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25th July 1952, Page 46
25th July 1952
Page 46
Page 47
Page 48
Page 49
Page 46, 25th July 1952 — MONEY SAVED AT EVERY MILE
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By Laurence J. Cotton,


FROM • the fuel-consumption returns obtained during a test of a Bedford-Scan-men articulated outfit converted to oil-engine operation, 1 estimate that the cost of modification Would be recovered in 30,000 to 40,000 miles: and. that afterwards there would be a saving of about 41d. a mile in fuel cost alone. The Arlington Motor

Co.,' . Ltd:, 'Ponders End, Enfield, Middx., has pioneered the conversion of ;Bedford S-type chassis to oil-engine operation. The cost of conversion is approxirnately E700, the petrol engine remaining the property of the. operator. • , By the Arlington conversion the former power unit in any of the Bedford • ,S range is replaced by the Leyland Comet direction-injection oil engine, which is installed complete with clutch unit and snubber mounting arrangements and the rear engine cross-member is modified to suit the chassis frame. An,auxiliary battery is added for 24-volt starting equipment and alterations are made to the exhaust system, clutch housing and second frame cross-member to accommodate the Bedford standard silencer, gearbox and selector tUrret. A brake vacuum reservoir is added to the medium and long-wheelbase models, but the tractor braking system is unchanged.

In outward appearance there is no indication that an alternative power unit has been installed and in the cab there is only a slight bulge revealing the position of the exhaust manifold and fuel-injection pump. There is a slight difference in torque between the original engine and the Comet power unit, which develops 20 lb.-ft.. more, its maximum net output being 240 lb.-ft. at 1,300 r.p.m. • There is also a difference in the torque characteristics, the curve of the oil engine being comparatively

flat throughout the governed range, ‘vhich is restricted at the top to 2.200 r.p.m. Secatise of the higher speed range afforded-by the petrol engine, the gearbox. ratios. are much more widely spaced than those associated

ith most oil engines, wheie it' is customary to employ a five-speed gearbox and possibly a two-speed axle in a tractor of moderate engine capacity ,hauling a 10-ton payload.

When considering the performance put up by the converted outfit, which as a tractor had an especially low power-weight ratio, an expensive multi-speed transmission system does not appear to be justified. The acceleration rate with the Bedford four-speed synchromesh gearbox, having moderately spaced ratios, was reasonable, and although heavy traffic was encountered on the road, the only vehicles overtaking the Bedford-Comet conversion during the consumption tests were empty or lightly loaded 5-tonners and the like.

Conditions governing the trials of the outfit differed from usual

because the tractor and semi-trailer were the property of the operator and both units had covered appreciable mileages-11,000 in the ease of the prime mover and much more by the semi-trailer.

No Preparation

Apart from the engine change, no other maintenance or servicing had been done to bring the outfit up to peak performance, therefore the retardation, for example, was probably below the standard for a . new tractor and semi-trailer. The Bedford articulated tractor has a straightforward hydraulically operated braking system, whereas the semi-trailer brakes are applied through a vacuum servo and are arranged to work slightly in advance to prevent jack-knifing.

When I collected the Bedford at Ponders End, it had covered 40 miles since the oil engine was installed, and during the trials the frictional losses were lowered, slight adjustment being made to the governor venturi

to provide smooth idling. A 20-ft. semi-trailer with a 10-ton load was collected.from the loading bank and a check disclosed that it had the original brake drums. Because of the high speeds of the present-day prime mover, braking has been improved on the Scammell semitrailer by employing wider shoes and drums.

A noticeable point about the tractor was that the operator had fitted an auxiliary fuel tank so that. it could cover out-and-return journeys between London and northern depots without need for refuelling after leaving its base. The normal tank of 26-gallon capacity, and the auxiliary holding 16 gallons, afforded an effective range of over 600 miles.

