Camshaft and its drive. What is a camshaft (camshaft)? Camshaft How the camshaft is driven

Camshaft, shortened camshaft- the main part or timing, an important element car engine. Its task is to synchronize the intake and exhaust strokes of the internal combustion engine.

Design features

Location this mechanism depends entirely on ICE designs, because 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 drive is also carried out from the camshaft fuel pump and distributor.

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.

What is a camshaft (camshaft)? camshaft

What is a camshaft in a car

The camshaft is a functional element fuel system car, responsible for the correct sequential opening and closing of the engine valves. The fuel consumption, the developed power, the stability of its operation, and other driving characteristics depend on the correct functioning of it. Let's look at what a camshaft is in a car, what is its principle of operation and how improper operation affects the car.

What is a camshaft

What does the distributor look like?

The camshaft is a rod on which several so-called cams are located. These are irregularly shaped parts rotating on the axis of the shaft. They correspond to the number of inlet valves of the cylinders and are located exactly opposite them. The set of cams is selected so that the rotation guarantees stable and uniform combustion of fuel in the cylinders. And the operation of the entire camshaft is clearly synchronized with other engine mechanisms.

On both sides of the cams, bearing journals are put on the shaft, holding it in bearings. One of the most important shaft components are oil channels. The physical wear of parts, the power characteristics of the motor and the stability of its operation depend on their condition. To supply oil in the camshaft axis, a through hole was made with leads to the support bearings and cams.

How is the camshaft

Camshafts in the cylinder head.

The camshaft is a key functional component of the gas distribution mechanism, which determines the order in which the valves open to start the air fuel mixture inside the cylinders. The synchronous operation of this mechanism ensures continuous sequential combustion of fuel portions in the engine chambers. In some car models, the gas distribution mechanism has several camshafts.

The design, location, composition and characteristics of the camshaft cams are completely dependent on the engine model. In some machines, the camshaft is located in the cylinder head, while in others it is located at the base. The top location is currently considered optimal, as it facilitates repair and maintenance. The camshaft is connected by a belt or chain drive to crankshaft engine, because that is what drives it.

How a camshaft works

How a distributor works.

When viewed transversely, the cam has the shape of a drop. When rotated, the extended part of the cam makes money on the valve lifter and causes the valve to open. This provokes the supply of an air-fuel mixture for combustion. With further rotation, the cam “releases” the pusher, and it, under the action of a spring mechanism, returns the valve to the closed position.

The camshaft gear has twice as many teeth as the crankshaft gear. This is due to the fact that in one working cycle of the engine, the crankshaft makes 2 revolutions, and the camshaft - 1.

The engine configuration may include two camshafts. The layout of the gas distribution mechanism with one shaft is used in budget cars, where the cylinders have 1 pair of valves. Two camshafts are needed in models with two pairs of valves on the cylinders.

What is the camshaft sensor for?

The camshaft position sensor determines the angular positions of the timing relative to crankshaft and generates appropriate signals in the system electronic control engine. As a result, the ignition and fuel injection are corrected. On gasoline vehicles, a malfunction of this device blocks the operation of the computer and does not allow the engine to be started. In diesel models, starting is possible, but still difficult.

Like the crankshaft sensor, the camshaft sensor works on the basis of the Hall principle - the magnetic field in the device changes when the magnetic gap is closed by a special tooth that is located on the shaft or drive disk. When a tooth passes close to the transducer, a signal is generated and sent to the electronic unit management. The frequency of the pulses is directly related to the rate of rotation of the camshaft, on the basis of which the ECU makes adjustments to the operation of the engine. By constantly obtaining data on the position of the piston of the first cylinder, consistent and timely injection is ensured.

Breakdowns and their causes

A faulty camshaft most often gives out its condition with a characteristic knock, which occurs due to wear of bearings or cams, shaft deformation, mechanical failure of one of the elements. Such breakdowns occur, both due to factory defects, and as a result of natural wear and tear.

Camshaft knocking also occurs when using bad engine oil or due to unregulated fuel supply. Because of this, the cylinder valves and cams work out of sync - the engine loses power, consumes too much fuel and runs erratically.

All about the camshaft (camshaft)

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 arrangement (in the cylinder block) or with an upper valve arrangement (in the cylinder head).

