Auto mechanic MDK 01.02.
The gas distribution mechanism (GRM) serves for the timely release of exhaust gases and filling the cylinders with a fresh charge, in carburetor engines – with a combustible mixture of fuel with air, and for diesel engines – with air.
The intake of a fresh charge and the release of exhaust gases is ensured by the timely opening and closing of the intake and exhaust valves of the engine, which, when closed during compression and the working stroke of the piston, also ensure reliable sealing of the cylinders. There are valve and spool gas distribution mechanisms. In four-stroke engines, gas exchange is carried out using valves. In a two-stroke engine, gas exchange occurs under the action of a piston that opens and closes the intake and bypass valves, or through a mixed gas distribution system.
Valve timing mechanisms share:
• at the place of installation of the valves – the upper location of the valves in the cylinder head (Fig. 1 a) and the lower one – in the cylinder block (Fig. 1 e);
• at the place of installation of the camshaft – upper (Fig. 1 b and c) and lower (Fig. 1 a);
• according to the type of camshaft drive – gear or gear (Fig. 6), chain (Fig. 7) and belt (Fig. 2).
|Rice. 1 Types of gas distribution mechanisms||Rice. 2 The drive of the gas distribution mechanism of the ZIL-508 engine|
The most common gas distribution mechanism with an overhead valve arrangement, which facilitates access to the valves for their maintenance, allows you to get a compact combustion chamber and ensure its best filling with a combustible mixture or air.
The main components of the gas distribution mechanism:
• camshaft drive mechanism;
• valve mechanism.
We will consider the operation of the gas distribution mechanism using the example of an engine with a V-shaped arrangement of cylinders (Fig. 3 a).
The camshaft 6 is located in the “collapse” of the engine block, that is, between its right and left rows of cylinders. It is installed in the holes of the block with pressed bronze bushings 8 (sliding bearings) and is driven from the crankshaft through the block of timing gears 1, 19.
When the camshaft 6 rotates, the cam 4 runs on the pusher 18 and lifts it together with the rod 14. The upper end of the rod 14 presses on the adjusting screw 16 installed in the inner arm of the rocker arm 12. The rocker arm 12, turning on its axis 11, presses the rod with the outer arm valve 15 and overcoming the forces of the springs 10 opens the hole 24 of the intake or exhaust valve in the cylinder head 22 strictly in accordance with the valve timing and the order of operation of the cylinders.
The valve stems move in cast-iron guide bushings 9 pressed into the cylinder head 22. The valve guide bushing is held in the block head by a key ring 32. Valve seats 23 pressed into the cylinder head seat are made of heat-resistant cast iron. The axles of the rocker arms are installed in racks 13, which are fixed on the cylinder heads.
The valve mechanism of the ZIL-508 engine (Fig. 3 b) consists of valves 15 placed in cast-iron guide bushings 9 with external retaining rings 32, valve seats 23, springs 10 and spring fastening parts (lower support washer 26, upper plate 28, cone crackers 29). Typically, motors are equipped with two springs 10 with the opposite direction of the turns in order to prevent vibration of the valve.
The outer and inner springs are held by the support plate 28, fixed on the valve stem by detachable crackers 29 included in its groove 30, which have the shape of a truncated cone when folded.
A rubber oil scraper cap 27 is put on the inlet valve stem, which prevents oil from entering the combustion chamber. On ZIL-508 engines and some others, the upper part of the exhaust valve stem is hollow and filled with sodium 33, which melts when heated and effectively cools the valve by transferring heat from the head to the stem and further through the guide sleeve 9 to the head or cylinder block.
Rice. 3 a) the gas distribution mechanism of the ZIL-508 engine; b) valve mechanism 1 – camshaft gear; 2 – thrust flange; 3 – fuel pump drive eccentric; 4 – cams of exhaust valves; 5 – intake valve cams; 6 – camshaft; 7 – middle neck; 8 – sleeve of the neck of the shaft; 9 – valve guide sleeve; 10 – spring; 11 – the axis of the rocker arms; 12 – rocker; 13 — rack axle rocker; 14 – rod; 15 – exhaust valve; 16 – adjusting screw; 17 – gear drive oil pump and distributor; 18 — pushers; 19 – crankshaft gear; 20 – piston; 21 – cylinder liner; 22 – cylinder head; 23 – heat-resistant insert; 24 – outlet channel; 25 – chamfer of the valve; 26 – support washer; 27 – oil scraper cap; 28 – plate; 29 – crackers; 30 – groove on the valve stem; 31 – valve rotation mechanism; 32 – lock ring; 33 – sodium.
