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Full Details of the Construction of a New Product of

22nd October 1929
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Page 58, 22nd October 1929 — Full Details of the Construction of a New Product of
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Kerr Stuart and Co. (StOke-on-Trent). A 7-ton Chassis of Outstanding Merit and Extraordinary Fuel Economy Which Has Already Run for Over Six Months.

II OR a long time we have been in close touch with the technical department of Kerr Stuart and Co.. the famous locomotive builders of Stoke-on-Trent, and have watched the evolution of at. entirely new departure which has now taken concrete form. Our association with the company dates back to November, when we undertook a trial of a Diesel-eiagined locomotive on a section of line in the .Welsh hills. We Were then informed of the company's latest activity, i.e., the production of a medium-capacity lorry, powered by a Diesel engine, -which has proved to be the first British example of this type.

Although new in the sense that this particular chassis has no prototype it must not be thought that the production-model Kerr Stuart lorry is an untried proposition, for an experimental vehicle has, to out knowledge, been running for at least six months and has covered many thousands of miles. Naturally Many details have been found which have required rectification but one important point has emerged from the tests. Fundamentally, the design has proved to be correct and practicable, such alterations as have been found necessary being confined to details unconnected With the principle of employing a Diesel engine. The maker has, we think, very wisely chosen a power plant that has been tried out under arduous conditions in all quarters of the .world, consequently it has, right from the start, discounted all engine troubles. The transmission of the power is an entirely different subject, but as any engineer will realize, there are no insuperable difficulties in this.

We have prefaced our article dealing with the new Kerr Stuart product in this manner because we feel that a certain amount of trepidation is bound to exist in the minds of prospective purchasers as to the dependability of the machine under arduous service conditions. When it is realized, however, that the components used in the layout of the chassis are all of a type which has been thoroughly tested— admittedly in various spheres of activity—and combined in a unit, the matter assumes a different aspect.

Although the engine is the most interesting part of the layout, we will give a description of the vehicle as a whole, in order that our readers may obtain a mind-picture of the general layout. The illustrations-will convey, some idea of the proportions titi2

and general Shape, and we will noiv quote a few of the main dimensions. With wheelbase and track dimensions of 13ft. 6 ins., 6 ft. 2 ins, and .5 ft. 8i ins.. (for: the front and . rear wheels respectively), the vehicle can be. turned in a circle of. approximately,66 -It.: diameter.

_The.' photographs reproduced depict the rst experimental vehicle, Which is not exattly the same as the production model, for it has been found that by. moving the cab 6 ins, to the rear and the driver's seat 6 ins, to the off side, a better layout for the mechanical features can be obtained. The position of the hand-brake, too, has been modified to bring if within easy reach of the driver's hand, whilst several small details have been more tidily arranged. , •

With these alterations the overall length works out'at 22 ft. 3 ins, and the overall width 7 ft. 6 ins., whilst the height of the cab is no more than 8 ft. 6i ins. Again, in the matter of loading height, the main channels a the chassis now point downwards towards the front, so that the spring brackets are ShalloWer.With 36-in. by 8-in.tYres • instead of 40-in. .Liy S-in. the height to the top ,of., the frame (loaded) is slightly less than 3 -ft: 2 ins., Whilst the loading height for the platform is 3 ft: at ins.

The first question upon which discriminating operators will require satisfaction when they are contemplating the purchase of a new vehicle relates to the mattered loading capacity. In the Kerr Stuart lorry the standard platform, which is of maple wood, by the way, measures 15 ft. 3 ins, long by 7 ft. wide inside the chock rails. These rails are 4 ins, deep, the rear one being hinged to facilitate the removal of heavy loads, and ample oak cross-members are fitted to transmit all stresses to the chassis.

The first experimental model, i.e., the one under. review, Actually weighs, unladen, 5 tons 18 cwt. 3 qrs., and is capable of taking a pay-load of approximately

tons. From the experience gained, however, the production types will each be one ton lighter. This has been effected by making the main-frame channels .

in. thick and of 36-42-ton high-tensile steel, instead, of in. thick and from cold-flanging 26-30-ton steel. Again, the cast-iron gearbox has been pared and much weight has been saved in the rear-axle suspension, whilst hand forgings, as used on the experimental model; are heavier than the stampings employed 'In the production chassis. The pay-load capacity, bearing in mind the reduction in chassis weight, will be 7 tons in the finished product. The frame side members are 12-in. channels braced by strong boxshaped cross-members, and gusseted against rhomboidal _ distortion. Drawbar brackets are fitted.

