VALVE CONSTRUCTION, TYPES, AND OPERATION
The proper and accurate functional operation of the valves is as
necessary to successful motor operation as the proper adjustment of
a hairspring is to a watch, for if a hairspring becomes impaired in
any way, a watch will not keep correct time. This is the case in a
motor when a valve becomes impaired. The valves in a motor, therefore,
must be considered the most vital part conducive to successful and
economical operation of the motor.
The valves are manufactured from a high grade tungsten or carbon steel,
and are designed to withstand the intense heat which the heads located
in the combustion chambers are subjected to, without warping. A perfect
seat is required to prevent leaking, which will cause low compression
and a weak power impulse, thus reducing the power and harmony of
successful operation.
The poppet valve is used by about ninety-five per cent. of motor
manufacturers. This type of valve is mechanically operated from the
cam shaft at one-half the crank shaft speed, as a valve is lifted only
once in every four strokes, or two revolutions of the crank shaft. The
reduction in speed is accomplished by using a gear on the cam shaft,
twice the size of that on the crank shaft.
The heads and chambers must be kept free from carbon which forms
and bakes into a shale and has a tendency to crack and chip as the
temperature changes in the combustion chambers. These chips are
blown about in the cylinders until they lodge or are trapped by the
descending valves. It then forms a pit on the seat and prevents the
valves from seating properly. This leaves an open space which attracts
more carbon, and the entire functional action of the valve is soon
impaired, necessitating regrinding in order that it may properly seat
again.
Carbon is generated from a poor gas mixture or from excessive use of
lubricating oil and may be considered the chief cause of improper
functional action of the valves.
VALVE CONSTRUCTION, TYPES, AND OPERATION 8-CYLINDERED V-TYPE ENGINE
[Illustration:
Valve Head
Removable Plates
Cam Shaft
Valve Head
Valve Seat
Valve Seat
Tappet for Adjusting Valves
Tappet for Adjusting Valves
Fig. 11. 8-Cylinder Valve Arrangement]
Fig. 11 shows the location of the cam shaft, valves, and tappet
adjustment, on a V-shaped engine. The cylinders of this type of engine
are arranged in two blocks, consisting of four cylinders in each,
set directly opposite each other on an angle of 90°. The connecting
rods from opposite cylinders are yoked and take their bearing on the
same crank pin. This arrangement allows the intake and exhaust valves
of each opposite cylinder to operate from a single cam shaft, or in
other words the entire sixteen valves are operated by a single cam
shaft carrying eight cams. Consequently an eight or twelve cylindered
engine is identical in regard to valve timing to either a four or six
cylindered engine.
[Illustration:
Valve Head
„ Seat
„ Guide
„ Stem
„ Spring
Sp. Seat
Cap Screw
Tappet
Lock Nut
Guide Bushing
Push Block
Roller
Cam
Fig. 12. Poppet Valve]
Fig. 12 shows a poppet valve. This type of valve has only one
adjustment, called the tappet. The adjustment is made by turning the
cap-screw out of the push block until the head comes into contact with
the valve stem. The lock nut on the cap screw is then turned down
tightly to the push block to hold the adjustment. A strong spring is
placed on the valve stem which causes it to close quickly and remain
closed until it comes into contact with the cam.
Valves are set and operate in three different positions as shown in
Fig. 13. The exhaust valve in this case seats on the floor of the
combustion chamber and is operated by the stem which extends through
the casting to the tappet, while the intake valve seats on the upper
wall of the combustion chamber and is operated from over head by a
push-rod extending from the tappet to a rocker-arm. When both valves
are operated from above and seat on the upper wall of the combustion
chamber the motor is referred to as the overhead valve type of motor.
In the majority of motors both valves seat on the floor of the valve
chamber.
[Illustration:
Rocker Arm
Valve Stem
Valve Open
Valve Seat
Combustion Chamber
Tappets
Cam
Cam Shaft
Overhead Type Valve
Push Rod
Poppet-Type Valve
Fig. 13. Valve Types, Location and Operation]
=Valve Timing.=--Valve timing is usually accomplished by setting the
first, or exhaust valve cam, to correspond with a mark on the flywheel
and cylinder (shown in Fig. 14).
This is accomplished by lining up the ¹⁄₄, or ¹⁄₆ D-C mark on the
flywheel rim with the center mark on the cylinder block, and means
that ¹⁄₄, or ¹⁄₆, pistons are on upper dead center of the compression
stroke, the flywheel is then turned a trifle until the marks E-C,
or Ex-C, is at upper dead center and in line with the mark on the
cylinder block. This means that the exhaust valve closes at this point.
