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Figure 9. Gas distribution mechanism: 1. Pulley on the crankshaft to drive the generator; 2. Toothed pulley on the crankshaft to drive the camshaft; 3. Timing belt of the camshaft drive; 4. Coolant pump pulley; 5. Tension roller; 6. Eccentric axis of the tension roller; 7. Timing mark (antenna) on the rear cover of the timing belt; 8. Timing mark on the camshaft pulley; 9. Camshaft pulley; 10. 5° ignition advance mark on the front timing belt cover; 11. Ignition advance mark at 0°; 12. TDC mark on the generator drive pulley; 13. Installation mark on the oil pump cover; 14. TDC mark on the crankshaft toothed pulley; 15. Front camshaft bearing housing; 16. Rear camshaft bearing housing; 17. Eccentric on the camshaft for driving the fuel pump; 18 Camshaft; 19. Valve crackers; 20. Valve plate; 21. Outer valve spring; 22. Inner valve spring; 23. Spring support washer; 24. Inlet valve; 25. Valve guide bushings; 26. Exhaust valve; 27. Retaining ring; 28. Oil deflector cap; 29. Valve tappet; 30. Adjusting washer; 31. Cylinder head; 32. Valve seat; 33. Distance ring; I. Checking the belt tension; II. Cylinder head bolt tightening procedure; III. The order of tightening the nuts securing the camshaft bearing housings.
The valve timing mechanism ensures that the engine cylinders are filled with a fresh charge of combustible mixture and exhaust gases are released in accordance with the requirements of the working process in each of the engine cylinders. This mechanism is characterized by the upper in-line arrangement of the valves.
The camshaft 18, which controls the opening and closing of the valves, is located in the cylinder head and is driven from the crankshaft by a toothed belt 3. The valves are driven directly by the camshaft cams through cylindrical tappets 29 without intermediate levers. In the tappet seat there is a washer 30, the selection of which regulates the clearance in the valve mechanism.
The elastic toothed belt also rotates pulley 4 of the coolant pump. Roller 5 serves to tension the belt. It rotates on an eccentric axis 6 attached to the cylinder head. By turning axis 6 relative to the fastening pin, the belt tension is changed. The belt tension is considered normal if in the middle part of the branch between the pulleys of the camshaft and crankshaft the belt is twisted with a finger force of 1.5-2 kgf.
Due to the strict orientation of the keyways in the leading 2 and driven 9 pulleys relative to the teeth and their corresponding engagement with the toothed belt, the required valve timing phases are ensured. The correct mutual arrangement of the drive pulleys is checked as follows: the crankshaft is rotated to a position in which the piston of the first cylinder is at TDC of the compression stroke (both valves are closed and the mark on the crankshaft pulley is aligned with mark 13 on the oil pump cover). In this case, mark 8 should coincide with mark 7 on the rear cover of the toothed belt, and the mark on the flywheel should be opposite the middle division of the scale on the clutch housing.
If the marks do not match, then loosen the belt with a tension roller, remove it from the camshaft pulley, adjust the pulley position, put the belt back on the pulley and slightly tighten it with a tension roller. Again check the coincidence of the installation marks by turning the crankshaft two turns clockwise.
It is not allowed to turn the crankshaft and camshaft of engines 2108 and 21081 if the camshaft drive belt is not installed, because the pistons at TDC will rest against the valves, and the engine parts will be damaged. In addition, the crankshaft is allowed to turn only by the side of the generator drive pulley mount and only in the direction of tightening the bolt (clockwise). Do not turn the crankshaft by the camshaft pulley or by its mounting bolt.
The camshaft, cast from cast iron, has five bearing journals that rotate in seats made in the cylinder head and in the camshaft bearing housings 15 and 16. The shaft has an eccentric 17 for driving the fuel pump. The rear end of the camshaft has a groove for connection with the engine ignition distributor sensor.
The camshaft is held from axial movements by a thrust flange of the shaft, located between the end of the rear support of the shaft and the housing of the auxiliary units. To increase wear resistance, the working surfaces of the cams, eccentric and the surface under the oil seal are bleached. The depth of the bleached layer is not less than 0.2 mm.
Valves (inlet 24 and outlet 26), used for periodic opening and closing of the inlet and outlet channels, are located in the cylinder head at an angle in a row.
The intake valve is made of chromium-silicon steel. Its head has a larger diameter for better cylinder filling. The exhaust valve is made of composite materials: the stem is made of chromium-nickel-molybdenum steel with better wear resistance to friction and good thermal conductivity for heat removal from the valve head to its guide bushing, and the head is made of heat-resistant chromium-nickel-manganese steel. In addition, the working chamfer of the exhaust valve, which operates at high temperatures in an aggressive exhaust gas environment, has a heat-resistant alloy surfacing
The valve guide bushings are made of cast iron, pressed into the cylinder head and are held against possible loss by retaining rings 27. The holes in the bushings are finally machined together with the cylinder head, which ensures a narrow tolerance on the hole diameter and the accuracy of its location relative to the working chamfers of the seat and valve. The holes in the guide bushings have spiral grooves for lubrication. The inlet valve bushings have grooves cut to half the length of the hole, and the outlet valve bushings have grooves cut along the entire length of the hole.
Caps 28 made of fluororubber with a steel reinforcement ring are placed on top of the guide bushings; they cover the valve stem and serve to reduce the penetration of oil into the combustion chamber through the gaps between the guide bushing and the valve stem.
Springs (outer 21 and inner 22) press the valve to the seat and do not allow it to come off the drive. The lower ends of the springs rest on the support washer 23. The upper support plate 20 of the springs is held on the valve stem by two crackers 19, which have the shape of a truncated cone when folded. The crackers have three internal flanges that fit into the grooves on the valve stem. This design ensures both a reliable connection and rotation of the valves during operation, due to which they wear more evenly.
Tappets 29 are designed to transmit force from the camshaft cams to the valves. The tappets are made in the form of cylindrical cups and are located in the cylinder head guides. An adjusting washer 30 of a certain thickness is placed in the end recess of the tappet, providing the necessary clearance between the camshaft cam and the tappet with the washer.
The washers are made of 20X steel and are nitrocarburized to increase the surface hardness. Spare parts include adjusting washers with a thickness of 3 to 4.5 mm, spaced every 0.05 mm. The thickness of the washer is marked on its surface. The washer must be installed in the pusher with the marking facing down.
When the engine is running, the tappets rotate around their axes all the time, which is necessary for their uniform wear. The rotation of the tappets is achieved by shifting the cam axis relative to the tappet axis by 1 mm.