The front axle is carried on semi-elliptic springs with extra-wide leaves of ample strength for the load and braking stresses. At the rear the layout is of more than passing interest, for two lateral semielliptic springs are carried on a cross-member and bear at both ends on prepared pads incorporated lathe brake-housings. Now there are many advantages to be obtained from such a layout as this. The maximum amount of weight is "sprung "—a very important feature in a heavy vehicle such as this Kerr Stuart product. Next, the vehicle is in reality three-point suspended, for the rear wheels can articulate to a large degree without affecting the alignment of the chassis as a whole. Finally, as the movement of the axle-assembly is controlled by combined radius and torque arms, the two springs tend •to give an. enharmonic differential action which is to some extent self damping..

We can now turn to a consideration of the power plant. It should be unnecessary to state that the unit is much heavier than a petrol counterpart, bY virtue of the greatly increased loads on all journals occasioned by the high cylinder pressures -which are a sine qua non to the efficient operation of the Diesel cycle. The unit employed is a product of J. and H. AfacLaren, Ltd., of Leeds, and is of almost identical..

type to that used for railway work.The conditions df service are somewhat comparable, for even the locomotives have to start from .rest through the medium of a clutch, and a gearbox is also necessary to maintain efficiency under varying conditions.

The weight of the unit in question is 23 cwt. It presents a neat appearance, despite its robustness, the entire absence of such items as an inlet manifold, exposed 'valve gear and so on giving the exterior of the cylinders unusually clean lines. With a cylinder bore of 135 mm. 'and a piston stroke of 200 mm.' the four cylinders .have a total piston-swept volume slightly -greater than 11 litres,. and with a compression ratio of 14 to 1 they provide a maximum power output of 00 b.h.p. stt 800 r.p.m.

It may be of advantage to divert our attention from a description of the power plant and describe the principle upon which the Diesel 'engine operates. The great difference between the Diesel engine and the ordinary petrol power unit is that the former is what is known as a con, stant-pressure engine, whereas the latter works on the constant, volume principle.

To appreciate this it is necessary to consider the cycle of operations: A four-cycle Diesel engine has four strokes as in the ordinary petrol engine, i.e., induction; ' compression, power and exhaust, but on the induction stroke air only is drawn into the cylinders, and is compressed on the second stroke, the ratio being 14 to I. So high is the compression that the air becomes very hot in the process. Just before the maximum compression pressure, i.e., a little beforeathe top of the .stroke, a small charge of oil in the form of a very finely 'divided spray is injected into the cylinders, and the heat of the air is sufficient to gnite the charge. Naturally, the introduction of the oil is so timed that, as it burns, the maximum cylinder pressure is reached while the piston is travelling downwards. This characteristic is exploited so that a steady push is given to the piston, instead of an almost instantaneous '' explosionlike" force.

In the ordinary Way the petrol engine is varied for power output by " throttling " the in-going mixture. In the Diesel engine the amount of oil injected at each stroke is controlled, and from this follows an important feature. Since the full quantity* of air is taken into the cylinders at each stroke the compression pressure remains constant, irrespective of load or speed. As a resultthe efficiency of the engine remains practically constant throughout the whole speed range. In other words, the fuel consumption of the Diesel engine is nearly proportional to the load.

We have purposely gone into details regarding the functioning of a Diesel 'power plant in 'order that our readers may comprehend fatly the factors involved.

The Kerr-Stuart Engine. .

• The power unit of the lorry .in question follows much upon normal lines. . The cylinders, however, are built up with strong cast-iron liners which are pressed into an outer water-jacket casting.' In this way the bores can be renews4 when, after prolonged set-vice, Wear has taken place.

Following the 'best petrol-engine .practice, the Cylinder heads. are detaohieble; anctbach-one carris the inlet and exhaust valves as Weltsiaa special antichamber intowhich the fuel is -injected: • The 'provision of, this chamber is ins

portant, as it gives aregulated fuel ignition and makes the engine easier to start from cold. Actually, the antichamber communicates with the cylinder proper by means of a somewhat restrieted opening—a flame plate—which is so arranged that it ,remains hotter than the rest of the cylinder, .thus aiding the combustion of the fuel. The overhead valves are operated by push-rods and rockers.

Robustness of Construction.

The crankcase is of cast-iron, in which webs are formed to carry the live large bearings of the crankshaft. _ This component is drilled throughout for forced lubrication to the big-ends of the II-section rods. Even the gudgeon-pin bearings are supplied with oil from the main system. All bearing surfaces are of ample proportions to permit the bearing pressures per square inch of surface to be relatively low even under overload conditions. The pistons are of the usual east-iron trunk pattern, but in view of the high working pressure to which they are subjected they have specially strengthened crowns and each has five rings.

