What is the system of changing the phases of gas distribution
The efficiency of any internal combustion engine, Engine efficiency, power indicator, torque characteristics and fuel efficiency directly depend on a number of factors. One of the important components in the list are the phases of gas distribution. Answer the question, what is the phase of the engine timing, can be as follows. Such phases should be understood as timely opening and closing of inlet and outlet valves.
Most modern internal combustion engines are increasingly receiving a system of changing the phases of gas distribution, although still close 20 years ago, a four-stroke engine was not widely available. In a conventional engine, the valves open due to the action of camshafts. The shape of the camshaft cam profile determines the moment and duration of valve opening.
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These parameters are the so-called width of the gas distribution phase. An additional parameter is also the value of the valve stroke (the height of its rise). It is worth considering, that the fuel-air mixture and the exhaust gases under the intake, in the cylinder of the internal combustion engine and at the outlet behave differently, depending on the different modes of its operation. The flow velocity changes dynamically, gas fluctuations appear, which lead to resonances or stagnation. All this affects the efficiency of filling the cylinders and their purge in different modes of operation of the power unit.
Fixed phases of gas distribution force designers of internal combustion engines to design the engine so, to be sure of traction in the range of low and medium speeds, but at the same time there was a reserve of power for maintenance of the typed speed and the further acceleration of the car at an exit of the internal combustion engine on the modes about a zone of the maximum turns. Additionally, it is necessary to ensure stable operation of the power unit at idle, elasticity in transient modes, as well as the efficiency and environmental friendliness of the power plant. If the timing phases are fixed, then the improvement of some parameters will naturally lead to the deterioration of others. To solve this problem, a system of changing the phases of gas distribution was developed, which flexibly and dynamically changes the basic parameters of the timing system depending on the mode, in which the engine is running at a certain point.
VVT gas distribution phase change system (English. Variable Valve Timing) created for dynamic adjustment of operating parameters of the gas distribution mechanism. This control is carried out taking into account the different modes of operation of the power unit. Use of the specified system of regulation of phases of gas distribution allows to achieve increase in power of the motor and moment characteristic. The VVT system saves fuel, and also reduces the toxicity of exhaust gases during engine operation.
The system of changing the gas distribution phases affects the basic parameters of the gas distribution mechanism. Such parameters include the opening and closing moments of the inlet and outlet valves, the duration of the valve opening time and the height of its rise. These parameters are ultimately the phase of gas distribution, as the duration of an input and release stroke depends on them, which is expressed by that angle, on which the engine crankshaft is rotated relative to the dead points (TDC and BMT) during the movement of the piston in the cylinder. The shape of the camshaft determines the phases of the timing, since the specified cam has a direct effect on the intake or exhaust timing valve.
What is the need for a system of changing the phases of gas distribution
Different values of gas distribution phases are required to achieve the greatest efficiency in relation to dynamically changing modes of operation of internal combustion engines.. In idle mode, the most rational are the "narrow" phases of gas distribution, by which is meant the later opening and earlier closing of the valves. This eliminates the overlap of the phases, which means the time of simultaneous opening of the inlet and outlet valve. This is necessary for that, to exclude the ingress of exhaust gases into the intake and exhaust of the fuel-air mixture into the exhaust manifold.
The output of the engine to maximum power means increased speed, since the camshaft rotates faster and the valve opening time is reduced. For, so as not to lose power and torque is maintained at high speeds, the engine must receive much more fuel-air mixture, and the release of exhaust gases must be implemented as efficiently as possible. The problem is solved by opening the valves early and increasing the time of their opening, making the phase "wide". The overlap phase also expands to a maximum with increasing speed, that is necessary for high-quality purge of cylinders.
If the engine is running at low speed, the shortest possible phases of gas distribution are required. This means, that the opening time of the valves should be minimal in duration, providing the so-called "narrow" phases. High engine speeds require the complete opposite in the form of "wide" phases of gas distribution. The valve opening time must be increased to the maximum, while providing intake and exhaust stroke, as well as effective overlap.
The camshaft cam itself is shaped, which is able to ensure a narrow implementation, and wide phase. The problem is, that the fixed shape of the cam does not allow to achieve both narrow and wide phases of gas distribution. It turns out, the shape of the cam is selected based on the possible optimal balance between high torque at low engine speeds and the maximum power of the unit in the high speed of the crankshaft. The system of change of phases of gas distribution allows to change these parameters more flexibly, literally "adjusting" the timing belt to the specific mode of operation of the engine to achieve better efficiency of the engine and fuel economy.
Gas distribution phase change systems are represented by several types. The main differences are in certain parameters of timing control during its operation. Today, the following solutions are used to control the phases of gas distribution:
- camshaft rotation system;
- cams of a camshaft with a different profile;
- valve lift height change system;
System on the basis of the hydraulically operated coupling
Widespread phases change systems have become widespread, the principle of operation of which is based on the implementation of the rotation of the camshaft. Such gas distribution phase control schemes include: Japanese VVT-i system, Dual VVT-i, decision of the German concern BMW called VANOS, Double VANOS, VVT scheme from Volkswagen, control of VTEC gas distribution phases from Honda, CVVT system of Hyundai brands, Kia and GM, regulation of VCP phases from Renault, etc.. d.
