ECONOMY

HIT & MISS ENGINE

By R Greenwell

A casting and drawings kit for this Economy model was purchased from the Engineers Emporium but the builder prepared his own drawings from sketches and graphic illustrations in a book.

The crankshaft and connecting rod were machined from EN8 flame cut profiles; the fuel mixer and non return system, tank filler, rocker and governor were all machined from bronze and steel bar stock; piston rings were machined from scrap car brake discs; spark plugs with spares have cores made from machinable ceramic with the earth points machined integral (not bent to suit), and the cores are detachable, not glued.

All nuts were made and all studs and bolts down to No 2 A.N.C. were single point screwcut, with the only exception being the brass screws in the magneto and the decals. Engraving was done by a local company.

The Magneto is approx. 1:3 scale copy of the Wico Impulse Magneto type EK, and was made using five 10mm dia x 5 thick neodymium 45 magnets are arranged in series between two soft iron laminated cores. These magnets are extremely powerful having a flux density of 13500 gauss.

The soft iron cores each have 20 laminations .020" thick and were machined from redundant transformer laminations as is the armature. Each core carries a coil with primary windings 200 turns of 1/64” (O.4mm) enameled copper wire and a secondary winding of 15000 turns of .002" (O.O5mm) wire. The points have tungsten contacts and bridged with an O5uf condenser.

In operation, as the piston rises on the compression stroke the first lobe on the cam advances the push bar. The strength of the magnets is such that the armature is held in contact compressing the spring on the impulse mechanism. When the point to fire is reached the push rod inside the spring reaches the stop and the armature is mechanically pulled from the magnet cores. The instant magnetic contact is broken the spring very rapidly pulls the armature clear and opens the points bringing the condenser into circuit. This sudden collapse of the magnetic field causes the primary winding to generate the electrical pulse which is transformed to several kilovolts through thro' the secondary.

The drip feed oilers are authentic, machined from solid, each having 23 components. Bowl glasses were cut from old laboratory tube, while the small sight glasses were cut from circuit board fuses. The top cup rotates l/4 turn clockwise for filling, and the control needles are adjustable.

This beautifully finished model was exhibited at a National Model Engineering and Modelling Exhibition in its Harrogate days.

https://www.sarikhobbies.com/model-engineer-builder/

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