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Engine Design

A-series Cutaway

The leader of the engine design section for the, A30 was W.V.Appleby who was later to become Chief Designer of engines and gearboxes for BMC. The photograph below shows him inspecting an example of an engine he designed 30 years ago.

Engine Design

Reference has already been made to experiments with a twin cylinder version of the A40 engine, but Bill Appleby says that this engine was almost impossible to balance satisfactorily and difficult to mount in the car if vibrations were to be to an acceptable level. In any case, the cost of producing a vertical twin in water cooled form would only have been about 12.5% cheaper than a four cylinder engine of twice the capacity. This is because the twin cylinder engine need a larger flywheel and large balance weights have also to be built into the crankshaft. Furthermore, all engine ancillaries are just as expensive as in the four cylinder unit.

A three cylinder in-line unit gave similar problems, and so the four cylinder in-line unit was favoured because the two firing strokes per revolution were the least required for the stand of smoothness they were wanting. As Bill Appleby put it, they had come to precisely the same decision as had Herbert Austin when he designed the original Seven.

Having determined the type of engine, the next step was to decide on the stoke/bore ratio. The A40 County engine had proved a great success and so the same stroke/bore ratio was retained (1.3:1) giving a bore of 59mm and a stroke of 76mm in the case of the A30 and since the actual stroke was shorter this meant a useful reduction in piston speed of some 14%.

The electrical equipment was fitted on the opposite side of the engine to the carburettor in order to keep it away from the heat of the exhaust manifold and also from possible petrol drips. The oil pump was to be driven from the rear of the camshaft in order to reduce the cost of providing another gear but this resulted in the pump being above the oil level in the sump and so the pump would require priming after an engine strip down. (Take note you engine strippers!) Bill Appleby was not happy about this arrangement but Johnnie Rix, Chief Designer, insisted that it was a necessary economy.

Austin were proud of the superior pulling power and general efficiency of their engines compared to say the equivalent Fords of the time and careful attention was given to the features that produced these characteristics such as large valves and comparatively heavy flywheels wheels.

cylinder head

The cylinder head incorporated the Weslake form of combustion chamber which is heart-shaped and directs the incoming gas towards the plug due to the plug being situated at the apex. Several other details of the chamber led to excellent efficiency and Zenith remarked more than once that combustion in the A-Series engines was more constant than any other engines they were testing at the time. In 1951 only Pool petrol was available in this country and so the compression ratio was restricted to 7.2:1. In the A35 the C.R. was raised to 8.3:1 as Premium petrol was then available.

The A-Series engine must surely rank as one of the greatest small engines ever produced and since its introduction in the A30 it has been used in a whole series of BMC and BL small cars right through to its present day use in the A-Plus form installed in the Austin Metro. Its designers have good reason to feel fairly satisfied that they 'had just about got it right'.

© Reproduced by kind permission of the author, Barney Sharratt


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