Road Tests Of The DOE System ,
Page 74
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QHORTLY before the paper announcing this Perkins develop ment was read, I tested two vehicles equipped with prototype DDE power units and transmissions. The results obtained are recorded in the accompanying panel, from which it will be seen that of the two units the one in the E.R.F.-York articulated vehicle (grossing nearly 25 tons) performed in a far more satisfactory way than that in the lighter Dodge-York conversion, although some allowance must be made for the rather high tractive resistance of the smaller vehicle.
Perhaps the two most important features of both the vehicles cannot be put in tabular form. One of these is the extremely high average speeds which were set up—particularly over the "
country" route, which consisted of narrow and hilly roads. Most heavy vehicles would have been unable to reach 30 m.p.h. at all over much of this route, let alone achieve average speeds of at least this figure.
Similarly, the high average speeds obtained on simulated motorway running are indicative of the DDE system's ability to maintain a heavy vehicle at a high cruising speed, and the result of 8.95 m.p.g, obtained with the attic is most satisfactory, giving a time-load-mileage factor not far short of 10,000. Had the unit in the Dodge been in better tune a figure of more than 9 m.p.g. would have resulted, and whilst there are six-wheelers with conventional diesel engines that could better this fuel performance, it must be remembered that these would have bigger, heavier and more expensive units.
The other " hidden " advantage is that no gear changing at all was involved during any of these tests (except when making the gradient restart trials), and this is a tremendous contribution towards the reduction of driver fatigue. The DDE system of two-pedal control is, to my mind, a bonus effect of the principle and, of course, adds appreciably to the weightand costsaving advantages when comparing the DDE system with a conventional diesel engine and automatic gearbox. Furthermore, driving is simplicity itself, the driver having virtually nothing to do but keep the throttle wide open and only ease off when the desired cruising speed is reached. This means also that the DDE system would be ideal for a stage-carriage passenger vehicle: presumably a bus "test bed" will be one of Perkins' next moves, for the good acceleration rate and ease of driving control are desirable characteristics for a city bus.
Initially the power-unit noise heard in the cab seems somewhat excessive, but this is because it is easy to forget the size of engine (and the noise it would produce) that would normally go into a 24-ton vehicle. The noise effect is deceptive also in that when accelerating the engine speed goes up to about 1,400 r.p.m. and stays there until the lock-up clutch cuts in, which is usually between 25 and 30 m.p.h., but can be varied to suit individual applications. The constant engine note under such conditions gives the impression that the vehicle is not accelerating, but the figures in the data panel show how false this is! After all, the average 150 b.h.p.-engined, eight-wheeled attic takes about 50 sec, to reach 30 m.p.h. from a standstill.
When cruising in the 50s the engine never reaches its true
governed speed, whilst at such road speeds the engine note is little more than a quiet hum. Fan and supercharger noise are fairly prominent when accelerating hard, but these die away as cruising speed is reached: at their worst the noise levels are very similar to those experienced in the cabin of a turbo-prop air liner such as the Vickers Viscount. In any case, neither vehicle had an insulated engine cowl.
Standing-start gradient tests were made on a 1-in-9 slope, and the Dodge accelerated away slowly but comfortably in high ratio with a maximum boost pressure of 21 p.s.i. The theoretical gradability in high is 1 in 7, whilst in low it is 1 in 4. A reverse restart was made even more easily, and the Dodge tests were spoilt only by the emission of black smoke for the initial second or two when restarting. This smoke effect was much less in the case of the E.R.F., because of variations in the hand-built fuel-injection pumps. The E.R.F. outfit almost restarted in high ratio on the I-in-9 test slope, and made it easily in low ratio. The reason for the high-ratio failure was that the gearbox temperature rose too quickly because the engine had a 20-in, fan instead of the 22-in, fan which should have been -fitted. Unfortunately, slipping of the " experimental " reverse-gear clutch made a reverse restart impossible.
The smooth progress of both vehicles under all road conditions was highly satisfactory, and even when making the transition into and out of the lock-up clutch there was no jerking or pause. When decelerating or when the speed drops on a hill the clutch cuts out at around 20 m.p.h., and the smoothness of this change results from backing-up the boost pressure to 25 p.s.i, just before the clutch disconnects: at this stage the engine m.e.p. is probably well over the quoted figure of 255 p.s.i.
The test Dodge's transmission included the optional hydraulic retarder, this being controlled by a pedal located where the clutch pedal of a normal vehicle would be. The effect of the retarder was quite remarkable, and it could be used to descend steep hills without touching the wheelbrakes and also to provide varying degrees of retardation to suit traffic conditions, again without using the wheeibrakes. The effect increases with road speed, but even at 20 m.p.h. it could produce a deceleration rate of about 10 per cent, and later versions of the retarder are to have a 30° blade angle instead of the 45° angle at present used, and this will increase the present retardation and improve the performance at low road speeds. So attractive is the idea of this compact retarder, in fact, that I feel sure Perkins would find a considerable market for it if they were to sell it separately.
'My experience with these two DDE units convinced me that there is a definite future for this Perkins-Glamann system. It can enable an engine to perform quite happily at up to twice the gross weight for which it was originally designed, and this is particularly important in view of the rising speeds of heavy traffic throughout the world and the greater loads being carried_ When applied to an engine with a nominal horsepower of 200 b.h.p. it could lift its output to nearly 300 b.h.p. without weight or bulk penalties, and this aspect becomes vitally important as the horsepower scale is ascended. In fact, Perkins have already made provision for use of ihe principle on a new power unit still in the development stage. The additional advantages of two-pedal control and reduced wear and tear on driver, engine and transmission make this "
package" even more attractive. J.F.M.