There are three important characteristics of camshaft design that govern the power curve of an engine: camshaft timing, valve timing, and valve lift. Later in the article we will tell you what the design of camshafts and their drive is.
valve lift usually calculated in millimeters and represents the distance that the valve will move as far as possible from the seat. Opening time valves is a period of time, which is measured in degrees of rotation of the crankshaft.
Duration can be measured in a variety of ways, but due to maximum flow at low valve lift, duration is usually measured after the valve has already moved up from the seat by some amount, often 0.6 or 1.3 mm. For example, a particular camshaft may have an opening duration of 2000 turns with a lift of 1.33 mm. As a result, if you use a 1.33mm pushrod lift as the stop and start point for valve lift, the camshaft will hold the valve open for 2000 crankshaft rotations. If the duration of the valve opening will be measured at zero lift (when it just moves away from the seat or is in it), then the duration of the crankshaft position will be 3100 or even more. The moment when a particular valve closes or opens is often referred to as camshaft timing. For example, the camshaft may act to open the intake valve at 350 BDC and close it at 750 BDC.
Increasing the valve lift distance can be a beneficial step in increasing engine power, as power can be added without significantly interfering with engine performance, especially at low rpm. If you delve into the theory, then the answer to this question will be quite simple: such a camshaft design with a short valve opening time is needed in order to increase the maximum engine power. It will theoretically work. But, the drive mechanisms in the valves are not so simple. In such a case, the high valve speeds that these profiles produce will greatly reduce the reliability of the engine.
As the valve's opening speed increases, there is less time to move the valve from the closed position to full lift and return to its starting point. If the driving time becomes even shorter, valve springs with more force will be needed. Often this becomes mechanically impossible, let alone to move the valves at fairly low RPMs.
As a result, what is a reliable and practical value for maximum valve lift? Camshafts with a lift greater than 12.8 mm (the minimum for a motor driven by hoses) are in an impractical area for conventional motors. Camshafts with an intake stroke duration of less than 2900, which are combined with a valve lift of more than 12.8 mm, provide very high valve closing and opening speeds. This, of course, will create an additional load on the valve drive mechanism, which significantly reduces the reliability of: camshaft cams, valve guides, valve stems, valve springs. However, a shaft with a high valve lift speed may work very well in the beginning, but the life of the valve guides and bushings will most likely not exceed 22,000 km. The good news is that most camshaft manufacturers design their parts to offer a compromise between valve opening times and lift values, with reliability and long life.
The duration of the intake stroke and the discussed valve lift are not the only design elements of the camshaft that affect the final power of the engine. The timing of valve opening and closing relative to camshaft position is also such an important parameter for optimizing engine performance. You can find these camshaft timings in the datasheet that comes with any quality camshaft. This datasheet graphically and numerically illustrates the angular positions of the camshaft when the exhaust and intake valves open and close. They will be precisely defined in degrees of crankshaft rotation before top or bottom dead center.
Angle between cam centers is the offset angle between the exhaust valve cam center line (called the exhaust cam) and the intake valve cam center line (called the intake cam).
Cylinder angle is often measured in "camshaft angles", as Since we're discussing cam offsets, this is one of the few times the camshaft characteristic is given in degrees of shaft rotation rather than degrees of crankshaft rotation. The exception is those engines where two camshafts are used in the cylinder head (cylinder head).
The angle chosen in the design of the camshafts and their drive will directly affect the valve overlap, that is, the period when the exhaust and intake valves are open at the same time. Valve overlap is often measured by SB crank angles. When the angle between the centers of the cams decreases, the intake valve opens and the exhaust valve closes. It must always be remembered that the valve overlap is also affected by the change in opening time: if the opening duration is increased, the valve overlap will also become greater, while ensuring that there are no angle changes to compensate for these increases.
Camshaft and its drive
The camshaft ensures the timely opening and closing of the valves. The shaft has inlet D and outlet B cams, support journals L, gear D for driving the oil pump and distributor of the ignition system, and eccentric B for driving the fuel pump in carburetor engines.
Rice. 1. Types of camshafts
The shaft is stamped from steel; its cams and necks are subjected to heat treatment to obtain increased wear resistance, after which they are ground. The cams are made as one piece with the shaft. Cast iron camshafts are also used.
