THE OPERATION OF A 4-CYCLE, 4-CYLINDERED ENGINE
The four-cycle or Otto stroke type of gasoline engine should rightly be
called the four-stroke-cycle engine, as it requires four strokes and
two revolutions of the crank shaft to complete one cycle of operation.
This type of motor is used almost universally by the manufacturers
of pleasure cars due to its reliability, and to the ability it has
to furnish continuous power at all speeds with the minimum amount of
vibration.
[Illustration:
Firing
Stroke
Exhaust
Stroke
Intake
Stroke
Compression
Stroke
1
2
3
4
Fig. 17. 4-Stroke Cycle. 1--Cylinder in Action]
Fig. 17 shows a diagram of one cylinder in the four strokes of the
cycle, and the distance traveled by the crank shaft during each stroke.
No. 1 begins with a charge of compressed vapor gas in the cylinder and
is called the firing or power stroke. The ignition system (explained
in a later chapter) furnishes a spark at from five to fifteen degrees
early or before the piston reaches top dead center. Although the stroke
theoretically starts before the piston reaches its highest point of
ascent, the actual pressure or force of the explosion is not exerted
until the piston has crossed dead center. This is due to the fact that
the piston travels very rapidly, and that it requires a small fraction
of a second for spark to ignite the compressed charge of gas. It may,
therefore, be easily seen that, if the spark did not occur until the
piston is on or has crossed dead center, the piston would have traveled
part of the distance of the stroke, and as it is moving away from the
highest point of compression the pressure is reduced by allowing more
volume space which causes a weak explosion and a short power stroke.
The intake and exhaust valves are closed through the duration of the
power stroke.
No. 2. The exhaust stroke begins from fifteen to thirty degrees early,
or before the piston reaches lower dead center on the firing stroke.
The exhaust valve opens at the start of this stroke allowing the
pressure of the burnt or inert gas to escape before the piston begins
to ascend on the upward part of the stroke, and closes seven to ten
degrees late to allow the combustion chamber to clear out before the
next stroke begins.
No. 3. The intake or suction stroke begins with the piston descending
from its highest level to its lowest level. The intake valve opens ten
or twenty degrees late, and as the piston is traveling on its descent,
considerable vacuum pressure has formed which draws suddenly when the
valve opens and starts the gas from the carburetor in full volume. The
entire length of this stroke creates a vacuum which draws a full charge
of vaporized gas into the cylinder through the open intake valve. The
intake valve closes from ten to twenty degrees late in order that the
full drawing force of the vacuum may be utilized while the piston is
crossing lower center.
No. 4. The compression stroke begins at the end of the intake stroke
with both valves closed. The piston ascends from its lowest extreme to
its highest level, compressing the charge of gas which was drawn into
the cylinder on the intake or suction stroke; and at the completion
of this stroke the cylinder is again in position to start No. 1, the
firing stroke, and begin a new cycle of operation. The cam shaft is
driven from the crank shaft through a set of gears or a silent chain,
and operates at one-half the speed of the crank shaft as a valve is
lifted once through the cycle of operation, or two revolutions of the
crankshaft.
[Illustration:
1
2
3
4
Firing
Val. Closed
Compressing
Val. Closed
Exhausting
Ex. Val. Open
Intake
In. Val. Open
Fig. 18. Diagram of Action, 4-Cylinder 4-Cycle Engine]
Fig. 18 shows the operation of a four-cylindered motor as it would
appear if the cylinder block were removed. The timing or firing order
of the motor shown in this diagram is 1-2-4-3. No. 1 cylinder is always
nearest the radiator and on the left in this diagram. No. 1 cylinder is
firing. The intake and exhaust valve remain closed while this stroke
is taking place. This causes the entire force of the explosion to be
exerted on the head of the receding piston. The cylinders, as may
be seen in the diagram, are timed to fire in succession, one stroke
behind each other. While No. 1 cylinder is on the firing stroke, No.
