Japanese Honda is a pioneer in variable valve technology using variable cams. In the late 1980s, it released its famous VTEC (Valve Timing Electronic Control = electronic control valve timing technology). It was first configured on Civic, CRX and NS-X, and then gradually became standard equipment on most models.
In the picture we can see that there are two sets of cams with different shapes to achieve different valve timing and lift. One group works under normal speed conditions, which is below 4500rpm for daily use. The other group alternates actions at high speeds. Obviously, this design cannot achieve continuous variable valve timing, so the VTEC engine behaves quite mildly below 4500rpm (the high-speed cam opening speed varies depending on different models and engine models), but once it exceeds 4500rpm, it switches to high speed. When the cam is turned on (commonly known as "turning on TEC"), the engine will perform like a beast with surging power.
This system can indeed increase the peak horsepower, allowing the engine to easily reach close to 8,000 rpm (the engine on the S2000 can even reach 9,000 rpm), just like a car equipped with a competition cam The shaft can increase the horsepower of a 1.6-upgrade engine by a huge 30 horses! ! However, to take full advantage of this power, the engine speed must be kept above the "TEC on" point, so you'll need to shift gears frequently. In addition, the horsepower performance in the low-speed range is not so impressive, because the low-speed cam must work from zero to 4500rpm, and the driving pleasure is not satisfactory. Overall, variable cam technology is ideal for sports cars.
Figure: Basic principle diagram of variable cam valve technology.
Honda has improved the 2-stage VTEC into a 3-stage VTEC on some engine models. Of course, the higher the segment number, the more complete its performance. However, it always only provides a less broad torque band than other continuously variable valve technologies. However, since other VVT systems cannot change the valve lift, the variable cam system is still the most powerful VVT system.
Advantages:
The rear section of the engine is more powerful, suitable for high-speed oriented engines
Disadvantages:
Only 2-3 sections Variable, discontinuous variable; limited improvement in torque distribution; complex structure
Technology with similar principles:
Honda VTEC, Mitsubitshi MIVEC, Nissan Neo VVL< /p>
Honda’s latest three-stage VTEC has been loaded with the Civic? Sohc? engine sold in Japan. Each camshaft section of this mechanism has three cams with different timing and lift shapes. Pay attention to the difference in their appearance - the cam in the middle (fast timing, high lift), as shown in the picture, is the largest; the cam on the right hand side (slow timing, medium lift) is in the middle?;? The left cam (slow timing, low lift) is the smallest.
The operating principle of this mechanism is as follows:
Figure: 3-section VTEC structure diagram
Figure: 3-section VTEC? 3D diagram
Stage 1 (low speed): 3 rocker arms move independently. Therefore, the left rocker arm that controls the left intake valve is driven by the slow cam, and the right rocker arm that controls the right intake valve is driven by the medium speed cam. The timing of both cams is slow relative to the middle cam, which, of course, does not control any valves.
Stage?2?(medium speed)?:?Hydraulic pressure connects the left and right rocker arms into one, but the middle rocker arm continues to move independently and cannot control any valves. Since the right cam is larger than the left cam, the connected rocker arm is actually driven by the right cam. As a result, both intake valves get low speed timing and medium travel.
Stage?3?(High Speed)?:?Hydraulically connects the 3 rocker arms together. Since the middle cam is the largest, both intake valves are controlled by the middle cam. Both intake valves are in high-speed timing and large-stroke conditions.
Very similar to Honda's three-stage i-VTEC, Neo?VVL also uses cams of different shapes to act on the valve rocker arms to achieve corresponding control under different engine speed conditions. The valve lift and opening duration are optimized to optimize the engine's power and fuel consumption performance. But the Nissan's left and right cam profiles are the same. ?In low-speed operating conditions, their corresponding rocker arms are independent of each other and controlled by their respective low-speed timing and low-speed lift cams. When it comes to high-speed operation, the three rocker arms are connected together by hydraulic pressure, and are controlled by a high-speed cam with high-speed timing and high-lift placed between the low-speed cams. The high-speed cam drives the valve rocker arm.