The cam shaft is then turned in the running direction and the cam shaft
gear meshed at the valve closing or seating point. This is all that is
necessary as the other cams take up correct operation when any one cam
is set properly.
Another method of valve timing used by some motor manufacturers is
shown in Fig. 14. It is simply necessary in this case to line up the
prick punch marks on the timing gears--after getting the first position
on upper D-C of the compression stroke--to acquire correct valve time.
No definite or average scale can be given for valve timing, as all
different types of motors are timed differently. These instructions
must be secured from the manufacturer when the motor is not marked.
[Illustration:
Cylinder Marks
Camshaft Gear
1-4 Pistons on Upper Dead Center
FLY-WHEEL MARKS
Running Direction of Fly Wheel
MARKS LINED UP Timing Gear Punch Marks
Crankshaft Gear
Fig. 14. Valve Timing Marks]
=Valve Grinding.=--A valve-grinding compound can be purchased at any
garage or service station or one may be compounded by mixing emery
dust with a heavy lubricating oil until a thin paste is formed. The
valve spring is released next by forcing up the tension with a screw
driver or valve lifter. A small H-shaped washer is drawn from a groove
near the end of the stem, which frees the valve; it can then be pushed
up and raised through the guide. A small spring is placed over the
valve stem. This spring should be strong enough to raise the valve
one-half inch above the seat. A thin film of the grinding compound is
evenly applied to the seating face of the valve head, a screw driver
or ratchet fork is set in the groove on the head of the valve, and the
handle rolled between the palms of the hands, covering about one-third
of the distance around the valve seat; the valve is let up after the
motion has been repeated four or five times, and repeated at another
angle until the entire surface of the valve is smoothly ground and
allows the valve to seat perfectly.
=Valves.=--The sleeve valve type of motor was invented several years
ago by Charles A. Knight. He met with some difficulty in having it
manufactured in this country because the lubrication system was thought
to be inadequate and the poppet valve was then at the height of its
popularity with the manufacturer of engines.
Knight took his engine to Europe and made some slight improvements
on it. It was then taken over and manufactured by one of the large
automobile manufacturing companies of that continent and is now being
used by many of the celebrated automobile manufacturers of every
country.
The principle of operation does not differ in any respect from the
ordinary type of four cycle motor, except, that instead of having the
poppet type of valves it has a set of sleeves which slide up and down
on the piston. The sleeves are operated from an eccentric shaft by a
short connecting rod and carry ports which are timed to line up with
the ports of the intake and exhaust manifold ports at the proper time
in the cycle of operation.
Fig. 15 shows the method of timing the sleeves on the four cylinder
engine. First, turn the motor over in the running direction until the
marks (I-4-T-C) on the flywheel are in alignment with the marks on the
cylinder casting. Turn the eccentric shaft in the running direction
until the marks A, B, C, shown in Fig. 15 are lined up, and then apply
the chain.
[Illustration:
Timer
Shaft
Sprocket
Crank Shaft Sprocket
Fig. 15. Knight Valve-Timing Marks--4-Cylinder]
To check up on the timing, back the flywheel up an inch or two and
insert a thin piece of tissue paper into the exhaust port and turn
the engine in the running direction until the paper is pinched, which
signifies that the valve is closed. The marks on the flywheel, timing
gears, and the crank case should be in alignment. Fig. 16 shows a
diagram of the timing marks on the eight cylinder Knight engine. The
method of timing this engine is as follows: (1) Turn the engine over
until the marks I-4-R-H--D-C align with the marks on the crank case.
(2) Turn the eccentric shaft and sprocket until the arrows shown in
Fig. 16 are in line with the guide marks on the front end of the chain
housing. Then put on the chain and check up the timing, using the thin
piece of tissue paper.
[Illustration:
Eccentric Shaft
Sprocket Hub
Mark on
Eccentric Shaft
Sprocket
Guide Mark on
Crank Case
Crank Shaft
Sprocket
Fig. 16. Knight Valve-Timing Marks--8-Cylinder]
VALVE CONSTRUCTION
If the sleeve rods are removed for some reason, the bearings should be
fitted very loosely to the eccentric shaft when they are put back. A
looseness of about .008 of an inch is permissible.