Two New Bicera Devices to Cut Engine Noise
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TWO devices which, when applied to representative oil engines, ,had been found to reduce the overall peak sound levels by 24 decibels were described by W. P. Mansfield to the Diesel Engineers and Users Association in London yesterday. He was speaking about inventions by the British Internal Combustion Engine Research Association.
Small projections on the heads of the exhaust valves, to divide the gas flow into several streams, produced a reduction of 12 dB., whilst a large simple box between the exhaust ports and the normal silencing system gave a further improvement of 10 dB. When the two means were used together, both gave slightly improved efficiency to achieve the final figure.
It was important to raise the speed of small oil engines to make them more comparable with petrol units, but it was equally essential to reduce the noise produced by the small oiler, The Bicera Mark IT pump-injector system, which operated satisfactorily at speeds of up to 3.000 r.p.m., was capable of giving a modified injection pattern to match engine requirements and provided good combustion over a wide speed range without "knock."
The Bicera blade-type supercharger, which gave pressure-charging at up to about 5 p.s.i„ was quieter than the commonly used Roots-type blower and produced less vibration. Mr. Mansfield thought that it would he cheaner to produce than the Roots unit, Peak cylinder, pressures tended to rise with pressure charging, and the Bicera variable-ratio 'piston had been developed to offset this. The piston employed lubricating oil to vary its dimensions and, therefore, the cylinder pressure level.