2 cylinder is compressing with both valves closed and will fire and
deliver another power impulse as soon as No. 1 cylinder completes and
reaches the lowest extreme of its firing stroke. No. 3 cylinder, being
fourth in the firing order, has just completed the firing stroke and is
starting the exhaust stroke which forces the burnt and inert gases out
of the cylinder through the open exhaust valve. No. 4 cylinder which
is third in the firing order has just completed the exhaust stroke and
is about to start the intake or suction stroke with the exhaust valve
open. This diagram should be studied and memorized as it is often
necessary to remove the wires which may easily be replaced if the
firing order is known, or found by watching the action of the exhaust
valves and made to conform with the distributor of the ignition system.
(Note the running direction of the distributor brush and connect the
wires up in that direction.) For the firing order given above connect
No. 4 wire to No. 3 distributor post, and No. 3 wire to No. 4 post, as
this cylinder fires last.
[Illustration:
1-CYL.
2-CYL.
4-CYL.
8-CYL.
Fig. 19. Power Stroke Diagram]
Fig. 19 shows a diagram of the power stroke impulse delivered to the
cycle in a one, two, four, and eight cylindered motor. A complete cycle
consists of 360 degrees, and as there are four strokes to the cycle an
even division would give a stroke of ninety degrees, which is not the
case, however, owing to the fact that the valves do not open and close
at the theoretical beginning and ending point of each stroke which is
upper dead center and lower dead center. The firing or power impulse
stroke begins at approximately five to seven degrees before the piston
reaches upper dead center on the compression stroke and ends from
fifteen to thirty degrees before the piston or cycle of rotation of the
crankshaft reaches lower dead center. This results in a power impulse
of less than ninety degrees, which varies accordingly with valve
timing in the different makes of motors. Consequently we have a power
stroke of a little less than ninety degrees in a one-cylinder motor;
two power strokes of a little less than 180 degrees in a two cylinder
motor, while the power impulse of the four-cylinder motor very nearly
completes the cycle. In the six, eight, and twelve cylinder motor the
power strokes overlap, thereby delivering continuous power of very
nearly equal strength.
=Twin, Four, and Six Cylindered Motors.=--The operation of the twin
cylindered motor varies very little from the single four or six. It
is simply a case where two, four, or two six cylindered motors are
set to a single crank case at an angle which will allow the piston or
connecting rods from the opposite cylinders to operate on a single
crank shaft. When the cylinders are set directly opposite each other
the connecting rods are yoked and take their bearing on a single crank
pin of the crank shaft. This, however, is not always the case, for in
some motors the connecting rods take their bearing side by side on the
crank pin. The cylinders in this case are set to the crank case in a
staggered position to allow the connecting rods from each cylinder to
operate in line with the crank shaft.
The cylinder blocks are usually set to the crank case at an angle of
ninety degrees and are timed to furnish the power impulse or stroke
opposite each other in the cycle of operation. The advantage of this
formation is that two power strokes are delivered in one cycle of
operation, which increases the power momentum and reduces the jar or
shock of the explosion causing a sweet running vibrationless motor.
The valves are usually operated by a single cam shaft located on the
upper inside wall of the crank case. Valve timing is accomplished by
following the marks on the flywheel or lining up the prick punch marks
on the gears, as shown in Chapter II on valves.
When a magneto is used to furnish the current for ignition on an eight
cylinder motor it has to be operated at the same speed as the crank
shaft, as a cylinder is fired at each revolution of the crank shaft and
an interruption of the current is required at the breaker points to
produce the secondary or high tension current at the spark plug gaps.
Twelve cylindered motors are usually equipped with two distributors
or a dual system, or two magnetos driven separately through a set of
timing gears.
=Knight or Sleeve Valve Motor.=--The Knight or sleeve valve motor
operates on the same plan as the ordinary type of motor except that
the valves form a sleeve and slide over the piston. The sleeves are
operated by an eccentric shaft and are provided with ports which are
timed to conform with the ports of the intake and exhaust manifolds at
the proper time.
MOTOR HORSEPOWER
S. A. E. SCALE
FOUR-CYCLE HORSEPOWER RATING
------+-------+-------+-------+------
Bore | 1 cyl.| 2 cyl.| 4 cyl.| 6 cyl.
