Motorway Record by Foden
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Experimental Foden Eight wheeler Fitted with Mark IV 210-b.h.p. Diesel Engine Averages 60.5 m.p.h. and 8.9 m.p.g. on Motorway
By John F. Moon
A.M.I.R.T.E.
FOR some months now motorists on the LondonBirmingham Motorway , have been startled to find themselves being passed by a cream n and red Foden eight-wheeler, and some of them must have had grave misgivings about the accuracy of their speedometers. This eight-wheeler has been no mirage, however: it has been making regular runs up and down the motorway at average speeds of up to 60 m.p.h, with, moreover, quite exceptional fuel economy.
Last week I travelled as " third man " in the cab of this Foden and took performance figures for myself. The vehicle, running at a gross weight of 244tons, completed the 130 miles of motorway between Dunchurch, the end of MIO and back again at an average speed of 60.5 m.p.h.. and returned an overall consumption rate of 8.9 m.p.g. These figures give a gross ton-m.p.g. figure of 218, whilst the time-load-mileage factor is I 3,322—the highest figure ever recorded by The Commercial Motor.
This sort of high-speed performance is exactly what the engineers of Fodens, Ltd., were aiming. at when they developed their Mark IV six-cylindered diesel engine, details of which were given in our issue of September 23, 1960. They were of the opinion that high-speed cruising with acceptable fuel economy would not be attainable
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Merely by increasing engine speed or raising rear-axle ratios.
Thus, their Mark IV unit has a governed speed of 2,200 r.p.m., whilst the experimental motorway eightwheeler has. 5.2-to-I rear axles. An overdrive ratio is incorporated, however, and this alone is felt by Fodens to be more advantageous than either a high-revving engine or over-geared axles.
Undoubtedly the sort of performance which I recorded last week is due almost entirely to the power unit, one particularly important characteristic of which is that, at a constant engine speed, the specific fuel-consumption curve remains almost completely flat between b.m.e.p. value of 80 and 160 p.s.i. The curve thus plotted is quite different from that of a more conventional engine, which reaches its minimum specific consumption figure at anything between 50 and 75 per cent. b.m.e.p., and which rises sharply again as the b.m.e.p. approaches its maximum.
What this " load loop" characteristic means in effect is that this latest Foden engine will propel a vehicle along a level stretch of road at. say, 40 m.p.h. at a certain consumption rate. KeePing the road and engine speeds constant. the vehicle can then be driven up a gradient without affecting the specific consumption. This appears to be the key to high-speed economy, although it can only be used to maximum effect provided' the transmission is well matched to the engine-power characteristics.
The Foden Mark IV diesel is based on their Mark III 4.09-litre supercharged two-stroke unit, which develops 150 b.h.p. (gross) at 2,400 r.p.m. and 365 lb.-ft. at 1,500 r.p.m. By adding a C.A.V. turbocharger to boost the air pressure into the normal Roots blower, these outputs have been raised to 210 b.h.p. at 2,200 r.p.m. and 580 lb.-ft. at 1,400 r.p.m. Air-chest pressure derived from the supercharger of the Mark Ill engine is about 7 p.s.i., and the maximum operating b.m.e.p. is 105 p.s.i, The combined effects of the C.A.V. blower and the supercharger raise the air-chest pressure to 22.5 p.s.i. at maximum power, the peak b.m.e.p. being in the region of 160 p.s.i.
The actual boost pressure coUld be higher than the present figure of 22.5 p.s.i., but this is not felt to be necessary, and air is bled back into the Roots blower as maximum engine • speed is approached, with a sligI# resultant power saving. The speed of the turbocharger roltor at full power and an engine speed of 2,200 r.p.m. is 751000 r.p.m.
Maximum blower efficiency has been achieved by the use of an air-to-air intercooler carried ahead of the normal vehicle radiator. This reduces the temperature of the air leaving the turbocharger compressor, so raising the charge density. Including the intercooler and the attendant ducting (for which plastics mouldings are used), the complete Mark IV engine weighs only 84 lb. more than the Mark III, its total weight of 1,420 lb. giving a specific figure of 6.75 lb. per b.h.p.