In order to reduce the burning of the landing chamfers of the exhaust valves, some engines install special devices for turning the valve around its axis. In the ZIL-508 engine, this device (Fig. 4) consists of a housing 1, in the inclined grooves of which five balls 3 with return springs 2 are installed. Above the balls there is a disk spring 4, a support washer 6 and a lock ring 5. When the valve is closed, the working pressure the springs are small, the disk spring 4 does not rest on the balls and they are pressed to the extreme position under the influence of the return springs 2.
When the valve opens, the working spring is compressed and the pressure on the disk spring 4 increases. It, bending, acts on the balls, which, under load, mix into the recess of the grooves, causing the disk spring and the support washer 6 to rotate, and with it the entire valve with the spring.
When the valve is closed, the force of its spring decreases, the disc spring 4 returns to its original position, the balls are released and, under the pressure of the springs 2, roll into their original position.
On the engines of KamAZ, GAZ-53A and others, the valve is rotated by installing an intermediate conical bushing between crackers and a thrust washer. This is because the conical surfaces of the crackers and the sleeve do not match over the entire area.
The camshaft gears are made of cast iron or textolite, the crankshaft drive camshaft gear is made of steel. The teeth of the gears are oblique, which causes axial movement of the shaft. To prevent axial displacement, a thrust flange 2 is provided (Fig. 3 a), which is fixed on the cylinder block between the end of the front shaft support journal and the timing gear hub.
For proper engine operation, the crankshaft cranks and camshaft cams must be in a strictly defined position relative to each other. Therefore, when assembling the engine, the timing gears are engaged according to the marks on their teeth: one on the tooth of the crankshaft gear, and the other between the two teeth of the camshaft gear. On engines with a block of timing gears, they are also installed according to the marks (Fig. 2).
The valve timing is the opening and closing times of the valves, expressed in degrees of the angle of rotation of the crankshaft relative to dead points. The quality of gas exchange is determined by the parameters of valve opening: the duration of opening and the flow area of the valve gap. Structurally, this is achieved by a given profile (shape) of the cam and the location of the intake and exhaust cams at appropriate angles to each other.
|Rice. 4. Device for turning the valve around its axis: 1 – housing; 2 – spring; 3 – ball; 4 – disc spring; 5 – lock ring; 6 – support washer.||Rice. 5. Diagram of the gas distribution phases of the ZIL-508 engine|
To get the most power from the engine, it is necessary to ensure a more complete cleaning of the cylinders from combustion products and more filling them with a combustible mixture. To do this, the valves open at moments that do not coincide with TDC and BDC, but with some lead at the beginning and a delay at the end of the intake and exhaust process (Fig. 5).
The intake valve opens ahead of time at the end of the exhaust stroke, when the piston has not yet reached TDC, and closes late at the beginning of the compression stroke, when the piston moves away from BDC.
Early opening and late closing of the intake valve provide a better filling of the cylinders with a combustible mixture due to the inertial pressure in the intake manifold. The exhaust valve opens in advance at the end of the stroke to BDC, which allows the exhaust gases to exit the cylinder under their own excess pressure. It closes after TDC, at the beginning of the intake stroke, which provides better cleaning of the cylinder, since the exhaust gases at this time will continue to exit the cylinder by inertia.
The angle of rotation of the crankshaft during which both valves in the cylinder are open is called valve overlap. The valve timing is selected empirically depending on the engine speed, maximum torque, maximum power and the design of the intake and exhaust pipes empirically as a result of lengthy finishing tests. Manufacturers indicate the valve timing for their engines in the form of tables or diagrams.
The correct installation of the gas distribution mechanism is determined by the installation marks, which are located on the timing gears or the drive pulley of the engine block.
Deviation in the installation of the phases leads to failure of the valves or the engine as a whole. The constancy of the valve timing is maintained only if the regulated thermal clearance in the valve mechanism of this engine model is observed. Violation of the value of this gap leads to accelerated wear of the valve mechanism and loss of engine power.
The sequence of alternation of the same cycles in different cylinders is called the order of operation of the engine cylinders, which depends on the location of the cylinders and the design of the crankshaft and camshaft.
For four-cylinder single-row engines, the cycles alternate through 180 ° and the order of operation of the cylinders can be 1-3-4-2 (AZLK, VAZ) or 1-2-4-3 (GAZ).
In V-shaped eight-cylinder four-stroke engines, the connecting rod journals of the crankshaft are located at an angle of 90 °, and at an engine camber angle of 90 °, the same cycles will overlap in the left row of cylinders with respect to the right row by 90 ° or 1/4 crankshaft turn. These engines have the following cylinder order: 1 – 5 – 4 – 2 – 6 – 3 – 7-8.
