The Sentinel Scores on
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Accessibilit and Performance
AFTER spending a day in trying to find a weakness in the Sentinel 7-8-ton chassis with horizontal oil engine, I was forced to conclude that the vehicle scores at all points for maintenance accessibility, general performance and driving comfort. The model I tested, a drop-sided lorry, had covered_ over 18,000 miles, but showed no sign of chassis wear or looseness of the body
A high grade of craftsmanship is apparent in the assembly and finish of the vehicle, which attains pre-war standards and should be free from teething troubles. With the engine suspended in the chassis in a horizontal position and immediately behind the cab, it follows that there are many unusual features of design. Balanced loading between the axles is possible, and any weighbridge observations showed that the ratio of loading was precisely one-third load on the front and two-thirds on the rear axle.
Ricardo Comet Heads
The four-cylindered oil engine is fitted with twin heads incorporating the Ricardo Comet Mark III combustion system, which is noteworthy for performance. With a bore of
ins, and a stroke of 51 ins., the engine develops 90 b.h.p. at the governed speed of 2,000 r.p.m. Crankcase and cylinder block form a monobloc iron casting, and Austenitic cast-iron liners are fitted, the inner surface of which is plated with a porous chrome deposit.
The crankshaft is retained by five large main bearings, the bearing caps being located in position by ferrules and retained by two studs on all bearings except the centre and rear, which have fciur studs. Simi 134 la rl y, the connecting-rod big-end bearings are secured by four bolts and all bearings are of lead-bronze steel-strip-shell pattern.
The engine is housed well within the frame line, protected against possible damage which might be caused in a collision. All component parts can be dismantled without removing the engine from the chassis. Normal maintenance is speeded up by easy access to the overhead valve gear and other parts.
Unrestricted in access to the engine, a fitter can sit on his toolbox at the side of the vehicle and adjust the valve clearances, change injectors, or remove the heads and main bearing cover to withdraw the pistons and all main bearings.
The fuel-injection pump is driven through bevel gears, and a spur reduction gear which by means of a duplex chain also operates the camshaft. The water pump is driven direct from the crankshaft through bevel gearing. The drive assembly is encased in an aluminium cover which carries the transversely mounted fuel-injection pump. A standard C.A.V. centrifugally governed fuel pump with pintle injectors is employed. •
Operating at a pressure of 60 lb. per sq. En., the gear-type lubrication pump passes oil through a full-flow fabric filter to the connecting-rod and main bearings, Camshaft bearings and rocker gear are supplied with oil at a pressure of 10 lb, per
sq. in., the pistons being splash lubricated from the big-ends..
The filter is accessibly mounted on the near side on the crankcase main bearing cover, in the vertical ,position. Oil filler and dip-stick guide form a single casting, which. is also located on the near side of the power unit.
The starter motor is conveniently mounted cri top of the engine, and may be reached or removed through a trap in th:: floorboards of the body. A combined air filter and silencer is bracketed from the off-side side frame member. Radiator, fan and dynamo are housed towards the front of the chassis, the fan and dynamo being driven through an oilsubmerged chain drive which is driven by a front end extension of the crankshaft.
The heavy-duty dry-plate clutch is placed between the engine and gearbox, which are bolted together • to form a unit. The hydraulic pump of the brake .system is bolted to the rear of the gearbox and is driven from the output
• shaft. By this means, although the engine may be stalled, brake servo pressure is always available while the chassis is moving.
A single-piece propeller shaft equipped with Hardy-Spicer needleroller bearings carries, the transmission to the overhead worm drive of the fully floating rear axle. The axle casing is formed from an alloy-steel casting and, like the driving shafts, is of generous proportions_
With many special features, the
cab is an all-steel spot-welded jigassembled structure, insulated from the frame by flexible mountings. Both doors slide on runners, thus reducing the space required for parking, and with no engine in the cab, a full-width seat is provided The fan tunnel bulges up in the centre of the floor boards, but the cab is spacious and excellent visibility is afforded.
With an 8-ton payload of iron
ingots, spare wheel, tools, crew and an extra 10 gallons of fuel in cans, the weighbridge showed our, running weight to be 12 tons 9 cwt. Although the load had been dropped on at
random, the distributed weights proved that the Sentinel could produce perfect balance in loading between the axles.
As it was a cold day and the atmospheric temperature Was in the
region of 35 degrees F., we started our journey towards Much-Wenlock with the radiator blind covering over half the cooling area. This appeared ideal for normal running and the radiator temperature remained at 176-178 degrees F.