In modern engine building, preference is given to the upper timing. This allows you to simplify the process of maintenance, adjustment and repair of the camshaft, due to the ease of access to the 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 most effective is the camshaft pulley - a split gear, which is used to tune the camshaft in order 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 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 the order of operation of the cylinders.

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: intake and exhaust. 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.
The rocker is used to transfer force to the valve from the rod. In the short arm of the rocker there is a screw for adjustment thermal gap.
The rod is 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 correct installation valve timing exist installation marks located on the timing gears or on 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 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 the engine configuration. 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-rev different engines have one camshaft in the collapse, or two, one camshaft in each head of the block. 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).

The modern market offers the consumer different engines with different systems for changing the valve timing. The most characteristic of them:

VTEC- technological development Honda company. Phase adjustment occurs through the use of adjustable valve 2 cams.

VVT-i - from Toyota. The phases are adjusted by turning the camshaft relative to its drive sprocket.

Valvetronic is a technological development of BMW. The valve lift height is adjusted by changing the position of the axis of rotation of the rocker arms.

Good luck with your car engine.

carnovato.ru

Camshaft - Auto Mechanic's Dictionary

The camshaft, in an abbreviated form the camshaft, is the main part of the main camshaft or timing mechanism, 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 depends entirely 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 the cylinder head. 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 the crankshaft. 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.

Camshaft device.

It should be noted that the fuel pump and oil pump 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.

Camshaft failure

There are quite a few reasons why camshaft knocking is woven into the operation of the engine, which indicates the appearance of problems with it. Here are just the most typical ones:

The camshaft requires proper care: replacement of oil seals, bearings and periodic troubleshooting.

wear of the cams, which leads to the appearance of a knock immediately only at startup, and then all the time the engine is running;
bearing wear;
mechanical failure of one of the shaft elements;
problems with adjusting the fuel supply, which causes asynchrony in the interaction of the camshaft and cylinder valves;
shaft deformation leading to axial runout;
poor quality motor oil, replete with impurities;
lack of engine oil.

According to experts, if a slight knock of the camshaft occurs, the car can drive for more than one month, but this leads to increased wear of the cylinders and other parts. Therefore, if a problem is found, it should be addressed. The camshaft is a collapsible mechanism, so repairs are most often carried out by replacing it all or only some elements, such as bearings.

Related terms

etlib.ru

Timing camshaft

The camshaft (camshaft) is a key element of the gas distribution mechanism, which is responsible for the timely opening and closing of the intake or exhaust valve to supply the fuel-air working mixture or exhaust gases.

The camshaft is used to synchronize the intake and exhaust on the cycles of the internal combustion engine. The part ensures the functioning of the entire gas distribution mechanism, taking into account the order of operation of the cylinders and the valve timing in relation to a particular engine.

The camshaft is a shaft with cams located on it. The camshaft rotates in plain bearings, which are made in the form of supports. Engine oil under pressure from the lubrication system enters the camshaft bearings through the channels. The number of cams on the camshaft corresponds to the number of intake and exhaust valves of the engine. One valve receives its cam, which opens it by pressing the pusher. At the moment when the camshaft cam comes off the pusher, then the valve closes under the powerful action of the return spring.

The valve timing depends on the shape of the camshaft cams. Such phases are understood as the moments of opening and closing of the valves, as well as the duration of the valve in the open or closed state. Modern power units also have a variable valve timing system to increase the overall efficiency of the timing and improve the performance of the internal combustion engine.

in engines modern cars The camshaft is located at the top of the cylinder head. The camshaft is connected to the sprocket or crankshaft pulley of the engine by means of a belt or chain drive. The camshaft is driven by the crankshaft.

On four-stroke engines, the entire timing rotates twice as slowly as the crankshaft, since the full working cycle of such internal combustion engines is carried out in two revolutions of the crankshaft. In these two revolutions, the intake and exhaust valves should only open once. It turns out that the camshaft that controls the opening of the valves must complete only one revolution per cycle.

More than one camshaft may be present in the timing design. Often this is due to the number of valves per cylinder. Today, the most widely used scheme is four valves per cylinder and a two-shaft timing (one camshaft drives the intake valves, and the other interacts with the exhaust). For V-shaped internal combustion engines, four camshafts are installed, since each row of cylinders has a separate cylinder head with two shafts. The single-shaft timing system is called SOHC (Single OverHead Camshaft), the twin-shaft is called DOHC (Double OverHead Camshaft).

