Lesson 1/Learning Event 1
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Valve Timing. From information already presented in this lesson, you can see valves will have to
open and close at certain times in relation to piston position. This is called valve timing. Since the
piston position is determined by the crankshaft position and the valves by the camshaft, the relative
position of the two shafts will determine the valve timing. Once positioned properly, the relative
position of the two shafts is maintained by the meshed teeth of the timing gears. The gears
generally have timing marks stamped on them that must be aligned during assembly of the engine
to obtain the correct valve timing or positioning of the crankshaft and camshaft.
Accessory Systems
If the engine is to give satisfactory service, it will need several accessory systems. Complete understanding of
these systems is not needed to understand how the engine parts work. Therefore, we will do no more than
identify the needed accessories at this time so you will be aware of their existence.
Fuel System. There must be a way of storing fuel, mixing it with the air, and then carrying the mixture to the
cylinder.
Ignition System. There must also be a method of producing a high-voltage spark to jump across the spark plug
gap at the proper time for igniting the fuel-air mixture.
Exhaust System. Once the fuel-air mixture is ignited and burned, there must be some means of carrying the
hot exhaust gases away from the cylinder.
Cooling System. Combustion of the fuel-air mixture creates so much heat that the engine parts will be ruined
if cooling is not provided. Cooling may be done by causing either air or a liquid to flow over the heated
surfaces.
Lubrication System. Last, but by no means least, there must be a system to supply lubrication to points where
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