- Purpose and types of timing:
- Valve group device:
- Valve group operation:
- Diagnostics, maintenance, repair:
- Timing diagnostic methods:
Purpose and types of timing:
1.1. Purpose of the gas distribution mechanism:
The purpose of the gas distribution mechanism is to pass the fresh fuel mixture into the engine cylinders and release the exhaust gases. Gas exchange is carried out through the inlet and outlet openings, which are hermetically closed by the timing belt elements in accordance with the accepted engine operation procedure.
1.2. Valve group assignment:
the purpose of the valve group is to hermetically close the inlet and outlet openings and open them at a specified time for a specified time.
1.3. Timing types:
depending on the organs by which the engine cylinders are connected to the environment, the timing is valve, spool and combined.
1.4. Comparison of timing types:
the valve timing is the most common due to its relatively simple design and reliable operation. Ideal and reliable sealing of the working space, achieved due to the fact that the valves remain stationary at high pressure in the cylinders, gives a serious advantage over a valve or combined timing. Therefore, valve timing is increasingly used.
Valve group device:
2.1. Valve device:
Engine valves consist of a stem and a head. The heads are most often made flat, convex or bell-shaped. The head has a small cylindrical belt (about 2 mm) and a 45˚ or 30˚ sealing bevel. The cylindrical belt allows, on the one hand, to maintain the main diameter of the valve when grinding the sealing chamfer, and on the other hand, to increase the valve rigidity and thereby prevent deformation. The most widespread are valves with a flat head and a sealing bevel at an angle of 45˚ (these are most often intake valves), and to improve filling and cleaning of cylinders, the intake valve has a larger diameter than the exhaust valve. Exhaust valves are often made with a domed ball head.
This improves the flow of exhaust gases from the cylinders and also increases the strength and rigidity of the valve. To improve the conditions for heat removal from the valve head and to increase the overall non-deformability of the valve, the transition between the head and the stem is made at an angle of 10˚ - 30˚ and with a large radius of curvature. At the upper end of the valve stem, grooves are made of conical, cylindrical or special shapes, depending on the adopted method of attaching the spring to the valve. Sodium cooling is used in a number of engines to reduce thermal stress on burst valves. For this, the valve is made hollow, and the resulting cavity is half-filled with sodium, the melting point of which is 100 ° C. When the engine is running, sodium melts and, moving in the valve cavity, transfers heat from the hot head to the cooler rod, and from there to the valve actuator.
2.2. Connecting the valve to its spring:
the designs of this unit are extremely diverse, but the most common design is with half-cones. With the help of two half-cones, which enter the channels made in the valve stem, the plate is pressed, which holds the spring and does not allow disassembling the unit. This creates a connection between the spring and the valve.
2.3. Valve seat location:
In all modern engines, the exhaust seats are manufactured separately from the cylinder head. These are also used for suction cups when the cylinder head is made of aluminum alloy. When it's cast iron, the saddles are made in it. Structurally, the seat is a ring that is attached to the cylinder head in a specially machined seat. At the same time, grooves are sometimes made on the outer surface of the seat, which, when pressed on the seat, are filled with cylinder head material, thereby ensuring their reliable fastening. In addition to clamping, fastening can also be done by swinging the saddle. To ensure tightness of the working space when the valve is closed, the working surface of the seat must be machined at the same angle as the sealing chamfer of the valve head. For this, the saddles are machined with special tools with sharpening angles not 15 not, 45˚ and 75˚ in order to obtain a sealing tape at an angle of 45˚ and a width of about 2 mm. The rest of the corners are made to improve the flow around the saddle.
2.4. Valve Guides Location:
the design of the guides is very diverse. Most often, guides with a smooth outer surface are used, which are made on a centerless plumbing machine. Guides with an external retaining strap are easier to fasten but harder to make. For this, it is more expedient to make a channel for the stop ring in the guide instead of the belt. Exhaust valve guides are often used to protect them from the oxidative effects of the hot exhaust gas stream. In this case, longer guides are made, the rest of which is located in the cylinder head exhaust channel. As the distance between the guide and the valve head decreases, the hole in the guide on the side of the valve head narrows or widens in the region of the valve head.
2.5. Springs device:
in modern engines, the most common cylindrical springs with a constant pitch. To form the supporting surfaces, the ends of the coils of the spring are brought together against each other and are lapped with their foreheads, as a result of which the total number of coils is two to three times greater than the number of working springs. The end coils are supported on one side of the plate and on the other side of the cylinder head or block. If there is a risk of resonance, the valve springs are made with a variable pitch. The stepped gearbox bends either from one end of the spring to the other, or from the middle to both ends. When the valve is opened, the windings closest to each other touch, as a result of which the number of working windings decreases, and the frequency of free oscillations of the spring increases. This removes the conditions for resonance. For the same purpose, sometimes conical springs are used, the natural frequency of which varies along their length and the occurrence of resonance is excluded.
2.6. Materials for the manufacture of valve group elements:
• Valves - Suction valves are available in chrome (40x), chromium-nickel (40XN) and other alloy steels. Exhaust valves are made of heat-resistant steels with a high content of chromium, nickel and other alloying metals: 4Х9С2, 4Х10С2М, Х12Н7С, 40СХ10МА.
