The gas distribution mechanism of a car is one of the most complex mechanisms in engine design. The control of the intake and exhaust valves of the internal combustion engine lies entirely on the timing. The mechanism controls the process of filling the cylinders with a fuel-air mixture by opening the intake valve on the intake stroke in time. The timing also controls the removal of already exhaust gases from the internal combustion chamber - for this, the exhaust valve opens on the exhaust stroke.
Timing mechanism parts perform different functions:
- Camshaft opens and closes valves.
- The drive mechanism drives the camshaft at a certain speed.
- Valves close and open the intake and exhaust ports.
The main parts of the timing are the camshaft and valves. The camshaft, or camshaft, is the element on which the cams are located. It is driven and rotates on bearings. At the time of the intake or exhaust stroke, the cams located on the shaft, during rotationpress on the valve lifters.
The timing mechanism is located on the cylinder head. The cylinder head has a camshaft and bearings from it, rocker arms, valves and valve lifters. The upper part of the head is closed with a valve cover, which is installed using a special sealing gasket.
Timing mechanism operation
Timing is fully synchronized with ignition and fuel injection. Simply put, at the moment you press the gas pedal, the throttle valve opens, letting air flow into the intake manifold. As a result, a fuel-air mixture is formed. After that, the gas distribution mechanism starts to work. Timing increases throughput and releases exhaust gases from the combustion chamber. For the correct performance of this function, it is necessary that the frequency with which the inlet and outlet timing valves open is high.
Valves are driven by the engine camshaft. When the crankshaft speed increases, the camshaft also begins to rotate faster, which increases the frequency of opening and closing the valves. As a result, engine speed and output from it increase.
Combining the crankshaft and camshaft makes it possible for the internal combustion engine to burn exactly the amount of air-fuel mixture that is necessary for the operation of the engine in one mode or another.
Timing drive features, chain and belt
The camshaft drive pulley is outside the cylinder head. In order to avoidoil leaks occurred, an oil seal is located on the shaft neck. The timing chain drives the entire timing mechanism and is worn on one side on the driven sprocket or pulley, and on the other hand transmits the force from the crankshaft.
The correct and constant arrangement of the crankshaft and camshaft relative to each other depends on the valve belt drive. Even small deviations in position can cause the timing, the engine to fail.
The most reliable is considered to be a chain drive using a timing roller, however, there are some problems with ensuring the required level of belt tension. The main problem that drivers face and which is typical for the chain of the mechanism is its breakage, which is often the cause of the bending of the valves.
Among the additional elements of the mechanism include the timing roller used to tension the belt. The disadvantages of the timing chain drive, in addition to the risk of breakage, also include a high level of noise during operation and the need to change it every 50-60 thousand kilometers.
The design of the valve mechanism includes valve seats, valve guides, valve rotation mechanism and other elements. The force from the camshaft is transmitted to the stem or to the intermediate link - the valve rocker, or the rocker.
It is not uncommon to find timing models that require constant adjustment. Such designs have special washers and bolts, the rotation of which is setnecessary clearances. Sometimes the gaps are maintained automatically: their position is adjusted by hydraulic compensators.
Management of gas distribution stages
Modern engine models have undergone significant changes, having received new control systems based on microprocessors - the so-called ECU. In the field of engine building, the main task was not only to increase the power, but also the efficiency of the produced power units.
It was possible to increase the performance of engines, while reducing fuel consumption, only with the use of timing control systems. The engine with such systems not only consumes less fuel, but also does not lose power, due to which they have become used everywhere in the production of cars.
The principle of operation of such systems is that they control the speed of rotation of the timing camshaft. Essentially, the valves open a little earlier due to the fact that the camshaft rotates in the direction of rotation. Actually, in modern engines, the camshaft no longer rotates relative to the crankshaft at a constant speed.
The main task remains the most efficient filling of the engine cylinders, depending on the selected mode of its operation. Such systems monitor the state of the engine and correct the supply of the fuel mixture: for example, when idling, its volumes are reduced to a near minimum, since fuel is not required in large quantities.
Depending ondesign features of the car engine and gas distribution mechanism, in particular, the number of drives and their type may vary.
- Chain drive. Somewhat earlier, this drive was the most common, however, it is now used in diesel timing. With this design, the camshaft is located in the cylinder head, and is driven by a chain leading from the gear. The disadvantage of such a drive is the difficult process of replacing the belt, since it is located inside the engine in order to ensure constant lubrication.