Knowing how the cylinders work is necessary for the correct connection of wires to the spark plugs of carbureted engines or high pressure pipelines of diesel engines, as well as for adjusting the thermal clearances of the valve mechanism.
Thermal gap. During engine operation, the valves and valve drive parts heat up, their length increases. As a result, a gap can form between the valve seat and head during compression and expansion strokes, which leads to burning of the valve and seat chamfers, their erosive wear and, ultimately, to deterioration of the cylinder tightness, and, consequently, to a sharp decrease in the technical and economic performance of the engine.
To prevent these phenomena, the kinematic chain of the valve drive is opened when it is closed, i.e., a gap is set between the end of the valve and the drive part acting on the valve (rocker arm or pusher). On average, thermal gaps, depending on the type of engine, are 0.15–0.30 mm for the intake valve and 0.15–0.35 mm for the exhaust valve.
On the VAZ-2106 engine, the camshaft 7 is driven from the sprocket 2 (Fig. 3) of the crankshaft 1 by a double-row roller chain 3, which rotates the camshaft sprocket 6 and simultaneously through the sprocket 16 – the drive shaft of the oil pump, ignition distributor and fuel pump.
Rice. 6. Gas distribution mechanism of VAZ-2106 engines: 1 – crankshaft; 2 – crankshaft sprocket; 3 – chain; 4 – shoe; 5 – tensioner; 6 – camshaft sprocket; 7 – camshaft; 8 – lever; 9 – adjusting bolt; 10 – bushing of the adjusting bolt; 11 – valves; 12 – outer spring of the valve; 13 – an internal spring of the valve; 14 – hairpin spring of the lever; 15 – chain damper; 16 – an asterisk of a drive of the oil pump; 17 – locknut; 18 – locking plate; 19 – lock ring; 20 – valve guide sleeve; 21 – oil scraper cap; 22 – washer; 23 – lower support washer; 24 – plate; 25 – crackers.
The lever 8 is pressed against the end of the valve stem and against the spherical surface of the adjusting bolt 9 screwed into the steel sleeve 10 of the cylinder head by the elastic force of the hairpin spring 14.
When the camshaft 7 rotates, its cam runs onto the lever 8, which, turning on the spherical support of the adjusting bolt 9, presses the valve stem 11 with the other end and opens a hole that communicates the combustion chamber of the cylinder with the inlet (inlet valve) or outlet (exhaust valve) pipeline .
With further rotation of the shaft, the cam descends with a convex part from the lever, which returns to its original position with the help of a pin spring 14, and the valve closes under the action of spring 12. In this case, a thermal gap is formed between the lever and the back of the cam, which ensures tight closing of the valve when its stem is extended due to heating during engine operation. The chain tension is carried out using a tensioner. When the chain is lengthened during operation, the shoe 4 of the tensioner will automatically move under the action of the tensioner 5. To reduce the noise of the chain drive, the working surfaces of the damper 15 and shoe 4 are covered with rubber with a low coefficient of friction.
The gas distribution mechanism with an overhead camshaft and belt-driven valves is shown in fig. 7.
Rice. 7. Gas distribution mechanism with an overhead camshaft and belt-driven valves: a) Schematic diagram of the drive; b) diagram of the camshaft drive of the VAZ-21083 engine; c) valve mechanism: 1 – crankshaft toothed pulley; 2 – toothed belt; 3 – tension roller; 4 – a gear pulley of a camshaft; 5 – camshaft cam; 6 – camshaft; 7 – pusher; 8 – outer spring of the valve; 9 – an internal spring of the valve; 10 – valve 11 – rear protective cover; 12 – crankshaft; 13 – toothed pulley of the coolant pump; 14 – adjusting gasket; 15 – crackers; 16 – plate; 17 – thrust washer.
From the toothed pulley 1 of the crankshaft, the rotation is transmitted by the belt 2 to the toothed pulley 4 of the camshaft. When the camshaft rotates, its cam 5 runs into the pusher 7, which presses the valve stem 9 and opens a hole that communicates the combustion chamber of the cylinder with the inlet (inlet valve) or outlet (exhaust valve) pipeline. With further rotation of the shaft, the cam descends with a convex part from the pusher and the valve closes under the action of spring 8. In this case, a thermal gap is formed between the pusher and the back of the cam.
The camshaft drive has a belt tensioner with roller 3, and a toothed pulley 13 can also be installed to drive auxiliary engine devices.