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New Voith box for buses has hydraulic retarder
Page 28
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by Martin Hayes
BRITISH bus operators this week had their first opportunity to sample a new bus transmission manufactured by Voith, the West German gearbok maker. Known as the DIWA D851, the new transmission includes an integral hydraulic retarder which is claimed to have trebled city bus brake life in European trials. On Monday I was able to drive the vehicle over a varied route near Earls Court.
The DIWA D851 was first introduced at the Frankfurt show last year and this is its first showing in the UK. (There are more than 100 DIWA bus transmissions in service with CIE, the Irish State operator, but apart from a few trial units with the Greater Glasgow PTE, there are virtually none in the UK. The new D851 model is in service in cities like Brussels, Stockholm and Berlin).
The new model is a threestage unit controlled in normal Voith manner by three push-buttons. These give fully automatic control, reverse and a hold on the two lower ratios. The first ratio is linked to the torque converter while the upper two are direct drive. Change points are governed by throttle position. By using planetary gears fore and aft of the hydraulic turbine, the turbine is driven in the opposite direction to travel and so used as a retarder. i Retardation is n three braking stages and is controlled by micro-switch operating on the brake pedal movement before engagement of the service brakes. A hand control lever to operate the retarder is available if required. The retarder is switched off when engine speed drops to idling in second gear.
Voith Engineering Ltd, of Thornton Heath, Surrey, Voith's UK subsidiary, this week demonstrated the unit on a Van Hool integral bus belonging to SNCV, the Brussels operator, and fitted with Fiat 409 running units. This engine develops 128 kW (172 bhp) and 651 Nm (480 lb ft) at 2,200 rpm (though the D851 can accommodate engines with outputs of up to 180 Nm (240 bhp) if desired). In this application change-up points at maximum engine speed take place at 34 km/ h (32mph) and 52 km/ h (32 mph) and maximum speed is 76 km/ h (47mph). Maximum retardation available from the retarder is 16 per cent, though this can be increased if required. Some assistance from the air brake system is used in the third stage.
On the road . . .
On the road, the DIWA system is simple to operate. There was occasionally a trace of surging when changing from the torque converter to direct drive, that is from 1st to 2nd, but this was not unacceptable. Full power down-changes produced more of a jerk than I would have liked, but during upward changes the relatively high speed at which gear changes take place should reduce the risk of upsetting passengers.
Use of the retardation technique was both simple and near impossible to detect to the driver. On a specially chosen 1 in 10 hill in Wimbledon ample retardation was possible without use of the service brake. Most important was that operation of the brake pedal was unaffected and most drivers could drive the vehicle and never realize that there was anything special about it.
An important part of the D851 is an oil-water heat exchanger which transfers heat from the gearbox during retardation to the engine cooling system. Even with this device, allup weight of the transmission is 250 kg (114 Ib). Price is said to be within five per cent of the DMA unit without the retardation facility.