- Gear drive. It was installed on the engines of tractors and some cars. Very reliable, but extremely difficult to maintain. The camshaft of such a mechanism is located below the cylinder block, due to which the camshaft gear clings to the crankshaft gear. If a timing drive of this type became unusable, the engine was changed almost completely.
- Belt drive. The most popular type, installed on gasoline power units in passenger cars.
Belt drive pros and cons
Belt drive gained its popularity due to its advantages over similar types of drives.
- Despite the fact that the production of such structures is more difficult than chain, it costs much less.
- Does not require constant lubrication, due to which the drive was placed on the outside of the power unit. Replacing and diagnosing the timing as a result of this was greatly facilitated.
- Because the belt drive does not have metal partsinteract with each other, as in a chain, the noise level during its operation has decreased significantly.
Despite the large number of advantages, the belt drive also has its disadvantages. The service life of the belt is several times shorter than the chain, which causes it to be replaced frequently. In the event of a broken belt, it is highly likely that the entire engine will have to be repaired.
The consequences of a broken or loose timing belt
If the timing chain breaks, the noise level increases while the engine is running. In general, such a nuisance does not cause something impossible in terms of repair, unlike the timing belt. When the belt is loosened and it jumps over one gear tooth, a slight violation of the normal functioning of all systems and mechanisms occurs. As a result, this can provoke a decrease in engine power, an increase in vibration during operation, and difficult starting. If the belt jumped over several teeth at once or broke completely, the consequences can be the most unpredictable.
The most innocuous option is a collision between a piston and a valve. The impact force will be enough to bend the valve. Sometimes it is enough to bend the connecting rod or completely destroy the piston.
One of the most serious car breakdowns is a broken timing belt. In this case, the engine will either have to be overhauled or completely changed.
Timing Belt Service
The level of belt tension and its general condition is one of the most frequently checked whencar maintenance factors. The frequency of inspection depends on the specific make and model of the machine. Timing belt tension control procedure: the engine is inspected, the protective cover is removed from the belt, after which the latter is checked for twisting. During this manipulation, it should not rotate more than 90 degrees. Otherwise, the belt is tensioned using special equipment.
How often is the timing belt replaced?
The belt is completely replaced every 50-70 thousand kilometers of the car. It can be carried out more often in case of damage or signs of delamination and cracks.
Depending on the type of timing, the complexity of the belt replacement procedure also changes. To date, two types of gas distribution mechanism are used in cars - with two (DOHC) or one (SOHC) camshafts.
Replacing the gas distribution mechanism
In order to replace the SOHC timing belt, it is enough to have a new part and a set of screwdrivers and wrenches on hand.
First, the protective cover is removed from the belt. It is attached either with latches or bolts. After removing the cover, access to the belt opens.
Before loosening the belt, timing marks are set on the camshaft gear and crankshaft. On the crankshaft, marks are placed on the flywheel. The shaft is rotated until the timing marks on the housing and on the flywheel coincide with each other. If all the marks matched with each other, proceed to loosen and remove the belt.
In order to remove the belt from the crankshaft gear, it is necessary to dismantle the timing pulley. To this end, the car is jacked up and the right wheel is removed from it, which gives access to the pulley bolt. Some of them have special holes through which the crankshaft can be fixed. If they are not there, then the shaft is fixed in one place by installing a screwdriver in the flywheel crown and resting it against the housing. After that, the pulley is removed.
Access to the timing belt is fully opened, and you can start removing and replacing it. The new one is put on the crankshaft gears, then it clings to the water pump and is put on the camshaft gears. For the tension roller, the belt is wound up at the very last turn. After that, you can return all the elements to their place in the reverse order. It remains only to tighten the belt with the tensioner.
Before starting the engine, it is advisable to rotate the crankshaft several times. They do this to check the coincidence of the marks and after turning the shaft. Only then does the engine start.
Features of the timing belt replacement procedure
On a car with a DOHC system, the timing belt is replaced a little differently. The very principle of changing the part is similar to that described above, however, access to it is more difficult for such machines, since there are protective covers attached to the bolts.
In the process of aligning the marks, it is worth remembering that there are two camshafts in the mechanism, respectively, the marks on both must completely match.
For such cars, in addition toguide roller, there is also a support roller. However, despite the presence of the second roller, the belt winds up behind the guide roller with the tensioner at the very last turn.
After the new belt is installed, the marks are checked for compliance.
At the same time as the belt is replaced, the rollers are also changed, since their service life is the same. It is also advisable to check the condition of the fluid pump bearings so that after the procedure for installing new timing parts, the failure of the pump does not become an unpleasant surprise.