2³⁄₄ | 3.00 | 6.00 |12.00 |18.00
2⁷⁄₈ | 3.00 | 6.50 |13.00 |20.00
3.00 | 3.50 | 7.00 |14.50 |21.50
3¹⁄₄ | 4.00 | 8.50 |17.00 |25.50
3¹⁄₂ | 5.00 |10.00 |20.00 |29.50
3³⁄₄ | 5.50 |11.00 |22.50 |34.00
4.00 | 6.50 |13.00 |25.50 |38.50
4¹⁄₄ | 7.00 |14.50 |29.00 |43.50
4¹⁄₂ | 8.00 |16.00 |32.50 |48.50
4³⁄₄ | 9.00 |18.00 |36.00 |54.00
5.00 |10.00 |20.00 |40.00 |60.00
5¹⁄₄ |11.00 |22.00 |44.00 |66.00
5¹⁄₂ |12.00 |24.00 |48.00 |73.00
5³⁄₄ |13.00 |26.50 |53.00 |79.50
6.00 |14.50 |29.00 |57.50 |86.50
------+-------+-------+-------+------
This scale gives the nearest equivalent to the whole or half
horsepower, as is required by State where licenses are paid at so
much per horsepower.
D² times N
Formula--S. A. E. ------------ equals horsepower.
2.5
For sleeve valve timing see Chapter II on Valves.
DISPLACEMENT
There are probably few men operating cars to-day who fully understand
what is meant by the term displacement, often used in referring
to automobile races. It is one of the main factors or points in
determining the class in which a car is qualified to enter under the
laws that govern races. In looking over a race program, you will note
that there are usually two or more classes, one of which is open, and
another with a limited piston displacement, which gives the smaller
cars a competing chance in their class.
Consequently piston displacement is merely the volume displaced by
all the piston in moving the full length of the stroke. The volume of
a single cylinder is equal to the area of the bore multiplied by the
length of the stroke, and the total displacement of a four cylinder
motor will be four times this and that of a six cylinder motor, six
times this.
Piston displacement:
D² times S times N times 3.14
-----------------------------
4
Where D equals bore in inches
S „ stroke in inches
N „ number of cylinders
Example: Required to find the piston displacement of a 3¹⁄₂ × 5 inch
four-cylindered motor. D equals 3.5 S equals 5. and N equals 4.
Piston Displacement
3.5² times 5 times 4 times 3.14
-------------------------------
4
3.5 times 3.5 times 5 times 4 times 3.14
----------------------------------------
4
equals 173.58 cubic inches.
[Illustration:
IGNITION COIL
DELCO GENERATOR
DISTRIBUTOR
CONTROL LEVER
PEDALS
FAN
BRAKE LEVER
FAN BELT
STARTER SLIDING GEAR CASE
UNIVERSAL HOUSING
STARTING CRANK SHAFT
TRANSMISSION END PLATE
TIMING GEAR CASE
TRANSMISSION
TIMING GEAR HOUSING
CLUTCH RELEASE BEARING RETAINER GREASE CUP
WATER PUMP
MOTOR ARM
FLY WHEEL HOUSING
LOWER CRANK CASE
DRAIN COCK
Fig. 20. Buick Engine--Parts Assembly]
[Illustration:
VALVE KEY
VALVE ROCKER ARM PIN
OIL FILLER WING PLUG
VALVE ROCKER ARM
VALVE SPRING CAP
VALVE ROCKER ARM WICK
WATER OUTLET
VALVE SPRING
SPARK PLUG
VALVE
FAN
VALVE GAGE
VALVE PUSH ROD
WATER JACKET
COMBUSTION SPACE
WATER INLET
VALVE LIFTER
VALVE LIFTER GUIDE
PISTON PIN
PISTON
VALVE LIFTER CLAMP
OIL PUMP DRIVING GEAR
FAN BRACKET STUD
FAN BELT
CONNECTING ROD
CRANK SHAFT
TIMING GEARS
CONNECTING ROD BEARING
FAN PULLEY
CAM SHAFT
CRANK SHAFT BEARING
CAM SHAFT BEARING
STARTING NUT
OIL PUMP
GEAR COVER
UPPER CRANK CASE
FLY WHEEL
TIMING GEAR HOUSING
FLY WHEEL HOUSING
CHECK VALVE
WATER PUMP
DRAIN PLUG
OIL DIPPER
SPLASH OIL TROUGH
VALVE ROLLER
LOWER CRANK CASE
CRANK CASE OIL PIPE
Fig. 21. Buick Engine--Location Inside Parts Assembly]
[Illustration:
ROCKER ARM
OIL WICK
WING PLUG
VALVE STEM
ROCKER ARM COVER
VALVE SPRING
ADJUSTING BALL
VALVE CAGE NUT
LOCK NUT
VALVE CAGE
WATER JACKET
VALVE
SPARK PLUG COVER
EXHAUST MANIFOLD
COMBUSTION SPACE
INTAKE MANIFOLD
PUSH ROD
HOT AIR CHAMBER
VALVE PUSH ROD COVER
WRIST PIN
CYLINDER
VALVE LIFTER CAP
PISTON
VALVE LIFTER GUIDE CLAMP