With the blind in this position we made an assault on Harley Hill, a mile gradient of 1 in 14, with a section of 1 in 6i towards the end. This was too severe a test for the radiator and it began to boil on the last stage of the climb.
Descending the hill for a second run, the blind was lowered to expose two-thirds of the cooling area. On this attempt the temperature rose by 15 degrees F. during the mile climb, the final reading being 169 degrees F. The hill was negotiated without difficulty and on a further climb a stopstart test was made on the 1-in-6i gradient, using low gear. Adjusting the blind to the original position, we continued on our way to the village of Much Wenlock.
During this outward journey experimented with the cab heating and Ventilating sysam and found that a temperature range from atmospheric to 65 degrees F. could be maintained. Thus the Sentinel assures not only a quiet and fumefree journey, but also provides a reasonable temperature in the cab.
I drove the Sentinel on the return journey. Descending Harley Hill, the speed was restricted to 30 m.p.h. by using the brakes—not a procedure to be generally advocated, but one which provided me with an opportunity of testing the brakes under adverse conditions. As may be judged, the drums were well heated up by the time we reached the foot of the hill, but there was remarkably little fade in their efficiency.
B6
After this test, we changed over from the main tank to run on the fuel from our calibrated measure. The fuel-consumption route was planned over an eight-mile out-andreturn journey on the main Shrewsbury road. This test was run off without hindrance from traffic or other delays and a quick return turn was made at the end of the outward journey.
At the end of the test it was found that 5.4 pints of oil fuel had been used, which corresponds to 11.85 m.p.g.—a very reasonable consumption for the load carried and high average speed maintained throughout the test. With the maximum speed restricted to 35 m.p.h., the average speed worked out to 30.5 m.p.h.
Cab Comfort By this time I had had a fair opportunity of judging the Sentinel and without question I would say that the cab is one of the most pleasant in which to travel, because of the lack of engine noise or vibration. Although this particular model had been through some stringent cross-country running during its short career, there was no trace of body movement or rattle from any point in the chassis.
We returned to the "mile stretch" on the Shrewsbury-Wenlock road, which is noted in the area for its smooth and level surface. This proved to be suitable for both acceleration and braking tests. By this time the brake drums had cooled to a reasonable temperature for testing and consisteot stopping distances were obtained. From 20 m.p.h. the Sentinel was brought to rest in 23 ft. (58 per cent. efficiency), and from 30 m.p.h. in 62 ft. (49 per cent. efficiency).
This was the first time I had tested a fully laden vehicle equipped with the hydraulic servo braking system. Judged by conventional air-operated servo systems, the brake effort appeared to be heavy, but continued use of the brakes dispersed any idea that the hydraulic system might be below standard. In subsequent driving in the congested thorough
fares of Shrewsbury, I soon gained confidence in the system and found the braking equal to all occasions.
With a head wind from the east, acceleration figures varied up to 5 secs. in each direction, but from an average of the results it was ascertained that from 0 to 20 m.p.h. and 30 m.p.h., through the gears, could be attained in 19.2 and 41 secs. respectively.
During top-gear acceleration, which had been more greatly affected by the head wind, 20 m.p.h. and 30 m.p.h. were reached from 10 m.p.h. in 17.5 and 38.5 secs. respectively. I had expected to find considerable transmission rattle in the 10-15 m.p.h. range with a fourcylindered power unit and using direct drive, but the Sentinel appeared to be worthy of the occasion and I was surprised to find no complaint from engine or transmission. The ascent of Harley Hill on a straight run did not deter the Sentinel, which proved that hill-climbing ability was one of its main features. With the radiator blind in a half-raked position, the radiator temperatures were normal.
Our return journey to Shrewsbury was accomplished at a fair average speed, and in travelling through the narrow congested streets of the town, the good driving visibility and light steering were particularly welcome.
Temperature readings were taken on arrival at the works. With an atmospheric reading of 36 degrees F., the engine-oil temperature was 150 degrees F., gearbox 108 degrees F., and back axle 140 degrees F., proving that the components were suitably ventilated for operation in this country.
Much may be said for the design of the comfortable cab. Another feature that should appeal to the driver is the large tool locker which is located beneath the driving seat and is protected from theft when the cab doors are locked. The sliding doors provide further means for ventilation in hot weather.