The main parts of the camshaft. Internal combustion engine camshaft: purpose, device, principle of operation

Oct 26 2014

Camshaft design: device and principle of operation

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 trouble-free 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 is put on the sprocket crankshaft or on the 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?

Structurally, the camshaft is located in the collapse of the cylinder block. The crankshaft gear or chain drive drives the camshaft.

When the camshaft rotates, the cams affect the valve operation. This process will occur correctly only in the case of strict compliance with the order of operation of the engine cylinders and with the valve timing.

In order to establish the appropriate valve timing, special timing marks are applied to the drive pulley or timing gears. In addition, it is necessary that the camshaft cams and crankshaft cranks are in a strictly defined position in relation to each other.

When the installation is made according to the marks, it is possible to achieve compliance with the correct sequence of strokes - the order of operation of the engine cylinders, which, in turn, depends on the location of the cylinders themselves, as well as on the design features of the crankshaft and camshafts.

Engine duty cycle

The engine duty cycle is the period during which the intake and exhaust valves are opened once. As a rule, the period passes in two crankshaft revolutions. During this time, the camshaft, whose gear has twice as many teeth as the crankshaft gear, makes one revolution.

Number of camshafts in the engine

The number of camshafts is directly influenced by the engine configuration. Engines, which differ in in-line configuration, and also have one pair of valves per cylinder, are equipped with one camshaft. If there are four valves for each cylinder, the engine is equipped with two camshafts.

Opposed and V-shaped engines are distinguished by the presence of one camshaft in the camber or have two camshafts, each of which is located in the block head. There are also exceptions to the generally accepted rules associated primarily with design features engine.

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, in 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.

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.

The important elements are the camshaft cams. In terms of quantity, 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 by pressing the pusher. By releasing the follower, the cam allows the spring to unfold, returning the valve to the closed state. The camshaft design assumes the presence of two cams for each cylinder - according to the number of valves.

It should be noted that the fuel pump and the oil pump distributor are also driven from the camshaft.

The principle of operation and the device of the camshaft

The camshaft is connected to the crankshaft using a chain or belt fitted over the camshaft pulley and crankshaft sprocket. The rotary movements of the shaft in the supports are provided by special plain bearings, due to which the shaft acts on the valves that trigger the cylinder valves. This process takes place in accordance with the phases of formation and distribution of gases, as well as the engine duty cycle.

The gas distribution phases are set according to the alignment marks that are located on the gears or pulley. Correct installation ensures compliance with the sequence of the onset of engine operating cycles.

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.

Cams are installed between bearing journals, two for each engine cylinder. 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 loss.

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.

Camshaft breakage

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

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

1. wear of the cams, which leads to the appearance of knocking immediately only at start-up, and then all the time the engine is running;
2. bearing wear;
3. mechanical failure of one of the shaft elements;
4. problems with the regulation of the fuel supply, due to which there is an asynchronous interaction between the camshaft and cylinder valves;
5. shaft deformation leading to axial runout;
6. poor quality motor oilreplete with impurities;
7. 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 you find a problem, you should start fixing it. The camshaft is a collapsible mechanism, therefore, repair is most often carried out by replacing it all or only some of its elements, for example, bearings. exhaust gases, it makes sense to start opening the intake valve. This is what happens when using a tuning camshaft.

MAIN CHARACTERISTICS OF THE CAMSHAFT

It is known that among the main characteristics of a camshaft, designers of uprated engines often use the concept of opening time. The fact is that it is this factor that directly affects the produced engine power. So, the longer the valves are open, the more powerful the unit. This gives the maximum motor speed. For example, when the opening time is longer than the standard, the engine will be able to generate the additional maximum power that will be obtained from operating the unit at low rpm. It is known that for racing cars the maximum engine speed is the priority target. When it comes to classic cars, the engineering force is focused on low-rev torque and throttle response.

The increase in power can also be affected by an increase in valve lift, which can add maximum speed. On the one hand, additional speed will be obtained by the short opening times of the valves. On the other hand, valve actuators are not so simple. For example, at high valve speeds, the engine will not be able to generate additional top speed. In the corresponding section of our website you can find an article about the main features of the exhaust system. So, with a short opening time of the valve after the closed position, the valve has less time to get to the starting position. After that, the duration becomes even shorter, which mainly affects the generation of additional power. The fact is that at this moment valve springs are required, which will have as much force as possible, which is considered impossible.