The device and principle of operation of the camshaft

IN modern engine The camshaft (most often there are two) is located at the top of the cylinder head.

The camshaft is connected to the crankshaft of the car's engine. The connection is made by a timing chain (or belt). For reliable transmission of force, a driven gear is attached to the end of the camshaft, resembling an "asterisk" on the rear wheel of a bicycle.

The camshaft cams are responsible for adjusting the valve timing and the firing order of the cylinders - there are exactly as many of them as there are intake and exhaust valves used in the timing mechanism. The work is organized as follows: the camshaft cam "runs" on the valve follower, presses on it and opens the valve. After the cam comes off the pusher, the valve closes under the action of a tight return spring.

The more valves in the gas distribution mechanism, the more camshafts are installed in it. Bugatti Veyron has four camshafts and 64 valves

So, the camshaft rotates, which ensures the impact of the cams on the operation of the intake and exhaust valves. The location of the cams relative to each other is carefully calculated in strict accordance with the valve timing and the firing order of the cylinders. In other words, while the intake valve (or two valves) of one cylinder is open, all other intake valves are at rest.

The number of camshafts in the engine is determined by the configuration of the motor itself: if the engine has an in-line design and one pair of valves per cylinder, then one camshaft is sufficient. If there are 4 valves per cylinder, it is advisable to use 2 camshafts - one of them serves only the intake valves, the other only the exhaust ones. Among other things, the system with twin shafts has another plus - speed.

As for V-shaped and boxer engines, they can have either one camshaft in the place of the so-called "camber" of the cylinders (the base of the imaginary letter V), or two - one on each cylinder head. It is possible to try to implement a complex scheme for opening and closing 16 valves using one camshaft, but it is not rational - the part will turn out to be too complicated. Such schemes are rare, but Honda nevertheless decided to adopt one of them: an in-line engine with four cylinders and one camshaft is installed, for example, on the popular Honda Fit / Jazz model. The undoubted advantage of such a system is the ability to make the engine compact and light.

Camshaft specifications

It is customary to distinguish three important characteristics camshaft: this is the amount of valve lift, the duration of valve opening and camshaft phase.

For the sake of maximum valve opening period, when designing sports engines, engineers sacrifice idling. At racing cars it rarely goes below 2000 rpm

Valve lift is measured in millimeters. This value measures the maximum distance that the valve moves away from the so-called "saddle", in which it is located at the moment of closing. The valve opening time is the length of time the valves remain open. It is customary to measure this value in degrees of rotation of the crankshaft. At the same time, each of the listed criteria can affect the operation of the engine: with an increase in valve lift, the duration of its opening, or optimization of the valve timing, the engine power increases. It is worth noting that it is the opening time that is the main parameter with which the designers of forced motors work.

So, for example, camshafts used on sports cars, provide a longer duration of valve opening, compared to standard ones. This means that the valves remain open for as long as possible, allowing you to burn the maximum dose of fuel for such a volume of the combustion chamber in one cycle. Unfortunately, in technology, to achieve one, you have to sacrifice something else: the installation of sports camshafts does not allow you to keep the momentum idle move below 2000 rpm. Naturally, with such work, the engine consumes a huge amount of fuel.

If we talk about the phases of the camshaft (the moments when the valves open and close in relation to the position of the camshaft), then all information about them is usually contained in the data table that is attached to the camshaft. The table shows the angular positions of the camshaft, as well as information about when the intake and exhaust valves open and close.

Modern engines are often equipped with variable valve timing systems. For example, some cars Toyota brands have a VVT-i system. the valve timing is adjusted by turning the camshaft relative to its drive sprocket. Another example is the development of the Japanese manufacturer Honda, designated VTEC - it allows you to change the phase using two cams for an adjustable valve.

blamper.ru

What is a camshaft (camshaft)?

The camshaft in the engine is a finger-shaped axial mechanism driven by the crankshaft and having several elliptical protrusions (cams) on its surface - one for each intake and exhaust valve located in the engine. As the camshaft rotates (under the action of the crankshaft), these elliptical lugs control the opening and closing of the valves by pushing them in sequence.

The first symptoms of a camshaft failure are:

Abnormal valve noise
Engine cuts.

The maintenance of the camshaft consists in regular checking and, if necessary, replacing its seals. So, replacement is usually carried out when the timing belt is replaced.

