AT THE HEART OF THE ROAD TRANSPORT INDUSTRY.

Call our Sales Team on 0208 912 2120

A NEW PETROL-ELECTRIC CHASSIS.

19th December 1918
Page 12
Page 13
Page 14
Page 12, 19th December 1918 — A NEW PETROL-ELECTRIC CHASSIS.
Close
Noticed an error?
If you've noticed an error in this article please click here to report it so we can fix it.

Which of the following most accurately describes the problem?

The Latest Production of a Pioneer in the Transmission of Motor Power by Electrical Means.

IN THE FIRST three issues of THE COMMEECIAL MOTOR in the month of November we dealt briefly with the general principles underlying transmission by electrical means of the power produced by an internal-combustion engine and designed to propel a road vehicle.

Falling within the category 'of the first system there to be outlined, in which the engine drives a dynamo which excites a motor positioned at the forward end of the propeller shaft, comes the Stevens petrol-electric commercial vehicle, an opportunity for a Works examination and a, road test of which was .afforded us recently. This vehicle is-manufactured by the Stevens Petrol-Electric Vehibles, Ltd., of 82, Victoria, 'Street, London, S.W. 1, in the works of Messrs. Dennis Bros., Ltd. The inventor is Mr. W. A. Stevens, who formerly was chairman and managing director of Messrs. W. A. Stevens, Ltd., who produced the Tilling-Stevens electrical transmission. Since the severance !of his connection with the latter company, its name has been changed to TillingStevens, Ltd. Mr. Stevens has devised a new system, offering, in his opinion, certain advantages over any system previously on the market, and, as managing director of Stevens Petrol-Electric Vehicles, Ltd., anticipates an extensive field of usefulness for the vehicles on which he tuts expended so much pioneer effort.

Produced, as it was, in war time, Mr. Stevens submitted his vehicle to the military authorities, and, convincing them of its merits, secured first orders for 80. When the armistice was signed the company had an order for a further 250, of which 20, following upon the original 80, had been delivered. So successful had the first deliveries been that one of the stipulations in the later and larger order was that the vehicles were in no wise to differ from those already supplied. In the course of our recent visit to the works at Guildford we saw 50 of these vehicles in various stages of construction being assembled upon the plan which has been adopted in the Dennis! works of bringing the partly-finished chassis from section to section, at each of which there is added certain details in the fitting of which the particular section is exclusively trained. Thus, the frame is drilled on a, swivelling jig bed, the services of three big radial drills being utilized to drill the holes to a number exceeding 200 and of various sizes. The accuracy thus obtained in the location of the holes materially assists in the later stages of assembly. The next operation. is the fitting of brackets, dumb irons, springs, shackles and towing hooks. Following the fitting of the axles is that of the wheels and steering gear, when the chassis becomes a mobile unit, and at

intervals of hours it, moves forward to a new sec

tion, its in turn its engine, its electric genera tor, controller gear, its electric motor, its pro

peller shaft and torque tube, its dashboard, petrol tank and wings, the final operation before it is wheeled to the paint shop and the running bay being the filling of all Freasers and the supply of lubricant.

To deal now with the mechanical and electrical features of this chassis, the War Office Subvention " A " type, as it is known, is a 3-ton chassis with a 14 ft. 6 in. wheelbase. whioli is 18 ins, longer than the minimum allowed under the W.D. subsidy scheme of 1912. The frame therefore, is a special construction, giving greater room between the engine flywheel and the torque tube cross girder. An nnderframe carries the engine and the electric generator and electrie motor, being adequately stayed to the main frame. Except for this greater frame length and for a larger radiator, the chassis is the standard Dennis subsidy type chassis, capable of handling 3-ton to 4-ton loads. It has a four-cylindered engine, 115 mm. bore by 150 mm. stroke, with a three-bearing crankshaft, its pump and magneto being driven from a cross shaft housed at the forward end. The pump is of large capacity, as is also the radiator, as one of the uses to which the vehicle is being put by the War Department is for mobile searchlight work, where it may have to stand still for hours with the engine running virtually at full power.

The engine is controlled, as usual, through the medium of a throttle lever and acceletator pedal, and an ignition advance lever. It is also equipped with a governor which acts on an additional butterfly valve in the induction pipe above the throttle. If, therefore, does not interfere in any way with the mixture, but merely serves to restrict the gas passage when the engine speed reaches the pre-designed limit. It is capable of being set at any required point by the operation of a small hand wheel mounted in a bracket on the dashboard within reach of the driver's left hand.

To deal with the last feature of the normal chassis, the power is transmitted through a worm-driven live axle with a gear ratio of 10 to 1, the torque being taken through a torque tube enshrouding the propeller shaft to a torque ball housed in the cross girder. in place of the gearbox there are mounted in line on the underframe an electric generator and a motor, the former being coupled up to the engine and the latter to the intermediate shaft, which is keyed at its rear ,end to one member of the ring type universal joint, the other member of which is keyed to the forward end of the propeller shaft. The shoes of the foot brake are hinged on carriers, which are mounted on pins extending from the rear end of the casing of the electric motpr.

