Speed, Quietness and Economy
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Sentinel Passenger Chassis has Direct. injection Oil Engine, Combined withWellchosen Gear Ratios
By
Laurence J. Cotton
THE latest version of the Sentinel coach chassis, as opposed to its predecessor, has a separate frame, a direct-injection oil engine and a triple-servo vacuum braking system. With long springs and shock absorbers at both axles it rides smoothly, whilst its power unit is practically devoid of the characteristic oil
engine noise. On test it put up fast times on the fuel-consumption trial, with reasonable economy, and climbed some of the steepest gradients in Denbigh without taxing the engine.
Conforming to the major overall dimensions permitted for coach operation in this country, the Sentinel has a 15-ft. 7-in, wheelbase with a 6-ft. 51-in. front overhang, and measures 7 ft. 51 in. over the rear wheels when equipped with 10.00-20-in. tyres. As a chassis it weighs 4 tons 13 cwt. but with a Plaxton 41-seat coach body, and carrying a spare wheel and other equipment, the unladen weight is about 7 tons 14 cwt. In bus service the spare wheel is mounted at the rear, but where a coach body is specified it is mounted forward so as to provide the maximum luggage-locker space. The frame is parallel, and apart from a slight slope on the top flange at the front, is level from front to rear.
Horizontal Engine
The engine is carried horizontally amidships inside the main frame members on a suspension having three Metalastik rubber-bonded units at the front and two similar units at the rear. In layout, the components are sited identically with the former indirect-injection engine, the fuel injection pump, with feed pump attached, being mounted transversely at the front where it is in an accessible position for rapid servicing. A vacuum-servo motor is an additional unit and this is mounted at the front and operated by a belt-and-pulley drive from the crankshaft.
A separate clutch-housing casting is used to link the gearbox and engine as a unit. The Meadows five-speed overdrive-top unit which is employed, has direct gearto-gear engagement. Drivers who have been accustomed to constant-mesh or syncromesh gearboxes might
find slight difficulty initially, but little experience is required to make a noiseless change. There is a single Hardy-Spicer propeller shaft connecting the gearbox to the rear axle which has a central overhead-worm drive of fl-in, centres. \
In the change-over to a vacuum braking system the main servo motor for rear brake operation, is outrigged on the frame behind the off-side front wheel with the reservoir attached on the opposite side. An additional brake cross-shaft has been introduced to bring the separate hand and foot linkage within legal requirements for public service vehicle operation.
No Overloading A bare chassis, with ballast, to represent a 41-seat coach carrying a full complement of passengers and an allowance of 10 cwt. for luggage, was provided for the test. The distribution of weight, with 41 tons on the front axle, indicated that the 14-ply-rating tyres. were not overloaded and gave promise of light steering.
. Negotiating the sharp turns in Shrewsbury to reach the by-pass confirmed this premise, and before reaching the outskirts of the town I had mastered the gear change. After covering seven miles on the Llangollen road, a stop was made to connect the fuel test tank direct to the injection pump, any fuel leakage from the injector nozzles being returned to the main tank.
The Sentinel was driven at normal coach speed for 12 miles, which constituted the first leg of the consumption-trial route. With adequate power and good low-speed torque characteristics of the engine, there was little need to use direct drive or third gear and the course was covered at 35.5 m.p.h. average speed. The fuel consumption rate was 14.25 m.p.g. Obviously, the direct-injection unit is more efficient than the engine formerly employed, which, in addition to the improved fuel-consumption figures, was noticeable in the heat dispersed to the cooling water.
Then followed the climb to Chirk, which was negotiated in third gear. In the drive to Llangollen I sat above the engine, where, when cruising at 30-45 m.p.h., the sound level from the fan at the front of the
chassis was more noticeable than the induction or ignition noise of the power unit. 1 consider the Sentinel direct-injection engine to be one of the quietest running oil engines produced in this country.
The engine suspension is also effective, and whether idling or running at high speed there was practically no vibration to be discerned at any part of the chassis frame. In its general handling I liked the Sentinel, the steering was light and positive, the clutch was relatively light and the brake-pedal effort about average.
