The Supercharged BMW

The Supercharged B.M.W.

The machine whicAh has been causing much interest in its recent record attempts. THE Bayerische Motorenwerke, Munich, had long desired to test their newly designed supercharged machine on a long straight track, at maximum speed, but had not succeeded in getting the machine placed according to its capabilities in the main racing events of the season, as a few teething troubles made themselves apparent in the design. A suitable track was selected on a part of the State Road from Munich to Ingolstadt, which here has a straight section of about 5 km., also the maximum permissible gradient of 1,1/0, and is ‘ in good condition. The run was made with a flying start of 1 kilometre with 1 kilometre for slowing down, the timed stretch of one kilometre or one English mile lying between both. The timing was carried out with a Loebner watch. The record machine, which was ridden by the Munich rider Henue, was completely clad with sheet metal on the streamline principle with a view to reducing the air resistance. The rider’s seat was placed above the axle of the rear wheel, to provide the necessary loading on this wheel. In the design, the possibility of easy interchangeability of the two engines of 500 and 750 c.c. was specially taken into account. The ratio was stepped down as follows by suitable

graduation of the bevel wheels on the cardan shaft drive : 1st speed =110 km., 2nd speed —180 km., 3rd speed—,approx. 220 km. (speed per hour).

Technical Details.

With a view to attaining the maximum speeds, it was necessary to raise the output of the motors to a considerable extent. To achieve this, certain technical points of refinement are nowadays practised on racing machines of which the following may be mentioned : polishing of the induction pipe, combustion space and crank-case interiors for the purpose of reducing fric

tional resistances, then the fitting of additional return springs on the rocking levers and thrust rods to reduce the inertia of the moving parts. A further method is to change over to rodless control of the valves, or overhead camshaft, which possesses important advantages by avoiding the mass inertia of the tappets and thrust rods, and is therefore gaining much ground recently. Not much regard was paid to these possibilities on the part of the Bayerische Motoren Werke ; at the most there was contemplated a test with camshaft drive above the two cylinders, but this would have led to serious constructional modifications, which would have compelled the abandonment of the traditional design hitherto used with transverse horizontal cylinders. The B.M.W. thus decided upon the incorpoiation of a compressor, in order to achieve an increase of power output. This has, in fact, been contrived without any considerable structural alterations to the existing model, and the 500 c.c. motor of 29 h.p. was in fact increased to a power output of 55 h.p., while the’ 750 c.c. motor was increased from 36 to 75 h.p. capacity. Tests which were instituted with an existing Zeller compressor showed the practicability of the principle, but the design had to be im

proved. instead of electrou. for example, duralumin was used, the shafts were strengthened and made of special steel. The incorporation of the supercharger in the whole set is shown in Fig. 1. The impression of block construction is still further increased by the incorporation of the compressor. The compressor is mounted between the carburetter (V) and the two long induction pipes, and is driven from the crankshaft by spur gearing with a reduction ratio of 1: 1.2 through the intermediate piece Z. The transmission from the intermediate shaft Z to the actual compressor shaft takes place through a set of bevel wheels. The block shown is of the 500 c.c.

motor, as being brake tested on the test bed. All the basic elements of construction were left in their normal dimensions ; even the valve mechanism worked entirely satisfactorily under the unusual working conditions. The dismantled compressor is shown in Fig. 2. The open induction slot allows one of the lotating blades to be clearly seen. The shaft placed eccentrically in the housing strikes the eye at once. The front side is screwed to the cover of the intermediate piece Z (fig. 1). Work is done by the compressor only in the lower half of the housing, the upper is ineffective. At 6,000 r.p.m. of the engine and therefore 5,200 r.p.m. of the compressor the latter delivers a superpressure of 1.2 atmospheres. It is interesting to note that any alteration of power output, with constant maximum engine speed, can be brought about solely by variation of the compressor transmission ratio. From this results the fact that with the 750 c.c. model, if it must be done, speeds

of 230 km. per hour and more can be attained. . Notwithstanding the heavy stlain on the engine, this only had to be modified to a very small extent, thus, for example, steel cylinder covers were used, similar to the B.M.W. aviation engine construction. Fig. 3 gives a clear idea of the appearance of the internal rotary drum with the blades. The blades are fitted in the compressor casing to a clearance of 1/100 min, and naturally require the most extreme precision in manufacture ; indeed, the compressor as a whole, in its present form, is the product of lengthy and careful works tests. The spindle end clearly visible

opposite the bevel gem drive serves to operate a supplementary oil pump, which forces the oil through the hollow shaft into the compressor blades. This supplementary lubrication, however, is in the first place provided for really long distance racing, and is not necessary for splint work.