The electric generator is four-pole compound wound and fitted with commutating poles. It is capable of a continuous output of 25 kilowatts at from 80 volts to 110 volts. It is direct coupled to the engine flywheel by way of three Hardy flexible coupling discs, and is ventilated by a fan bolted to the face of the flywheel. An adjustable resistance in series with the shunt windings is fitted on the engine side of the ieuttledash, and is operated by means of a hand wheel on the driver's side of the dash. This resistance is not used when driving, but is only operated when the vehicle is used for electric lighting °I-melding. A two-way switcn connects tiihs adjmitable resistance alternatively to one brush of the generator or to one brush of the motor for lighting er driving purposes respectively.

The controller box bolted to the chassis frame and fitted with an easily-removable cast-aluminium cover contains (a) a reversing switch for the series winding of the electric motor, operated by a side lever corresponding to the change-speed lever on an ordinary car, and (b) a resistance in series with the series winding of the generator. This consists of 12 resistance units capable of being successively connected in parallel by a suitably-shaped drum carried on, but insulated from' a spindle operated 'by means of control rods from ahand lever on the steering column.

The electric motor is of 'similar construction to the generator; but is fitted with series instead of compound field winding.

With the controller in the forward position, the connections of the electrical equipment are as follow : From the interpole windings of the generator two circuits are taken the first through the series winding of the,generator taken, the set of resistance units in the controller. From the .movable contact of this resistance a circuit is taken to one end of the series winding of the motor. The second circuit is taken to the other end of the series winding of the motor' so that the series winding of the generator, with its resistance in series, forms a variable shunt to the series winding of the motor, thus providing a very effective means of varying the speed and torque between the engine and the back axle. The remaining connections are shown in diagram Fig. 1 of the edheme of electrical connections. With the controller in the neutral position, the circuits are as shown in Fig. 2. In this position the circuit to the motor armature is broken and the motor series winding is short-circuited. The lighting terminals are shown in the centre of the lower diagram. It will be seen that the variable resistance in series with the generator series field winding now acts as an ,adjustable shunt, enabling the operator to adjust the amount of compounding by the movement of the lever on the steering column. By suitable'operation of this lever, combined with the adjustment of shunt resistance by a hand wheel on the dashboard, any desired characteristic, level, over or under compounding can be obtained.

The governor of the engine can be adjusted so that the engine will run at any speed between 800 and 1200 revolutions, and a range of voltage between 70 and 120 can be obtained.

The ability to adjust the characteristic of the Stevens electrical generator so that it will either under compound, level compound or over compound is very valuable, and, by means of a "Whitest, ' we were given a very convincing demonstration of this capability. The machine gave an output of 25 kilowatts at 110 volts from no load to full load with practically no variation of voltage, this 'factor being snecially useful when the chassis is fitted as a portable searchlight carrier or is equipped as a portable arc welding plant. We look to mobile arc welding sets as a very useful field for commercial vehicles, not only at home but for repairs in situ in the reconstruc tion work in Belgium and France. •

In the course of a run over the Hog's Back between Guildford and Farnham, we were struck not only with the smoothness with which the vehiole moved away from a standstill with 3 tons of metal and. six people on board, but with the absolute ease of control which the electrical transmission made possible as compared with a gearbox set. The gradual advance )or retardation of the speed-regulating controller which operates the drum of the resistance gave all the advantages and attributes of an infinitely-variable gear, so that the engine was always and at every point pulling at its greatest efficiency. It was neither being slowed down by a delayed change from, a gear that was becoming too high, nor racing through an early change into a lower gear. As all these gear change losses were eliminated, we were not surprised to learn that the petrol consumption of the Stevens petrol-electric vecncle is no higher than a gear-driven vehicle doing the same work, whilst the speed generally was higher, as 25 miles per hour is easily attained.

Then, again, the smooth torque of the petrol-electric makes for longevity of the tyres, oven although the net weight of the vehicle is greater than if a gearbox were employed.

On a falling gradient the throttle can be moved back to the point of minimum fuel supply, so that the engine is just ticking over, and the vehicle then coasts in a manner which couldnot be expected ofia gear. driven car without setting the gears in neutral (which involves trouble in re-engaging. at, the end -of the coast), or without the clutch -being held out (which is exeeedingly bad practice).

An accumulator can be charged from the generator if desired without the need for fitting a special generator, as the voltage remains constant, although the amperage may vary. The temperatures under full load are considerably less than for those ordinarily specified for a dynamo and motor. This is due to the fact that the machines have a very low resistance.

Our test of the Stevtns petrol-electric vehicle impressed us with the ease of control, and we were, at the end of a very interesting day's experience, quite prepared to accept the statement that it is one of the easiest of vehicles to manage and. control, and that it was as nearly absolutely foolproof as any piece of mechanism could be.

Tags

Organisations: War Department
Locations: Victoria, London

comments powered by Disqus