ESSENTIALS OF SUPERCHARGING

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53

G. E. T. EYSTON DISCOURSES ON

ESSENTIALS OF SUPERCHARGING

IT is interesting to study the application of” boosting” to high performance cars, and the methods employed to effect this with maximum efficiency. The choice of the blower unit is, of course, the primary consideration, and with this end in view it is necessary to examine the considerations leading to the selection of the best type of compressor.

It must be compact, light in weight, and absolutely reliable. Silence is also of primary importance, and although a slight whine is sometimes fascinating to some owners, in principle this should be eliminated as far as possible. For this purpose the drive for the blower should be direct off the crankshaft, without either drive, chain, or spur wheels. “

conversion” installations to be dpne, or where unit is definitely restricted considerations.

If the blower is mounted end of the crankcase, the induction ports is still of for the purpose of obtaining velocity of flow at low engine anything but a direct drive then an additional source to be faced by the designer.

As far as the blower is there are two sources of pneumatic and the other blower which does not casing before delivery is at a as compared with one in pressure rises to that discharge pipeline. Balance of all the angle is only on this has of power important the front line to the length necessary speeds. If is utilised, noise has concerned, noise, one A in the which the in the parts and

careful workmanship and design will look after the mechanical side. It is absolutely necessary to guard against any distortion of the mechanism or casing when functioning at maximum output. The lubrication of the machine must be well thought out and, as in an engine, directed to those parts only which are comparatively highly stressed.

The blower has to handle not only the normal air requirements of the engine, but supercharge in addition, so that the duty is heavy for so small a device, and selected materials are essential. The ‘ Powerplus ‘ design has been chosen for the M.G. Midget, which has been consistently successful, both in International races and record attempts. The engine is only 750 c.c.’s capacity, so there is nothing to give away when, say, achieving a speed officially timed over the measured mile of 119.48 m.p.h. Apart from this sprint, winning the Tourist Trophy and covering over 101 miles in the hour are proofs of reliability. There is, in fact, no doubt that for all purposes it is possible to const ru,ct a blower to-day which will give equal or better gear to front end case of power at gear for the clear that system with With the a certain not be obtaining ency. The with the but rotary automobile efficiency a good starts to and the that this permissible ency. The course, the charge air available with the possible to mixture in Unless a board for a the engine of the will be front of recourse has

service than the engine and transmission. The method of mounting on the front end of the crankcase by spigot will be obvious, such as has been done by Sunbeam, the old racing Talbots, Alfa Romeo, Maserati, etc. The drive is taken from the crankshaft direct in prolongation with same. If it is required to insert a reduction gear so that the blower can run at less than engine speed, this can easily be arranged by means of an internal toothed wheel and pinion, wttich does not take up much space, since one element slides in the other, and, as is well kn,own, this form of gearing is one of the most silent.

Airther,it is possible easily to arrange for the reduction in an oil bath at the the crankcase. If in the which develop their revolutions an increasing blower is required, then it is can be incorporated in the equal facility.

supercharger there is always speed which should from the point of view of highest volumetric efficivolumetric efficiency falls off of back pressure or boost, in positive displacement such as are required for work, the drop in volumetric boost is about constant in At the point where it the speed should be noted, geared to the engine, so coincides witn maximum engine revolutions and efficicause of this sagging is, of inability to aspirate and dismixture in the short time running at high speed— available it is only draw and, discharge air or time. is laid out on the drawing blower fitted to the nose of to be driven in prolongation it is unlikely that space between radiator and on a conversion. Then to be made to methods of

drive shown in the accompanying photographs. One shows the installation of a Powerplus blower on the Ridley Special, with which Lord Ridley took various 750 c.c. records last season at Brooklands. In this case a shaft with flexible coupling drives a sprocket at the top of the chaincase shown in the photograph. From this the blower itself, which is slung below crankshaft level, is driven by roller chain. This system makes changing of the gear ratio very simple by the use of alternative sprockets on the blower driving shaft.

The tensioning of the chain is dealt with by a jockey sprocket, the bearing of which can be seen in the illustration. Other details of the layout can be seen. Another shows the layout of the Powerplus on the latest 850 c.c. M.G. Midget, which sells at £250. Here the drive is direct from the crankshaft, and the blower is rigidly mounted on two cross-frames. The twin pipes for the induction can be plainly seen. The third photo shows a typical example of a layout alongside the cylinder

block. In this case the drive is by chain from the front end of the crankshaft, and makes a very neat layout. The only disadvantage is the noise of the drive, but the short induction pipe is a decided advantage. The application of supercharging of sports cars seems likely to become more general, as the improvement in performance is accompanied by improved distribution and consequently greater smoothness of running at lower

speeds. —G. E. T. EYSTON.

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