Figure 4-2. Ignition System Schematic

TM 10-3930-623-12

4-21. The capacitor is 'connected acros s the distributor

through their speed ranges. The centrifugal advance for

contact points to produce a quick collapse of the

any particular engine is determined by operating that

magnetic field in the coil so that a high voltage will be

engine at wide-open throttle on a dynamometer and

produced. In doing this, the capacitor protects the

varying the spark advance at each engine speed until

distributor contact points from arcing. After the high

the advance is found which will give maximum power at

tension surge is induced in the coil, it is conducted

that speed. Centrifugal advance weights, and weight

through the coil high tension lead to the center terminal

springs are designed to supply this advance through the

of the distributor cap (figure 4-2). From there it passes

engine speed range. Some centrifugal advance curves

down to the rotor and to the outer cap electrode under

are straight lines when shown in graph form relating

which the rotor segment is positioned at the moment that

speed to degrees of advance. In others the spark

the surge occurs. This outer terminal is connected by a

advance does not increase proportionally with engine

high tension lead to the spark plug in the cylinder which

speed increase so that the curve breaks or "dog-legs" at

is ready to fire. The cap, rotor and high tension leads

some intermediate speed. Maximum advance provided

form a system which distributes the high voltage surges

for some engines is only a few degrees. Other engines

to the spark plugs.

can use as much as 42 degrees advance (spark occurs

42 degrees of engine crankshaft rotation before piston

4-22. SPARK ADVANCE. The timing of the spark to

reaches top dead center) at high speed. Maximum

piston position in the cylinder must vary if best engine

advance for the distributor used on this truck is 15

performance is to be obtained. Thus, at high speed, the

degrees to 19 degrees (crankshaft) obtained at 2000

spark must occur at the plug earlier in the compression

engine rpm.

stroke in order to give the fuel-air mixture ample time to

ignite, burn and give up its power to the piston as it

4-24. COOLING SYSTEM.

starts down on the power stroke.

4-25. In liquid cooling systems, the heat from the

4-23. CENTRIFUGAL ADVANCE MECHANISM. The

cylinders is transferred to a liquid contained in jackets

distributor centrifugal advance mechanism advances the

surrounding the cylinders. This liquid then passes

breaker cam ahead of the distributor shaft (and thus the

through a radiator designed to expose as large an area

camshaft, crankshaft and pistons). This mechanism

as possible to the air. Air

consists of an advance cam which is integral with the

breaker cam, a pair of advance weights, springs and a

weight base which is integral with the distributor shaft.

Different spark advance is required on different engines

Figure 4-2. Ignition System Schematic

4-3