GOVERNORS FOR HEAVY VEHICLES.
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A Discussion on the Advantages and Disadvantages of Various Methods of Speed Control.
AFEATURE of many _petrol lorries of 2-ton capacity and over is the governor control which limits the speed attainable. The ways in which this control can be carried out form an interesting subject for impartial discussion, and are of considerable importance to designers and users alike.
Two methods can be adopted. In the first, the maximum speed which the engine can attain is limited by a governor operating a throttle ; in the second, a brake can be brought into operation by centrifugal action at a pre-determined road speed. Of these, the
first is by far the most widely used, owing to the comparative simplicity with which the control can be carried out. There are, however, many disadvantages in this system.
In the first place, the aim of the control should be to prevent the exceeding of a certain road speed in order that troubles such as spring, axle, and frame breakages shall not occur. Hence, it would appear that the limitation of engine speed is open to serious objections. Suppose, for example, that the governed speed of a 3-ton lorry engine is 1,000 r.p.m., and that on top gear this corresponds with a road speed of 12 m.p.h. Then, if a three-speed box be fitted, the maximum road speeds on first and second gear will he about 3 m.p.h. and 7 m.p.h. This is a great disadvantage on a medium gradient, such as can be taken on second gear: The driver is unable to accelerate, and cannot, in consequence, pass slower vehicles.
If a steeper gradient involving the use of the first speed is followed by a milder slope, the driver is unable to change up because he cannot accelerate beyond 3 m.p.h. on the first speed, and if a change be attempted the engine will stall, as. the vehicle will not have sufficient momentum. When downward grades are encountered the makers' intention is frustrated alto
gether. If the driver be short of time, he can take the hill with the clutch disengaged at anything up to 30 m.p.h., with results on the chassis which can hardly be imagined.
-There is also the question of how best to arrange the governor control. In many designs it is a simple matter to disconnect the governor entirely, a fact whichis well known to many drivers. The writer remembers a certain firm in the north which sent a convoy of 3-ton W.D. lorries to the south by road every
1335 week-end. The drivers of this convoy made a regular practice of stopping five miles from the works, disconnecting the governors, proceeding at an average speed of 20 m.p.h., and utilizing the time saved in entertaining themselves in London before returning.
The type of governor control used on the engine of a famous commercial vehicle is shown in the bottom sketch. Two independent throttles are employed, the outer one being controlled by a pedal in the usual way. The inner one is mounted on a completely enclosed vertical rod, which carries a lever at the lower
end. This lever is enclosed in the crankcase and is operated by a rod from the governor at the front end of the crankshaft. It is a difficult matter to render this governor inoperative. Even if the carburetter be rewired and the screw securing the inner butterfly valve detached, the vertical rod cannot be, got out ; consequently the valve will continue to operate.
The second sketch shows the Goes engine governor, which is specially designed for application to the engine of the Ford one-tormer. It is enclosed in n. neat circular casing and is driven by the timing wheel at the front of the engine. The plunger, operated by the weights, actuates a butterfly throttle valve which is placed between the carburetter and the induction pipe. This constitutes a reasonably driver-proof" arrangement and is said to give good results in practice.
The design of e petrol engine governor must be very carefully carried out. It is desirable that the _governor should have no effect until
speed within about 50 r.p.m. of the maximum is reached ; the valve should thee close positively and rapidly. Another trouble. to be avoided • is "hunting." Some designs are very bad in this respect ; if the throttle be opened wide with the engine running light, the speed fluctuates widely for some time before the engine settles down.
Many of the disadvantages inherent in engine governor designs could be obviated if a centrifugal brake were used, although this would involve some additional complications. The centrifugal brake could be arranged so that it would not come into operation until -a reasonably high speed was reached and, by fitting it behind the gearbox, this speed would remain the same no matter what gear was being
used. To obviate a duplication of brakes, the shoes of the centrifugal brake could be independently controlled by the pedal or hand lever. A sketch of such an arrangement is shown, based on the design of the "Broadbent " centrifugal clutch. Four sliding shoes are normally held in position by flat springs, the cage in which they are mounted being secured to the shaft behind the gearbox. When a certain speed is exceeded, the centrifugal force overcomes the springs and the shoes moVe outwards, coming into contact with the fix8d drum. The action of
such a brake would be smooth and progressive, and could be so arranged that the shoes would cornmence to move at, say, 18 na:p.h., and would produce their maximum effect at, say, 20 m.p.h.
An independent control actuated by the pedal or hand lever could be arranged as shdwn, operating the shoes through the medium of a sliding collar, four radial arms and a slotted link. The wear on the fabric linings might be somewhat excessive, but a readily detachable cover would enable the shoes easily to be reached and removed for re-lining.