Four-stroke engines have two cams for each cylinder: an intake cam and an exhaust cam. The shape (profile) of the cam ensures smooth lifting and lowering of the valve and the corresponding duration of its opening. Cams of the same name are located in a four-cylinder in-line engine at an angle of 90° (Fig. 1, a), in a six-cylinder engine - at an angle of 60° (Fig. 1, b). Opposite cams are set at an angle, the value of which depends on the valve timing. The tops of the cams are located in the order of operation adopted for the engine, taking into account the direction of rotation of the shaft. The intake and exhaust cams alternate along the length of the shaft in accordance with the arrangement of the valves.
In V-shaped engines, the location of the cams on a camshaft common to both sections of the block depends on the alternation of strokes in the cylinders, the camber angle and the adopted valve timing. The camshaft of a Y-shaped eight-cylinder carburetor engine is shown in fig. 1, c.
In two-stroke diesel engines (YaAZ-M204 and YAAZ-M206), for each cylinder there are two exhaust cams facing the tops in one direction, and one cam that controls the operation of the pump-injector.
With the lower location of the camshaft, it is installed in the crankcase on supports, which are holes in the walls and partitions of the crankcase, into which steel thin-walled bimetallic or trimetallic bushings are pressed. The shaft is sometimes also installed in special liners. The number of camshaft bearings for different types of engines is different.
The axial movements of the camshaft for most engines are limited by a thrust flange (Fig. 2), fixed on the block and located with a certain clearance between the end face of the front shaft journal and the gear hub; the gap between the support flange and the end of the shaft neck is set for engines of different brands in the range of 0.05-0.2 mm; the size of this gap is determined by the thickness of the spacer ring fixed on the shaft between the neck end and the gear hub. For YaMZ two-stroke diesel engines, the axial movement of the shaft is limited by bronze thrust washers installed on both sides of the front bearing.
The camshaft is driven from the crankshaft by a gear or chain drive. With a gear train, timing gears are fixed at the end of the crankshaft and camshaft.
To increase noiselessness and smoothness of operation, gears are made with oblique teeth; the camshaft gear is usually made of plastic - textolite, and the crankshaft gear is made of steel.
With a chain transmission that provides greater noiselessness of operation (ZIL-111 cars), sprockets connected by a steel flexible silent chain are fixed at the end of the crankshaft and at the end of the camshaft. The chain teeth engage with the sprocket teeth.
Rice. 2. Types of camshaft drives: a - gear; b - chain drive
Distribution gears or sprockets during assembly are installed one relative to the other according to the marks on their teeth.
On new engine models, the upper camshaft (on the head of the block) is used. The shaft is driven by a chain transmission (Moskvich-412 car).
The gas distribution mechanism ensures the timely entry of a combustible mixture (or air) into the engine cylinders and the release of exhaust gases.
Engines can have a lower valve arrangement (GAZ -52, ZIL -157K, ZIL -1E0K), in which the valves are located in the cylinder block, and an upper one (ZMZ -24, 3M3-S3, ZIL -130, YaMZ -740, etc.) when they are located in the cylinder head.
With the lower valves, the force from the camshaft cam is transmitted to the valve or through the pusher. The valve moves in a guide sleeve pressed into the cylinder block. The valve is closed by a spring resting against the block and a washer fixed with two crackers at the end of the valve stem.
With an overhead valve arrangement, the force from the camshaft cam is transmitted to the pusher, rod, rocker arm and valve. The overhead valve arrangement is predominantly used, since this design allows for a compact combustion chamber, provides better filling of the cylinders, reduces heat loss from the coolant and simplifies valve clearance adjustment.
The camshaft ensures the timely opening and closing of the valves. It is made from steel or cast iron.
During assembly, the camshaft is inserted into the hole in the end of the crankcase, so the diameters of the bearing journals are successively reduced, starting from the front journal. The number of bearing journals is usually equal to the number of crankshaft main bearings. The bushings of 8 bearing journals are made of steel, bronze (YaMZ-740) or cermet.
The inner surface of the steel bushings is filled with a layer of babbit or SOS-6-6 alloy.
On the camshaft are cams that act on the pushers; oil pump drive gear and breaker-distributor; fuel pump drive eccentric. There are two cams for each cylinder. The angles of their mutual arrangement depend for the same cams - on the number of cylinders and the alternation of strokes in different cylinders, for opposite cams - on the valve timing. The cams and necks of steel camshafts are hardened with high-frequency currents, and cast-iron ones are bleached. During grinding, the cams are given a slight taper, which, in combination with the spherical shape of the end of the pushers, ensures that the pusher rotates during operation.
