Good on-line prospects for lower price autos
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feature also eliminates the "jerk" from the first gear into second which is wide spaced and thus the most likely to cause passenger discomfort with this type of gearbox.
Allison currently offers four or five-speed versions right across the range with one exception and this in conjunction with the torque converter is claimed to give sufficient ratio coverage.
Although Allison admits that the ideal transmission would have no shifts at all", there is no sign of a constantly variable transmission in the short-term development plan.
One feature which is standard on Allison boxes is a downshift inhibitor which stops the transmission going into a lower gear unless the speed is right, whatever the driver does with the selector. This is also used to prevent reverse gear coming into operation until the vehicle is stationary.
The long awaited take-off of automatic transmissions has never actually happened although the penetration is now increasing, albeit slowly. It is a classic chicken and egg situation in that most operators are put off by a £2,000-plus cost penalty The only way to get the cost down is to go on-line and the vehicle manufacturers won't do this until the demand is there. But the demand isn't there because the cost penalty is too high. And so on to infinity....
One of Allison's rivals in the transmission market is Voith, and the German company's views on future developments are, not surprisingly, very similar. The constantly variable transmission is a case in point.
Voith likes the idea in principle but is not proposing to do anything about it in the short term. Voith argues that the CVT is fine for cars where the torque requirement is not so great. For a commercial vehicle the horsepower range can be similar but the transmission will need to cope with far more torque. The ratio spread requirement would mean a fairly bulky unit as Voith claims it would be difficult to cope with a 10 to 1 ratio spread within a physically compact unit.
Thus Voith believes that the "mechanical" gearbox is going to be around for a long time yet.
Another point of agreement concerns control systems, as Voith see solid-state technology moving in more and more.
As far as operator requirements are concerned, Voith experience puts long life at the top of the list and by this "long life" means first life and overhaul life. Voith claims that the semi-automatic box can have a perfectly satisfactory first life but not so good during post-overhaul operation.
Other major requirements are that the transmission should match the engine parameters and that it should be easy to use by the driver and workshop alike. Fuel consumption is also an important factor and Voith argues that it is not necessary for an automatic transmission to "automatically" imply a fuel penalty. According to Voith "we have got over the effect of the torque converter on fuel consumption".
The Voith gearbox range uses a mixture of a torque converter and a differential gear set. At the pull-away point there is a pure hydraulic power flow, then the torque converter cuts out and there is a mechanical transmission. Voith claims that with the Diwa 851, for example, from the start up to 40 per cent of the vehicle's top speed, a gear cal be saved and thus a shift can 131 eliminated.
The maximum tolerated impu speed for Voith is 2,500rpri which easily takes care of all thi UK bus rated engines, such at Gardner, Rolls-Royce and thi Leyland TL11 (and also the Mer cedes engine in the Dennis Fal con).
For transmission fluids, Volt) uses a GM spec fluid which i has found gives a better shif quality. The static coefficient o friction is lower than the dy namic and thus as the plates locl there is less shift disturbance.
Voith relies on the oil corn panies for fluid development. A the design stage certain frictior materialsare looked at alonc with the torque and power re quirements. Thus Voith car come up with a brake hors( power/square inch figure anc then approach the oil company.
The operator wants to cu' down on the number of fluids he has to stock but Voith insists or its own testing before an ap proval is given for a fluid. This if. taken up to 100,000 miles m. Voith has found in many case. "rapid deterioration after a promising start".
For the future, Voith sees thE major contribution being in thE field of production rather thar design technology. As a furthet pointer to the future, Voith admits that the company is nol involved in the lorry market al the moment but "we see the short-haul vehicle as being similar in operation to that of a bus ..."