"A FUEL CONSUMPTION consistently better than 31.4 lit/100km (9mpg) at
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60mph on motorways with a fully laden 32.5 tonnes gcw articulated combination with a box-bodied semi-trailer". Such a claim would normally be dismissed as either the dream of a very hopeful haulier or one of the wilder promises of a manufacturer's marketing department.
In fact in this case it is neither, but a considered forecast for the imminent future (that is within the next twelve months) based on some intensive test and development work made by engineers at Leyland's Technical Centre.
They are prepared to stick their necks out still farther and predict confidently that by the year 1990 the same 32-tonner with even the tallest boxvan semi-trailer will be achieving 12mpg.
Leyland plans to reveal the full details of its 9mpg 32-tonner later this year so at present its specification cannot be described in detail, but there is no secret about the development work on fuel economy which the technical centre has been involved in over the past year or two.
In the short term, that is up to 1990, there is no doubt that conventional diesel engines will continue to be the power units for heavy commercial vehicles. The engine in the 9mpg vehicle, for instance, is an in-line six, Leyland's own TL11, with no feature more technologically advanced than charge cooling.
Leyland is preparing for the degradation in diesel fuel qual ity, which virtually everybody in the industry sees as inevitable over the next few years, by assessing fuels with cetane numbers as low as 40 (the standard minimum in the UK at present is 50) and measuring the effect on engine power, specific fuel consumption and exhaust emissions.
Tests have also been carried out with a fuel that Leyland's engineers believe will be typical of a future European diesel. It is a wide cut fuel with a cetane number of 45 and with more aromatic content than current grades.
While the results show a worsening in engine performance and exhaust emissions, they are not so serious as to warrant a major redesign of the engine.
However, naturally aspirated engines are more affected than turbocharged models by degraded fuels and so are likely to continue to lose favour.
The higher temperature allowed by ceramic components in the combustion chamber helps efficient combustion with a wide range of fuels and this feature has given added impetus to work on thermally insulated engines.
Leyland's target for a practical insulated engine is to reduce the heat lost to coolant from the normal 22 per cent to 14 per cent and to this end a minimally cooled cylinder head has been designed and manufactured and an engine fitted with such a head has been run on a test bed.
It is clear that this work and the parallel testing of ceramic materials for valves and pistons is leading to small improvements in thermal efficiency in the short term and providing a foundation for the more radical step forward to an adiabatic turbo-compound engine perhaps in the 1990s.
But the results of Leyland's fuel economy vehicle project have shown that significant improvements in mpg can be achieved with a current production vehicle before any of this advanced technology is applied.
The model chosen by the technical centre for its testing was a day-cabbed 17.28 Roadtrain with a Rolls-Royce 290 engine. Roadtrain was used in preference to a Cruiser model because the former currently outsells the latter and the gcw was kept at 32.5 tonnes because that is still the most frequently specified plated weight.
The day cab's tractive unit offers greater potential for aerodynamic efficiency improvement than a sleeper cab because its mismatch with a high load or box van is greater.
Like many other manufacturers, Leyland has a computer program (this one is called RSVP — route simulation vehicle program) for simulating on-road testing but actual on-road testing techniques were used for the economy vehicle project so that predicted results could be correlated against actual results.
A section of M6 motorway was used as a typical motorway route while the flat M55 motorway was used for realistic constant speed testing at 60mph.
Leyland has its own test track and this too was used for con stant speed fuel consumption testing but the engineers were unhappy with the results. Be cause the vehicle is constantly negotiating bends on the track, its tyrelosses (due to either centrifugal force on banking or slip angle) and aerodynamic drag due to articulation were abnormally high.
It was found that a deviation of as little as half a lane from the test track's neutral line would increase fuel consumption by as much as 10 per cent.
A so-called endurance route in West Yorkshire was used to check that driveline modifications were not too seriously affecting the vehicle's driveability.
Leyland set about improving the fuel economy of the base line vehicle by concentrating on three broad areas: aerodynamic efficiency, rolling resistance and driveline matching.
A whole range of aerodynamic aids for tractive units and trailers was tested and the accompany ing diagram shows how each affected wind-averaged drag coefficient (ed) on half-scale models in a wind tunnel. Windaveraged drag coefficient is the factor defined by TRRL staff, which takes into account the probability of wind direction in this country.
The combined effect of all the devices was found to be a reduc tion in wind-averaged drag coef ficient by 40 per cent and in practice this gave a fuel eco nomy improvement of 16-20 per cent at 60mph, reducing to 7-13 per cent at 40mph.
On the M6 test section a typical mpg improvement with the full aerodynamic package was from 6.45mpg to 7.64. Not surprisingly it was also found that the effect of the various deflectors, skirts, gap seal and under-bumper spoiler was to noticeably lower in-cab noise and suppress spray from the tyres.
It is interesting that the chassis filler panel which had shown considerable promise with oneeighth scale models proved to have very little effect on larger scale models.
On the subject of rolling resistance Leyland is quite strongly critical of the lack of information provided by tyre manufacturers, describing the detailed data available for commercial vehicle tyres as "incomplete and inconsistent".
The engineers at the technical centre were surprised at the magnitude of fuel economy improvement that could be achieved by substituting wide singles on the semi-trailer for the standard twinned 11R-22.5s. The improvement of six to eight per cent suggested a reduction in rolling resistance of at least 20 per cent.
In the area of driveline match ing, Leyland is involved in some particularly interesting work with a continuously variable transmission for urban delivery vehicles but no benefit is seen in this type of transmission for 32 tonnes.
Leyland has repeatedly argued with the widely held view that single reduction drive axles have significantly reduced losses compared with hub re duction units and the manufacturer is not making a complete U turn on that subject now, maintaining that their relative efficiences over the range of operating speeds is "remarkably similar".
However, Leyland's practical test results confound that theory to an extent, for a fuel economy improvement of six to nine per cent was measured on the Roadtrain when the standard 4.43:1 hub reduction axle was replaced by a 4.11:1 single re duction unit, though it is re ported that on the endurance route in West Yorkshire "drivea bility was slightly impaired".