Key Points of the valve mechanism in aviation piston engines
The key working points of the valve mechanism of an aviation piston engine are as follows:
First, the function of the valve mechanism
The valve mechanism is an important component of an aviation piston engine. Its main function is to control the opening and closing timing of the intake and exhaust valves, ensuring that fresh mixtures (or air) enter the cylinder at the appropriate time and that the exhaust gas after combustion and power work is discharged from the cylinder in a timely manner.
Second, the main components of the valve mechanism
Valve mechanisms are typically composed of the following main components:
CAM disc (or camshaft) :
There are many protruding parts on the CAM disc (or camshaft), which are used to drive the operation of the valve mechanism.
The shape of the protruding surface on the CAM disc determines the lift of the valve (the distance the valve rises from the valve seat) and the duration for which the valve remains open.
Roller (or follower wheel) :
Contact with the protruding part on the CAM disc to reduce friction and wear.
Tappet
Convert the rotational motion of the CAM disc (or camshaft) into linear reciprocating motion.
There are two types of tappets: mechanical and hydraulic. The hydraulic tappet can automatically keep the valve stem closely attached to the rocker arm under any heating degree and working conditions of the engine, that is, to maintain the valve clearance at zero.
Putting:
Transfer the movement of the tappet to the rocker arm.
Push rods are usually tubular, which are both lightweight and strong. They also allow engine lubricating oil to pass through the hollow push rods under pressure to lubricate rocker arms and other components.
Rocker arm
Transfer the lifting force of the CAM disc (or camshaft) to the valves.
One end of the rocker arm is sometimes slotted to facilitate the installation of the rotating wheel. The other end is made into a slotted chuck with threads and a locking bolt, or a threaded hole is tapped.
Valve spring
Close the valves and press them tightly against the valve seats.
Generally, two or three valve springs are installed on each valve to reduce vibration.
Valve
It is divided into intake valves and exhaust valves.
Valves are composed of valve heads, valve stems, valve necks and valve tops.
The valve head has a ground surface. When the valve is closed, this surface is in close contact with the ground valve seat, forming the valve sealing surface.
Third, the working principle of the valve mechanism
Intake stroke
The protruding part on the CAM disc (or camshaft) drives the rollers (or follower wheels), tappets and push rods, and thereby drives the rocker arm.
The rocker arm transmits the lifting force of the CAM to the intake valve, causing it to open.
Meanwhile, the exhaust valve remains closed under the action of the valve spring.
The piston slides down from the top dead center to the bottom dead center. The volume inside the cylinder gradually increases, the air pressure decreases, and fresh mixture is drawn into the cylinder through the open intake valve.
Compression stroke
Both the intake valve and the exhaust valve remain closed.
The crankshaft continues to rotate due to inertia, pushing the piston upwards from the bottom dead center. The volume inside the cylinder gradually decreases, and the mixed gas is strongly compressed by the piston.
Working stroke (expansion stroke) :
The spark plug generates an electric spark through high voltage to ignite the mixed gas.
The gas expands violently, pushing the piston to move rapidly towards the dead center and simultaneously driving the crankshaft to rotate.
Both the intake valve and the exhaust valve remain closed.
Exhaust stroke
The protruding part on the CAM disc (or camshaft) pushes the rollers (or follower wheels), tappets and push rods, which in turn push the rocker arm to open the exhaust valve.
Meanwhile, the intake valve remains closed under the action of the valve spring.
The piston moves upward from the bottom dead center, expelling the burned exhaust gas from the cylinder through the open exhaust valve.
Fourth, key points of the valve mechanism's work
Timing
The valves must be opened and closed at the specified time to ensure the normal operation of the engine.
The timing of the valves depends on the intervals of the protrusions on the CAM disc (or camshaft) and the rotational speed and direction relative to the crankshaft.
Lift and duration:
The lift and duration of the valve determine the flow volume and velocity of the gas in the cylinder.
The shape of the protruding surface on the CAM disc (or camshaft) determines the lift and duration of the valve.
Sealing property
A good seal must be formed between the valve and the valve seat to prevent gas leakage in the cylinder.
The valve head has a ground surface. When the valve is closed, this surface is in close contact with the ground valve seat, forming the valve sealing surface.
Lubrication and cooling
The valve mechanism generates friction and heat during operation and requires appropriate lubrication and cooling.
Lubricating oil can enter the valve mechanism through components such as push rods to lubricate and cool it.
Adjustment and maintenance
The valve mechanism needs to be inspected and adjusted regularly to ensure its normal operation.
Common adjustment items include valve clearance adjustment, valve seat grinding, etc.
Fifth, Summary
The key points of the operation of the valve mechanism in an aviation piston engine involve multiple aspects, including the function of the valve mechanism, its main components, working principle, and key points of operation. Understanding these key points helps to better comprehend the working principle and maintenance methods of the valve mechanism, thereby ensuring the normal operation and stable performance of the engine.