VALVE LIFTER SPRING
VALVE LIFTER GUIDE
VALVE LIFTER
CAM ROLLER PIN
CAM ROLLER
CONNECTING ROD
CAM SHAFT
CRANK CASE
CRANK SHAFT
Fig. 22. Buick Motor--End View]
[Illustration:
Fan Belt Adjustment
Split Collar with Locking Cup
Valve Tappet Adjustment
Cam Shaft End Thrust Adjustment
Shims for Adjustment of Connecting Rods
Oil Passage to Connecting Rod
Oil Pipe to Piston Ring
Oil Pump Filter Screen
Oil Sump Filter Screen
Oil Pump
Felt Gasket
Oil Drain Plugs
Fig. 23. Liberty U. S. A. Engine]
LUBRICATION SYSTEMS, OILS, AND GREASES
Special attention should be given to regular lubrication, as this, more
than any one thing, not only determines the life but also the economic
up-keep of the car.
Whenever you hear an owner say that his car is a gas eater, or that
it uses twice or three times as much oil as his neighbor’s, which is
the same model and make, you know at once that he, or some one who has
used the car before him, either did not give sufficient attention to
lubrication, or used a poor grade of oil. It is almost impossible to
impress the importance of the foregoing facts upon the minds of the
average motorist, and we have, as a direct result, a loss of millions
of dollars annually through depreciation.
The manufacturers of automobiles and gasoline engines have earnestly
striven to overcome this negligence by providing their products with
automatically fed oiling systems which alleviate some of the former
troubles. These systems, however, also require some attention to
function properly.
=Grease.=--A medium grade of light hard oil grease is best adapted for
use in grease cups, universal joints, and for packing wheel bearings
and steering gear housings. The transmission and differential operate
more successfully when a lighter grade of grease is used, such as a
graphite compound, or a heavy oil known as 600 W.
=Oils.=--Great care should always be exercised in purchasing
lubricants. None but the best grades should be considered under any
circumstances. The cheaper grades of oil will always prove to be the
most expensive in the end. The ordinary farm machinery oils should
never be used in any case as an engine lubricant, for in most cases
they contain acids, alkalies, and foreign matter which will deteriorate
and destroy the bearings of the motor.
An oil to give the best satisfaction must be a purely mineral or
vegetable composition which will flow freely at a temperature of 33°
Fahrenheit and also stand a temperature of 400° Fahrenheit without
burning. Always choose an oil which is light in color as the darker oil
usually contains much carbon.
=Lubrication= (Lat. _Lubricus_, meaning slippery).---Lubrication is
provided on all types of automobile engines, and at various other
places where moving parts come in contact or operate upon each other.
Three different types of lubricating systems are found in common use.
Fig. 24 shows the splash system. The oil is placed into the crank case
and maintained at a level between two points, marked high and low, on a
float or glass gauge at the lower left-hand side of the crank case. The
oil is usually poured directly into the crank case through a breather
pipe provided to prevent excessive vacuum pressure.
The lower end of the connecting rod carries a spoon or paddle which
dips into the oil at each revolution and splashes it to the cylinder
walls and various bearing surfaces within the motor.
[Illustration: Fig. 24. Splash Oiling]
=Care of the Splash System.=--This type of oiling system does not
require any adjustments, or special care, except that the oil level be
constantly kept between the high and low level marked on the gauge.
