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Sevcon conversion gives smooth changes and improves fuel consumption
by Martin Watkins
Photographs by Harry Roberts A FULLY automatic transmission that improves fuel consumption—is popular with drivers—gives longer engine life —and costs from £200 to £300 per vehicle. These are the claims made by Sevcon, of Kingsway, Gateshead, Tyne and Wear, about its new fully automatic transmission conversion for buses fitted with Wilson type semi automatic gearboxes.
I was able last week to try out the system fitted to a Daimler Fleetline operated by Northern General. This bus was four years old and had been fitted with the Sevcon unit for about one year. In order to try to simulate service operation with the bus I followed a Gateshead Atlantean round that company's route 17 serving the hilly Gosforth estate.
Responsive
T h e Sevcon conversion works on four different throttle ranges so that on light throttle changes take place at low engine speeds, but when accelerating hard the changes take place at just below the maximum governed engine speed. Using these four ranges gives a much more responsive system than can be achieved with automatic systems using only two ranges.
I found when driving the bus that all changes both up and down the box were extremely smooth and that no jerks occurred either under braking or under hard acceleration. Neither was there any loss of performance caused by the conversion, as upward changes under hard acceleration all occurred near peak engine revs. When braking, gear changes took place at very low speeds, from 4th to 3rd at 13mph, from 3rd to 2nd at 7mph, and from 2nd to 1st at 2mph.
Overspeed protection, a n optional extra, was fitted to the bus I drove and this prevented any possibility of drivers making downward changes at too high road speeds. For normal automatic operation all that was required was to select the 4th gear slot on the selector. If the lever was then moved into any other gear, whether 3rd, 2nd or 1st at a speed that would have resulted in the engine exceeding its maximum governed revs then this change did not take place. When the correct road speed for a down change was reached then the control circuit allowed that change to take place.
The Sevcon system is very biased towards selecting high gears. When going into a roundabout the bus remained in fourth gear until I started to accelerate away, when it quickly dropped down into third.
Going around the service route was easy as the gearbox behaved exactly as a good driver should use a semi-automatic box. I found I was always in the right gear whatever the circumstances and at no time did the bus hunt between gears. For demonstration purpc the bus was specially flu with a set of eight indiu lights to show what thro range and what gear it wa: at any time. The total thro movement was divided i four ranges, set so that a thing less than half thro signalled range 1; range 2 C2 in at half-throttle; range 3 threequarters and range 4 before full throttle.
A digital counting sysi made it possible to obi these speeds exactly, the pr being aimed at the props!. nuts. All the speeds at wl changes occur could be rai or lowered proportionately that the same unit co equally well be used for hi geared motorway coaches.
In each throttle range all down-shifts occurred at 3n less than the correspond upshifts. This difference s pressed any tendency to h between gears on hills. chosen speed ranges also I vented changes at any sp except between two adjac gears. There was no spf where it was possible to ki down from top to second g(
Smooth
The smooth downshifts w made possible by making time delay a function of throttle range. This resulted a downchange taking place the least throttle open needed to trigger it. At m than the minimum throi opening the downshifts w slightly over-revved.
Upshifts were instantane, with the lower gear-band h 1 the higher gear-band had hold.
Then accelerating at full tttle the upward changes .e triggered at different me speeds to allow for conted acceleration of the bus ing the response time of the r. For the governor set at ■ Orpm at full acceleration 1st to 2nd change was ggered at approximately lOrpm to prevent governor :rference during level-road aleration with an empty ice. The full throttle nge from second to third ; triggered at about iOrpm and from third to rth at about 1,680rpm. At thing less than half throttle change from first to second r took place at about rpm at which speed any jerk ; stopped by the softness of fluid flywheel.
rifty
'he package offered by Sev:, and designed by its conant Gavin Martin, involves ,ng four units to the vehicle h small alterations to the ing. The control unit, which orporates all the logic cirts, is usually fitted under the :k seat. In addition, there is .peed-sensing unit aimed at propshaft nuts, a throttlesing unit on the fuel pump ttrol lever on the engine and ;mall amplifier for the road ted signal.
rhe bus I tested had been in normal service from Chesterle-Street garage for about one year on various routes with different drivers. London Transport and Eastern Scottish have also tried the system. Northern General chief engineer David Cox told me that throughout this period the fuel consumption both for this bus and for five similar vehicles without the Sevcon control system had been monitored.
These records kept by Northern showed that the Sevcon bus returned an average fuel consumption of 28.2 lit/100km (10.2mpg) whereas the other five averaged 31.71it/100km (8.9mpg)—an improvement of about 12 per cent. He put this improvement down largely to the high-gear bias on the unit. He also said that the overspeed protection device could have saved at least some of the 15 engines written off by Northern in the week of my test. This optional extra adds about 12 per cent to the basic price of the unit.