The same operations occur in the fourcycle type of motor; however, they require two complete revolutions of the crank shaft or four piston strokes. This comprises the following operations as mentioned below: (1) Admission of charge to the cylinder, (2) compression of charge within the cylinders, (3) ignition and expansion of charge, (4) exhausting spent gases.
Fig. 4. Crank Case Inlet and Cylinder Compression.
Fig. 5. Expansion and Exhaust.
Fig. 6. Exhaust and Transfer of Gas.
In four-cycle motors the inlet and exhaust of the charge is entirely controlled by valves instead of the piston and ports in the two-cycle motor. During the past few years numerous types of valves have been used, such as the piston type, which may be compared with a small piston opening and closing the ports at the proper time, sliding sleeve, operated on the same principle, and poppet valves which are opened by a cam mechanism. The sleeve and piston valve motors have been experimented with for pleasure car work: however, in commercial cars poppet valve motors have been almost exclusively used in this country, and the writer will confine his discussion to this type of four-cycle motor. As mentioned above, the first stroke of a four-cycle motor is the admission or intake stroke.
The piston travels downward in the cylinder and at some point in the wall of the combustion chamber an inlet valve is located, which at the proper time opens and places the combustion chamber in communication with the carburetor (see Fig. 7). As this valve opens the piston has moved downward a short distance, and a vacuum has been formed with the cylinder which creates a suction and permits the charge to enter. These valves were formerly operated by this vacuum within the cylinder and a light spring to close them when suction ceased. This type was known as the automatic intake valve, but was discarded some years ago, as it presented numerous disadvantages. At the present time mechanically operated intake valves are used, which are opened and closed by mechanical connection with the motor crank shaft at the proper time.
This valve remains open until piston has passed the bottom of its stroke, and shortly after the piston has started on its up stroke it closes and remains closed during the completion of this up stroke. During this up stroke the charge is compressed within the combustion chamber (which is sometimes termed the compression space) and prepared for ignition. So far the piston has made two complete strokes, and the crank shaft one complete revolution, while two operations have been completed within the cylinders, this second operation being shown in Fig. 8.
Upon the completion of the compression stroke the gas is ignited by introducing an electrical spark to occur within the combustion chamber. When the charge is ignited the pressure rises to between four or five times what it was previously, causing the gases to expand and thus forcing the piston downward and converting the heat of these gases into useful energy or power. This stroke, known as the power stroke, is illustrated in Fig. 9.
When the piston reaches the bottom of its stroke the exhaust begins to open and the spent gases escape rapidly. This exhaustion of spent gases continues all through the following return stroke of the piston, the valve remaining open until the piston has again moved downward a slight distance and then both valves remain closed for a short period to create the vacuum which causes the suction of gases from the carburetor on the intake stroke, and permitting the motor to again resume its cycle of operations.
The exhaust valves are always operated by mechanical connection with the motor crank shaft, similar to the intake valve; however, timing is different, as they must remain open longer than the intake. In two-cycle motors the exhaust and transfer ports must be placed opposite each other; however, the valves of the four-cycle type of motor may be placed in various positions, as both valves are never open at the same time. Fig. 10 clearly illustrates this point. The exhaust valve is shown and the piston is traveling upward, while when the intake valve is open the piston must travel downward.
In presenting these illustrations the writer has depicted valves on both sides, so as to simplify the illustrations as much as possible. In the various motors the following valve arrangements may be found: The valves may be on opposite sides of the cylinder, which is called the T-head type while in others they are located side by side, and known as the L-head type; they are also located in the cylinder heads, and by combination of one valve in the head and the other in the side; however, the first three are the most popular types.
Fig. 7. Intake Stroke.
Fig. 8. Compression Stroke.
Fig. 9. Power Stroke.
Fig. 10. Exhaust Stroke.