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FINAL FIFTY: but ore speed governors on acceptable answer?
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FIFTY miles an hour (80 km/h) is the cruising speed for optimum fuel consumption, delegates at CM's Fleet Fuel Economy seminar were told last week. Keith Roberts of British Leyland showed how this speed produced the best payload tonmiles per gallon with heavy trucks, and Robert Davis of Perkins Engines illustrated his paper with slides which revealed that for medium! heavy vehicles, too, 50mph was an economical target.
Mr Roberts suggested that road speed governors set a little above the critical' 50mph would produce the right results, but he was not alone in recognizing that drivers and unions might need convincing of the value of such devices.
Elaborating on his written paper (CM last week) Mr Roberts, who is chief engineer, power units and transmissions, truck and bus division, showed how a 32ton artic needed 108kW (145 bhp) to cruise at 80 km/h (50mph) but because of air drag this rose to 153 kW (205 bhp) at 96 km/h (60 mph).
He told operators that only modest fuel savings could be obtained by adding equipment to existing vehicles or adjusting engine settings. In payload terms the best specification was one with around 6.4 kW/ tonne (8.5 bhp/ ton) using a high-torque-rise turbocharged engine; he suggested a 35 per cent torque rise.
It paid to go for the largest payload capacity that one could genuinely use, and extrapolated CM road test figures suggested that a gcw above the present 32 tons — perhaps around 38 — would provide an optimum in payload ton-miles per gallon.
With big tractive units, use of boxvans or containers did not increase drag measurably — they matched well — but tests had shown that poorly sheeted, humpy loads on platform trailers could increase air drag by 30 to 40 per cent.
Streamlined cabs could perhaps be expected to save 25 per cent in fuel consumption at cruising speeds, while a tractive unit with hightensile steel frame, integral fifth wheel, lighter engine cab and suspension could be 685 kg (15001b) lighter than now and lop 3 per cent off mpg.
Having enough gear ratios to enable the optimum ratio to be selected for each road condition could yield a5 per cent gain in economy. Single reduction axles matched to lirect-top gearboxes were nore economical than ndirect-top boxes and iouble-reduction axles. In total, changes in design and the limiting of maximum speed to 50 mph for 20 per cent of a journey could reL duce fuel usage by 20 per cent.
Mr Roberts quoted results from some very recent Leyland tests comparing a bread-and-butter vehicle With one specified for good fuel consumpton. The first, a 32-ton artic with naturally aspirated engine, fixed fan, overdrive gearbox, doublereduction axle and 6 bhp/ ton achieved 7.12 mpg and 149 payload ton-miles per gallon on a motorway journey without special restrictions. The other had a turbocharged high-torquerise engine, thermostatically controlled fan, direct-top gearbox, singledrive axle and was limited to 50 mph for the test it returned 9.31 mpg, a 31 per cent improvement. The vehicle was 2mph slower over the journey but its payload ton-miles per gallon rose to 196. Put over CM's 730-mire test route it was 25 per cent more economical than a similar, undoctored vehicle tested earlier.
Mr Davis, an engineer in the design application department of Perkins Engines, said that it certainly paid to increase weight, within the legal limit, so long as excess power was available. In some cases gvw could be raised by 25 per -cent for only a 6 per cent increase in mpg. He stressed that careful matching of engine and transmission could pay dividends, as could the study of crosscountry conditions with engine torque and specific fuel consumption characteristics in mind.
Frontal area was important. At around 84 km/h (52 mph) a 20-ton boxvan had the same resistance to motion as a 32-ton platform truck; the convenient matching of cab to container profile mentioned by Mr Roberts applied to high "Continental" vehicles but was not true of smallercabbed typically UK outfits.
While empty vehicles had less rolling resistance than loaded ones, the mpg benefits could be negated if the driver drove faster; an empty boxvan running 19 km/h (12mph) faster than when loaded could use just as much fuel because air drag rose sharply.
Mr Roberts had shown how much more economical were direct-injection than indirect-injection engines — down to quite low power ranges — but the indirect type was "cleaner". Mr Davis explained how the new Perkins "squish-lip" combustion chamber would enable some engines to remain DI without emission problems.
Costly drivers
A vehicle needed to be in excellent order to be economical but a proficient driver would always return a fuel consumption 0.7 kpl (2 mpg) better than computer predictions normally allowed; alterna tively, a driver could cost his employer an extra £600 a year in fuel.
Perkins had a computer system for assessing the effects of different engine / transmission / vehicle specifications, said Mr Davis. In reply to a question from the seminar chairman (C.M's editor) he said this could be used to give answers to specific requests from operators, through the chassis makers or distributors.
The advantages of thermostatically controlled radiator shutters and engine cooling fans and radial steelcord tyres were explained by manufacturers in the seminar's closing session. Mr Humphrey Kennerell of Michelin said that the mpg gain with this tyre construction averaged around 10 per cent.
Gordon Goddard, Whitbread's transport projects engineer, said Kysor shutters had improved heavy vehicles' consumption by 7.5 per cent and lighter vehicles' by 9.5 per cent over a fivemonth test period. There was a faster engine warm-up, and the shutters avoided the sudden cold-air impact which had been found when a vehicle which had laboured up an incline breasted the summit and eased off.
The general manager of Holt's Coaches and J. A.
Holt and Son Ltd, hauliers, told delegates of experience with Dynair fans, adding that both drivers and passengers had commented on the reduction in noise when these had been fitted.