The car engine is a complex mechanism, one of the most important elements of which is the camshaft, which is part of the timing. The correct operation of the engine largely depends on the accurate and smooth operation of the camshaft.
One of the most important functions in the operation of a car engine is performed by the camshaft, which is an integral part of the gas distribution mechanism (timing). The camshaft provides intake and exhaust strokes of the engine.
Depending on the design of the engine, the gas distribution mechanism can have a lower or upper valve arrangement. Today, timing belts with overhead valves are more common. This design allows for faster and easier maintenance, which includes adjusting and repairing the camshaft, which requires camshaft parts.
Camshaft device
From a structural point of view, the engine camshaft is connected to the crankshaft, which is ensured by the presence of a chain and a belt. The camshaft chain or belt slides over the crankshaft sprocket or camshaft pulley. A camshaft pulley such as a split gear is considered the most practical and effective option, therefore it is often used to tune engines in order to increase their power.
The bearings, inside which the camshaft bearing journals rotate, are located on the cylinder head. If the journal mountings fail, repair camshaft liners are used to repair them.
In order to avoid axial play, special retainers are included in the camshaft design. Directly along the axis of the shaft there is a through hole designed for lubrication of rubbing parts. This hole is closed at the rear using a special camshaft plug.
The most important part of the camshaft is the cams, the number of which indicates the number of intake and exhaust valves. The cams are responsible for performing the main function of the camshaft - adjusting the valve timing of the engine and regulating the order of operation of the cylinders.
Each valve is equipped with a cam. The cam runs onto the tappet, helping to open the valve. After the cam leaves the tappet, a powerful return spring closes the valve.
The camshaft cams are located between the bearing journals. The gas distribution phase of the camshaft, which depends on the engine speed and on the design of the intake and exhaust valves, is determined empirically. Similar data for a specific engine model can be found in special tables and diagrams that are specially compiled by the manufacturer.
How does a camshaft work?
The main function of the camshaft (camshaft) is to ensure the opening / closing of the intake and exhaust valves, with the help of which fuel assemblies (air-fuel mixture) are supplied and the resulting gases are removed. The camshaft is the main part of the timing (gas distribution mechanism), which takes part in the complex process of gas exchange in a car engine.
The modern timing belt can be equipped with one or two camshafts. In a single-shaft mechanism, all intake and exhaust valves are serviced at once (1 intake and exhaust valve per cylinder). In a mechanism equipped with two shafts, one camshaft drives the intake valves, the other shaft drives the exhaust valves (2 intake and exhaust valves per cylinder).
The location of the gas distribution mechanism directly depends on the type of car engine. Distinguish between timing with an upper valve arrangement (in the cylinder block) and with a lower valve arrangement (in the cylinder head).
The most common option is the overhead location, which makes it possible to efficiently adjust and maintain the camshaft.
The principle of operation and the device of the camshaft
The gas distribution phases are set according to the alignment marks that are located on the gears or pulley. Correct installation ensures that the engine cycle is in sequence.
The main part of the camshaft is the cams. In this case, the number of cams with which the camshaft is equipped depends on the number of valves. The main purpose of the cams is to adjust the phases of the gassing process. Depending on the type of timing design, the cams can interact with a rocker arm or a pusher.
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Cams are installed between bearing journals, two for each cylinder of the engine. During operation, the camshaft has to overcome the resistance of the valve springs, which serve as a return mechanism, bringing the valve to its original (closed) position.
Overcoming these efforts requires the useful power of the engine, so designers are constantly thinking about how to reduce power losses.
In order to reduce friction between the tappet and the cam, the tappet can be equipped with a special roller.
In addition, a special desmodromic mechanism has been developed, in which a springless system is implemented.
The camshaft supports are equipped with covers, with the front cover being shared. It has thrust flanges that are connected to the shaft journals.
The camshaft is manufactured in one of two ways - forging from steel or casting from cast iron.
Timing systems
As mentioned above, the number of camshafts corresponds to the type of engine.
In in-line engines with one pair of valves (one intake and one exhaust valve each), the cylinder is equipped with only one shaft. Inline engines with two pairs of valves have two shafts.