Both tanks were filled to overflowing point and the fuel-consumption trial started from the Arlington depot at Ponders End. The 411-mile circuit covered embraced traffic work .in Enfield, Potters Bar, South Mimms, London Colney, Palmers Green and Ponders End, combined with trunk operation and a fair prow-11On Of hills between the starting point and South Mirrims.

After leaving the depot we were

soon competing with other traffic along Southbury Road, Enfield, and, after a couple of short delays in the town, started climbing Windmill Hill. This was,the only part of the day's work where I felt any need for closer-spaced gear ratio s, because it was an easy climb in second gear but just too steep to hold in third. Had this climb been tackled at the end of the day or after a little more work to ease the initial friction in the engine, the hill might have been climbed successfully in third gear.

Restriction of space in the region of the coupling gear. prevents the employment of a two-speed axle and the cost of conversion would be much increased if a five-speed gearbox were employed. In my opinion, the four-speed box used gives a reasonable performance under most conditions.

A fair amount of gear work on the undulating roads brought to notice the position of the lever, which, in its present standard form, is too far back in the cab for comfortable manipulation. I would prefer it to be extended and cranked to lie almost horizontal along the engine cowl.

There were short delays at Potters Bar and South Mimms. A 'stop watch was used to measure all stationary time, which was totalled anti deducted from the overall time, thus arriving at an average running speed. Altogether there were 17 stops in the circuit totalling 5 mins.

Although legally, in this country, the speed of an articulated combination is restricted to 20 m.p.h., the Bedford kept pace with other goods vehicles on the main trunk roads, often exceeding 30 m.p.h. With the low-ratio final drive (6.8 to.1) which is standard in the Bedford _tractor, the maximum governed speed of the oil-engined chassis is approximately 34 m.p.h. in top gear.

The course was completed at an average speed of 23.5 m.p.h. and 3.2 gallons of fuel were used. The consumption rate works out to approximately 13 m.p.g., which, in .conjunction with the total gross weight and speed, gives an exceptional time-load-mileage factor for a vehicle of this class.

The engine coolant was well below average temperature during the consumption trials, because there is less heat dispersed to the cooling water in an oil engine and the Bedford radiator is retained. As a tentative measure, the diameter of the fan is reduced, but a partial blanking of the radiator would have been more beneficial and a slight improvement in fuel economy might have been obtained. Even when climbing a 1-in-7 gradient for several minutes, on a day when the ambient reading was over 80 degrees F., the engine was still below optimum warmth.

With a bottom-gear ratio of 7.06 to 1, 'giving an overall reduction of 48 to 1, the Bedford had ample hill climbing capacity. In addition to making comfortable ascents of the local gradients, some of which measured up to I in 5 on the Tapley meter, stop-start tests were accomplished with equal ease. The rubbercushioned centre plate in the clutch employed with the Comet engine affords a smooth take-up in transmission and prevents propeller-shaft chatter at low speed.

1 had to drive a fair distance before finding a suitable stretch for the short performance trials, and, with little space to manceuvre the outfit at each end of the road, the 28-ft. turning circle proved a great advantage. Acceleration trials were made from a standing start, employ ing second gear, subsequent changes being made at governed engine speed. The average of six tests indicated that the maximum legal 'speed of 20 m.p.h. could be reached in 23.5 secs. and the total time taken to accelerate to 30 m.p.h. was 40 secs.

I was pleasantly surprised at the braking effort, remembering that neither section of the unit was new and that improvements have since been made in the semi-trailer braking system. Tests were made from 20 m.p.h. and with emergency applica

lions of the pedal alone, the outfit was brought to rest in 32 ft. This was the best effort in a series of trials which gave stopping distances ranging up to 38 ft., the average being 36 ft., equal to a deceleration rate of 0.36g.

From independent tests it has been found that used vehicles of all types, with a fair amount of service, have an average deceleration rate of 0.25g. From this it would appear that the Bedford-Scammell is well equipped in its present standard braking arrangements.


Locations: Arlington, London

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