Because of the high cruising speed the Horseshoe Pass was reached in advance of schedule, and a brief halt was made at the foot to let the photographer go ahead to find a vantage point. Up to this point the Sentinel had kept_ close on the tail of his car. Temperatures before starling the climb were, ambient. 45 F., engine oil 138° F. and the radiator top-tank 150° F.
Then came the long and winding ascent with continued use of second and third gears, and a brief spell of the lowest ratio when rounding the 1-in-5 bend just above the Britannia Inn. The distance from start to finish of the climb was 4.2 miles, which took slightly over 14 min., but there was no appreciable rise in operating temperatures. The Pass is shielded against wind, but at the top there was a stiff cold breeze of near freezing point, and snow was heaped to a depth of about 4 ft alongside the road. The oil and water temperatures
had risen by 8' and 6° F. respectively, and
the oil in the axle registered 178° F.
The works representative drove on the return to Llangollen, and I was apprehensive when he engaged overdrive and used the brakes for the descent. The smell of brake-facing material was soon noticeable, and by the time four miles had been covered heavy smoke was belching from the drums.' Although the pedal travel had increased, the brakes provided sufficient retardation effort to check or stop the vehicle in an emergency.
On the return leg of the fuel consumption test route there were slight checks due to traffic, but as the territory was generally more favourable the average speed was not impaired. There was only 5 sec. difference between the times of the two runs, the fuel consumption on the return journey being at the rate of 16.8 m.p.g.. making an overall average of 15.5 m.p.g.
The stretch of road used for the o5
acceleration and braking tests was level, but sufficient in length to permit of speeds exceeding 40 m.p.h. Using Atcond, third, fourth and overdrive gears, it required 23:4 sec. to reach 30 m.p.h. from rest and 42.5 sec. to attain. 40 m.p.h. If a driver prefers to use the full tiYiAtie%range of the engine the Sentinel will accelerate, lir xlireet drive, from 10-30 m.p.h. in 32 sec.
Appreciated the combination of gear ratios and available engine power in the test vehicle because there 'as no occasion during 100 miles running, when the eligine could not be used to full advantage. It is not unusual to find, in a power unit governed to 1,800 r.p.m., that there are occasions where one gear is too low, and the next too high, to meet the demands on a long climb. Again, from the fast acceleration rate it is obvious that the ratios are well:chosen.
The wheels did not lock on full brake-aedal pressure from 30 m.p.h., but the marking at the liar was more distinct. In underfloor-engined chassis, where perhaps 45% of the total weight is imposed on the front axle, it is not uncommon, where a triple-servo braking system is employed, for the rear wheels to lock, and the front ones to run free. With 41% of the load on the front axle the Sentinel escapes such criticism giving as it does a smooth and straight deceleration path under
06 emergency brake-application conditions. Stopping distances of 53 to 57 ft. were recorded from 30 m.p.h. with a 150 lb. pedal pressure.
Concerning hand-brake efficiency, a matter of considerable importance in Ministry of Transport tests, I have found, as a rough guide, that a Tapley reading of 20% on a passenger vehicle carrying a load of 2f tons, corresponds to a 30% reading with the vehicle unladen. The latter is the efficiency normally expected by the Ministry's examiners with an unladen vehicle. The Sentinel was carrying a load of 3 tons 1 cwt., in addition to the chassis, body and crew, and Tapley readings recorded during my trials ranged from 21 to 22%. In view of the way the brakes were abused when descending the Horseshoe Pass before these tests were made, with correctly adjusted shoes, there should be no difficulty in meeting the requirements of the certifying officers.
• Were I deputed to be a driver of this Sentinel model I would take kindly to it for its ease of handling and its ability to get off the mark quickly, and with an overdrive for cruising above 30 m.p.h. it is almost effortless to drive. The suspension gives "magic carpet" travel over all but the most rutted road surfaces, and it corners without squeal or steering difficulties.