Rice. 3. Gas distribution mechanism with lower valves: a-scheme, 6-details; 1 - camshaft, 2 - pusher, 3 - locknut, 4 - adjusting bolt, 5 - crackers, b - thrust. spring washer, 7 - valve spring, 8 - exhaust valve, 9 - valve guide, 10 - exhaust valve seat insert, 11 - intake valve
A spacer washer and a thrust flange are installed between the camshaft gear and the front support journal, which is bolted to the cylinder block and keeps the shaft from axial movement.
The camshaft receives rotation from the crankshaft. In four-stroke engines, the duty cycle occurs in two revolutions of the crankshaft. During this period, the intake and exhaust valves of each cylinder must open once, and therefore the camshaft must rotate one revolution. Thus, the camshaft must rotate twice as slowly as the crankshaft. Therefore, the camshaft gear has twice as many teeth as the gear at the front end of the crankshaft. The crankshaft gear is steel, the gear on the camshaft is cast iron (ZIL-130) or textolite (ZMZ-24, 3M3-53). Gear teeth are oblique.
Rice. 4. Gas distribution mechanism with overhead valves (ZIGMZO): 1 - camshaft gear, 2 - thrust flange, 3 - spacer ring, 4-support necks, 5-eccentric fuel pump drive, 6 - exhaust valve cams, 7 - intake cams valves, 8 bushings, 9 - inlet valve, 10 - guide bushing, 11 thrust washer, 12 - spring, 13 - rocker arm axis, 14 - rocker arm, 15 - adjusting screw, 16 rocker axle post, 17 - mechanism turning the exhaust valve, 18 - exhaust valve, 19 - rod, 20 pushers, 21 - oil pump drive gear and breaker-distributor
The distribution gears of the YaMZ -740 engine are located on the rear end of the cylinder block.
The timing gears engage with each other at a strictly defined position of the crankshaft and camshaft. This is achieved by combining the marks on the tooth of one gear and the cavity between the teeth of the other gear.
In high-speed engines (Moskvich-412, VAZ-2101 Zhiguli), the camshaft is located in the cylinder head and its cams act directly on the rocker arms, which, turning on the axles, open the valves. In such a valve mechanism, there are no pushers and rods, the casting of the cylinder block is simplified, and noise during operation is reduced.
The camshaft driven sprocket is driven by a roller chain from the crankshaft drive sprocket. The chain tensioner has a sprocket and a lever.
Rice. 5. Gas distribution mechanism with an overhead camshaft ("Moskvich-412"): a - gas distribution mechanism, b - gas distribution mechanism drive; 1 - valve tip, 2 - exhaust valve rocker axle, 3.6 - rocker arms, 4 - camshaft, 5 - intake rocker arm axle, 7 - locknut, 8 - adjusting screw, 9 - cylinder head, 10 - valves, 11 - drive sprocket, 12 tensioner sprocket, 13 - lever, 14 - driven sprocket, 15 - chain, 16 - crankshaft
TO category: - Design and operation of the engine
Good day, dear motorists! Let's try together to put on the shelves, in the literal sense of the word, the device of one of the important components of the gas distribution mechanism (timing) of the engine - the camshaft.
Camshaft device
The camshaft performs far from the last function in the operation of a car engine - it synchronizes the intake and exhaust cycles of the engine.
Depending on the type of engine, the timing can be with a lower valve position (), and with an upper valve position (at).
In modern engine building, preference is given to the upper timing. This allows you to simplify the process of maintenance, adjustment and, thanks to ease of access to timing parts.
Structurally, the camshaft is connected to the crankshaft of the engine. This connection is made by means of a belt or chain. The camshaft belt or chain is put on the camshaft pulley and crankshaft sprocket. The camshaft is driven by the crankshaft.
The camshaft pulley is considered the most effective, which is used to increase the power characteristics of the engine.
Bearings are located on the cylinder head, in which the camshaft bearing journals rotate. In case of repair, camshaft repair bushings are used to fasten the bearing journals.
Camshaft end play is prevented by camshaft retainers. A through hole is made along the axis of the camshaft. Through it, the rubbing surfaces of parts are lubricated. On the rear side, this hole is closed by a camshaft plug.
camshaft lobes- the most important component. Their number corresponds to the number of intake and exhaust valves of the engine. It is the cams that perform the main purpose of the camshaft - adjusting the valve timing of the engine and.
Each valve has its own, individual cam, which opens it, "running" on the pusher. When the cam comes off the pusher, under the action of a powerful return spring, the valve closes.