What is a camshaft (camshaft)? Video

howcarworks.ru

Engine Camshaft

For all its external complexity and seeming inaccessibility to understanding, the internal combustion engine is a surprisingly rational and expediently designed device. The purpose of any of its parts is to ensure proper operation and maximum efficiency from the engine. At the same time, literally all its elements are interconnected, but nevertheless, the operation of the timing (gas distribution mechanism), as well as its basis - the camshaft, should be considered separately.

About cycles and operation of internal combustion engines

The internal combustion engine is a four-stroke power unit, which means that all the processes associated with its operation are carried out in four cycles. Their sequence is strictly defined, and if it is violated, the operation of such a motor is impossible. Sequence, i.e. opening the valves at the right time to remove the exhaust gases and start combustible mixture, defines the camshaft, which can be seen in the figure.
Its main working element must be considered cams. It is they who, through the drive system, which includes pushers, rocker arms, springs and other parts determined by the design of the timing, open the valves at the right time. Each valve has its own cam, when it presses on the valve through the pusher, it rises, and either a fresh mixture can enter the cylinder or its combustion products are removed. When the protrusion leaves the pusher, the valve closes under the action of the spring.

The bearing neck of the camshaft is designed for its installation in predetermined places, on which it rotates during operation. Friction parts are hardened with high frequency currents and lubricated in the process.

About the design of the camshaft

The timing device and drawing, including the camshaft, are shown below.
Structurally, the camshaft can be located either in the cylinder block or in the block head power unit. Depending on its location, the drive also changes, due to which the force from the cams is transmitted to the valve. The camshaft drive is connected to the crankshaft. The drive can be made both with the help of a chain drive (see the drawing above), and with the help of a flexible belt. In addition, there may be other ways to transfer the control force to the valves, but this is already determined by the drawing and documentation of the motor.

Which is better to use the camshaft drive, determines the design of the engine. In cases where the camshaft is located in the cylinder block (the so-called bottom location), a gear drive may even be involved. The latter, however, has not been used recently because of its bulkiness and increased noise during operation. Both the chain drive and the belt drive are quite reliable, but each of them has its own operating features that must be taken into account when servicing the engine.
Its device may provide that there may be more than one camshaft in the motor. As a rule, in modern multi-valve engines, it is located as close as possible to the valves to reduce the load on it. The design and drawing, for example, of a V-shaped engine, provides for at least two shafts, while in a conventional in-line engine, as a rule, there is one camshaft. Although for multi-valve engines their purpose will be decisive - there may be separate exhaust and intake camshafts, i.e. they control the operation of the exhaust or intake valves.

About joint work with the crankshaft

Do not forget that for the camshaft the main purpose is to ensure the correct gas distribution during engine operation. To do this, the operation of the camshaft and crankshaft must be coordinated, i.e. the opening and closing of the valves must occur at the right moments - at the TDC or BDC position of the piston, or in accordance with the lead set by the drawing or design documentation.

To perform such a connection, special marks are made on the timing gears, the coincidence of which means ensuring the desired position of the camshaft and crankshaft. To achieve this, a special technique for adjusting their position is used.

Camshaft position sensor

With the transition to injection engines for these purposes, a special camshaft position sensor began to be used. So, on VAZ cars, a Hall sensor serves for this. Its work is based on a change in the magnetic field, for the creation of which the sensor device provides a magnet. When the magnetic field changes, which occurs when the camshaft is in the desired position, the sensor determines that in the first cylinder the piston is in the TDC position, and transmits this data to the controller. In accordance with them, it ensures fuel injection and its combustion, as provided by the order of operation of individual engine cylinders in a drawing or documentation.

Camshaft Maintenance

First of all, when carrying out maintenance work affecting the camshaft, it is necessary to pay attention to the condition of the belts or the chain of its drive. The point is not so much that the entire gas distribution mechanism provided by the camshaft will be disrupted, but that mechanical damage to both the valves and the piston is possible.

Sometimes the cause of failure or improper operation of the engine is the position sensor. This manifestation may be bad dynamics vehicles and significant fuel consumption, as well as the ignition of the engine health indicator light on the instrument panel. Detecting a malfunction and determining its source - whether it is a sensor or not, is performed using a multimeter. Often possible cause It's not the sensor itself, it's the wiring. If the defect indicates that the sensor is faulty, then it must be changed.

