What are valve timing and how they work
Auto Terms,  Car device,  Engine unit

What are valve timing and how they work

The design of the four-stroke engine, which works on the principle of energy release during the combustion of a mixture of fuel and fuel, includes one important mechanism, without which the unit cannot function. This is a timing or gas distribution mechanism.

In most standard engines, it is installed in the cylinder head. More details about the mechanism structure are described in separate article... Now we will focus on what the valve timing is, as well as how its work affects the power indicators of the motor and its efficiency.

What is engine valve timing

Briefly about the timing mechanism itself. The crankshaft through a belt drive (in many modern internal combustion engines, a chain is installed instead of a rubberized belt) is connected to camshaft. When the driver starts the engine, the starter cranks the flywheel. Both shafts begin to rotate synchronously, but at different speeds (basically, in one revolution of the camshaft, the crankshaft makes two revolutions).

What are valve timing and how they work

There are special droplet-shaped cams on the camshaft. As the structure rotates, the cam pushes against the spring loaded valve stem. The valve opens, allowing the fuel / air mixture to enter the cylinder or exhaust into the exhaust manifold.

The gas distribution phase is precisely the moment when the valve begins to open the inlet / outlet before the moment when it completely closes. Each engineer working on the development of a power unit calculates what the valve opening height should be, as well as how long it will remain open.

Influence of valve timing on engine operation

Depending on the mode in which the engine is operating, the gas distribution should start either earlier or later. This affects the efficiency of the unit, its economy and maximum torque. This is because the timely opening / closing of the intake and exhaust manifolds is key to making the most of the energy released during the combustion of the HVAC.

If the intake valve starts to open at a different moment when the piston performs the intake stroke, then an uneven filling of the cylinder cavity with a fresh portion of air will occur and the fuel will mix worse, which will lead to incomplete combustion of the mixture.

What are valve timing and how they work

As for the exhaust valve, it should also open no earlier than the piston occupies bottom dead center, but not later than after it starts its upward stroke. In the first case, the compression will drop, and with it the motor will lose power. In the second, the combustion products with a closed valve will create resistance for the piston, which has begun to rise. This is an additional load on the crank mechanism, which can damage some of its parts.

For adequate operation of the power unit, different valve timing is required. For one mode, it is necessary that the valves open earlier and close later, and for others, vice versa. The overlap parameter is also of great importance - whether both valves will be opened simultaneously.

Most standard motors have a fixed timing. Such an engine, depending on the type of camshaft, will have maximum efficiency either in sport mode or with measured driving at low revs.

What are valve timing and how they work

Today, many cars of the middle and premium segment are equipped with motors, the gas distribution system of which can change some parameters of valve opening, due to which there is a high-quality filling and ventilation of the cylinders at different crankshaft speeds.

Here's how the timing should be done at different engine speeds:

  1. Idling requires so-called narrow phases. This means that the valves begin to open later, and the time of their closing, on the contrary, is early. There is no simultaneous open state in this mode (both valves will not be open at the same time). When the rotation of the crankshaft is of little importance, during phase overlap, the exhaust gases can enter the intake manifold, and some volume of the VTS into the exhaust.
  2. The most powerful mode - it requires wide phases. This is a mode in which, due to the high speed, the valves have a shorter open position. This leads to the fact that during sports driving, the filling and ventilation of the cylinders is performed poorly. To rectify the situation, the valve timing must be changed, that is, the valves must be opened earlier, and their duration in this position must increase.

When developing the design of motors with variable valve timing, engineers take into account the dependence of the valve opening moment on the crankshaft speed. These sophisticated systems allow the motor to be as versatile as possible for different riding styles. Thanks to this development, the unit shows a wide range of possibilities:

  • At low revs, the motor should be stringy;
  • When the revs increase, it should not lose power;
  • Regardless of the mode in which the internal combustion engine is operating, fuel economy, and with it the environmental friendliness of transport, should have the highest possible level for a particular unit.
What are valve timing and how they work

All these parameters can be changed by changing the design of the camshafts. However, in this case, the motor efficiency will have its limit only in one mode. How about the motor can change the profile on its own depending on the number of revolutions of the crankshaft?

Variable valve timing

The idea itself of changing the valve opening time during the operation of the power unit is not new. This idea periodically appeared in the minds of engineers who were still developing steam engines.

So, one of these developments was called a Stevenson gear. The mechanism changed the time of steam entering the working cylinder. The regime was called "steam cut-off". When the mechanism was triggered, the pressure was redirected depending on the design of the vehicle. For this reason, in addition to smoke, old steam locomotives also emitted puffs of steam when the train stood still.

What are valve timing and how they work

Work with changing valve timing was also carried out with aircraft units. So, an experimental model of the V-8 engine from the Clerget-Blin company with a capacity of 200 horsepower could change this parameter due to the fact that the design of the mechanism included a sliding camshaft.

And on the Lycoming XR-7755 engine, camshafts were installed, in which there were two different cams for each valve. The device had a mechanical drive, and was activated by the pilot himself. He could choose one of two options depending on whether he needed to take the plane into the sky, get away from the chase, or just fly economically.