=Cleaning the Splash System.=--Lubricating oils lose their
effectiveness and become thin and watery after a certain period of use
due to a fluid deposit called residue which remains in the combustion
chambers after the charge of gas has been fired. This fluid generally
works its way into the crank case, thinning the oil.
The crank case should, therefore, be drained, cleaned, and refilled
with fresh oil every fifth week or thousand miles that the car is
driven. This will prevent much wear and give a quiet and satisfactory
running motor. Draining and washing out the crank case is accomplished
by removing a drain plug at the bottom of the crank case. The old oil
is drained off and thrown away. Kerosene is then poured into the crank
case through the breather pipe until it flows out of the drain clear in
color. The plug is then replaced and the crank case replenished with
fresh oil until the three-quarter from low level is reached on the
gauge. The oil level should be carried as near this point as possible
to obtain the most satisfactory result.
Fig. 25 shows the plunger or piston pump pressure system usually
used in conjunction with the splash system. The oil is carried in a
reservoir at the bottom of the crank case and is drawn through a fine
meshed screen by the oil pump, which is of the plunger type operated
off the cam shaft. It forces the oil through copper tubes in the three
main bearings. The front and center bearings have an outlet which
furnishes the oil to the gears in front and to the troughs in which the
connecting rods dip. The troughs have holes drilled to keep the level
of the oil, the surplus being returned to the reservoir.
[Illustration:
PLUNGER PUMP AND STRAINER
OIL PRESSURE ADJUSTMENT
FRONT BEARING LINE
REAR BEARING LINE
CENTER BEARING LINE
OIL FLOAT LEVEL
Fig. 25. Plunger Pump Oiling System]
There is a pipe line running from the pump to the gear case with a
screw adjustment to regulate the oil pressure by turning either in or
out. There is a pipe line running to a gauge on the dash which gives
the pressure at all times. The cam shaft and cylinder walls get the
oil by the splash from the connecting rods. The bottom rings of the
pistons wash the oil back into the crank case. The overflow from the
front bearings falls into a small compartment immediately under the
crank shaft gear where it is picked up by this gear and carried to the
other gears and the bearings of the water pump shaft. A small oil throw
washer on the pump shaft prevents any surplus oil from being carried
out on the shaft or the hub of the fan drive pulley. Any overflow from
the gear compartment is carried immediately to the splash pan where it
provides for the splash lubrication of the connecting rod bearings and
the cylinder walls. The dippers on the connecting rod bearings should
go ¹⁄₄ in. beneath the surface of the oil. The upward stroke of the oil
pump plunger draws the oil through the lower ball check into the pump
body and the downward stroke discharges it through the upper ball check
into the body of the plunger which is hollow and has outlets on either
side. This allows the oil to flow from the plunger into the by-pass in
the oil pump body and then into the lines running to the main crank
shaft bearings. The next upward stroke forces the oil through the lines
to the main bearings.
The oil pressure regulator is located on the body of the pump and
connects to the by-pass. It consists of a hollow sleeve screwed into
the body of the pump which has a small ball check held by a short
coiled spring the tension of which determines the oil pressure. The
tension and the pressure may be increased by turning the nut to the
right. The nut should not be given more than one turn at a time in
either direction as it is very sensitive. A loose main bearing will
allow more oil to pass through it. Consequently the pressure registered
on the oil gauge will be reduced. This will come about gradually. It
is not advisable to attempt to adjust the oil pressure without first
noting the condition of the main crank shaft bearings.
The most common cause of failure to operate is the collection of
dust and dirt on the sleeve at the lower end of the pump or from an
accumulation of sediment back of the ball check. This needs to be
cleaned from time to time.
=Force and Gravity Oiling System.=--The force and gravity oiling
system operates in much the same manner as the plunger pump system,
except that the oil is pumped into an elevated tank from which it
flows through lines by gravity to the various bearings. Oil pumps,
however, differ in construction and some manufacturers use eccentric,
centrifugal, and gear pumps. Oil pumps are very simple in construction
and action and can be readily understood by recalling their functional
action.
Oil pumps rarely give any trouble, and if they fail to function
properly, dirt should be immediately suspected, and the ball valves and
pipes inspected and cleaned.