The camshaft cams are located between the bearing journals. Two cams: inlet and outlet for each cylinder. In addition, a gear is attached to the shaft to drive the breaker-distributor and the oil pump. Plus an eccentric to actuate the fuel pump.
The gas distribution phase of the camshaft is selected empirically, and depends on the design of the intake and exhaust valves and the engine speed. Manufacturers for each engine model indicate the camshaft phases in the form of diagrams or tables.
The camshaft cover is mounted on the camshaft bearings. The front camshaft cover is common. It has thrust flanges included in the grooves in the necks of the camshafts.
The main parts of the timing
- valves: inlet and outlet. The valve consists of a stem and a disc plane. The valve seats are plug-in for ease of replacement. The intake valve head is larger than the exhaust valve.
- rocker serves to transfer force to the valve from the rod. In the short arm of the rocker there is a screw for adjusting the thermal gap.
- Barbell designed to transfer force from the pusher to the rocker. One end of the rod rests against the pusher, and the other end against the rocker arm adjusting bolt.
The principle of operation of the camshaft
The camshaft is located in the collapse of the cylinder block. By means of a gear or chain drive, the camshaft is driven by the crankshaft.
The rotation of the camshaft provides the effect of the cams on the operation of the intake and exhaust valves. This happens in strict accordance with the valve timing and the order of operation of the engine cylinders.
For the correct installation of the valve timing, there are installation marks located on the timing gears or on the drive pulley. For the same purpose, the crankshaft cranks and camshaft cams must be in a strictly defined position relative to each other.
Thanks to the installation, made by marks, the sequence of cycles is observed - the order of operation of the engine cylinders. The order of operation of the cylinders depends on their location and design features of the crankshaft and camshaft.
Engine duty cycle
The period when the intake and exhaust valves in each cylinder must open once is the engine's duty cycle. It is carried out in 2 revolutions of the crankshaft. At this time, the camshaft should make one revolution. It is for this that the camshaft gear has twice as many teeth.
Number of camshafts in the engine
This value usually depends on . Engines with in-line configuration and one pair of valves per cylinder have one camshaft. If there are 4 valves per cylinder, then two camshafts.
Boxer and V-twin engines have one camshaft in the collapse, or two, one camshaft in each block head. There are also exceptions related to the design features of the engine model. (for example, an in-line arrangement of four cylinders - one camshaft with 4 valves per cylinder, like the Mitsubishi Lancer 4G18).
Automotive expert. Graduated from IzhGTU named after M.T. Kalashnikov with a degree in Operation of Transport and Technological Machines and Complexes. Over 10 years of professional car repair experience.
Modern engines rarely have one camshaft, most often there are two, which ensures quieter operation of the engine, increases efficiency and increases power due to more valves (the intake-exhaust cycle is accelerated). One camshaft controls the intake valves and the other the exhaust valves. For more powerful vehicles with V-engines, four camshafts are used due to the design features of the power plant. The gas distribution mechanism with one camshaft is called Single OverHead Camshaft (SOCH), the system with two shafts is called Double OverHead Camshaft (DOCH). With proper operation, camshafts rarely fail, their main malfunction is the natural wear of rubbing parts or deformation of the assembly due to cracks. Wear accelerates significantly in the following cases:
- low oil pressure (insufficient level);
- ingress of antifreeze or fuel into the oil;
- burnout of valves or malfunctions of hydraulic lifters;
- violation of the valve timing.
Good luck with your car engine.
Camshaft, shortened camshaft- the main part or timing, an important element of an automobile engine. Its task is to synchronize the intake and exhaust strokes of the internal combustion engine.
Design features
The location of this mechanism entirely depends on the design of the internal combustion engine, since in some models the camshaft is located at the bottom, at the base of the cylinder block, and in others, at the top, right in. At the moment, the top location of the camshaft is considered optimal, since this greatly simplifies service and repair access to it. The camshaft is directly connected to. They are interconnected by a chain or belt drive by providing a connection between the pulley on the timing shaft and the sprocket on the crankshaft. This is necessary because the camshaft is driven by the crankshaft.
The camshaft is installed in bearings, which in turn are securely fixed in the cylinder block. Axial play of the part is not allowed due to the use of clamps in the design. The axis of any camshaft has a through channel inside through which the mechanism is lubricated. At the back, this hole is closed with a plug.
Important elements are the camshaft cams. In number, they correspond to the number of valves in the cylinders. It is these parts that perform the main function of the timing - regulating the order of operation of the cylinders.