Reasons for sensor failure can be:

failure of the gear disk of the pulse sensor;
its displacement due to a violation of fastening;
short circuit in the internal circuit of the sensor;
the effect of increased temperature from engine overheating.

Properly performed fault detection will avoid the failure of a new sensor installed instead of the old one.

The camshaft is the main assembly that ensures proper gas distribution when the engine is running, and often mainly ensures its efficient operation. Its timely maintenance and control technical condition will allow you to operate the car correctly and at no additional cost.

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 useful action in increasing the power of the motor, since power can be added without significantly interfering with the characteristics of the engine, especially at low revs. 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 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, the shaft high speed valve lift 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 moments, closing and opening of valves relative to the position of the camshaft, are also so important parameters to optimize motor 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 are discussing cam offsets, this is one of the few times that the camshaft characteristic is given in degrees of shaft rotation and not in 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 valve overlap is also affected by changes 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.

The four-stroke D0HC timing mechanism is an improvement on the SOHC design and is designed to eliminate the only remaining reciprocating mass of the rocker arms (although this would require the pushrods to be returned). Instead of a single central camshaft, a pair is used, placed directly above the valve stems (see fig. 1. (see below)
1.Typical design of a gas distribution mechanism with two overhead camshafts

This design uses two camshafts, one above each valve or row of valves. The valve is opened by means of a "bowl-shaped" type pusher, while the clearance is adjusted using washers. In this design, only the most necessary parts of the gas distribution mechanism drive remained.

To drive the gas distribution mechanism, a chain drive is used - the most traditional and cheap to manufacture, although a design is known (but not yet widespread) following trends in the automotive industry, in which a pulley and a timing belt are used instead of a chain drive. Examples of the use of this design are the Honda JGoldwing, Pan European, Moto Guzzi Daytona, Centauro and a number of Ducati motorcycles. The advantages of belt drives include: they are less noisy, they don't stretch like chains, and the pulleys don't wear like sprockets, although the belt needs to be replaced more frequently.

Another way to drive the camshafts is used on Honda VFR models and is a crankshaft driven gear train (see Fig. 2). This design eliminates the need for a tensioner and is also quieter than a chain, although the gears are subject to wear.

2. Gas distribution mechanism with gear drive .

Camshaft tappets, made in the form of a "bowl". work in cylinder head bores. When using "bowl-shaped" tappets, valve clearance is adjusted using small round shims called shims. Since the washers themselves are not adjustable, they must be replaced with washers of various thicknesses until the correct gap is restored. On some engines, the washer practically coincides with the diameter of the pusher and is installed in the socket, which is located at the top of the pusher; such a design is called a “pusher with shims on top” (see Fig. 3). The washer can be replaced by holding the tappet in the down position using a special tool so that there is enough clearance between the tappet and the camshaft to remove and install the washer.

3. A typical DOHC camshaft drive mechanism in a section showing the device of cup-shaped pushers with shims on top

On other engines, the washer is much smaller and is located under the tappet in the center of the valve spring retainer. At the same time, it rests directly on the end of the valve stem: this design is called a “pusher with shims from below” (see Fig. 4).

4. A typical DOHC camshaft drive mechanism in section showing the device of cup-shaped pushers with shims from below

Thus, the weight of the reciprocating parts is reduced even more when using small spacers, but it becomes necessary to dismantle the camshaft with each valve clearance adjustment procedure, which increases the cost and laboriousness of maintenance. To avoid the hassle of having to use special tools or remove the camshaft, some DOHC engines use small, lightweight rocker arms instead of "bowl-shaped tappets" (see Figure 5).

5. DOHC valve timing mechanism showing non-direct valve action with short rocker arms or rockers that allow easier valve clearance adjustment

On some engines with a similar layout, the rocker arms are equipped with a traditional adjusting screw and locknut. On others, the rocker arms rest on a small washer located in the center of the valve spring holder, and the rocker arms themselves are mounted on shafts whose length exceeds the width of the rocker arm. To hold the rocker over the valve, a spring is located on the shaft. To replace the adjusting washer, the rocker arms are moved towards the spring so that the washer can be removed…….

……continued in the next article

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 (in).

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.

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 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 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.

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.

Engine duty cycle

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. An experience professional repair vehicles for over 10 years.

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 cars with V-shaped motors, four camshafts are used due to design features 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). At correct 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.

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