The operation of the above systems is based on a small rotation of the camshaft during its rotation. This method allows to achieve early opening of the valves compared to their basic starting position. This type of systems of change of phases of gas distribution constructively consists of the special coupling, which is operated hydraulically, as well as an additional control system for the specified coupling. Hydraulically operated clutch among car mechanics is called phaser.
The camshaft is rotated by control electronics and hydraulics, and the system itself often affects only the intake valves. The increase in the speed of the internal combustion engine leads to that, that the phaser turns the camshaft in the course of its rotation, the intake valves open earlier and the cylinders are filled much more efficiently with the working mixture at high speed.
It turns out, the hydraulically operated coupling realizes turn of a cam-shaft of a timing belt. This coupling constructively includes:
- rotor, which is connected to the distribution;
- frame, which is the camshaft drive pulley;
In certain cavities, which are located between the rotor and the pulley housing, gets engine oil from the engine lubrication system. Oil in the coupling is fed through special channels. When the engine oil fills one or the other cavity of the clutch, the rotor is rotated relative to the housing. This rotation of the rotor means, that the camshaft will be rotated to the required angle.
Most often, the place of installation of the hydraulic clutch is the drive of the camshaft, which is responsible for the operation of the intake valves. There are also internal combustion engine designs, when such phase-rotary clutches stand as on the intake camshaft, and at graduation. This solution allows you to more widely and efficiently adjust the parameters of the timing at the inlet and outlet, but complicates the mechanism.
Electronic control automatically regulates the operation of the hydraulic clutch. The system of such management includes:
- group of input sensors;
- electronic control unit;
- list of actuators;
The control system receives readings from the Hall sensor, which assesses the condition of the camshafts. Additionally, other sensors are involved, which are used by the ECU to control the operation of the entire engine.
These include the sensor, measuring the crankshaft speed, coolant temperature sensor (OS), air flow sensor and others. Signals from these sensors are fed to the ECU, which then sends the appropriate signal to a special controller (executive) device.
Such a device, which is affected by the electronic engine control unit, there is a solenoid valve (electrohydraulic distributor). The valve is a distributor, which, if necessary, opens access to the flow of engine oil to the hydraulic clutch, and also implements the removal of oil from the phase shifter. It depends, in what mode the power unit works.
This scheme of change of phases of gas distribution with use of the coupling is involved at the moment of work of the engine at idling, (the motor runs at low speed), in the mode of the maximum power at high turns, as well as in that mode, when the output of the internal combustion engine at maximum torque.
System of step change of gas distribution phases
The evolution of gas distribution phase change systems has allowed engineers not only to shift phases, but also effectively perform their expansion and contraction. The next type of gas distribution phase change systems is solutions, based on the use of camshafts of various shapes. Thanks to this method it is possible to achieve a gradual change of time, on which the valve opens, as well as change the lifting height of the valves. The list of such systems includes VVTL-i from the auto giant Toyota, Japanese Honda VTEC and MIVEC from Mitsubishi, solutions from called Audi Valvelift System and others.
These systems are similar to each other in design, and on the principle of action. Only the German Valvelift System is slightly different. The most popular system was VVTL-i, VTEC and MIVEC. At the heart of such systems of change of phases of gas distribution are cams with a different profile, as well as a management system. The camshaft in such control systems of the gas distribution phases is made as follows, having two small cams at once, as well as one larger cam. Smaller cams with a special rocker (коромисла) connect to the intake valves. The big cam is responsible for moving one unused rocker.
This system of changing the phases of the timing allows you to switch from small cams to large depending on the mode of operation of the internal combustion engine. The transition between modes is achieved due to this, that a special locking mechanism is triggered. This locking mechanism is based on a hydraulic drive.
When the engine is running at low speed and at low load, the intake valves are actuated by small cams of the camshaft, phases of gas distribution in this mode have a short duration (narrow phase).
If the engine spins to a certain speed, the control system activates the locking mechanism. The result is the connection of the rocker arms of the small and large cams, which provides rigidity of a design. The connection is made with a special locking pin, and the force on the intake valves begins to come from a single large cam. Small camshafts at high engine speeds become inactive.
Existing varieties of VTEC systems can have three modes of timing control. In this modification, one small cam of the camshaft operates at low speeds of the internal combustion engine, which opens only one inlet valve. Two small cams are engaged in the mode of average loadings and turns of the engine, ensuring the opening of the two intake valves. The big cam comes into action at an exit of power plant on a mode of turns, close to the maximum.
I-VTEC gas distribution phase change system, which is presented by the manufacturer Honda, combines the main advantages of solutions like VTC, and VTEC. Three-stage adjustment provides significant fuel savings. At low speeds, half of the intake valves have virtually no activity. Increasing the speed to medium speed connects the deactivated valves, and the height of their rise does not imply full opening.