It is worth noting that today there is a concept of a reliable and practical valve lift. In this case, the amount of lift should be more than 12.7 millimeters, which will ensure a high speed of opening and closing the valves. The length of the cycle starts from 2 850 rpm. However, these values \u200b\u200bput stress on the valve mechanisms, which ultimately leads to short service life of valve springs, valve stems and camshaft cams. It is known that a shaft with a high valve lift speed works without failure for the first time, for example, up to 20 thousand kilometers. Nevertheless, today automakers are developing such propulsion systems, where the camshaft has the same indicators of the duration of valve opening and valve lift, which significantly increases their service life.

In addition, the opening and closing of valves in relation to the position of the camshaft affects engine power. So, the phases of the distribution of the camshaft can be found in the table that is attached to it. According to this data, you can find out about the angular positions of the camshaft at the time of opening and closing the valves. All data is usually taken at the moment the crankshaft turns before and after the top and bottom dead points, indicated in degrees.

As for the opening times of the valves, it is calculated according to the gas distribution phases indicated in the table. Usually, in this case, you need to add up the opening moment, the closing moment and add 1 800. All moments are indicated in degrees.

Now it is worth dealing with the ratio of the phases of the distribution of the power gas and the camshaft. In this case, imagine that one camshaft is A, the other is B. It is known that both of these shafts have similar inlet and outlet valve shapes, as well as a similar valve opening time, which is 2,700 revolutions. In this section of our website, you can find an article about engine troit: causes and methods of elimination. These camshafts are commonly referred to as single profile designs. There are still some differences between these camshafts. For example, at shaft A, the cams are located so that the intake opens 270 before top dead center and closes at 630 after bottom dead center.

Shaft A exhaust valve opens at 710 BDC and closes 190 BDC. That is, the valve timing looks like this: 27-63-71 - 19. As for the shaft B, it has a different picture: 23 o67 - 75 -15. Question: How Shafts A and B can affect engine power? Answer: Shaft A will create additional maximum power. Still, it is worth noting that the engine will have worse characteristics, in addition, it will have a narrower power curve compared to shaft B. It should be noted right away that such indicators are not affected in any way by the duration of opening and closing of valves, since it, as we noted above, is the same. In fact, this result is influenced by changes in the phases of the gas distribution, that is, in the corners located between the centers of the cams in each camshaft.

This angle is the angular displacement that occurs between the intake and exhaust cams. It should be noted that in this case the data will be indicated in degrees of camshaft rotation, and not in degrees of crankshaft rotation, which were indicated earlier. Thus, valve overlap depends mainly on the angle. For example, when the angle between the valve centers decreases, the intake and exhaust valves will overlap more. In addition, at the time of increasing the duration of the opening of the valves, their overlap also increases.

The D0HC valve timing mechanism of the four-stroke engine is an improvement on the SOHC scheme and is designed to eliminate the only remaining reciprocating mass of the rocker arms (although this would require the return of the tappets). Instead of a single central camshaft, steam is used, mixed directly above the valve stems (see Fig. 1 (see below)
1.Typical double overhead camshaft timing design

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

To drive the timing mechanism, a chain drive is used - the most traditional and cheapest to manufacture, although there is a known (but not yet widespread) design, following the trends in the automotive industry, in which a pulley and a toothed belt are used instead of a chain drive. Examples of the use of this design include the Honda JGoldwing, Pan European, Moto Guzzi Daytona, Centauro and a number of Ducati motorcycles. The advantages of a belt drive include the following: they are quieter, do not stretch like chains, and pulleys do not wear out like sprockets, although belt replacement should be done more often.

Another method of camshaft drive 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 of the gear train are subject to wear.

2. Gear-driven gas distribution mechanism .

Bowl-shaped camshaft pushers. work in the cylinder head bores. When using "cup" tappets, the 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 clearance is restored. On some engines, the washer practically coincides with the diameter of the pusher and is installed in a socket located in the upper part of the pusher; this is referred to as a “pusher with shims on top” (see Figure 3). The washer can be replaced by holding the pusher in the down position using a special tool so that there is enough clearance between the pusher and the camshaft to remove and install the washer.

3 Typical DOHC Timing Actuator Sectional View showing cup-shaped tappets 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 face of the valve stem: this design is called "pusher with shims from below" (see Fig. 4).

4 Typical DOHC Timing Actuator Sectional View showing cup-shaped tappets with shims underneath

Thus, the mass of parts moving back and forth is reduced even more when using small gaskets, but it becomes necessary to dismantle the camshaft with each procedure for adjusting the valve clearance, which increases the cost and labor intensity of maintenance. To avoid the hassle of having to use special tools or dismantling the camshaft, some DOHC engines use small lightweight rockers instead of "cup tappets" (see Fig. 5).

