Higher torque or more gears for heavies?
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by Brian Cottee
MERE 1 was, as the story goes, not .ipside down at 30,000 ft perhaps but :ertainly at an impossible angle at 300 ft, uith nothing but sky and pine-tops in /iew, while the Scania test driver inched )ur 4 x 4 military vehicle over a rounded )oulder the size of a bungalow. He was tot inching through lack of power but ;imply to demonstrate how easy it is vith an automatic gearbox (and fourvheel drive, but that is just a luxury for he military), to stop and restart on mpossible gradients which a contentional clutch wouldn't look at.
I was getting this demonstration on a boulder-strewn hillside in a wood overlooking Saab-Scania's Sodertalje plant, south of Stockholm, as a bit of practical to back up the theory which earlier in the day I had been hearing from Mr' Folke Karlsson, the group's chief engineer, engines and transmissions.
In the past year or so, several Continental manufacturers and some prominent British transport engineers have forecast the wider use of multispeed gearboxes, with as many as 15 or 16 ratios for the really high-powered trucks, as the way to give heavy vehicles the performance to keep up with modern traffic at high gross weights.
Mr Karlsson, I found, does not share this view. The present Scania heavies have a 10-speed gearbox (five, with range change) and he sees no need for more than this in fact, forecasts that eight may be enough, depending of course on terrain and traffic conditions. But he believes that the alternative answer — the development of higher-torque, or, more accurately, sustained-torque, engines is not only the better solution but will be given a new boost by the fuel situation.
He took as an example the US Mack engines, a 172kW (230 bhp) "six" and a 243kW (325 bhp) V8 which have been designed with high torque back-up; in fact giving 50 per cent more torque at 1,300 rpm than at maximum revs (2100). This deliberate "distortion" of the torque curve results in higher bmep, which can cause smoke problems at low rpm but Mack claims to have overcome this.
One of the practical results is that the high-torque Macks can, says Mr Karlsson, perform satisfactorily in American conditions with as few as five gears. And with a high-torque back-up engine the driver can run at perhaps 1800 rpm at the same power output as at 2000 rpm, but more economically in fuel terms, and at a throttle setting which feels more natural and is easier to maintain for long periods of driving.
It would take no great leap of imagination to suppose that, in developing this theme, Mr Karlsson was revealing thinking which will be evident in future Scania diesels. And Mack, of course, are not the only people who have developed high-torque engines — both Cummins and Berliet are among the makers who have done much work in this field.
Automatics
While many long-distance vehicles are likely to keep the familiar manual gearbox for many years to come, Scania believes that we may now be entering the period of a real swing towards automatics or semi-automatics, especially for urban and suburban work, and the company has certainly been devoting a lot of development time to varieties of transmission which it is, or soon will be, offering. As reported in CM on May 17, a fully automatic gearbox is going into series production next year, but Mr Karlsson admitted that there was 'still some doubt about the market's readiness for this development.
Saab-Scania's programme on automatics has been helped enormously by work done for a Swedish government contract, under which the Army will get 2000 trucks initially — and all these 4 x 4 and 6 x 6 load-carriers will have automatic transmission.
This Scania automatic is a development of a type which the company has offered on buses for some years, this version having no epicyclic gearing.
The gearbox is kept small and efficient by splitting the drive with a differential immediately behind the engine; the sun gear (crownwheel side) of the differential drives the hydraulic input pump of a torque converter, while the ring gear provides a mechanical output drive.
Behind the torque converter are Simpson planetary gear sets — in this case two gears and four clutches — to provide three forward gears and one reverse'. But by using the facility of braking the hydro-mechanical input section of the gearbox it is possible to get six forward speeds and two reverse. This transmission provides a torque multiplication ,of 7 to 1 at stall, and the military version has take-off gearing to drive all axles — which gets me back to where I was sitting on a Swedish hillside in the cab of a Swedish Army 4x4.
The military people specified automatics for a variety of reasons, but one of them was tractability and controllability in steep, difficult terrain. Not many hauliers operate in such conditions, but the parallel becomes clearer when I record that a roadgoing civilian test truck with a simpler Scania automatic has been doing restarts on a 16 per cent gradient (1 in 6.25) at 40 tons gross without a sign of protest. And under these conditions the most hamfisted driver can make it creep away by inches — an attribute which could have icy-road advantages.
This particular version dispenses with the epicyclics. It is, Mr Karlsson explained, simply the basic five-speed Scania manual box with the torque converter added at the front to replace the clutch, but with a lock-up clutch to provide direct-drive cruising with no wasteful slip.
Folke Karlsson sees a growing European need for this sort of half-way house, and it will be considerably cheaper than a full automatic, and easier from a servicing and spares viewpoint. This is one of a variety of automatic and semi-automatic transmissions which the company will be offering truck and bus operators. They are already offering the GP Allison fully-automatic on the LB81 mainly for municipal vehicles, and ai finding it much more reliable tha earlier models. Mr Karlsson interested, too, in a form of Allisc automatic designed for use with gi turbine engines — it has no torque CO1 verter (the turbine is, in a sense, its ow torque converter) but has a hydr, mechanical clutch with an epicycl gearbox behind.
From transmissions we progressed 1 overall configurations for heavy true] -influenced by the fight over EE weights. Folke Karls§on is a man wt favours the simple solution when it giv good results, and he expressed 1surprise at the readiness with which least one UK manufacturer adopted ti relatively heavy and complex hu reduction axle for medium-Neig trucks.
He believes in sticking to a sins driving axle wherever possible, al points-out that this works at high grc weights in Sweden. In icy weather t driver may have to use his bogie lift, put more or all of the load on to t driving axle for starting, but vehic were seldom stuck.
Didn't the bogie lift mean that i driving axle was, at least momentari well over the legal axle limit?
An understanding smile, a tact motion of the hand. Message recei and understood.
For good measure, Mr Karlsson a thinks that the single-driving-axle ci figuration should be given special c sideration now, because it is v efficient and thus a fuel-saver.
But there is, of course, a limit to use of simple solutions. Folke Karls reckons the 460 mm-diameter cro, wheel used on the biggest roadge Scania is nearly the practical limit. beyond that one is forced to adopt al native solutions, such as hub-reduct with planetary gears — and, he a firmly, with roller bearings. efficiency, of course.