Each valve has a separate cam that opens it through pressure on the pusher. By releasing the pusher, the cam allows the spring to straighten, returning the valve to the closed state. The camshaft device assumes the presence of two cams for each cylinder - according to the number of valves.
Camshaft device.
It should be noted that the fuel pump and distributor are also driven from the camshaft.
Principle of operation
The engine camshaft, located in the cylinder block, is driven by a gear or chain drive from the crankshaft.
Rotating, the camshaft rotates the cams located on it, which alternately act on the intake and exhaust valves of the cylinders, ensuring their opening and closing in a certain order, unique to each ICE model.
The working cycle of the engine (alternate movement of each of the valves of the cylinders) is carried out in 2 revolutions of the crankshaft. During this time, the camshaft has to complete only one revolution, so its gear has twice as many teeth.
There can be more than one camshaft in one internal combustion engine. Their exact number is determined by the engine configuration. The most common budget in-line engines, which have a pair of valves for each cylinder, are equipped with only one camshaft. For systems with two pairs of valves, two camshafts must already be used. For example, power units with a different arrangement of cylinders have either a single camshaft installed in the collapse, or a pair - for each block head separately.
1. Rolling hydraulic jack. The regular jack of a VAZ 2107 car is often either inconvenient or simply useless when performing some work.
2. car support, adjustable in height and with a permissible load of at least 1 t. It is desirable to have four such stands.
3. wheel chocks(at least 2 pieces).
4. Double ended wrenches for 8, 10 and 13mm brake fittings. The two most common types of wrenches are the clamp wrench and the slotted box wrench. The clamping key allows you to unscrew fittings with worn edges. To put the wrench on the brake pipe fitting, it is necessary to unscrew the coupling bolt. A ring wrench with a slot allows you to work more quickly, however, such a wrench must be made of high-quality steel with appropriate heat treatment.
5. Special tongs to remove retaining rings. There are two types of such tongs: sliding - for removing circlips from holes, and sliding - for removing circlips from shafts, axles, rods. Forceps also come with straight and curved jaws.
6. Oil filter puller.
7. Universal two-jaw puller for removing pulleys, hubs, gears.
8. Universal three-jaw pullers for removing pulleys, hubs, gears.
9. Cardan joint puller.
10. Puller and mandrel for replacing valve stem seals.
11. Crusher for dismantling the valve mechanism of the cylinder head.
12. Tool for removing ball bearings.
13. Piston pin extractor.
14. Device for pressing and pressing silent blocks front suspension arms.
15. Device for removal of steering drafts.
16. Crankshaft ratchet wrench.
17. Spring puller.
18. impact screwdriver with a set of nozzles.
19. Digital multimeter to check the parameters of electrical circuits.
20. Special probe or test lamp for 12V to check the electrical circuits of a VAZ 2107 car that are energized.
21. pressure gauge to check the pressure in the tires (if there is no pressure gauge on the tire pump).
22. pressure gauge to measure the pressure in the fuel rail of the engine.
23. Compressometer to check the pressure in the engine cylinders.
24. Nutromer for measurement of diameter of cylinders.
25. Caliper with depth gauge.
26. Micrometers with a measurement limit of 25-50 mm and 50-75 mm.
27. Styli set to check the gap between the electrodes of the spark plugs. You can use a combination wrench to service the ignition system with a set of necessary probes. The key has special slots for bending the side electrode of the spark plug.
28. Flat feeler set for measuring gaps when assessing the technical condition of the units.
29. Wide probe 0.15mm to check valve clearances.
30. Mandrel for centering the clutch disc.
31. Mandrel for crimping piston rings when installing the piston in the cylinder.
32. Hydrometer to measure the density of a liquid (electrolyte in a battery or antifreeze in an expansion tank).
33. Special tool with metal brushes for cleaning wire terminals and battery terminals.
34. oil syringe for filling oil in the gearbox and rear axle.
35. injection syringe for lubricating the splines of the cardan shaft.
36. Hose with pear for pumping fuel. The hoses can be used to remove fuel from the tank before removing it.
37. Medical syringe or pear for the selection of fluids (for example, if it is necessary to remove the reservoir of the main brake cylinder without draining all the brake fluid from the system). The syringe is also indispensable for cleaning carburetor parts. When performing repair work on a VAZ 2107 car, you may also need: a technical hair dryer (thermal gun), an electric drill with a set of drills for metal, a clamp, tweezers, an awl, a tape measure, a wide metalwork ruler, a household steelyard, a wide container for draining oil and coolant with a volume of at least 10 liters.
