Wobble-plate Supercharger Demonstrated
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ATEST demonstration last week of two Leyland Comets, one equipped with a WellworthyRicardo wobble-plate supercharger, gave practical evidence of the blower's ability to provide a large increase in engine output, and at the same time virtually to eliminate excess smoke and to reduce" knock. The tests were arranged by the makers of the supercharger, Wellworthy, Ltd., Lymington, Hants. and were performed on the 1-in-6 Rufus Stone hill, near Cadnam in the New Forest, and on a near-by airfield, both vehicles carrying 8-ton loads.
The vehicle with the supercharged engine had covered over 30,000 miles on routine runs with full loads from Lymington to the Midlands, and comparison has been made between its operating characteristics and those of the second vehicle, engaged on similar work. These show that the blower provides a gain of about 1 m.p.g. when the vehicle is loaded. When running empty the consumption of the blown engine is slightly higher.
The engines of 17 lorries in the Wellworthy transport fleet, comprising a number of makes, are fitted with the supercharger, and some 20 units have been loaned for testing to vehicle manufacturers and public-service-vehicle operators. Over 100 blowers of an improved type will shortly be produced for further operational testing, and the production facilities will enable the maximum forseeable demand For original-equipment units to be satisfied.
16 sec. Saved In the four timed runs on the 500-yd. test hill, the supercharged Comet completed the climb in 44 sec. to 461 sec., whilst the time taken by the vehicle fitted with the normally aspirated engine exceeded 60 sec. on all the runs. Acceleration tests on the airfield provided comparable results, the time to reach 30 m.p.h. in top gear from 10 m.p.h. being reduced by supercharging from 38 sec. to 26 sec.
The output of the Comet engine has been increased 40 per cent, by supercharging, from 100 b.h.p. at 2,200 r.p.m. to 140 b.h.p. at the same speed, the blower operating at 14 times crankshaft speed.. At 3.500 r.p.m. on foil load the power absorbed by the supercharger is about 18 b.h.p., but about 50 per cent. of this power is .regained under load by virtue of the pneumatic pressure on the pistons during the intake stroke. The maximum boost pressure is approximately:9 p.s.i.
All Comet engines are of the standard type, and the back-axle ratio has not been changed to make optimum use of the greater power.
If _suitable modifications Were made to the engines and higher transmission ratios were employed, the resultant gains would, according to a technician of the company, be appreciably greater than those already obtained. • The advantage of eliminating soot. formation during combustion is particularly emphasized, both with regard to reducing the smoke' nuisance .to other road users and its favourable effect on the internal cleanliness of the engine.
Because the blower has 14 cylinders 4ith double-acting pistons, its drive torque is virtually uniform, and direct belt drive from the crankshaft (the Comet engine has three V-belts) is satisfactory. If the supercharger were fitted as original. equipment, however,
it would normally be driven through a gear train.
In laboratory tests of a 5.1-litre engine of another make, the blower increased the maximum output from 88 b.h.p. to 119 b.h.p. at 2,400 r.p.m., and, of greater importance, raised the output at 1,000 r.p.m. from 38 b.h,p. to 56 b.h.p., the corresponding improvement in b.m.e.p. being from 93 psi. to 137 p,s.i. The provision of a good low-speed torque was the main objective of the designer of the supercharger, Sir Harry Ricardo, who has repeatedly emphasized that the positive-displacement blower is the only type that can give a satisfactory performance at lower speeds without high mechanical and pumping losses.
The adiabatic efficiency of the blower is over 70 per cent., giving an overall efficiency in excess of 60 per cent. The 14 cylinders reduce pulsations to a minimum, and the type of pistons and valves employed enable oil-free air to he delivered. Neither the pistons nor the 'rotary valves are in contact with the casing and do riot, therefore, require lubrication.
By careful production' it has been
possible to design these-parts with small clearances, and internal leakage is low, thus reducing pumping losses. The elimination of contact between the pistons and valves and the casing was a prerequisite to producing the parts and the casing in light alloy. This has enabled the weight of the cOmplete unit to be reduced to 80 lb.
The succesiful application of the blower to the engines of vehicles engaged on routine service runs has largely been the result of modifications of the original design carried out by
the company, which include. the 'substitution of the master rod by a slidingblock anchor to restrain the wobble plate from revolving and redesigning the layout of the main bearing races_ The rigidity of the main shaft has been increased and the weight of the plate has been reduced by I lb., the latter being valuable in that it reduces inertia forces. No changes have been made to the overall dimensions, the diameter and length being about II in. and 1 ft. 9 in., respectively.
The main wobble-plate bearings are now preloaded to compensate for the differential expansion of the light-alloy casing and the steel shaft, and this modification, together with increasing the rigidity of the structure and reducing the inertia forces, has so improved the mechanical properties of the unit that 100-hr. overspeed type tests at 4,000 r.p.m. are now included in routine testing. This speed is far in excess of that at which former prototypes failed after a comparatively short period.