manufacturers, however, do not employ the ignition-valve on motors of less than 15 to 20 horse-power, chiefly because of the cheaper construction. The total consumption

is of less moment in a motor of small than of great power, and the loss due to the lack of an ignition-valve not so marked. In a high-power engine, premature explosion may be the cause of the breaking of a vital part, such as the piston-rod or the crank-shaft. For this reason, a valve is indispensable for engines of more than 20 to 25 horse-power. A breakage of this kind is less to be feared in a small motor, where the parts are comparatively stout. The gas consumption of a well-designed burner does not exceed from 3.5 to 5 cubic feet per hour.

Fig. 11.—Ignition-tube with valve.

Electric Ignition.—Electric ignition consists in producing a spark in the explosion-chamber of the engine. The nicety with which it can be controlled gives it an undeniable advantage over the hot tube. But the objection

has been raised, perhaps with some force, that it entails certain complications in installing the engine. Its opponents even assert that the power and the rapidity of the deflagration of the explosive mixture are greater with hot-tube ignition. This reason may have caused the hot-tube system to prevail in England, where manufacturers of gas-engines are very numerous and not lacking in experience.

Electric ignition is effected in gas-engines by means of a battery and spark-coil, or by means of a small magneto machine which mechanically produces a current-breaking spark.

Fig. 12.—Electric ignition by spark-coil and battery.

Fig. 13.—Spark-plug.

Electric Ignition by Battery and Induction-Coil.—The first system is the cheaper; but it exacts the most painstaking care in maintaining the parts in proper working condition. It comprises three essential elements—a battery, a coil, and a spark-plug (Fig. 12). The battery may be a storage-battery, which must, consequently, be recharged from time to time; or it may be

a primary battery which must be frequently renewed and carefully cleaned. The induction-coil is fitted with a trembler or interrupter, which easily gets out of order and which must be regulated with considerable accuracy. The spark-plug is a particularly delicate part, subject to many possible accidents. The porcelain of which it is made is liable to crack. It is hard to obtain absolutely perfect insulation; for the terminals deteriorate as they become overheated, break, or become foul (Fig. 13). In oil-engines, especially, soot is rapidly deposited on the terminals, so that no spark can be produced. In benzine or naphtha motors, such an accident is less likely to happen. In automobile-motors, however, the spark-plug only too often fails to perform its function. The one remedy for these evils is to be found

in the most painstaking care of the spark-plug and of the other elements of the ignition system.

Fig. 14.—Magneto ignition apparatus.

Fig. 15.—General view and details of a magneto ignition apparatus.

Ignition by Magnetos.—Magneto apparatus, on the other hand, are noteworthy for the regularity of their operation. They may be used for several years without being remagnetized, and require no exceptional care. Magneto ignition devices are mechanically actuated, the necessary displacement of the coil being effected by means of a cam carried on a shaft turning with half the motor speed (Figs. 14 and 15). At the moment when it is released by the cam, the coil is suddenly

returned to its initial position by means of a spring. This rapid movement generates a current that passes through terminals, which are arranged within the cylinder and which are immediately separated by mechanical means. Thus a much hotter circuit-breaking spark is produced, which is very much more energetic than that of a battery and induction-coil, and which surely ignites the gaseous mixture in the cylinder. The terminals are generally of steel, sometimes pointed with nickel or platinum (Fig. 16). The only precaution to be observed is the exclusion of moisture

and occasional cleaning. For engines driven by producer-gas magneto-igniters are preferable to cells and batteries. In general, electrical ignition is to be recommended for high-pressure engines.

Fig. 16.—Contacts of a magneto-igniter.

Fig. 17.—Device for regulating the moment of ignition.

In order to explain more clearly modern methods of ignition a diagram is presented, showing an electric magneto-igniter applied to the cylinder-head of a Winterthur motor, and also a sectional view of the member varying the make-and-break contacts which

are mounted in the explosion-chamber (Figs. 18 and 19)

1. The magneto A consists of horseshoe-magnets, between the poles of which the armature rotates. At its conically turned end, the armature-shaft carries an arm B, held in place by a nut.

Fig. 18.—Winterthur electric ignition system.

2. The igniter C is a casting secured to the cylinder-head by a movable strap and provided with two axes D and M, of which the one, D, made of bronze, is movable, and is fitted with a small interior contact-hammer, a percussion-lever, and an exterior recoil-spring; the other, M, is fixed, insulated, and arranged

to receive the current from the magneto A, by means of an insulated copper wire E.

3. The spring F comprises two continuous coils contained in a brass casing, and actuating a steel striking or percussion-pin.

4. The controlling devices of the magneto include a stem or rod G, slidable in a guide H, provided with a