Currently, modern engines can be equipped with various valve timing systems:
- VVT-i. In this technology, the phases are adjusted by turning the camshaft in relation to the sprocket on the drive.
- Valvetronic. The technology allows you to adjust the valve lift by shifting the axis of rotation of the rocker arm
- VTEC. This technology involves the regulation of the phases of the distribution of gases through the use of cams on an adjustable valve
So, to summarize ... the camshaft, being the main link of the gas distribution mechanism, ensures timely and accurate opening of the engine valves. This is ensured by precisely fitting the shape of the cams, which pushing the tappets, make the valve move.
Good day, dear motorists! Let us together try to put on shelves, in the literal sense of the word, the device of one of the important components of the engine's gas distribution mechanism (timing) - the camshaft.
Camshaft device
The camshaft performs far from the last function in the operation of the car engine - it synchronizes the intake and exhaust of engine strokes.
Depending on the type of engine, the timing can be with a lower valve arrangement () or with an upper valve arrangement (c).
In modern engine building, preference is given to the upper timing belt. This allows you to simplify the process of maintenance, adjustment and, due to ease of access to the timing parts.
Structurally, the camshaft is connected to the engine crankshaft. This connection is made by means of a belt or chain. The camshaft belt or chain is fitted to the camshaft pulley and crankshaft sprocket. The camshaft is driven by the crankshaft.
The most effective is the camshaft pulley, which is used to increase the power characteristics of the engine.
On the cylinder head there are bearings in which the camshaft journal journals rotate. In the case of repairs, repair camshaft liners are used to secure the bearing journals.
Camshaft end play is prevented by camshaft clips. A through hole is made along the camshaft axis. Through it, the rubbing surfaces of the parts are lubricated. On the rear side, this hole is covered by a camshaft plug.
Camshaft cams - 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 tappet, a powerful return spring closes the valve.
The camshaft cams are located between the bearing journals. Two cams: intake and exhaust 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 drive 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.
A camshaft cover is installed on the camshaft supports. The front camshaft cover is common. It has thrust flanges that enter the grooves in the camshaft journals.
The main parts of the timing
- Valves: inlet and outlet. The valve consists of a stem and a poppet. The valve seats are plug-in for easy replacement. The intake valve head is larger in diameter than the exhaust valve head.
- Rocker serves to transfer force to the valve from the rod. There is a screw in the short arm of the rocker to adjust the thermal gap.
- Barbell designed to transfer force from the pusher to the rocker arm. One end of the bar rests against the pusher, and the other against the rocker arm adjusting bolt.
How the camshaft works
The camshaft is located in the camber of the cylinder block. The camshaft is driven by a gear or chain drive from the crankshaft.
The rotation of the camshaft causes the cams to act on the operation of the intake and exhaust valves. This happens in strict accordance with the valve timing and the order of the engine cylinders.
To correctly set the valve timing, there are timing 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 according to the marks, the sequence of the alternation of strokes is observed - the order of operation of the engine cylinders. The order of operation of the cylinders depends on their location and the 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 takes 2 crankshaft revolutions. 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-shaped engines have one camshaft in the camber, 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 ISTU named after M.T. Kalashnikov with a degree in Operation of Transport and Technological Machines and Complexes. More than 10 years of professional car repair experience.
Modern engines rarely have one camshaft, most often there are two, which ensures quieter engine operation, increased efficiency and increased power due to more valves (the intake-exhaust cycle is faster). One camshaft is responsible for the intake valves and the other exhaust valves. For more powerful cars with V-shaped engines, four camshafts are used due to the design features of the power plant. The single-camshaft gas distribution mechanism is called Single OverHead Camshaft (SOCH), the two-shaft system is called Double OverHead Camshaft (DOCH). With proper operation, camshafts rarely fail, their main malfunction is natural wear of rubbing parts or deformation of the unit due to cracks. Wear is significantly accelerated in the following cases:
- low oil pressure (insufficient level);
- getting into the oil of antifreeze or fuel;
- burnout valves or malfunction of hydraulic lifters;
- violation of the valve timing.
