidentally, it is close to the engine-driven exhauster.

The primary features of the Scammell Scarab design are its small overall dimensions and excellent turning circle. It has an incomparable degree of manceuvrability, the driving seat can easily be reached from each side of the cab, and engine accessibility is excellent. Moreover, the complete engine-gearbox, rearaxle unit can' be withdrawn from the chassis without difficulty.

The pressed-steel channel-section frame is sharply swept down between the front wheel and the rear axle to give a low cab floor line. The front wheel suspension consists of two oil-damped concentric coil springs, and the steering is by skew gears.

A substantial rubber.bushed bearing, which is secured by a single bolt, supports the engine and transmission unit at its forward end. At the rear the road springs provide the

location. In effect, therefore, the front mounting absorbs all torque stresses and the complete assembly becomes one substantial torque tube.

Withdrawal of the engine and transmission entails disconnecting the control linkages, and .removal of the front-mounting bolt and the four security bolts at the spring-slipper brackets. Two men can remove the complete unit in under 1-1 hours.

A 10-in.-diameter dutch is used Awn the Perkins engine, and the gearbox is a four-speed constant-mesh' component with remote gearchange mechanism actuated by a lever to the right of the driver's seat. -The double-reduction 'rear axle has spiral-bevel primary reduction and

spur-gear secondary reduction.

Direct 'mechanical braking from the pedal is applied to the driving wheels, which have two-leading-shoe units (the front wheel is unbraked). Incorporated in the system is a vacuum reaction valve which controls a Clayton Dewandre vacuum servo for actuation of the semitrailer brakes. The vehicle chosen for test was the left-hand-drive unit which had previously been, exhibited on the Scammell stand at last year's Show. Its sole difference from the righthand-drive home version, other than the position of the controls, was the provision of twin head lamps, and export-type dual-rate rear springs,

The prime mover was tested in conjunction with a Scammell M.H. 6-ton 15-ft. semi-trailer with platform body, and a test load weighing 6 tons 1 cwt. had been added. A weighbridge check showed that with driver and passenger aboard, the outfit was running at a gross weight of 9 toils 6 cwt. The driving wheels were carry

ing 3 tons 19 cwt., whilst the front wheel had a 1-ton loading.

This made for light steering without loss of control on wet roads and foretold a long service life for the 8.25 x 10-in, front tyre. The oilengined tractor alone has a kerb weight of I ton 171 cwt., which is approximately 5 cwt. heavier than the petrol-engined version.

The Scarab was driven out to Batchworth Hill, Rickmansworth, which is a climb of just under half a mile with an average gradient of 1 in 10. The climb, which was started from rest, was made in an ambient temperature of 57° F. and occupied 4 minutes 55 seconds. Starting off in second gear, a quick change to third was just possible before the main part of the incline was reached, when it became necessary to drop into low ratio. Three and three-quarter minutes were spent in this gear at full throttle and the road speed was 3 m.p.h.

At the top of the climb a check showed that the coolant temperature had risen by only 1° F., the final temperature being 153° F., but this is not surprising, considering the fan speed during the ascent. I decided, therefore, to take the outfit to Harrow on the Hill, where there is a hill of less severe gradient which would allow the use of second gear.

Peterborough Road, which is about a third of a mile long with a general gradient . of 1 in 12, was chosen, and a second-gear climb was . made in 2 minutes 10 seconds. The engine pulled extremely well in this gear at low speed, and the climb produced a temperature rise of 8° F.

The engine thermostat is set to open at 165' F., so it becomes obvious that the coaling arrangements for this power unit are, if anything, on the generous side, and overheating would be unlikely to occur in normal service, certainly in this country. A blankingstrip for the air intake is provided in the tool kit for use in cold weather.

A descent of Peterborough Hill was then made in neutral, the brakes being applied to restrict the road speed to 20 m.p.h. After 11 minutes, a " crash " stop from 20 m.p.h. produced a Tapley meter reading of

40 per cent., which showed a 13-percent. drop in efficiency when compared with the braking figures taken later in the day: Substantial pedal effort was required to produce this figure, but the pedal is rather. heavy under normal conditions when braking hard.

West Street, Harrow, which has a maximum gradient of 1 in 51, was the scene of the gradient stop-start test. The outfit was stopped on this slope and as a precautionary measure I applied both hand brakes. I then found it was possible to release the trailer hand brake, which speaks well for the efficiency of the driving-axle brake units.

Smooth Starts Several smooth, low-gear re-starts using half-throttle were then made and I would imagine that it would require a gradient of at least 4 to make a re-start impossible when fully laden.

Acceleration and braking figures were obtained along a level quartermile stretch of the North Orbital Road, near Hatfield. The 0-30 m.p.h. acceleration runs were made using second, third and tbp gears, and the average 'figures obtained show Little variation from those recorded in 1948 with the petrolengined model, which was running 5 cwt. lighter.

