The Petrol Engine / A Text-book dealing with the Principles of Design and Construction, with a Special Chapter on the Two-stroke Engine

by Francis John Kean

[CHAPTER VI]
IGNITION AND IGNITION DEVICES

We have already stated that the charge of explosive mixture is ignited in the cylinder at the end of the compression stroke by means of an electric spark. The electric spark takes place as the result of an electric discharge across the gap between the electrodes of the sparking plug.

The Sparking Plug.—Two views of a typical sparking plug are shown in Figs. [45] and [46], in which A is the high tension electrode which is periodically charged with electricity at high voltage (or electrical pressure) from a high tension magneto or a high tension coil, and B₁, B₂ are electrodes which, being in metallic contact with the cylinders and framework of the engine, are thus at zero potential. The electric discharge occurs across the gap C₁, C₂ in the form of a spark or flash. The electrode A is heavily insulated from the metal casing D of the sparking plug by porcelain insulators E and F. The locknuts G and H serve to keep the plug gas-tight and hold the several portions together mechanically. The terminal K is used for clamping the wire (or lead) which brings the supply of high tension electricity. The high tension electric current may be supplied either by (1) a magneto machine or (2), a coil and accumulator ignition system.

Fig. 45.—Sectional Drawing
of a Sparking Plug.

Fig. 46.—A Sparking Plug.