Good luck in learning how your car's engine works.
Camshaft and its drive
The camshaft ensures timely opening and closing of valves. The shaft has inlet D and outlet B cams, bearing 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.
Figure: 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 manufactured in one piece with the shaft. Cast iron camshafts are also used.
Four-stroke engines have two cams for each cylinder: an intake and an exhaust. The shape (profile) of the cam ensures smooth lifting and lowering of the valve and the corresponding duration of its opening. The cams of the same name are located in an in-line four-cylinder engine at an angle of 90 ° (Fig. 1, a), in a six-cylinder engine - at an angle of 60 ° (Fig. 1, b). The 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 operating order adopted for the engine, taking into account the direction of rotation of the shaft. Along the length of the shaft, the intake and exhaust cams alternate according to the valve position.
In V-shaped engines, the location of the cams on the 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, with their tops facing 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 bushings. The number of camshaft bearings is different for different types of engines.
Axial movements of the camshaft in most engines are limited by a thrust flange (Fig. 2), fixed to the block and located with a certain gap 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 journal 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 journal end and the gear hub. In two-stroke YaMZ diesel engines, the axial movements of the shaft are limited by bronze thrust washers installed on both sides of the front bearing.
The camshaft is driven from the crankshaft using a gear or chain drive. With a gear train, the timing gears are attached to the end of the crankshaft and camshaft.
To increase the noiselessness and smoothness of operation, the 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 drive, which ensures great quiet operation (ZIL-111 vehicles), sprockets are fixed at the end of the crankshaft and at the end of the camshaft, connected by a flexible steel silent chain. The chain teeth mesh with the sprocket teeth.
Figure: 2. Types of camshaft drives: a - gear transmission; b - chain drive
When assembling, timing gears or sprockets are installed relative to one another according to the marks on their teeth.
On new engine models, an overhead camshaft (on the block head) is used. The shaft is driven by a chain drive (Moskvich-412 vehicle).
The gas distribution mechanism ensures the timely entry of the 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 tappet. The valve moves in a guide sleeve pressed into the cylinder block. The valve is closed by a spring resting on the block and a washer secured by two crackers at the end of the valve stem.
When the valves are located overhead, 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 a compact combustion chamber, provides better cylinder filling, reduces heat loss with the coolant and simplifies valve clearances adjustment.
The camshaft ensures timely opening and closing of valves. It is made of steel or cast iron.
When assembling, the camshaft is inserted into the hole in the end of the engine crankcase, so the diameters of the bearing journals gradually decrease, starting from the front journal. The number of journal journals is usually equal to the number of crankshaft main bearings. Bushings of 8 bearing journals are made of steel, bronze (YMZ-740) or cermets.
The inner surface of steel bushings is poured with a layer of babbitt or SOS-6-6 alloy.
There are cams on the camshaft that act on the pushers; gear wheel drive oil pump and breaker-distributor; fuel pump drive eccentric. There are two cams for each cylinder. The angles of their relative position for cams of the same name depend on the number of cylinders and the alternation of working strokes in different cylinders, for dissimilar cams - on the valve timing. The cams and journals of steel camshafts are hardened with high frequency currents, and cast iron ones are bleached. When 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.
Figure: 3. Gas distribution mechanism with lower valves: a-diagram, 6 - details; 1-camshaft, 2 - pusher, 3- lock nut, 4- adjusting bolt, 5-crackers, b - persistent. spring washer, 7- valve spring, 8- outlet valve, 9- valve guide sleeve, 10- insertable outlet valve seat, 11- inlet 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 movements.
The camshaft receives rotation from the crankshaft. In four-stroke engines, the operating cycle takes place 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 slow 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). The teeth of the gears are oblique.
Figure: 4. Gas distribution mechanism with overhead valves (ZIGMZO): 1 - camshaft gear, 2 - thrust flange, 3 - spacer ring, 4-bearing journals, 5-eccentric of the fuel pump drive, 6 - exhaust cams, 7 - intake cams valves, 8 - bushings, 9 - inlet valve, 10 - guide sleeve, 11 - thrust washer, 12 - spring, 13 - rocker arm axis, 14 - rocker arm, 15 - adjusting screw, 16 - rocker arm axis, 17 - mechanism turning the exhaust valve, 18 - the exhaust valve, 19 - the rod, 20-pushers, 21 - the gear wheel of the drive of the oil pump and the breaker-distributor
The distribution gears of the YaMZ-740 engine are located at the rear end of the cylinder block.
