Requirements in Buses for Mountain Routes
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A Practical Approach to the Design of Passenger Chassis Intended for Long-Distance Operation on Routes Embracing Alpine Passes
THE underfloor-engined bus was to be preferred for long-distance service, said Mr. J. A. Steenman, technical director, Kemperen Van Twist Diesel N.V., Schiedam, Holland, on Tuesday. He was reading a paper, Long-distance Diesel Buses on the European Continent," to members of the Automobile Division of the Institution of Mechanical Engineers in London.
An important feature of this type of bus, he said, was its good weight distribution whether laden or empty. Moreover, a body. with 45 seats facing forward could be built on such a chassis. This number could be increased to 52 if one took full advantage of Dutch regulations regarding seat spacing.
With the type of 40-45-seat body now in use on the Continent, the total laden weight was approximately 13 tons. The available output of the engine must make it possible to give such a vehicle a top speed on main roads of 60-65 m.p.h., whilst the transmission must be such as to enable gradients of 20-25 per cent. (I in 5 to 1 in 4) to be climbed:
Rolling and Air Resistance
Rolling resistance on macadam roads was, said the speaker, between 24 lb. and 30 lb. per ton, whilst on softer types of road it could be 45 lb. per ton or even higher. The average air resistance factor for a bus of modern shape was taken as 0.8.
Air resistance at speeds in excess of 40 m.p.h. had a big influence, and one way of reducing it was to cover-in the underside of the chassis. Mr. Steenman produced graphs which showed that with an air-resistance factor of 0.6 at a speed of 60 m.p.h., and a head wind of 20 m.p.h., the power required at the rear wheels was 27 b.h.p. less than with an airresistance factor of 0.8.
At 55 m.p.h., with no head wind, approximately 100 b.h.p. was called for, and with a head wind of 20 m.p.h. the power required was 132 b.h.p.
To maintain 50 mph. for several hours on the level against a head wind, the engine must have a continuous output of at least 110 b.h.p., with a maximum output of about 150 b.h.p. with a smoke-free exhaust.
The buses and coaches which formed the subject of the Speaker's paper cover distances of 1.000-2,000 miles, with daily mileages of 200-400.
In another graph Mr. Steenman showed that 100 b.h.p. was necessary to propel a 13-ton bus up a 1-in-7 gradient at about 7 m.p.h., the calculated rolling. resistance being 26.5 lb. per ton and the air-resistance factor 0.6. The direct-injection oil engine was preferred because it combined good starting characteristics with long life, a low fuel consumption and high thermal efficiency.
Maximum setting of the injection pump would be approximately 10 per cent. below the visible exhaust-smoke region. This would enable the engine to operate at full power at an altitude of approximately 5,000 ft. without signs of smoke. If an engine smaller size were used and its maximum output were more or less on the limit of its visible exhaust, the injection pump must be provided with a device which reduced the maximum setting of the pump at higher altitudes.
As the vehicles to which Mr. Steenman was referring had to operale for long periods at altitudes of 3,000 ft. and over, the thermostat in the watercooling system must have a low setting and must start opening at approximately 60 degrees C., whilst the by-pass pipe must be closed at 70 degrees C. Further temperature correction could be arranged by a shutter.
An oil cooler would be necessary where the engine was totally or partly enclosed. The fitting of a thermostat in the lubricating oil system was advocated.
Requirements of the transmission system were that a top speed of approximately 60 m.p.h. must he attainable, and gradients up to 25 per cent. (1 in 4) must be climbable. With a tractive effort in top gear•of approximately 920-1,000 lb., the tractive effort in low gear must be about 6,600 lb.
These requirements called for a difference between the highest and lowest ratios of 6.5 or even more, depending on the torque characteristics'. of the engine. To obtain a good road performance the difference between the ratios must not exceed 1.6, otherwise five ratios would become necessary.
In the speaker's opinion,the torque converter was not suitable for the kind of service with which he was dealing, because of its low efficiency on steep gradients. Both the constant-mesh gearbox and the preselective type, with fluid coupling, were considered suitable. The preselective gearbox was favoured for its easy and quick gearchange, features which were appreciated to the full when travelling on alpine passes.
Exhaust Brake Systems
It was necessary, said Mr. Steenman, for such vehicles to be fitted with exhaust-brake systems. • Such a brake consisted of a throttle valve arranged in the exhaust manifold. By closing this valve a pressure of 2-3 atmospheres was built up in the manifold, and the engine operated as a single-stage lowpressure compressor.
Correct use of such an exhaust brake relieved the normal braking system of considerable work, with the result that facings and drums had a much longer life. The braking capacity of an exhaust brake depended on the valve timing of the engine; the greater the overlap, the smaller was the braking effect produced.
Such a braking system, said the speaker, should form standard equipment on all vehicles which had to operate in hilly or mountainous areas. Similar views were expressed concerning powered steering.