(73) Principles of Ignition.
It is the purpose of the ignition system to raise a small portion of the mixture to the combustion temperature, or the temperature at which the air and fuel will start to enter into chemical combination. When combustion is once started in a compressed combustible gas it will spread throughout the mass no matter how small the original portion inflamed. The rate at which the flame spreads through the combustion chamber depends upon the compression pressure, the richness of the mixture, the nature of the fuel and upon the number of points at which it is ignited.
In practice perfect ignition is seldom realized. This is due not only to the ignition system itself but to poor mixture proportions, imperfect vaporizing of the fuel, and low compression; all of which tend to a slow burning mixture with the attendant losses.
The best ignition system will be that which will cause the ignition to occur invariably at the point of highest compression and which will supply ample heat to start the process of combustion with a cold cylinder, imperfect mixtures, and low compressions. An efficient and reliable ignition system is without a doubt the most important unit in the construction of a gas engine. As ignition systems have improved and become more reliable, so has the gas engine become more widely used and appreciated, and in almost a direct proportion to these improvements.
Many ingenious ignition systems have been proposed, but only two of these have met with any degree of success in practice; i. e., electrical ignition and ignition by means of the hot tube.
Sponge platinum has the peculiar property of igniting jets of hydrogen gas, or hydrocarbons, without the aid of heat; this is due to the condensing effect of the platinum on these gases.
It was proposed to ignite the gaseous charge of the gas engine by means of the platinum sponge (catalytic ignition) but the system proved a failure because of the clogging of the pores in the sponge by fine particles of soot.
Dr. Otto employed an open flame which was introduced into the mixture by means of a slide valve. This met with only a fair measure of success.
Cerium, Lanthum and several other rare metals cause a considerable spark when brought into contact with iron or steel. The objection to this method was the expense of the Cerium plugs which required frequent renewal.
The writer remembers a quaint attempt at firing the charge by means of a piece of flint and steel; the failure of this is obvious.
The Diesel Engine, a great success from a thermodynamic standpoint, is fired by means of the heat produced by the compression of air, the fuel being sprayed into air which is compressed to several hundred pounds pressure.
Mr. Victor Lougheed proposes ignition by means of a platinum wire rendered incandescent by a current of electricity. The plan sounds feasible, but we are still waiting to be shown.
Electric ignition is applicable to all classes of engines; in fact this system made the variable speed engine as used on automobiles, etc., a possibility, as accurate timing with the electric spark covers the range from the lowest possible speed to speeds of 4,500 revolutions per minute and over.