At the forward end of the crankshaft a train of gears operates the camshaft, the' fuel pumps, that for the water -• and the oil pump. This last-nanied unit is situated in the bottom of the sump and is always fully, submerged and, in consequence, is kept primed.

The fuel-pump assembly can be said to be the heart of the whole engine, for it has to perform the dual functions of carburetter and magneto. It has to inject into each cylinder at the exact moment of time when it is desirable a carefully determined. amount of fuel. It is clear that the plunger has to work ,against the full compression in ihe engine cylinders and the fuel must 1 injected with such force that it is broken up into a fine spray or mist so that combos-. tion will be rapid and complete. In the Kerr Stuart design the pump assembly contains four separate pumps, i.e.,

one for each cylinder. They are actuated by means ()f

rocking levers from a camshaft and function with extreme accuracy. They can be taken adrift and reassembled. • The rocker-arm driving mechanism of the fuel pump is arranged so that a variable stroke may be given to the plungers, which, in turn, varies the amount of fuel dealt with at each stroke.. Incorporated in the mechanism is a governor which restricts the speed to 800 r.p.m.; for idling and reduced speeds this governor is controlled by an accelerator which, in effect, alters the governor spring.

One more point remains—the .design of the atomizers. These components are of the spring-closed pattern, and are fitted into the cylinder-heads, where they spray into special Pre-ignition chambers retained normally at higher temperatures than the cylinders. These chambers, 'which are a basic feature of the engine, allow a heavier grade of fuel to be used than that permitted in a direct-injection type. Each atomizer is provided with a hand-operated by-pass and tell-tale.

In the main the transmission layout follows.accepted practice; The clutch, for example, is an inverted cone wAth Ferodo 'facing; and is operated by a pedal. All the operating gear is self-lubricated, and no pressure is applied to the end of the crankshaft when the clutch is out of engagement.

A short propeller shaft with -Hardy Spicer fabric, joints at each end transmits the Poiver to the gearbox, which is housed amidships. Four speeds are provided, the standard ratios being : top, 4.4 to 1; 3rd, 7.94 to 1 ; 2nd, 14.78 to 1 ; and reverse, 27.8 to 1. To promote rigidity the gearbox is made of cast-iron, the whole unit being suspended in the chassis at three points. The differential shafts emerge from each side of the casting, and on their ends chain sprockets are mounted as close to the frame as possible in order to avoid overhanging the bearings. Top gear is, of course, direct, the final reduction being effected by means of a steel worm and a phosphor-bronze. worm wheel, in which the differential is carried and lubricated from the gearbox.

Roller chains of 2i-in, pitch run from the differential shaft sprockets on each side of the gearbox to chain wheels on the brake drums of the back wheels. It is fairly obvious` that the chains themselves are well above their work, and since most of the reduction takes place in the gearbox the sprockets are of large diameter, .reducing chain wear to an insignificant amount. Mudguards are mounted over the top of the chains to prevent dirt and mud falling on them from the underside of the body. The rear axle itself is formed from a high-duty nickel-steel tube, which makes it pasible to combine lightness with strength ; the rear axle does not revolve, carrying only the dead weight of the vehicle. _

Brakes are provided for all four wheels ; they are of the internal-expanding type, two shoes being arranged in each front hub and four iu each of the rear hubs. The pedal operates the front brakes and one set of rear brakes, the hand lever applies the remaining set in the rear drums.

The cab is so arranged that it may be removed together with the forepart of the bonnet without disturbing any portion of the chassis, thereby providing facility for inspection of the power unit or, if necessary, its entire removal for overhauling. The forward part of the bonnet, integral with the cab, is provided with a removabletop and side .plates to enable the engine to be reached for minor adjustments. The rear portion of the bonnet is separate from the front part, and runs inside the cab, where it, insulates the engine from the driver and mate's seats.

As might be expected, a high-compression engine such as this K rr Stuart product cannot conveniently be swung by hand r by a,starter motor. Instead, a 4 h.p. aircooled sin le-cylindered petrol engine is installed, driving by means of a chain to a gearbox, thence to a friction ring which can be brought into contact with a set of grooves in the mainngine flywheel.

This Kerr Stuart vehicle is 'unquestionably one of the outstanding examples of automobile design for 1930.. We hope to publish in thd near future the results an extensive road test which we have carried out.


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