shows how the crankshaft changes the reciprocating (up
and down) motion of the piston and connecting rod into a
rotary motion.
The engine flywheel is a very heavy, round disk
mounted to the back of the crankshaft. It helps to keep the
crankshaft spinning between power strokes and smooth
engine operation. It also holds a large gear used by the
starter.
TDC and BDC
The abbreviation TDC stands for top dead center. This
is the point of travel where the piston is at its highest point
in the cylinder. The abbreviation BDC stands for bottom
dead center. This is the point of travel where the piston is
at its lowest point in the cylinder. Refer to Figure 1-11.
Cylinder Head
The cylinder head is bolted to the top of the block
deck to enclose the top of the cylinders and form the top
of the combustion chamber, Figure 1-12. Like the block,
the cylinder head contains water jackets for cooling and
oil passages for lubricating moving parts on or in the
cylinder head.
Valve guides are machined through the top of the
head for the valves. The valves slide up and down in these
guides.
Cylinder head ports are passages for the air-fuel
mixture to enter the combustion chamber and for exhaust
gasses to flow out of the engine. These are located in the
cylinder head.
Valve seats are machined in the opening where the
ports enter the combustion chamber. The valves close against
the seats to make a leakproof seal at high temperatures.
Engine Valves
Engine valves control the flow into and out of the
engine cylinder or combustion chamber. They fit into the
cylinder head, operate inside the valve guides, and close
on the valve seats. Valve springs fit over the top end of the
valves to keep the valves in a normally closed position,
Figure 1-13.
Figure 1-14 shows how a valve opens and closes the
ports in the cylinder head. When the valve slides down,
the valve head moves away from the valve seat and the
port is opened. When the valve slides up, the valve head
moves toward the valve seat until the valve face makes
contact with the valve seat. This seals the combustion
chamber from the port.
14 Auto Engine Repair
Top dead
center
(TDC)
Piston
Crankshaft
One-half
revolution
180°
One stroke
TDC
BDC
Connecting
rod
Bottom
dead center
(BDC)
Figure 1-11. TDC is when the piston is at the top of its stroke.
BDC is when the piston is at the bottom of its stroke. One stroke
is the piston movement from TDC to BDC or vice versa. (Ford)
Cylinder head
Valve guides
Water
jacket
Exhaust
port
Exhaust
valve seat
Intake
valve seat
Intake
port
Figure 1-12. The cylinder head is bolted to the top of the block.
It forms a cover over the cylinder. The head also holds the
valves that control flow into and out of the cylinder.