Another important device contributing to the potential success of this. Foden engine is a boost torque control incorporated in the fuel-injection pump. This limits the fuel delivery below an engine speed of 1,200 r.p.m. and so reduces the chance of over-fuelling at low engine speeds, with subsequent reduced risk of black-smoke emission.
Nevertheless, I noticed that, despite this precaution, slight exhaust smoking was occurring when travelling along normal roads at normal speeds, although the amount of Smoke emitted was only slight compared with the condition of some of the other vehicles on the same roads and could by no means be categorized as being objectionable.
The vehicle with which this experimental work is being carried out is in other respects 1 a standard Foden longwheelbase eight-wheeler. The'specification includes the Foden 12-speed gearbox, consisting of a .four-speed main section with integral three-speed auxiliary section. The highest overall ratio afforded is 0.77 to 1.
Normally the auxiliary section is controlled by a separate gear lever located to the rear of the driving seat; the gearselection mechanism being such as to give a preselection
effect. As an experimental installation, however, this particular test vehicle had air-pressure selection for the auxiliary gearing, the control lever being a neat unit mounted on the right of the steering column. The chassis had the now-familiar Foden-built plastics cab—a.strong, attractive assembly which saves weight and costs very little more than the alternative coach-built unit. The chassis-cab kerb weight on the test vehicle was remarkably low, being only 6 tons 9 cwt. with a full tank of fuel. Had a single-drive rear bogie been fitted, 4f cwt. would have been lopped off this kerb weight, but as it is a lightalloy platform body would have added only about ton, thereby permitting a genuine 17-ton payload to be carried.
A fuel-consumption check was taken during the journey from the Foden factory at Sandbach to the start of M45 at Dunchurch and the 81 none-too-easy miles were covered at an average speed of 30.5 m.p.h. for a consumption rate of 8.7 m.p.g., although for the most part the eight-wheeler was cruised at little more than 35 m.p.h.
Impressions of the Foden on the motorway were entirely satisfactory. The vehicle handled well and rode smoothly, and engine noise in the cab was decidedly less than is normally experienced with a 200 b.h.p. diesel engine. A staff photographer positioned at one of the service areas was passed by the vehicle twice—each time while travelling at more than 60 m.p.h.—and he was quite positive that the vehicle was no noisier than other heavies travelling at slower speeds and decidedly quieter than at least one make of medium-capacity vehicle.
The gearing of the test vehicle gave a maximum speed of about 64 m.p.h., and that the average speed for the round trip was only 3.5 m.p.h. lower than this indicates that upgrades have little effect oil the speed of this Foden. Rarely did the engine speed drop below 2,000 r.p.m., and only when ascending one gradient did the road speed fall below 50 m.p.h. To achieve an average speed so close to the vehicle's maximum speed is a remarkable indication of the potentialities of this design; almost as outstanding, in fact, as the consumption rate of 8.9 m.p.g. achieved despite over two hours at continuous full throttle. Earlier tests at lower average speeds have produced such figures as 11.9 m.p.g. at 41 m.p.h., and 10.4 m.p.g. at 49.9 m.p.h.
No extreme temperatures were recorded during this run, the coolant in the radiator top tank remaining at a steady 170° F., despite an ambient of over 70° F., whilst the oils in the main and auxiliary gearbox sections and in the first and second differentials finished at temperatures of 165° F., 186° F., 183° F. and 188° F. respectively.
After the round trip up and down the motorway the brief acceleration tests were carried out. By split shifting 20, 30 and 40 m.p.h. were reached from a standstill in 19.4, 37.0 and 70.5 seconds respectively. Direct-drive tests could not be taken from 10 m.p.h. because of the high gearing, but 20 to 30 m.p.h. took 34.0 seconds, a further 26 seconds being required to reach 40 m.p.h.