Subtleties of Governor Throttle Design
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Position of Throttle Valve has Important Bearing on Physical Constitution of Mixture Charge
SPEED limits, whilst intended to promote safety, may easily have the reverse effect. When exacting time schedules compel drivers of fast passenger and goods vehicles to keep as nearly up to the limit as possible, the difficult conditions of modern road traffic and the irresponsibilities of pedestrians, call more urgently for the attention of the driver's eyes than does the dial of the speedometer. The necessity of being obliged continually to observe the speed-indicating needle, automatically constitutes a distraction.
-Need for a Governor.
One solution of the problem that thus arises is the inclusion in the engine equipment of some form of governor, which, by limiting the engine revolutions, makes it impossible for, a predetermined maximum road speed to be exceeded.
For a variety of reasons, a common practice is to employ a means for governing which comprises, broadly, a throttle valve that is brought into operation by the flow of gas mixture in the induction pipe and automatically impedes the passage of-gas when a certain speed of flow is attained. In considering methods of adopting this system, several important and highly interesting technical points arise.
Secondary Throttle Valve.
One widely used arrangement is to provide, in addition to the usual throttle, a second valve, on the engine side of the first, off-setting it and balancing it by a spring in such a way that an adjustable characteristic can be matched up with the sharply varying curves of minus pressures and charge velocity changes, so that the desired cut-off can be obtained.
This type of governor is, clearly, simple and cheap to produce, and easier to add to an engine not originally equipped with a governor, than the type which is operated by the centrifugal action of rotating masses. In practice it serves its purpose with success, and consequently enjoys considerable popularity.
Although the off-set secondary valve functions satisfactorily in so far as the accommodation of the charge weight to the r.p.m. is concerned, it is bound, especially when B40 applied to static carburetters, to fall short of the engine's needs in certain subtle respects which should not be difficult to understand.
For perfect flexibility throughout the throttle and load curve, not only must the charge weight conform to the requirements of the motor, but its physical constitution must also be maintained, both proportionately, in the air-petrol ratio sense, and in respect of the homogeneity of the mixture—that is—the correct disintegration and suspension of the fuel component.
Uniform Gas Speed Important.
It is well known that, to preserve this, it is essential that the fuel, after leaving the intentional and calculated restrictions in the carburetter by which it is disintegrated and suspended, must be kept at uniform velocity right up to the inlet valve. Any sudden change, en route, even inside the inlet port, which will cause a local alteration in the velocity, will reflect generally upon the performance.
Even a carburetter with a throttle chamber appreciably larger than the subsequent manifoiding and eventual inlet port will, in most cases, have its effect. It may be on the power, but, as a rule, it is either on part-load economy or low-throttle flexibility.
At full throttle the waist diameter of the venturiforrn choke, and the spraying arrangements, are generally dimensioned and designed, respectively, to spray adequately over a fair range of charge-velocity variation, provided there is no considerable restriction en route. When, however, the throttle is partly closed, this will, of course, progressively interfere with the predetermined functioning of the choke and main-jet assembly in the production of a suspended spray.
It does not matter, however, in this case, for the distance from the main spraying orifice to the throttle is carefully determined in a welldesigned carburetter, so that as the main efflux becomes progressively coarser, the maximum-velocity point, originally in the choke• waist, is gradually transferred to the throttle edge, where the lumpy output is caught up and finally disintegrated.
The distance from the main spraying orifice to the throttle edge is, in fact, very important, and is one of those subtleties of design seldom suspected by those not concerned therein. If it be a little too great, or if, perhaps, the direction of throttle operation be unsuitable, difficulties may be experienced in obtaining low-throttle flexibility or a clean transfer from the main to the pilot jet, owing to the choke velocity having been reduced to below the critical minimum by the closing throttle before the auxiliary or pilot jet is ready to take over the delivery.
Effect of Remote Valve.
Consider now the action of a secondary governor throttle situated on the engine side of the carburetter and the reason for stressing these points will be clear. When the cutout speed is approached, this throttle will commence to close, gradually at first, but much more suddenly as the critical speed gets nearer.
The carburetter throttle, however, is fully open and, accordingly, there is no peripheral velocity to come to the aid of the failing main jet and to pick up and disintegrate its output which is quickly becoming impossibly coarse. Nor is there any pilotjet help, for this comes into operation only by virtue of the high velocity or depression imposed upon it by the closing of its own throttle. The governor throttle is too far away for its peripheral velocity to"pick up and spray the main-jet output, and this is therefore wasted.
Qualitative Constancy Essential.
The consequence is that although the charge weight may be adjusted to produce automatically and correctly the r.p.m. limitation, the physical constitution of the charge cannot be maintained and, therefore, a cleah and unobtrusive flattening out of the speed curve is not possible. In order to achieve this end, qualitative constancy must always accompany quantitative governing, which it does not and cannot do unless the governing takes place on the carburetter throttle itself.
Theory and practice do not always agree, but in this case it has been found, both on road and bench tests, that, when the carburetter throttle is governed, the r.p.m. limit is imposed unobtrusively.