Volvo comes up with fly bus scheme
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IN ORDER to keep up with the demands of urban driving — continual acceleration, and so on — buses in use today are generally fitted with more powerful engines and larger 'brakes.
This in turn has meant an increase in deadweight and fuel consumption, but not in the number of passengers carried.
The energy crisis of 1974 made road transport engineers acutely aware for the first time of the need to conserve fuel and to control noise and exhaust emission in cities.
Meanwhile, fuel prices have reached a level undreamed of five years ago, and during this time overseas manufacturers began developing vehicles which would meet the new energy saving criteria.
Although not supported by the funds which are available in some European countries for such research and development schemes, Volvo actively pursued an energy storage project for city buses and in 1977 began tests of a prototype vehicle incorporating an engine/ driveline system in which braking energy — dissipated in normal vehicles — is utilised in acceleration.
There are a number of different ways of recuperating braking energy and these include flywheels and hydraulic acaimulators, The latter system was applied in an experimental scheme used in two Berlin city buses, while at the Hamburg transport exhibition of 1979 both Daimler-Benz and MAN exhibited proposals or psv drivelines using flywheel storage,
Although Daimler-Benz has a small 307-based bus on the road using a modified flywheel braking energy recuperator, Volvo Bussar, of Sweden, has now chalked up a world "first". It has presented a –second phase" prototype flywheel system in a new Hogglund-Saffle 12m city bus of the design introduced only recently on the
novel Halmstad busway system.
In the first prototype, the novel propulsion system was carried in a small trailer connected by hydraulic hoses to the drive axle of the bus. Tested over a period of almost three years, the bus has provided a mass of data and has justified the development engineers' estimates of fuel savings between 15 and 25 per cent, depending on driving and traffic conditions.
The main features of the second prototype — with the complete engine/flywheel/ hydraulics pack installed in a normal underfloor location at the rear of the chassis — include three driving modes.
First, with engine and flywheel engaged; second, using the diesel engine only — when it is necessary to move the bus quickly and the flywheel is at a standstill the diesel engine is used to power the bus: and third, the flywheel only, enabling the vehicle to be driven short distances with the diesel engine cut out past such areas as those with hospitals.
The new prototype, codenamed B6RH — that is, rearengined bus with TD60 engine and hydraulic transmission — is fitted with a power unit rated at only 140bhp against the normal 230bhp.
From the passenger's viewpoint the most impressive aspect of the two Volvo prototypes demonstrated earlier this month was the perfectly smooth stepless acceleration, and the correspondingly smooth and silent retardation to a complete stop. This contributes to a high degiee of ride comfort, particularly when driving under flywheel power only when noise emission is dramatically lowered.
The ability to drive on flywheel-stored energy only gives a range of around 1200m including three stops with the engine switched oft
Wear of brake linings is reduced considerably as normal continued overleaf Above left: The Utile-bodied Mk II prototype has engine/flywheel pack fitted at the rear of the chassis.
Len: The first prototype, with hydraulicb and flywheel accommodated in a trailer, has now completed almost three years of tests. braking is by hydraulically recycling brake energy to the flywheel with service brakes applied only after a certain retardation level has been reached.
Volvo engineers envisage that flywheel buses would approach stops with the diesel engine switched off. A few seconds after leaving the stop on flywheel power the engine is re-started automatically and run at full output until sufficient • energy has been fed back into the flywheel to compensate for real and estimated losses. Energy lost by drag and rolling resistance is in any case compensated for by the diesel engine.
The heart of the new Volvo propulsion system is based on technology developed by an associated company, Volvo Flygmotor, founded in 1930 and engaged since 1970in the production of hydrostatic drive systems.
Although in the new prototype the hydraulic motor supplies power to a conventional drive axle, there is considerable potential for redesign of psv chassis by using hub motors which 4vould allow a lower floor line. The flywheel, weighing 300kg, is located in a safety cage under the rear-mounted power unit from which it is driven by a clutch-controlled transfer box.
The diesel engine has been optimised to operate in the 1 600-2000rev /min speed range, and this relatively narrow band has been chosen on the basis of the best specific fuel consumption and low exhaust emissions. The hydraulic pump "regulates" the output to and from the drive wheels.
The flywheel reaches a speed of 10,000rev / min when its braking energy storage capacity has been reached. In combination with the engine output of 105kVV, the flywheel output of 125kVV (at full charge) can provide an acceleration value of 2300.N.
This allows acceleration of cm/s up to 1.62 throughout the entire acceleration phase as against the norms in conventional city buses which do not have enough power to sustain high acceleration at speeds of over 25-30km /h (15.5-18.5mph). Maximum speed of the Volvo B6FiH flywheel bus is 80km / h (50mph).
As already mentioned the new Volvo achievement, though aided by a Swedish government loan, is financed entirely as a private venture.
Following a full test programme under simulated and actual city traffic conditions, the company proposes to manufacture a limited number of flywheel /hydraulic-drive buses which could go into revenue service with selected operators by 1984-5.