Going to maximum speed makes the intake valves work from the central cam of large size. This cam has a special profile, which is specially selected to achieve maximum lift of the valves, which means increasing the recoil of the internal combustion engine at maximum operating modes of the unit. This approach has significantly expanded the ability to control the timing parameters for effective control of the engine in different modes.
If we consider the example of the VVTL-i system from Toyota, then after the output of the motor with this solution, the speed is close 6000 rpm the standard camshaft cam is switched off and replaced by a cam with a changed profile. The specified cam provides an arc of the valve operation algorithm, shifts (expands) phase and increases the height of its rise. In practice, this will mean, that when the engine enters the high speed mode, the engine will have a sharp increase in thrust, necessary to ensure further reliable acceleration.
The scheme of operation of the VVTL-i system is based on the following algorithm. The opening time and lifting height of the intake valves are regulated similarly to other solutions. When the motor is running in rotation mode 6000 rpm, then the smaller camshaft cam acts on the valve, which puts pressure on the rocker and thus opens the valve. After a set of turns above the set mark the high cam with a special profile starts to control opening of valves. To activate it, a special rusk is moved under oil pressure.
The control system is responsible for the timely supply of engine oil on a special line at the right time. The oil pressure and the movement of the rusk allows the camshaft cam through a special rod, who was previously in a free position, start to affect the valve with the rocker arm.
Valve lift height adjustment system
Further development of gas distribution phase change systems has led to complex solutions, which are based on the control of the lifting height of the valves. An innovator in this field was BMW introduced a system called Valvetronic on its engines 2001 year.
Adjusting the lifting height of the valve additionally allowed to exclude from the circuit the throttle valve in relation to the main modes of operation of the engine. The presence of the valve significantly reduces the efficiency of filling the cylinders with fuel-air mixture at low and medium speeds. The reason lies in that, that in the intake manifold (in the field of the throttle) in the process of internal combustion engine there is a vacuum. The fuel-air mixture becomes inert under such dilution conditions, the cylinders are filled less efficiently, the reaction to depressing the accelerator pedal loses sharpness and becomes slow.
The best solution to this problem is to mechanically open the intake valve at this point in time, which is necessary for efficient filling of the cylinder of the working fuel-air combustible mixture. The duration of the intake phase (intake phase) in valve lift height adjustment systems varies accordingly, how hard the accelerator pedal was depressed. The throttle control system saves fuel significantly (to 15% compared to other solutions), and also increases the power characteristic by 10 % and more.
Structurally, the timing belt in such systems is able to control the operation of the power plant in different modes. Toyota's Valvematic solutions are based on a similar principle, Nissan's VEL solution, VTI from Peugeot and others. As for the Valvetronic valve height adjustment system, the ability to control this parameter is implemented through a special kinematic scheme. The Valvetronic solution is placed on the intake valves. Traditional design, which includes a camshaft cam, rocker (rocker) and valve, was developed in the form of installation of additional elements.
The system has an eccentric shaft, as well as an intermediate lever. The specified eccentric shaft begins to rotate by force, which creates an electric motor through a worm gear.
This rotation of the eccentric shaft affects the intermediate lever, as a result, its position changes (there is a shift of the fulcrum). Changing the position makes the rocker move like that, to move (open) the valve exactly to the required value.
The system of change of height of rise of the valve works constantly, and the lifting height of the valves directly depends on the mode of operation of the power unit. Valves can rise within 0,2 to 12 mm. Nissan's VEL system provides a valve lift height within 0,5 to 2 mm
Solenoid valve actuator
Today, engine designers almost fully use the potential of timing. The maximum possible number of valves per cylinder is designed, and the size of the valve itself has reached its limit. But the evolution of the engine at this stage continues. It is also possible to improve the filling and blowing of the engine cylinders due to the speed, with which it is possible to implement the opening and closing of valves. We are talking about timing, in which the valves have a solenoid (electromechanical) occasion, which replaces the mechanical with electronic control. Moreover, the camshaft in such timing is completely absent.
The electromagnetic timing drive is called EVA (English. Electromagnetic Valve Actuator) and allows you to change the phases of gas distribution as widely as possible. The electromagnetic drive system can only open the required valves (which is similar to the controlled shutdown of the cylinders), and do it at a specific time, depending on the mode of operation of the internal combustion engine. The solution is able to save fuel at idle, at the time of engine braking, etc.. P. The amount of air entering the cylinder of the engine is regulated by the time of opening the intake valve.
The valve stroke length itself is not an adjustable parameter. The valve is fastened by a spring, and also has an anchor. Such an armature of the solenoid valve is placed between two solenoids of a certain power. The task of such electromagnets is to keep the valve in one or another extreme position.
Position accuracy, in which it is necessary to fix the valve, is determined by a separate sensor designed for this purpose. Reduction of destructive loads on electromagnetic timing at the moment of approach of the valve to its extreme point (especially at the time of landing the valve in the saddle) is carried out by "braking" the valve.
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