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 center 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 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 a design is known (but not yet widespread), following 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 way to drive the camshafts is used on Honda VFR models and is a gear train driven by crankshaft(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 camshaft mechanism .

Bowl-shaped camshaft pushers. work in the bore of the cylinder head. When using "cup" tappets, the valve clearance is adjusted using small round shims called shims. Since the washers themselves are unadjustable, they must be replaced with washers of different thicknesses before reconditioning. correct clearance... 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 Fig. 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. In this case, it rests directly on the end face of the valve stem: this design is called a "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 laboriousness of maintenance. To avoid the hassle of having to use special tools or dismantling the camshaft, some DOHC engines use small, lightweight rocker arms instead of "cup tappets" (see Fig. 5).

5. The DOHC type timing drive 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 arrangement, 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

"Engine gas distribution mechanism"

Purpose of work: to study the purpose, device, principle of operation, design of the gas distribution mechanism (timing) of the engine.

Progress:

Purpose and characteristics

The gas distribution mechanism is called the mechanism that opens and closes the intake and exhaust valves of the engine.

The gas distribution mechanism (timing) serves for the timely admission of the combustible mixture or air into the engine cylinders and the release of the exhaust gases from the cylinders. Gas distribution mechanisms with overhead valves are used in car engines. The overhead valve arrangement increases the compression ratio of the engine, improves the filling of the cylinders with a combustible mixture or air, and simplifies the maintenance of the engine in operation. Car engines may have gas distribution mechanisms different types (picture 1), which depends on the layout of the engine and mainly on the relative position of the crankshaft, camshaft and intake and exhaust valves. The number of camshafts depends on the engine type.

At top location the camshaft is installed in the cylinder head where the valves are located. The valves are opened and closed directly from the camshaft through pushers or valve levers. The camshaft is driven by the crankshaft using a roller chain or a toothed belt.

Top location camshaft simplifies the design of the engine, reduces the mass and inertial forces of the reciprocating moving parts of the mechanism and ensures high reliability and noiselessness of its operation at a high engine speed.

The camshaft chain and belt drives also ensure quiet operation of the gas distribution mechanism.

In the lower position, the camshaft is installed in the cylinder block next to crankshaft... The valves are opened and closed from the camshaft through the rod and rocker pushers. The camshaft is driven by gears from the crankshaft. With a lower camshaft arrangement, the design of the gas distribution mechanism and engine becomes more complicated. In this case, the inertial forces of the reciprocating moving parts of the gas distribution mechanism increase. The number of camshafts in the timing mechanism and the number of valves per cylinder depend on the engine type. So, with a larger number of intake and exhaust valves, better filling the cylinders with a combustible mixture and cleaning them from exhaust gases... As a result, the engine can develop high power and torque. With an odd number of valves per cylinder, the number of intake valves is one more than the exhaust valves.

The design and operation of the gas distribution mechanism

Gas distribution mechanisms, regardless of the location of the camshafts in the engine, include valve group, transmission parts and camshafts with drive.

V valve group includes intake and exhaust valves, valve guides and valve springs with fastening parts.

Transmission parts are pushers, push rods guide bushings, push rods, rocker arms, rocker arm axle, valve drive levers, adjusting washers and adjusting bolts. However, with an overhead camshaft, tappets, guide bushings and tappet rods, rocker arms and rocker shaft are usually missing.

On Figure 2 The gas distribution mechanism of the engine is presented with an overhead valve arrangement, with an overhead camshaft with a chain drive and with two valves per cylinder. It consists of a camshaft 14 with a bearing housing 13, a camshaft drive, valve drive levers 11, support adjusting bolts 18 for valves 1 and 22, guide bushings 4, valve springs 7 and 8 with fastening parts.

Picture 2- Gas distribution mechanism of a passenger car with a chain drive

1, 22 - valves; 2 - head; 3 - rod; 4, 20 - bushings; 5 - cap; 6 - washers; 7, 8, 17 - springs; 9 - plate; 10 - biscuit; 11 - lever; 12 - flange; 13 - case; 14 - a camshaft; 15 - neck; 16 - cam; 18 - bolt; 19 - nut; 21 - plate; 23 - ring; 24, 27, 28 - asterisks; 25 - chain; 26 - sedative; 29 - finger; 30 - shoe; 31 - tensioner

Camshaft ensures timely opening and closing of valves. The camshaft is five-bearing, cast from cast iron. It has bearing journals 15 and cams 16 (inlet and outlet). A channel runs inside the shaft through which oil is supplied from the middle bearing journal to the other journals and cams. A driven sprocket 24 of the chain drive is attached to the front end of the shaft. The shaft is installed in a special case of 13 bearings, cast from an aluminum alloy, which is fixed on the upper plane of the cylinder head. From axial movements, the camshaft is fixed with a thrust flange 12, which enters the groove of the front bearing journal of the shaft and is attached to the end of the bearing housing.

