Double blow for Volvo trucks
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by Brian Weatherley • Put a six-litre engine up against a seven and the bigger diesel will always win on performance, right? Wrong! Volvo has re-written the rule book with its supercharged and turbocharged
TD6OMTE concept engine which Commercial Motor has been evaluating in Sweden. The effect was little short of astonishing.
Volvo's double-blown TD6OMTE—the initials stand for
Mechanical Turbocharged Engine—broke cover at last year's Hanover Show; we covered it in an Engineers Notebook (CM 11-17 June 1992) and were looking forward to comparing theory with reality The TD6OMTE is based on the long-established 5.5-litre chargecooled straight-six used in Volvo's FL6 17-tonner. A belt-driven supercharger upstream of the turbo provides a number of benefits, not least of which is overcoming low speed turbo lag,
providing better engine efficiency over a wider rev range.
Getting more air into the cylinder also reduces emission levels, particularly at start up. There is no visible smoke, even when idling. The Roots-type compressor comes in from idle and cuts out at 1,650rpm via an electromechanical clutch, just when the turbo is beginning to come on song.
That extra blower allows Volvo to screw a similar amount of power and torque out of the TD6OMTE as the 61-litre TD73E engine fitted in the FS7 chassis.
For our test Volvo provided us with a pair of FS7 rigids, both grossing around 15.6 tonnes. One was powered by the 6.7-litre TD73E engine developing 172kW (230hp) and 800Nm of torque; the other was fitted with the TD6OMTE with 187kW (250hp) and 850Nm on tap. Both trucks had the R1000 nine-speed synchro box and 315/80R22.5 tyres.
Performance The TD6O1'vlTE-powered FS7 had a slightly faster rear axle ratio (4.63 against 4.25:1). Volvo's engineers insist that while this evens out the top speed of both vehicles, it has little or no effect on performance.
Our test route was a 30-minute circuit round Gothenburg, including a fair amount of stop/start city work Volvo says its the ideal engine for a distribution truck—and a couple of tough hill climbs.The first lap was in the standard FS7.
With a powerto-weight ratio of over 11 kW/tonne (16hp/ton) it was certainly easy to drive. At 60krn/h it pulled well in top gear,but as its speed fell away on shallow inclines and the revs fell below 1,:300rpm it began to labour. When leaving a roundabout below 1,500rpm its throttle response was pretty pedestrian.
Over 1,500rpm the seven-litre engine pulled strongly and most gear-changes could be taken at around 1,800rpm.
When following the TD6OMTEengined truck we were being beaten away from the lights and other traffic hazards, as well as on the hills. Even revving the engine to 2,000rpm before changing up made little difference. Then we drove the concept truck.
As before the two trucks started off together with the doubleblown FS7 leading the way, and we immediately realised why it had been hard to keep up in the standard truck.
Acceleration from rest with the concept engine is spectacular. That's hardly surprising considering that the concept engine delivers 750Nm of clutch engagement torque, compared with 400450Nm from the TD73E.
The standard FS7 had struggled to block shift into high range; in its supercharged stablemate we could dispense with odd number gears right the way throughout the box, changing at 2.000rpm without ever straining the engine. In fact, the rev counter needle appears to moves around the dial much faster than the driver's right foot might suggest!
We were also able to tackle hills and roundabouts one gear higher than in the regular FS7 thanks to
the double blown engine's superb lugging power. It can pull down to, and recover from, 1,000rpm without breaking its stride.
The supercharger is clearly audible although its throaty whine is never intrusive. As the electromagnetic dutch disengages the blower the whine drops away and this is usually a good time to change up.
The transition from blower and turbo to just turbo is very smooth: the only hint of hesitancy is when changing up on hills when the supercharger can cut in with a slight pull. Otherwise you hardly notice it's there—apart from its exceptional performance.
Durabilty
Operator trials with the TD6OMTE are about to start and durability is obviously going to be closely monitored. We can't help but wonder what this system would do to a seven-litre lump in, say, a FL7-260 tipper. Volvo reckons it could deliver the same sort power as a 10-litre. If that's true, and it can be made to last, then tippermen looking for more performance from less weight will be queueing up to try it.