A similar state of affairs was brought to light when making the direct-drive tests. These figures testify to the success of the Scammell engineers in choosing a suitable axle ratio to produce similar "figures for the petroland oil-engined chassis.

Maximum of 32 m.p.h.

During these tests I noted that the maximum speed in top gear was 32 m.p.h., whilst the governed speeds in second and third were 9 m.p.h. and 18 m.p.h., respectively. When making the direct-drive tests, the unit pulled away smoothly from below 8 m.p.h.; showing more than adequate flexibility for delivery work in congested traffic.

The braking figures obtained on the dry asphalt surface indicated a slight improvement on those achieved eight years ago, but this would be because of the greater vacuum power obtainable from the exhauster, compared with that derived from the inlet manifold

of the .petrol-engined model. The outfit remained perfectly. stable when making these tests and no wheellocking occurred at any time.

A section of the North Orbital Road between the Al and AS was employed for the fuel-consutription tests. This out-and-returrr circuit of five miles is slightly undulating and reasonably traffic-free, and it was possible to conduct the non-stop runs at the maximum governed speed.

When making the stops-per-mile tests, the outfit was halted for 15 seconds on each occasion with the engine ticking over. The average speeds recorded are based on the time that the wheels were turning..

• No Fade Because of the relatively slow. acceleration rate, the outfit Was accelerating almost all the time between stops when making four stops per mile and only on down grades was it possible to exceed 25 m.p.h. At the end 'of this test the brakes smelt and felt quite hot, but fade was not noticeable. The engine temperature remained constant at 154° F. throughout this series of runs.

For the unladen test, the semitrailer was detached. The progressive rear springing (export specification), which incorporates helper. leaves, gave a smooth ride, which not only makes life more comfortable for the driver but reduces the wear on the running components.

The outfit handled extremely well during all these tests and the only criticism of its design concerns the slight vibration and noise produced by the engine and transmission, As this is a short-distance vehicle, the criticis.rt is not important.

Exceptional Manceuvrability Other than a rather heavy brake pedal, the controls are light in operation. The general degree of visibility is good and the high manoeuvrability puts the Scarab on a plane by itself.

Back at the Scammell works I conducted a few maintenance tests. The first was to check the water and engine-oil levels. This can be done without removing the engine cowl, and occupied 15 seconds and 21 seconds, respectively.

To check the gearbox level, the breather-tube assembly is removed and the engine dipstick is inserted, the correct level being 24 in. from the bottom of the box. This operation was carried out in 41 seconds.

Although in unit with the gearbox, the rear axle has a separate lubrication system. and a combined filler and level plug is located at the rear of .the differential pot. The oil level in the axle was checked in 30 seconds.

The large oil-bath air filter, which has a centrifugal pre-filter, is outside the right-hand frame member, and the 61 bowl is secured by three spring clips. Checking the oil level in this cleaner took only 30 seconds.

Four wing nuts secured the engine cowl, which was removed, singlehanded, in 18 seconds. One cannot fail to be amazed at the accessibility of the engine with the cowl removed.

I removed No. 4 injector element without any difficulty in 50 seconds and replaced it in 48 seconds. The complete .set could have been changed in under 7 minutes.

The fuel filter is equally easy to reach and its element was removed in 24 seconds arid replaced in 38 seconds.

then bled the fuel system at the injection, pump in 33 seconds. The use of plastics fuel lines between the tank, the lift pump, the filter and the injection pump makes it easy to see whether there is any air left in the system on occasions such as this Removing the rocker cover, which is secured by two nuts, took 28 seconds, and the tappets were adjusted in 4 minutes. Provision is made for the use of a starting handle and the engagement point is through a hole in the bodywork ahead of the right-hand door. The rocker cover was replaced in 50 seconds, and the engine cowl in 22 seconds.

As two final checks, the driving

axle brakes were adjusted in 65 seconds. This extremely short time is achieved by jacking up the axle centrally and having an assistant to rock the vehicle to lift each rear wheel in turn, the chassis pivoting .about its front wheel and the lifting jack.

The level of the electrolyte in the battery cells was then checked in minutes. The batteries are located in the cab between the seats, being secured by a wooden cover provided with two wing nuts.

Excellent Tool Kit There can be few vehicles easier to service than the Scammell Scarab. A comprehensive tool kit is supplied and, together with the driver's handbook and workshop maintenance manual, there should be no excuse, accidents barred, for any Scarab to be off the road for longer than a few hours.

The extra purchase price which has to be paid for the economy given by the oil-engined Scarab is not high. The basic increase in price is £160, and in Great Britain there is an additional £40 purchase tax to pay. An oil-engined version of the Scarab 3-ton prime mover is not available, and the petrol-engined version of the 6-tonner will remain in production.


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