Camshaft drive is carried out through the driven sprocket 24 installed on it by a double-row roller chain 25 from the driving sprocket 28 of the crankshaft. This chain also rotates the sprocket 27 of the drive shaft of the oil pump. The camshaft drive has a semi-automatic tensioning mechanism, consisting of a shoe and a tensioner. The chain is tensioned by a shoe 30, which is acted upon by the springs of the tensioner 31. A damper 26 serves to damp the vibrations of the driving branch of the chain. The shoe and the damper have a steel frame with a vulcanized rubber layer. The stop pin 29 prevents the chain from falling off when the camshaft sprocket is removed from the vehicle.

Valves open and close the inlet and outlet ports. The valves are installed in the cylinder head in one row at an angle to the vertical axis of the engine cylinders. Inlet valve 1 for better filling of the cylinders with a combustible mixture has a head with a larger diameter than the exhaust valve. It is made of special chromium steel with high wear resistance and thermal conductivity. Exhaust valve 22 operates in more severe temperature conditions than the intake. It is made composite. Its head is made of heat-resistant chromium steel, and the stem is made of special chromium steel.

Each valve consists of a head 2 and a stem 3. The head has a tapered surface (chamfer), with which the valve, when closed, fits snugly against a special cast iron seat installed in the cylinder head and also having a tapered surface.

The valve stem moves in a cast-iron guide sleeve 4, pressed and fixed by a retaining ring 23 in the cylinder head, which ensures an accurate valve fit. An oil deflector cap 5 made of oil-resistant rubber is put on the sleeve. The valve has two coil springs: an outer 8 and an inner 7. The springs are attached to the valve stem using washers 6, a plate 9 and a split cracker 10. The valve is actuated from the camshaft cam by a forged steel lever 11, which rests on one end of the adjusting bolt 18 and the other onto the valve stem. The adjusting bolt has a ball head. It is screwed into a threaded bushing 20, fixed in the cylinder head and locked by a plate 21, and fixed with a nut 19. The adjusting bolt sets the required clearance between the camshaft cam and the valve drive lever, equal to 0.15 mm on a cold engine and 0.2 mm on hot engine (warmed up to 75 ... 85 ° C). Spring 17 creates permanent contact between the end of the actuator arm and the valve stem.

In this article, we will consider the existing types of gas distribution mechanisms. This information will be very useful for motorists, especially those who repair their cars on their own. Well, or trying to repair them.

Each timing belt is driven by a crankshaft. Power transmission can be carried out by belt, chain or gears. Each of these three types of timing has both advantages and disadvantages.

Let's consider in more detail the types of timing drive

1. The belt drive has low noise during operation, but it is not strong enough and can break. The consequence of such a cliff is bent valves... In addition, a weak belt tension leads to the possibility of its jumping, and this is fraught with phase displacement, complicated start-up. In addition, knocked down phases will give unstable work on Idling and the engine will not be able to run at full power.

2. The chain drive can also make a "jump", but the probability of it is greatly reduced due to the special tensioner, which is more powerful in the chain drive than in the belt drive. The chain is more reliable, but has some noise, which is why not all car manufacturers use it.

3. The gear type of the timing belt has been massively used for a long time, in those days when the camshaft was located in the engine block (bottom engine). Such motors are now not widely used. Among their advantages, one can note the low cost of manufacture, simplicity of design, high reliability and a practical eternal mechanism that does not require replacement. Of the minuses - low power, which can only be increased by increasing the volume and, accordingly, the size of the structure (for example, a Dodge Viper with a volume of more than eight liters).

Camshaft

What is it and why? The camshaft is used to adjust the opening moment of the valves, which supply fuel to the cylinders at the inlet, and exhaust gases from them during the exhaust phase. On camshaft for these purposes, eccentrics are located in a special way. The work of the camshaft is directly related to work crankshaft, and due to this, fuel is injected at the most useful moment - when the cylinder is located in its lowest position (at bottom dead center), i.e. before the start of the intake tract.

The camshaft (one or more - it doesn't matter) can be located in the cylinder head, then the engine is called "upper shaft", or it can be located in the cylinder block itself, then the engine is called "lower shaft". It was written above about it. They are usually used in powerful American pickups, and some expensive cars with a gigantic engine displacement, oddly enough. In these powertrains, the valves are actuated by rods that run through the entire engine. These motors are slow and very inertial, and consume oil actively. Lower-shaft engines are a dead-end branch of the development of motor engineering.

Types of gas distribution mechanisms

Above, we examined the types of timing drives, and now we will focus specifically on the types of the gas distribution mechanism itself.

