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QUICK CHANGE ARTIST

The installation of a high-tech semi-automated transmission in the MAN 16.331 FTS makes for very smooth, slick and fast gear changes

• Eaton's SAMT is the latest development in semi-automated gearboxes. Although it is still on trial, MAN has supplied us with a prototype installation in its latest 16.331 tractor. In this exclusive test, we have been able to check out the SAMT's theoretical benefits over our Scottish test route: we must emphasise that this vehicle was not to production specification.

It is more than a year since we first drove a MAN development vehicle equipped with Eaton's semi-automated mechanical transmission (SAMT). In our original assessment we found that gear changing could be jerky and that nearly all of the ratios had to be used in succession — although some skip changes were possible and the engine was protected from overspeeding.

Although still at the preproduction stage, Eaton's micro-chip technology has become much more refined so that smooth, slick and fast single or multiple changes can be made at will.

As a starting point Eaton chose to automate its newest 12-speed heavy duty ISO 11612 Twin Splitter mechanical gearbox which, in manual control guise, went into full production earlier this year.

Further refinement of the control system, aimed at reducing driver effort, produced the SAMT, doing away with clutch pedal and gear lever movements almost completely. Through a column-mounted stalk, the driver retains control of gear changing but his effort is minimised to the flick of a switch for each gear transition.

The total command must be completed within 0.5 seconds, but it is possible for the more dextrous to effect block changes of up to three gears at one time simply by flicking the control switch three times in quick succession. The change is very smooth but more importantly it is positive and safe.

The on-board computer only accepts a command that is within the engine's capability; so if two or more gears are asked for when it is only practical to take one the signal is modified.

Moving off from rest may be executed in any of the five lowest ratios, depending upon load and gradient.

The clutch pedal is used to disengage the clutch for the initial gear selection, but that is the only time it is depressed until the vehicle comes to rest With only 300rpm steps between ratios it is extremely easy to contain engine speed to a very narrow band. At MAN's recommendation, we chose not to res trict power to the most economical engine speed during acceleration but drove through to maximum power at about 1900rpm, skipping gears where appropriate, to gain cruising speed in the shortest time.

• HANDLING

On entering a roundabout, or if negotiating some obstacle, engine speed can be allowed to slow to about 1,000rpm whereupon a touch down on the gear switch automatically selects the ratio most appropriate to road speed ready to drive on through.

In congested town traffic driving is easier and safer with both hands almost totally free to steer the vehicle. From the outside the exhaust note would flatter the most professional driver as down changes are accompanied by just the right amount of throttle blip, matching engine and transmission speed, automatically.

A display on the dash indicates which gear is in use and the number of options there are at any one time to change up or down.

While this seemed to be an important feature at first, after a very short period we found that we were driving almost exclusively on the rev counter. It can, however, be reassuring to note that the desired forward or reverse ratio has been selected before moving away from rest. After every up or down command the gear switch returns to a neutral position.

The speed at which gear changing is made, tailored to meet the requirements of MAN engineers, is quicker than with the base Twin Splitter set-up. Try as we might, we were unable to fault the system in any way. Indeed, many of the up changes executed on hill climbs might not have been possible with a slower change rate. On down grades we were more ready to change down an extra gear and make the best of exhaust braking to hold back the vehicle with less reliance on the service brake system.

The drive-by-wire accelerator system is devoid of any mechanical link between the pedal and fuel rack. The fuel injection pump is controlled electronically so that the driver need not even alter the attitude of his right foot during gear changing. The black box computerised control unit also monitors the upshift brake, the X-Y electro-pneumatic shift unit, splitter shift assembly, clutch actuation engine brake and gearbox input and output speeds.

In addition to gear changing the compu ter has the facility to manage other vehicle systems such as road speed control as well as engine speed where a PTO is fitted.

Eaton says the system is applicable to any of its mechanical transmissions in automotive use.

• CAB COMFORT

At the same time as we assessed SAMT we were able to comment on other aspects of the vehicle. The 16.331 tractive unit uses the older style of MAN cab as the new F90 is an addition to the range for the time being.

Big and square, it offers plenty of room inside and is well equipped with driver's air suspension seat and fixed adjustable passenger seat. The steering wheel is adjustable for height and rake and the park brake lever is located within the facia. Behind the seats there are two fullsize bunks and the top one hinges down for rear cab access.

Instrumentation is minimal with gauges for oil pressure, water temperature, air pressure and fuel level. Both tachograph and tachometer are prominently mounted and of comparable size.

Switching for the indicators, screen wash-wipe, lights and horn is all contained within a single stalk mounted diametrically opposite to the gear change switch.

Inside trim is a mixture of rubber well mats, running flush to the door lip, and carpeted engine cowl. A document storage box mounted between the seats is supplemented by a lockable compartment above the front windscreen.

Curtain rails running around the sides and front of the cab give privacy and screen off the bunks from the driving area. Two well placed steps with large runners give easy and sure access to the driving seat.

• SUSPENSION

On the road the cab suspension added to the soft ride but accentuated body roll more than necessary. The chassis suspension showed a tendency to creak and groan at times as it compensated for the transition in imposed load.

• DRIVELINE

At the heart of the vehicle the MAN nontuned induction engine pulled strongly over a wide rev band but proved to be more thirsty than we might have expected allowing for the transmission and low aspect tyres. In top gear at 60mph the engine speed was only 1585rpm while, to maintain 50mph, 11th gear was often engaged with a subsequent increase to 1750rpm and at 40mph it dropped to 1400rpm. Overall journey times are good but not exceptional.

Track test once more showed the inadequacy of standard vehicle braking systems because, during braking stops from both 20 and 30mph, the drive axle alone was made to lock on the dry surface.

Consideration for the tyres and a little discretion on the side of safety was sufficient for us to decide not to continue the exercise at a higher speed.

Factors influencing jack-knifing under braking are well documented. If load sensing valves are unable to compensate sufficiently for the effects of load transfer under heavy braking chassis manufacturers should be specifying one of the excellent anti-lock systems now available.

Large windows lighten the cab interior and provides good all-round visibility. On route the mirror support arm on the near side loosened and at speed folded back onto the door. We had the correct size of open-ended spanner to tighten it but the recessed bolt could only be reached with a socket.

Access to the engine, not needed for routine daily checks, is a manual operation assisted by a torsion bar counter balance but a hefty heave is still required.

• SUMMARY

Eaton's semi-automated transmission removes the effort from gear changing, yet leaves the control with the driver. As set up on the MAN 16.331 it gave smooth, quick and positive response.

The half second allowance to instigate a block change is a little short to change more than two ratios at a time but it is possible to make three with practice.

The beauty of the Eaton system is that it takes nothing away from the efficiency of the base Twin Splitter gearbox and there is little or no weight penalty.

Clean, perfectly executed changes should show benefits in extended clutch and drive line component life.

With gear changing literally at our finger tips we might have expected a better fuel consumption performance if it had been a factory-prepared vehicle, but the prototype was taken straight out of service with a customer assisting MAN with its fleet trials.

Eaton expects the SAMT to be made available on a production basis towards the end of next year and the control package will become available as a retrofit option for existing Twin Splitter gearboxes. The on-cost has yet to be disclosed but it is a system worth waiting for.

by Bill Brock.

D Pictures by Colin Sowman