the cylinder. As will be observed, one of the leads from the stationary primary winding goes to the ground while the other is led to the insulated contact point of the contact breaker. The condenser is interposed in the circuit so that it is in shunt or parallel connection with

FIG. 79.-Illustration Showing Advance and Retard Positions of K. W. Magneto Breaker Box.

the contact points. As the two members comprising the condenser are insulated from each other the primary current cannot flow through this device. One of the high tension leads from the secondary winding is directed

to the ground while the other goes to the revolving distributor brush of the distributing device. Four wires lead from the distributor to the various spark plugs of the engine.

When the points in the contact breaker are separated by the cam point a current of electricity is induced in the stationary winding because the inductor is at such a position between the pole pieces that a maximum current of electricity is being produced. The high tension current is distributed to the spark plug in the cylinder about to ignite, the spark plugs being brought into action progressively as the magneto distributor brush establishes contact with the insulated segments to which the plug wires are attached. The application of a typical true high tension magneto to a four cylinder gas tractor power plant is outlined at Fig. 78 and attention is directed to the simplicity of the external wiring.

Timing the Spark.-The contact breaker of either a magneto or battery ignition system should be arranged in some manner so that the time of sparking may be varied. In starting the engine by hand it is imperative that the spark be retarded, which means that it should occur later than the correct ignition point and only after the piston has started to go down again after it has reached the top of the compression stroke. When the spark takes place under these conditions the piston is always driven in the right direction and the engine cannot back fire. If the engine were started with the spark advanced the explosion would take place before the piston had reached the end of its compression stroke and it would be driven back in a direction opposite to that in which it should normally revolve. This will tend to knock the starting crank or flywheel out of the operators grasp and may result in a permanent injury

to the wrist or arm if this is not removed out of the path of the incorrectly rotating starting handle.

While a late spark is desirable for starting and running an engine at low speed it is necessary to advance the spark when it is desired to operate the motor at a higher speed. To do this the timer case is usually rocked so the contact will be broken sooner between the points of the magneto contact breaker or established earlier in a timer for a battery ignition system. The positions of a magneto contact breaker when in advance and retard positions are clearly shown at Fig. 79. It will be seen that to obtain an advance spark the timer case is rocked so that the roller on the movable timer lever is brought into contact with the point of the cam sooner. In other words, to advance the time of the spark, the timer case is rocked in a direction opposite to that of rotation of the circuit breaking cam, whereas to retard the spark the contact breaker casing is oscillated in the same direction as that of cam rotation so the points of the contact breaker would be separated after the piston has started to go down on its explosion stroke.

CHAPTER VI.

COOLING AND LUBRICATING THE POWER PLANT.

Reason for Cooling Engine-Thermo-syphon System-Forced Circulation Method-Oil Cooling Features-Parts of Cooling System-The Pump-The Cooling Fan-The Radiator-Why Engines are Oiled-Different Lubricating Mediums-Simple Gravity Oil Cup Method-Mechanical Oiling Systems-Constant Level Splash Systems.

Reason for Cooling Engine. It is apparent that power is produced in an internal-combustion engine by a series of explosions in the cylinder. As the temperature of the explosion is over 2,000 deg. Fahr. in some cases, the rapid combustion and continued series of explosions would soon heat up the metal parts of the combustion chamber to such a point that they would show color unless cooling means were provided. Under these conditions it would be impossible to lubricate the cylinder because even the best quality of lubricating oil would be burnt. The piston would expand sufficiently to seize in the cylinder and the valves would warp so that they could no longer hold compression. Premature ignition of the charge would probably take place long before the engine was put out of commission by the distortion of the parts.

The fact that the ratio of engine efficiency is dependent upon the amount of useful work delivered by the heat generated from the explosion makes it important that

the jackets be cooled to a point where the cylinder will not be robbed of too much heat. The losses through the water jacket of the average gas-tractor power plant are over 50 per cent of the total fuel efficiency. While it is very important that the engine should not get too hot,

it is equally desirable that it is not cooled too much. The object of cylinder cooling is, therefore, to keep the heat of the cylinder metal below the danger point but at the same time keep the engine hot enough to obtain maximum power from the gas burnt.

Two general systems of cylinder cooling are in general use on farm engines, though

the simpler air-cooled types are never found on gas tractors of regular manufacture, because air cooling is not considered practical for heavy-duty motors of the bore and stroke used as gas tractor power plants. Water