Page 58 - Mechanic Diesel - TT

MECHANIC DIESEL - CITS

Cylinder head assembly

Valves: To have heat energy, you must burn air and fuel. After you have burned the fuel, you must get rid of the
burned gases. Ports are provided in the engine for this purpose. In some engines, valves must open and close
these ports at a given time to allow raw fuel to enter and burned gases to leave the combustion chamber. While
the fuel is being burned, these valves help to seal the combustion chamber to allow the heat energy to move the
piston.

The valve is a long-stemmed metal piece with a circular top known as the valve head. The mechanism required to
operate the valve consists of a valve guide a valve spring, a valve spring retainer, valve spring locks, a camshaft,
and timing gears or sprockets and chain. Figure 1-15 illustrates a portion of the valve mechanism.

On the underside of the valve head is the valve face. This face is machined smooth and when the valve is in the
closed position, it seats firmly in the valve seat. The valve seat is a metal ring pressed into the cylinder block (or
the cylinder head, depending upon the engine design) around the valve port. The valve face and the valve seat
are machined smooth to ensure a pressure-tight fit. It is this portion of the valve mechanism that seals the ports
while the fuel is being burned.
The valve must not be allowed to wobble while it is opening and closing or it will not seat properly and pressure will
be lost. To prevent the valve from wobbling, a long tube called the valve guide is pressed or cast into the cylinder
block and/or cylinder head. The valve stem travels up and down inside the valve guides as the valve opens and
closes. A valve spring closes the valve. The spring seats against the cylinder block (or cylinder head) and the
valve spring retainer.
Cylinder Head Malfunctions: A cracked cylinder head will produce the same results as a cracked engine block,
and the same holds true for clogged oil passages in the cylinder head. A common problem in cylinder heads is a
“blown” head gasket (fig. 3-9). This is usually indicated by two adjacent (side-by-side) cylinders failing to deliver
power (misfiring)

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CITS : Automotive - Mechanic Diesel - Lesson 11 - 14