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DRIVING FOR

14th April 1978, Page 33
14th April 1978
Page 33
Page 34
Page 33, 14th April 1978 — DRIVING FOR
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Which of the following most accurately describes the problem?

ECONOMY

1E of the biggest single tors influencing the fuel isumption is the driver_ rget all the high techno-. y terms of high torque rise minimum specific fuel isumption — they are all :less if the man behind the .eel doesn't know how to ve.

And let me say straight ay that I am not implying lack of skill on the part of driver — far from it. I am king about not knowing N to drive to get the maxim economy from a partiar engine.

Consider the case of a fleet h a fair cross-section of 32tractive units. Its certainly uncommon to find, say, idon Atkinsons with Cumis engines rubbing shoulders h Gardner-engined ERFs all he same yard as a Scania and Iedford TM. If the company .rates a one man-one vehicle tem then in time the driver I probably slip into the way of ting the best from his vehicle experience.

But what if he had to move rn a Gardner engine to a Deit Diesel? If our driver leaps ) his TM and drives it like a rdner-powered Foden then will not be running at optim performance.

Different engines need difnt techniques if their design o be exploited to the full. e thrive on high revs while ers give of their best when wed to slog away down at bottom of the speed range.

one engine which benefits a high-revving technique is two-stroke Detroit Diesel fitto the top-weight TM Beds. These engines (in V6 and

versions) give of their best in bustion efficiency and ce fuel economy at the top • of the speed range, owever a problem arises as as the driver is concerned ause of the different engine e. As the two-stroke engine s twice to a conventional el's once, the exhaust note ompletely different. Thus a er used to a four-stroke en gine must reaccustom his ear to a different sound so, in the early stages especially, a careful eye on the rev-counter is necessary.

The normal operating range of the Detroit Diesel engine is between 1,200rpm and 2,100 rpm. For changing up, an engine speed of 2,000rpm is recommended while, on hilly terrain, a change down is advised before the revs have dropped below 1,600rpm.

The air-cooled Magirus Deutz engine, be it V8 or V10, is another high-revving engine with a 2,500rpm maximum, Magirus suggest that all gear changes should be made in the 1,8002,000rpm band.

At the other end of the scale in terms of engine speed comes the Gardner. Obviously how a driver drives a particular engine depends on a great many factors, not the least of which are axle ratio, number of gears and gross weight, but it is possible to give a few pointers.

With the Patricroft equipment, upward gearchanges should be made at or near the governed maximum speed for maximum performance, which is 1,700 or 1,850rpm depending on the engine type. But for good performance coupled with the best fuel consumption about 100rpm should be trimmed off these engine speeds.

Gardner engines are used with several different gearboxes, with the result that varying techniques are recommended for downchanges. With close-ratio multi-speed gearboxes such as the nine-speed constant-mesh Fuller, downchanges should be made between 1,400 and 1,500 rpm, whereas with a relatively wideratio box such as the David Brown six-speed unit, the recommended change point is between 1,200 and 1,300 rpm. This is to obtain the best road performance — for the maximum economy Gardner suggests using speeds 200 rpm below those just mentioned, Many drivers feel that for maximum acceleration, the throttle pedal should be pressed to the floor, Not to be recom mended with most engines anyway, this is definitely not a good idea with a Gardner, because opening the throttle wide rapidly over-advances the timing so the engine in fact gives a worse performance. Because of the Gardner injection control arrangement, the accelerator should be depressed proportionally with the engine speed, ie the foot should be only slightly ahead of engine speed.

The trend in engine design is now towards low-speed, hightorque-rise engines and nowhere is this better illustrated than by the latest products from Cummins and Rolls-Royce — the former with the new -big cam" Formula E290 and R-R with the L series.

To illustrate the effect of driving according to the manufacturer's recommendation, I can do no better than quote some fuel consumption figures for the Formula E290 obtained under road test conditions in a Seddon Atkinson.

Driving as I would have

continued overleaf driven a "normal" Cummins 290, ie changing up at around 2,000rpm and down at 1,300rpm, the fuel consumption was 44.1 lit/ 100km (6.4mpg). With the same vehicle over the same route but using the Formula E290 change points of 1,600 and 1,000rpm, .it improved dramatically to 38.71it/ 100km (7.3mpg) — merely by trimming the engine speed. In my experience, the Formula E290 -let it lugtechnique certainly works.

Moving down the power scale slightly, we come to the NHC 250 Cummins, which can be found in a great variety of machines including those from ERF, Foden, Leyland and Seddon Atkinson. With this particular engine, which develops its maximum torque of 895Nm (6601bft) at 1,500rpm, Cummins recommend that down changes be delayed until about 1,35Orpm. Upward changes should be made in the range 1,450 to 1 ,850rpm with progressive shifting, ie the higher the gear selected, the higher the revs within the range. Assuming a maximum geared speed of around 100km/ h (62mph), the best cruising revs for an NHC 250 are 1,900-2,000rpm.

As Cummins and Rolls-Royce have proved, it is not necessary to go for a high-revving engine just because the unit is turbocharged. Careful turbocharger matching can ensure satisfactory performance and fuel consumption even with engines rated at 1,900 rpm.

The Rolls-Royce turbocharged engines, be they rated at 2,100 rpm or 1,900rpm, can be allowed to lug down to 1,000rpm on the flat when conditions allow. Obviously this does not necessarily apply if there is heavy traffic or a gradient on the horizon, but it does indicate how low down a turbo engine can operate. With the naturally aspirated Eagle engines this bottom end speed is set at around 1,200rpm.

Most manufacturers provide a driver's handbook with the vehicle which gives hints on how to get the best from their particular product. Scania goes one better by publishing a series of small booklets for the driver on a variety of related subjects including braking techniques and fuel economy.

The fuel economy booklet lists, in a cartoon format, the various factors which influence the consumption for better or for worse. Scania suggests that the man behind the wheel should

drive -softlywith no unnecessary power-consuming acceleration or hard last-minute braking. Less frequent gear shifting and a lower cruising speed can save a lot of money — in wear and tear as well as derv.

Scania is another of the engine builders who follow the low revs-high torque concept. The big DS14 engine illustrates the point well. The maximum power from this 14 litre (865 cuin) V8 unit is 276kW (370bhp) but the important statistic is the torque — no less than 1,480 Nm (1,090Ibft) at 1 ,30Orpm. Obviously such an engine is not exactly overburdened when running at a puny 32 tons but, being designed for r higher weights, it illustrate particular design characte very well with its optimun consumption speed of 1,50Orpm.

Scania goes to great pa put over the correct dr technique for its engine they for an 81, 111 or After all, it's not a great d use designing an engin optimum economy and el up with a high fuel consurr merely because the driver familiar with its individua racteristics.

The Scania tachomete calibrated in different co with green for the best wc range, red for overspee( white for the -no man's la( between. Moral? As for lights, "drive on green"!

Apart from the ob possibilities of saving driving less energeticall save on another major cos — tyres. Increasing the v speed from 80 to 100km/ to 62mph) can mean uri per cent more tyre wear.

To sum up this artic driving for economy I qt recommendation from a E driver's handbook — "E SOFTLY".

Although it is easy to sa this implies the use of less it is misleading to apply th general rule because of such as the Detroit Diesel.

If there is one major that stands out, it is tl-N driver should be made aw the technique for gettin best from his own engine. in the driver's handbook.

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