5. DOHC type valve timing mechanism demonstrating an indirect effect on the valve using short rocker arms or rockers, which simplify the adjustment of clearances in the valve mechanism

On some engines with this design, the rocker arms are equipped with a traditional adjusting screw and locknut. In 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 that are longer than the width of the rocker arm. A spring is located on the shaft to hold the rocker arm over the valve. To replace the adjusting washer, the rocker arms are shifted towards the spring so that the washer can be removed …….

…… continued in the next article

The camshaft or simply the camshaft in the gas distribution mechanism provides the main function - the timely opening and closing of the valves, due to which the inflow of fresh air and the release of exhaust gases is produced. In general terms, the camshaft controls the gas exchange process in the engine.

To reduce inertial loads, increase the rigidity of the elements of the gas distribution mechanism, the camshaft should be located as close as possible to the valves. Therefore, the standard position of the camshaft on a modern engine in the cylinder head is the so-called. overhead camshaft.

The gas distribution mechanism uses one or two camshafts per cylinder bank. With a single-shaft arrangement, the intake and exhaust valves ( two valves per cylinder). In a two-shaft gas distribution mechanism, one shaft serves the intake valves, the other - the exhaust ( two intake and two exhaust valves per cylinder).

The design of the camshaft is based on cams... Typically one cam is used per valve. The cam has a complex shape that allows the valve to open and close at a set time and rise to a certain height. Depending on the design of the gas distribution mechanism, the cam interacts with either a pusher or a rocker arm.

When the camshaft is operating, the cams are forced to overcome the forces of the return springs of the valves and the frictional forces from the interaction with the pushers. All this consumes useful engine power. The springless system implemented in the desmodromic mechanism is devoid of these disadvantages. To reduce the friction between cam and follower, the flat surface of the follower can be replaced roller... In the long term, the use of a magnetic system to control the valves, ensuring complete rejection of the camshaft.

The camshaft is available in cast iron (cast) or steel (forged). The camshaft rotates in bearings, which are plain bearings. The number of supports is one more than the number of cylinders. The supports are mainly detachable, less often - one-piece (made as one piece with the block head). The supports, made in a cast-iron head, use thin-walled bushings, which are replaced when worn.

The camshaft is kept from longitudinal movement by thrust bearings located near the drive gear (sprocket). The camshaft is pressure lubricated. Individual oil supply to each bearing is preferred. The efficiency of the gas distribution mechanism is significantly increased with the use of various variable valve timing systems, which make it possible to achieve an increase in power, fuel efficiency, and a decrease in the toxicity of exhaust gases. There are several approaches to changing the valve timing:

• turning the camshaft in different operating modes;
• use of several cams with different profiles per valve;
• changing the position of the rocker arm axis.

The camshaft is driven by the engine crankshaft. In a four-stroke internal combustion engine, the drive ensures that the crankshaft rotates at twice the speed of the crankshaft.

On engines passenger cars the camshaft is driven by a chain or belt drive. These types of drives are used as equals both in gasoline enginesand diesel engines. Previously, a gear drive was used for the drive, but due to its bulkiness and increased noise, it was no longer used.

Chain drive combines a metal chain that runs around the sprockets on the crankshaft and camshaft. In addition, a tensioner and a damper are used in the drive. The chain consists of links connected by hinges. One chain can serve two camshafts.

The camshaft chain drive is quite reliable, compact, can be used at large center distances. At the same time, wear of the hinges during operation leads to stretching of the chain, the consequences of which can be the most sad for the timing. Even a tensioner with a damper does not help. Therefore, the chain drive requires regular condition monitoring.

IN belt drive The camshaft uses a timing belt that spans the corresponding timing pulleys on the shafts. The drive belt is equipped with a tension roller. The belt drive is compact, almost silent, reliable enough, which makes it popular with manufacturers. Modern timing belts have a significant resource - up to 100 thousand kilometers and more.

The camshaft drive can be used for drive and other devices - oil pump, high pressure fuel pump, ignition distributor.

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 gases formed. 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 position, 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 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 or pusher.

"Nockenwelle ani". Under the Public domain license from Wikimedia Commons - https://commons.wikimedia.org/wiki/File:Nockenwelle_ani.gif#mediaviewer/File:Nockenwelle_ani.gif

Cams are installed between bearing journals, two for each engine cylinder. 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 loss.

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.

Valve 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 push the tappets and force the valve to move.