Tilling' s New Petrol-electric Chassis.
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The First Example of this Interesting New Model Has Now Been Approved by Scotland Yard.
On Friday of last week, in common with the representatives of several other technical journals, we were bidden to participate in a demonstration run of a new motorbus model; which had been arranged by Mr. Percy Frost-Smith, the chief engineer to Thomas Tilling, Ltd. The first of this company's new model of petrol-electric chassis, of which a batch is now completing in the shops at Bull Yard, Peckham, is now ready for service, subject to formal acceptance by the Noise Committee of Scotland Yard.
It will be remembered that, for some time past, Thos. Tilling, Ltd.. has been conducting most-conmrehensive trials on the road with a petrol-electric model, and the particular vehicle which has been used as a basis for these prolonged trials is familiarly known to those who have been interes:ed. as the " S.B.S." It is still 'doing satisfactory duty on the road and has at the present time completed a total mileage, approximately of 106,000 miles.
The new machine, which last Friday was the subject of an interested and critical examination by our representative, is a very distinct departure from the original machine with which Mr. FrostSmith and his associates commenced experiments. As owners of motorbuses; on a large scale, Tilling's naturally had to take some steps to provide themselves with a practical machine which should conform to the latest regulations of the Public Carriage Office, especially in the matter of weight reduction and of silent running ; therefore, improvements 'mid io be made.
Whilst other contractors had tackled this problem by the direct evolution of the gear-driven chassis, Thomas Tilling, Ltd., decided that its ends would he best served by the endeavour to perfect a simple form of petrol-electric chassis. 'It would be foolish to pretend that, the designers of this new machine have not had many debatable problems to ponder, but, taking everything into Consideration, they came to the conclusion that, although a petrol-electric system might involve somewhat increased prime
Coat, its undoubted advantages in the matters of simplicity of control, of sweetness of running, and of regular acceleration, outweighed any other objections there might be.
It cannot be denied that the petrolelectric proposition for motorbus service in London has in the past received scant encouragement from most of the owners and proprietors, but, so far as could he gathered from this demonstration with the first of Tilling's new models, it would appear that this company's persistence should go far to disarm the critics of the petrol-electric method of transmission per se.
We are happy to be in a position to offer our readers the first reproductions that have been published of certain drawings and photographs of the new vehicle. Examination of these illustrations shoulc: render the method of construction that has been adopted quite clear. Other reproductions appear on page 282.
We may briefly describe the system as consisting of a petrol engine which is spring-coupled to a generator, whilst on the front end of the cardan shaft is a series-wound electric motor ; the drive from this shaft is by worm-gearing to a live back axle; between the generator and the motor there is a powerful fan. The outstanding features of this interesting machine, in addition to the adoption of the special form of electric transmission, are the facts that the Renault type of bonnet and cooler has been embodied and that the duplicate method of driving by twin motors through independent worm gears to road-wheels running on a dead axlo has been abandoned in favour of a well-designed type of self
contained worm-driven live axle. This last feature is now of special interest, in view Of the fact that it was principally the method of driving by twin independent, worm gears, which is used on the standard K.P.L. motorbus, which was the subject of objection to the proposed new Premier company, by the owners of the Patents embodied in Tilling's first petrolelectric omnibus, and which the London General Omnibus Co. had purchased.
• The engine is a four-cylinder one, and, owing to the adoption of the dashboard position for the radiator, it is naturally very accessible. The design of the power unit is a straightforward one and calls for little comment, other than that the designer has quite evidently learned many lessons from current practice. The lubrication is of a well-thought-out trough type. the troughs being fed by a small Albany type of pump situated at the side of the sump on the bottom of the engine case. The cylinders are cast in pairs, and both the inlet and exhaust valves are operated by a single camshaft on one side. The camshaft itself is diiven from the crankshaft by means of a silent chain. High-tension magneto ignition alone is fitted and the magneto is driven from one end of a skew-driven cross-shaft that runs across the front of the engine ; the other end of this shaft drives an old-type Daimler-pattern water pump. The magneto is completely enclosed by an aluminium case and is padlocked in position.