• Valve seats — use heat resistant steels, alloy cast iron, aluminum bronze, or cermets.
• Valve guides are difficult to manufacture and require materials with high thermal and wear resistance and good thermal conductivity, such as gray pearlitic cast iron and aluminum bronze.
• Springs - made by winding a wire from a spring ostomy, for example 65G, 60C2A, 50HFA.
Valve group operation:
3.1. Synchronization mechanism:
the synchronization mechanism is kinematically connected to the crankshaft, moving synchronously with it. The timing belt opens and seals the inlet and outlet ports of the individual cylinders in accordance with the accepted operating procedure. This is the process of gas exchange in cylinders.
3.2 Action of the timing drive:
The timing drive depends on the location of the camshaft.
• With a lower shaft - through cylindrical gears for smoother operation are made with inclined teeth, and for silent operation the toothed ring is made of PCB. A parasitic gear or chain is used to provide the drive over a greater distance.
• With top shaft - roller chain. Relatively low noise level, simple design, low weight, but the circuit will wear out and stretch. Through a neoprene-based toothed belt reinforced with steel wire and covered with a wear-resistant nylon layer. Simple structure, quiet operation.
3.3. Gas distribution scheme:
The total flow area provided for the passage of gases through the valve depends on the duration of its opening. As you know, in four-stroke engines, for the implementation of the intake and exhaust strokes, one piston stroke is provided, corresponding to the rotation of the crankshaft by 180˚. However, experience has shown that for better filling and cleaning of the cylinder it is necessary that the duration of the filling and emptying processes be longer than the corresponding piston strokes, i.e. the opening and closing of the valves should not be carried out at the dead points of the piston stroke, but with some overtaking or delay.
The valve opening and closing times are expressed in angles of rotation of the crankshaft and are called valve timing. For greater reliability, these phases are made in the form of pie charts (Fig. 1).
The suction valve normally opens with an overtaking angle φ1 = 5˚ - 30˚ before the piston reaches top dead center. This provides a defined valve cross-section at the very beginning of the filling stroke and thus improves the filling of the cylinder. The closing of the suction valve is performed with a delay angle φ2 = 30˚ - 90˚ after the piston passes the bottom dead center. The intake valve closing delay allows the intake fresh fuel rate to be used to improve refueling and therefore increase engine power.
The exhaust valve is opened with an overtaking angle φ3 = 40˚ - 80˚, i.e. at the end of the stroke, when the pressure in the cylinder gases is relatively high (0,4 - 0,5 MPa). An intense ejection of a gas cylinder, started at this pressure, leads to a rapid drop in pressures and their temperature, which significantly reduces the work of displacing working gases. The outlet valve closes with a delay angle φ4 = 5˚ - 45˚. This delay ensures good cleaning of the combustion chamber from exhaust gases.
Diagnostics, maintenance, repair:
- •Reduced power of the internal combustion engine:
- Reduced clearance;
- Incomplete valve fit;
- Seized valves.
• Increased fuel consumption:
- Reduced clearance between valves and lifters;
- Incomplete valve fit;
- Seized valves.
• Wear in internal combustion engines:
- Camshaft wear;
- opening the camshaft cams;
- Increased clearance between valve stems and valve bushings;
- Large clearance between valves and lifters;
- fracture, violation of the elasticity of the valve springs.
• Low pressure indicator:
- The valve seats are soft;
- Soft or broken valve spring;
- Burnt out valve;
- Burnt or torn cylinder head gasket
- Unadjusted thermal gap.
• High pressure indicator.
- Decreased head height;
Timing diagnostic methods:
• Measurement of pressure in the cylinder at the end of the compression stroke. During the measurement, the following conditions must be met: the internal combustion engine must be heated to operating temperature; The spark plugs must be removed; The center cable of the induction coil must be oiled and the throttle and air valve open. Measurement is performed using compressors. The pressure difference between individual cylinders must not exceed 5%.
4.2. Adjusting the thermal clearance in the timing belt:
Checking and adjusting the thermal gap is carried out using the pressure gauge plates in the sequence corresponding to the order of engine operation, starting with the first cylinder. The gap is properly adjusted if the thickness gauge, corresponding to the normal gap, passes freely. When adjusting the clearance, hold the adjusting screw with a screwdriver, loosen the locknut, place the clearance plate between the valve stem and the coupling, and turn the adjusting screw to set the required clearance. Then the lock nut is tightened.
4.3. Valve group repair:
• Valve repair - major failures are wear and tear of the tapered working surface, stem wear and cracking. If the heads burn or cracks, the valves are disposed of. Curved valve stems are straightened on a hand press using a tool. Worn valve stems are repaired by chronizing or ironing and then sanded to their nominal or oversized size. The worn working surface of the valve head is ground to repair size. The valves are lapped to the seats using abrasive pastes. The grinding accuracy is checked by pouring kerosene onto the hinged valves, if it does not leak, then the grinding is good for 4-5 minutes. Valve springs are not restored, but replaced with new ones.