Patent GB-583995-A (GB)
Title: An electromagnetic pickup unit for determining the intensity of engine detonation
IPC: G01L23/22
Filed: 1944-09-15
Abstract: 583,995. Detecting vibrations. STANDARD OIL CO. OF CALIFORNIA. Sept. 15, 1944, No. 17725. Convention date, Oct. 23, 1943. [Class 40 (iv)] [Also in Groups XIX, XXXVI and XXXVIII] Apparatus for detecting detonation in a cylinder of an internal-combustion engine comprises a rigid housing of magnetic material adapted to be rigidly attached to the outer wall of the cylinder, a rod of magneto-strictive material secured in the housing and carrying a weight and a winding enclosing a part of the magnetic circuit including the rod, so that the flux in the magnetic circuit varies with stresses due to forced vibrations in the rod caused by detonation in the and induces correspondingly varying electromotive forces in the winding. A base 10 is provided with a threaded stud 11 which is screwed into a tapped hole in the cylinder. Into the base 10 is screwed a housing 13 having cooling fins 14 and closed at its upper end by a plug 16 of magnetic material which supports within the housing 13 a magneto-strictive element 17. As shown, this is supported at its upper end only, the free or unsupported end being provided with an enlargement or weight 18 of magnetic material. The element 17 and weight 18 are so proportioned that longitudinal vibrations will be most readily set up therein by the detonation frequency of the engine, usually about 6000 cycles per second. The element 17 comprises a tube formed from an alloy of 52 per cent nickel and 48 per cent iron heat treated so that its permeability. varies with stresses caused by forced longitudinal vibrations. A winding 19 surrounds the tube 17. An initially constant magnetic flux is impressed on the element 17, for example by making the cylinder 13 of a ferrous or other alloy which is permanently magnetized. The weight 18 is spaced a short distance, e.g. 0.002 inch, above the upper face of the base 10. A thin corrugated strip of magnetic material may be placed between the periphery of the weight 18 and the lower bore of the cylinder 13 to provide a closed magnetic circuit, or the clearance may be so small, e.g. 0.0005 inch, as to provide a magnetic flux path having a lower reluctance than that of the remainder of the magnetic circuit. The element 17 may be heat treated in such a manner that it is sensitive to compression but not to tension, thus making the unit sensitive to transverse as well as longitudinal vibrations. The space surrounding the winding 19 may be filled with a plastic or semi-plastic insulating compound. In a modification, the magnetostrictive element 17 has its upper end supported by the plug 16 and its lower end by the base 10. Its upper and lower portions are of different magneto-strictive characteristics, e.g. the upper part may be of pure nickel, which has a higher permeability when under compression than when under tension, and the lower part may be composed of an alloy of equal parts of nickle and iron, which has a higher permeability when under tension than when under compression. Alternatively, the element may be of homogeneous material having its upper and lower parts subjected to different heat treatments. The weight is connected to the junction between the upper and lower parts of the element. The winding is divided into two portions, one above and one below the weight. The weight may alternatively be included within the body of the element.