It, is interesting to note that. Mr. FrostSmith is of opinion that the balance of argument for motorbus work is in favour of positive-driven water-circulation systems, and in this connection he considers that the old long-sleeve Daimler pattern water-pump easily holds its own. The carburetter is a Solex, and our readers will remember that we have already described this efficient and simple contrivance in our pages in full detail. In connection with the oiling system of the engine, we may add that troughs are also formed over each main-basting; these troughs were at first also supplied with oil from the delivery-pump, but it has now been found that sufficient lubricant reaches them on account of the splash, and therefcre the pomp deliveries to them are to be abandoned. Delivery, however, is made direct on to tho chain which drives the camshaft to which we have already referred. The engine has a bore of 105 mm.. and its piston-stroke is 125 mm. The weight of the engine, with the flywheel, carburetter and magneto complete, is approximately 525 lb.
The dynamo is coupled tu the engine in an interesting manner by means of a pair of laminated springs lying transversely acrcss the flywheel ; this system will be remembered as having been successfully employed for the final drive on early Thernycroft wagons. This method allows for inaccuracy of alignment.
The cardan-shaft has a ball-bearing universal joint at its front end, and a castellated telescopic joint at its rear end the shaft is of unusual length and diameter.
The rear axle is. as we have already said, of Lite live worm-driven pattern, and its design embodies a number of minor improvements that are obviously based on current practice. The worm is placed above the worm whsel, and is carried in a special pot casting, which houses the ordinary ball-bearings as well as the composite bearing, which takes the thrust from both sides of the worm wheel. The whole of the worm drive, complete with all its necessary bearings, can be removed, therefore, through, the tau of the easing, when the castellated eras of the differential shafts are withdrawn. The mainaxle casing is a stiffly-ribbed steel casting, whose flanged ends are bolted to the steel spring-seating casings. Another special feature of the axle is the employment of high-grade steel sleeves which are pressed into the casing and through the spring-seat castings ; their outer ends support the steel road wheels, which are hushed for the purpose.
The frame is a first-class job, and is of pressed-steel throughout with the exception of the back tubular cross member. The axle springs are all anchored at their front ends, whilst their rear ends are carried on slides. No radius rods nor torque bars are employed, the hind springs lak• ing the whole of the drk,e. both brakes are on the back wheels, and act on separate drums in accordance with the latest police regulations; the hand brake is of the external type, whilst the foot brake actuates two pairs of internal-expanding shoes.
With regard to the electrical control apparatus. this is regulated by alteration of the field resistance of the generator ; this resistance, together with a reversing switch, is carried in the controller box on the off-side of the main frame. The generator is capable of an output of from 1 to 25 kilowatts at speeds varying from 350 to 1,400 revolutions. The design of the dynamo is so contrived that any in crease iu the demand for current when the engine is fully loaded is accompanied by a corresponding reduction of voltage. On level roads and on ordinary gradients the whole of the control is effected by the throttle pedal, but on stiff gradients the shunt resistance is introduced in order to allow the engine to speed up. An ingenious interlocking arrangement of the pedal control prevents the racing of the engine when the electrical circuit is broken.
The whole of the electrical equipment on these new machines has been manufactured under the Stevens patents by W. A. Stevens, Ltd.. of Maidstone.
The trial trip on Friday of last week afforded ample demonstration that the new machine is silent, powerful, and well sprung ; the ease with which the driver was enabled to control the vehicle, under all kinds of traffic conditions, was remarkable. The route traversed was from the R.A.C. new premises in Pall Mall through South London to Bromley, and en route (he worst hills ‘vhieh
regular services have to tackle were negotiated in fine. style. So easily indeed did the vehicle surmount Bromley Hill that most of the passengers were unaware that there had been any climbing at all! The entire absence of jar due to sudden starting away was noticeable, and the steady manner in which acceleration took place WAR also a matter for favourable comment.
Tilling's have tackled the problem of the new class of motorbus in their own way, and they deserve every credit for what appears to he a most-satisfactory result. Prolonged road service will probably confirm these preliminary good impressions.