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The Claims of the Controllable Infinitely Variable Gear

20th December 1935
Page 38
Page 38, 20th December 1935 — The Claims of the Controllable Infinitely Variable Gear
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Which of the following most accurately describes the problem?

By H. F. Hobbs

AUTOMATIC or semi-automatic gears, as such, may or may not possess the balance of advantage over the straightforward gearbox. They may not give that measure of control which can be considered desirable and necessary. What is, in theory, the ideal transmission would, I think, permit of constant engine speed, thus providing a perfect or nearly perfect tractive-effort curve. In practice, however, an infinitely variable and fully auto-. matic gear of this type would meet with a great deal of criticism.

The internal-combustion engine, being what it is, calls for some measure of control. It would not be desirable to run it continually at its maximumb.h.p. speed (when full power and best performance would always be available) or at its-maximum-torque speed (when economy would be good and upkeep low) or, for that matter, at any point intermediate. The driver would not be accustomed to, nor would he like the monotony of a constant-speed engine..

Desirable Characteristics.

I put forward the claim that the Hobbs system, whilst retaining most of the advantages of the theoretically ideal system, fully provides that measure of control which is desirable in practice. This system need possess no means for control other than that for reversing, for the various changes and effects are most readily produced by manipulation of the engine throttle or accelerator pedal.

The action can, perhaps, be illustrated, if I take the case of a coach chassis equipped with this gear, and compare it with one having the normal gearbox. It should be noted that the gear can be designed to suit any particular vehicle, a bus gear, for instance, requiring slightly different characteristics.

The engine of the vehicle under consideration develops its maximum power at. 2,125 r.p.m, and gives maximum torque at about 1.000 r.p.m. Top gear or direct ratio can be obtained, with the Hobbs gear, at any speed by momentarily fully closing the throttle, whilst the gear will remain in " top " with a fully opened throttle at any speed above 14 m.p.h. or 875 r.p.m. (125 r.p.m. below maximum-torque speed) and at lower speeds with a partially opened throttle. The ordinary gearbox will provide no more. No one, I think, would consider it desirable to continue in " top " under full throttle at any lower speed. There can B28 be no advantage fi doing so to driver, operator or passengers.

On the other hand, the indirect ratios can be retained in use up to 30 m.p.h. without substantially exceeding the maximum-b.h.p. speed of the engine. If 30 m.p.h. can be reached in third gear with the ordinary box, the engine will rotate at 3,200 r.p.m.—far above maximum-b.h.p, speed. The Hobbs system, therefore, provides a much wider range of indirect ratios than is obtainable with the ordinary gearbox.

Once in top gear, indirect ratios cannot be obtained at speeds above 20 m.p.h., except by incorporating a special control, as is intended for private-car gearboxes. To bring indirect gear into action, except at 14 m.p.h. when it will automatically come into operation under load, the driver closes the throttle and then suddenly opens it.

Little or nothing can be gained by changing at a higher speed with the normal box, because in third gear at 20 m.p.h. the engine rotates at 2,120 r.p.m., which is the speed giving the maximum power.

Gearbox Limitations, With the ordinary gearbox one can travel at say 12 m.p.h. in second gear, giving an engine speed of 2,200 r.p.m. (maximum-b.h.p. speed) or in third gear with the engine rotating at 1,300 r.p.m., and this is the measure of control that the ordinary four-speed gearbox gives. In second gear the full power of the engine is available for climbing or acceleration, whilst in "third," engine r.p.m. can be reduced and greater economy obtained.

Take the case at 16 m.p.h., however, and we see that the ordinary gear does not provide that same oppo4tunity for the driver to exercise his own judgment. In second gear, engine speed would be 2,875 r.p.m., which is far too high, and " third " is his only alternative at 1,700 r.p.m.

The Hobbs system provides two different sets of infinitely variable ratios, a partial closing of the throttle being all that is necessary to drop from one to the other.

At 16 m.p.h., for instance, the driver can have, at will, either 1,990 r.p.m. of the engine for accelerating or climbing, or 1,540 r.p.m. for more economical running. At 12 m.p.h. there are alternative engine speeds of 1,800 or 1,350 r.p.m.; at 14 m.p.h. 1,875 or 1,450 r.p.m.; at 18 m.p.h. 2,125 or

1,675 r.p.m., and so on; or any, other range that Might be provided for in the design.

This gear, therefore, although automatic in nature, does provide a large measure of control—taken over the full. range, more, I think, than the ordinary gear. Furthermore, greater power is available for acceleration when required. The feel of the vehicle is similar to that of a machine having an ordinary gear in that engine speed does vary with road speed, but not to the same extent in indirect gears, and this is, of course, where the infinitely variable gear has the advantage..

Elimination of Steps Up.

The drive is continuous, except when changing from one set of ratios to another, but this change is almost instantaneous, an appreciable pause being necessary to attain only top gear or direct running. Full engine power can be obtained at very low road speeds, and a steady and high torque is available for starting or climbing severe gradients. This shows .great advantage over the ordinary lowest fixed gear and clutch.

The infmitely variable gear not only affords the ideal starting ratio, but also eliminates the dead loss of power inseparable from the most efficient mechanical slipping device.

The only real limitations the gear imposes are those which prevent the driver from doing that which a good driver would not do, namely, making the engine labour at speeds well below the maximum torque speed, or race beyond the maximum b.h.p. speed. It is mainly because of these limitations that greater fuel economy and decreased maintenance can he claimed.

The driver must gain by the simplicity of control and absence of manual effort, and the public by increased safety and better riding comfort.

The weight of this alternative gear is less, and, quantity for quantity, the cost, at the most, no greater than the ordinary clash gear and clutch.


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