Camshaft gears mesh with each other at a strictly defined position of the crankshaft and camshaft. This is achieved by aligning 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 the noise during operation is reduced.
The driven camshaft sprocket is driven by a sleeve roller chain from the crankshaft drive sprocket. The chain tensioner has a sprocket and a lever.
Figure: 5. Gas distribution mechanism with an upper camshaft (Moskvich-412): a - gas distribution mechanism, b - gas distribution mechanism drive; 1 - valve tip, 2 - axle of exhaust rocker arms, 3,6 - rocker arms, 4 - camshaft, 5 - axis of inlet rocker arms, 7 - lock nut, 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: - Engine design and operation
There are three important characteristics of the camshaft design and they control the engine power curve: camshaft timing, valve opening time, and valve lift. Further in the article we will tell you what the design of the camshafts and their drive is.
Valve lift usually calculated in millimeters, it is the distance the valve will move farthest from the seat. Duration of opening valves is a period of time that is measured in degrees of crankshaft rotation.
The duration can be measured in various ways, but due to the maximum flow with a small valve lift, the duration is usually measured after the valve has already risen from the seat by some amount, often 0.6 or 1.3 mm. For example, a particular camshaft may have an opening time of 2,000 turns at a stroke of 1.33 mm. As a result, if you use the 1.33mm tappet lift as the stop and start point for valve lift, the camshaft will hold the valve open for 2000 cranks. If the duration of the valve opening is measured at zero lift (when it is only moving away from the seat or 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 called camshaft timing... For example, the camshaft can act to open the intake valve at 350 before top dead center and close at 750 after bottom dead center.
Increasing valve lift distance can be beneficial 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, 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 work theoretically. But, the actuator mechanisms in the valves are not so simple. In this case, the high speed of the valves, which are caused by these profiles, will significantly reduce the reliability of the engine.
When the opening speed of the valve increases, there is less time left for the valve to move from the closed position to full lift and return from the point of departure. If the travel times become even shorter, more force valve springs are needed. This often becomes mechanically impossible, let alone moving the valves at fairly low RPM.
As a result, what is a reliable and practical value for maximum valve lift? Camshafts with a lift greater than 12.8 mm (minimum for a motor in which the drive is carried out using hoses) are in an area impractical for conventional motors. Camshafts with an intake stroke of less than 2900, combined with a valve lift of more than 12.8 mm, provide very high valve opening and closing speeds. This will undoubtedly create additional stress 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 rate can work very well in the beginning, but the service life of the valve guides and bushings will most likely not exceed 22,000 km. It is good that most camshaft manufacturers design their parts in such a way that they provide a compromise between valve opening times and lift values, while ensuring reliability and long service life.
The timing of the intake and the valve lift discussed are not the only design elements of the camshaft that affect the final engine power. The moments, closing and opening of valves in relation to the position of the camshaft, are also such important parameters for optimizing the performance of the engine. You can find these camshaft timing 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 are closed and opened. They will be accurately measured in degrees of crankshaft rotation before TDC or TDC.
The angle between the centers of the cams is the offset angle between the center line of the exhaust cam (called the exhaust cam) and the center line of the intake cam (called the intake cam).
Cylinder angle is often measured in "camshaft angles" because we are discussing the offset of the cams relative to each other, this is one of the few times when the characteristic of the camshaft is indicated in degrees of rotation of the shaft, and not in degrees of rotation of the crankshaft. 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 simultaneously open. Valve overlap is often measured by SB crankshaft angles. When the angle between the cam centers decreases, the intake valve opens and the exhaust valve closes. It should always be remembered that valve overlap is also influenced by a change in the opening time: if the opening time is increased, the overlap of the valves will also become large, while ensuring that there are no angle changes to compensate for these increases.