What are valve timing and how they work

As for the automotive industry, engineers began to think about the application of this idea back in the 20s of the last century. The reason was the emergence of high-speed motors that were installed on sports cars. The increase in power in such units had a certain limit, although the unit could be unwound even more. In order for the vehicle to have more power, at first, the engine volume was only increased.

The first to introduce variable valve timing was Lawrence Pomeroy, who worked as chief designer for the auto company Vauxhall. He created a motor in which a special camshaft was installed in the gas distribution mechanism. A number of his cams had several sets of profiles.

What are valve timing and how they work

The 4.4-liter H-Type, depending on the speed of the crankshaft and the load it experienced, could move the camshaft along the longitudinal axis. Due to this, the time and height of the valves were changed. Since this part had limitations in movement, phase control also had its limits.

Porsche was also involved in this idea. In 1959, a patent was issued for the "oscillating cams" of the camshaft. This development was supposed to change the height of the valves, and at the same time, the time of their opening. The development remained at the project stage.

The very first workable valve timing control mechanism was developed by Fiat. The invention was developed by Giovanni Torazza in the late 60s. The mechanism used hydraulic pushers, which changed the pivot point of the valve tappet. The device worked depending on what the engine speed and the pressure in the intake manifold were.

What are valve timing and how they work

However, the first production car with variable GR phases was from Alfa Romeo. The 1980 Spider model received an electronic mechanism that changes phases depending on the operating modes of the internal combustion engine.

Ways to change the duration and width of the valve timing

Today there are several types of mechanisms that change the moment, time and height of valve opening:

  1. In its simplest form, this is a special clutch that is installed on the drive of the gas distribution mechanism (phase shifter). The control is carried out thanks to the hydraulic effect on the executive mechanism, and the control is carried out by the electronics. When the engine is idling, the camshaft is in its original position. As soon as the revs increase, the electronics reacts to this parameter and activates the hydraulics, which rotates the camshaft slightly relative to the initial position. Thanks to this, the valves open a little earlier, which makes it possible to quickly fill the cylinders with a fresh portion of BTC.What are valve timing and how they work
  2. Changing the cam profile. This is a development that motorists have been using for a long time. Installing a camshaft with non-standard cams can make the unit work more efficiently at higher rpm. However, such upgrades must be performed by a knowledgeable mechanic, which leads to a lot of waste. In engines with the VVTL-i system, the camshafts have several sets of cams with different profiles. When the internal combustion engine is idling, standard elements perform their function. As soon as the crankshaft speed indicator moves past the 6 thousand mark, the camshaft shifts slightly, due to which another set of cams comes into operation. A similar process occurs when the engine spins up to 8.5 thousand, and the third set of cams begins to work, which makes the phases even wider.What are valve timing and how they work
  3. Change in valve opening height. This development allows you to simultaneously change the operating modes of the timing, as well as exclude the throttle valve. In such mechanisms, pressing the accelerator pedal activates a mechanical device that influences the opening force of the intake valves. This system reduces fuel consumption by about 15 percent and increases the power of the unit by the same amount. In more modern motors, not a mechanical, but an electromagnetic analogue is used. The advantage of the second option is that the electronics are able to more efficiently and smoothly change the valve opening modes. Lift height can be near ideal and opening times can be wider than with previous versions. Such a development for the sake of saving fuel may even turn off some cylinders (do not open some valves). These motors activate the system when the car stops, but the internal combustion engine does not need to be turned off (for example, at a traffic light) or when the driver slows down the car with the internal combustion engine.What are valve timing and how they work

Why change the valve timing

The use of mechanisms that change the valve timing allow:

  • It is more efficient to use the resource of the power unit in different modes of its operation;
  • Increase power without the need to install a custom camshaft;
  • Make the vehicle more economical;
  • Provide effective filling and ventilation of cylinders at high speeds;
  • Increase the environmental friendliness of transport due to more efficient combustion of the air-fuel mixture.

Since different operating modes of the internal combustion engine require their own parameters of the valve timing, using the mechanisms for changing the FGR, the machine can correspond to the ideal parameters of power, torque, environmental friendliness and economy. The only problem that no manufacturer can solve so far is the high cost of the device. Compared to a standard motor, an analogue equipped with a similar mechanism will cost almost twice as much.

Some motorists use variable valve timing systems to increase the power of the car. However, with the help of a modified timing belt, it is impossible to squeeze the maximum out of the unit. Read about other possibilities here.

In conclusion, we offer a small visual aid on the operation of the variable valve timing system:

Variable valve timing system using the example of CVVT

Questions and answers:

What is the valve timing? This is the moment when the valve (inlet or outlet) opens / closes. This term is expressed in degrees of engine crankshaft rotation.

ЧWhat affects the valve timing? The valve timing is affected by the engine operating mode. If there is no phase shifter in the timing, then the maximum effect is achieved only in a certain range of motor revolutions.

What is the valve timing diagram for? This diagram shows how efficiently filling, combustion and cleaning in cylinders occurs at a specific RPM range. It allows you to correctly select the valve timing.

One comment

  • mudi

    please
    close the inlet valve if it gives compleshen problem what happens

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