Practical Hand Book of Gas, Oil and Steam Engines / Stationary, Marine, Traction; Gas Burners, Oil Burners, Etc.; Farm, Traction, Automobile, Locomotive; A simple, practical and comprehensive book on the construction, operation and repair of all kinds of engines. Dealing with the various parts in detail and the various types of engines and also the use of different kinds of fuel.

by John B. Rathbun

(78) Electrical Ignition.

Ignition by means of an electric spark is by far the most satisfactory method as it makes accurate timing and prompt starting possible. It is the most reliable of all systems and is easily inspected and adjusted by anyone having even a rudimentary idea of electricity or the gas engine. For this reason electric ignition is used on practically all modern engines (with the exception of the Diesel types). The spark is caused by the current jumping an opening or gap in the conducting path of the current, and the ignition of the charge is obtained by placing this cap in the midst of the combustible mixture to which the spark communicates its heat.

The method of producing the spark gap, and the method by which the current is forced to jump the gap, divides the electrical ignition system into two principal classes:

• (1) The MAKE AND BREAK, or LOW TENSION system.
• (2) The JUMP SPARK or HIGH TENSION system.

In either system the spark is produced by the electrical friction of the current passing through the high resistance of the gas in the spark gap. The incandescent vapor in the gap formed by this increase of temperatures causes the flash that is known as the spark. The temperature of the gap depends principally upon the current flowing through it, the amount of heat developed being proportional to the square of the current.

There is of course a practical limit to the amount of current used in the ignition apparatus to produce spark heat. The limit is generally set by considerations of the life of the battery furnishing the current, expense of generating the current, and the life of the contact points between which the spark occurs.

The heat developed by an electric current is proportional to the amount of resistance offered to its flow and the strength of the current employed. The greater the resistance, the more heat developed.

The resistance of copper wire (the usual conducting path), being very low causes little rise in temperature, but the air in the opening or break has a resistance of many thousands of times the resistance of the copper; hence the current passing across the opening spark or gap raises the air to an exceedingly high temperature.

With a comparatively heavy current flowing across the break, the temperature developed is high enough to boil or vaporize any metal in contact with the spark or flame, rendering the metallic vapors incandescent. With sufficient current, the ends of the wires which constitute the break may be melted away.

For the successful and continuous operation of the engine it is imperative that ends of the conducting path or terminals be made of a metal of a high fusing point in order to withstand the heat of the spark and also that the current be kept to as low a value as possible.

In actual construction the spark gap terminals are generally made of platinum or platino-iridium, or an alloy of high fusing point. Iron is sometimes used, but deteriorates rapidly. Nickel steel lasts longer than common iron or steel but is not as durable as platinum or its alloys.

As the temperature of the electric spark or arc is approximately 7,500° F., and the ignition temperature of an ordinary rich gas at 70 lbs. compression is 1,100° F., it is evident that the quantity of current for ignition may be kept to an exceedingly low value. High compression increases the resistance of the spark gap, and requires higher electrical pressure to force a given current across a gap of given length.