Automatic Compression Variation
Page 43
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A SCHEME by which the compression it ratio of an engine can be made to adjust itself to prevailing speed is shown in patent No. 902,707. Intended mainly for two-strokes, the scheme uses a piston that can vary its effective length. (The British Internal Combustion Engine Research Association, 111-112 Buckingham Avenue, Slough.)
Referring to the drawing, the piston body is not fixed to the gudgeon-pin assembly but is free to slide over it within the limits of the top space (1) and a stop ring (2). According to the patent, the piston is practically free of mechanical stresses and can in fact be made of ceramic material. If the top space be inflated with oil, the piston will lengthen and vice versa. The oil pressure necessary to fill it is obtained from the inertia of the column of oil in the long bore (3) in the connecting rod. When the pressure rises (at late upstroke), the oil passes a non-return valve (4) and is stored by a spring-loaded accumulator piston (5).
On the downstroke the stored oil passes a valve (6) and enters the head space, thus lengthening the piston. A blow-off valve limits the maximum pressure. The piston is shortened by discharge orifices but the specification gives no details of these.