SOHC mechanism

The name literally means "one overhead camshaft". Formerly called simply "OHC".

Such an engine, as the name implies, contains one camshaft located in the cylinder head. Such an engine can have either two or four valves in each cylinder. That is, contrary to various opinions, the SOHC motor can also be sixteen-valve.

What are the strong and weak sides for such motors?

The engine runs relatively quietly. The silence is precisely relative to the two-camshaft engine. Although the difference is not big.

Simplicity of design. And hence the cheapness. This also applies to repairs and maintenance.

But of the minuses (albeit very insignificant), one can note the weak ventilation of the motor, equipped with two valves per cylinder. This causes the engine power to drop.

The second disadvantage is in all sixteen-valve engines with one camshaft. Since the camshaft is one, all 16 valves are driven by one camshaft, which increases the load on it and makes the whole system relatively fragile. In addition, due to the low phase angle, the cylinders are less filled and ventilated.

DOHC mechanism

This system looks almost the same as SOHC, but differs in the second camshaft installed next to the first. One camshaft is responsible for actuating the intake valves, the second, of course, the exhaust valves. The system is not ideal, and, of course, it has its own disadvantages and advantages, a detailed description of them is beyond the scope of this article. DOHC was invented at the end of the last century, and has not changed since then. It is worth noting that the design of such an engine becomes much more complicated and costly with the second camshaft.

But for that, such an engine consumes less fuel due to better filling of the cylinders, after which almost all the exhaust gases go out of them. The advent of such a mechanism has significantly increased the efficiency of the engine.



OHV mechanism

Above in the text, this type of engine (lower shaft) has already been considered. It was invented at the beginning of the last century. The camshaft in it is located at the bottom - in the block, and rocker arms are used to bring the action of the valves. From the advantages of such an engine, a simpler cylinder head design can be distinguished, which allows V-shaped lower-shaft engines to reduce their size. We repeat the disadvantages: low speed, high inertia, low torque and low power, inability to use four valves per cylinder (except for very expensive cars).

Summarize

The mechanisms described above are not an exhaustive list. Motors spinning more than 9 thousand revolutions, for example, do not use springs under the valve plates, and in such engines one camshaft is responsible for opening the valve, and the second for closing, which allows the system not to hang at revolutions above 14 thousand. This system is mainly used on motorcycles above 120 hp.

Video about how the timing belt works and what it consists of:

The consequences of a broken timing belt on the Lada Priora:

Replacing the timing belt using the example of Ford Focus 2:

Camshaft, in an abbreviated form 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 entirely depends on the design of the internal combustion engine, since in some models the camshaft is located at the bottom, at the base of the cylinder block, and in others, at the top, right in. At the moment, the top location of the camshaft is considered optimal, since this greatly simplifies service and repair access to it. The camshaft is directly connected to. They are interconnected by a chain or belt drive by providing a connection between the pulley on the timing shaft and the sprocket on the crankshaft. This is necessary because the camshaft is driven by the crankshaft.

The camshaft is installed in bearings, which in turn are securely fixed in the cylinder block. Axial play of the part is not allowed due to the use of clamps in the design. The axis of any camshaft has a through channel inside, through which the mechanism is lubricated. At the back, this hole is closed with a plug.

Important elements are the camshaft cams. In 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 belt - regulating the order of operation of the cylinders.

Each valve has a separate cam that opens it by pressing on 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.

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-closing in a specific order, unique for each ICE model.

The working cycle of the engine (alternate movement of each of the cylinder valves) is carried out in 2 crankshaft revolutions. During this time, the camshaft only needs to make one revolution, so its gear has twice as many teeth.

One ICE can have more than one camshaft. The 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 be used. For example, power units with a different arrangement of cylinders, they have either a single camshaft installed in the camber, or a pair - for each block head separately.

There are three important characteristics camshaft design, they control the engine power curve: valve 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 might 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 2,000 crankshaft rotations. If the duration of the valve opening is measured at zero lift (when it is just 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 may 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 motor power, as power can be added without significantly interfering with engine performance, especially at low revs... If you delve into the theory, then the answer to this question will be quite simple: such a camshaft design with a short valve opening time is needed in order to increase the maximum engine power. It will 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 time becomes even shorter, more force valve springs are needed. This often becomes mechanically impossible, let alone driving 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 impractical area 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 closing and opening speeds. This, of course, will create an additional load on the valve drive mechanism, which significantly reduces the reliability of: camshaft cams, valve guides, valve stems, valve springs. However, a shaft with a high valve lift 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 power of the engine. 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 valve timing of the camshaft in the data table, which is attached to 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 moments 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 centers of the cams 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: in the event of an increase in the opening time, the overlap of the valves will also become large, while ensuring that there are no angle changes to compensate for these increases.