Do you think this is a two-stage system? No, at least not entirely. As shown in the figure, since Nissan Neo?VVL is equipped with the same system on the exhaust side camshaft, the working status of this system can be divided into the following three situations:
Stage?1? (Low speed )?:?Both the intake and exhaust valves use slow mode.
Stage?1?(Low speed)?: Both the intake valve and the exhaust valve use slow speed mode.
Stage?3?(High-speed)?: Both the intake valve and the exhaust valve use high-speed mode.
MIVEC (Mitsubishi Innovative Valve-timing-and-lift Engine Control, Mitsubishi's new valve timing and lift engine control technology) is an automotive variable valve timing technology developed by Mitsubishi Corporation . MIVEC achieves variable valve lift by using two sets of camshafts with different shapes. It is very similar to Honda's VTEC system, but in fact, the MIVEC system sacrifices the output performance at higher speeds and is relatively conservative in the design of the cam. In exchange, MIVEC is turned on at a relatively low speed. Makes the transition between high and low turning cams appear soft. However, the MIVEC system on the Lancer Evolution only controls valve timing but not lift. Similarly, MIVEC's working conditions are also divided into low-speed and high-speed modes:
Figure: MIVEC's working principle diagram.
Low speed mode:
Dual intake valves with different lifts (low and medium lift) and enhanced airflow in the cylinder further make combustion more stable without compromising fuel economy, emissions and torque.
High-speed mode:
Prolonging the opening time of the intake valve and expanding the valve opening degree can increase the amount of air charge and successfully approach the best performance of the same level of engine output.
MIVEC technology has been incorporated into the "World Engine" and has become a joint development project of Mitsubishi, Daimler Chrysler and Hyundai Motor.
BMW’s Valvetronic is actually a variable valve lift technology used for intake valves. Compared with other camshaft-rocker arm structures, Valvetronic has an intermediate rocker arm between the camshaft and each rocker arm to transform the motion trajectory of the camshaft into the valve rocker arm. This intermediate rocker arm introduces an electronic control and motor execution system. The system has a camshaft like a traditional engine, and also has an eccentric shaft, roller and ejector mechanism, and is driven by a stepper motor. By receiving the signal from the throttle position, the stepper motor changes the offset of the eccentric cam, and controls the motor to rotate at a certain angle to drive the movement of the threaded rod, thus changing the angle at which the intermediate rocker arm contacts the camshaft and valve rocker arm. A cam path is converted into a variable lift valve opening and closing process.
Compared with the traditional twin-convex engine, Valvetronic uses an additional eccentric shaft, stepper motor and some central rocker arms to control the opening or closing of the valve. If the rocker arm presses The deeper the intake valve will have a higher lift, Valvetronic has a way to freely control the valve lift. Long intake means a large valve lift, and short intake means a small valve lift.
Figure: The stepper motor controls the rotation angle of the eccentric wheel, causing the angle of the middle rocker arm to change, thereby changing the valve lift.
Picture: From the schematic diagram and the cross-sectional view of the Valvetronic system, you can see the corresponding movable parts and actuators of the system. The arrangement of the camshaft is completely different from the conventional sample.
Audi also released its own variable valve lift technology on the new generation 3.2 V6FSI engine (for A5), called "Audi Valve-lift System", abbreviated as AVS. As Audi's variable valve lift technology, AVS can effectively reduce fuel consumption while increasing engine output. It has a simple structure and reliable operation. It is no wonder that Audi has spent more than 6 years on this technology.
The principle of AVS is completely different from other variable valve lift technologies. Other valve lift technologies generally achieve variable valve lift by changing the action mode of the rocker arm; but what is special about AVS is that its valve lift is variable through the axial movement of the camshaft. Switching cam's.
The main component of this system is the hydraulic ejector pin - the metal jacket and black inner core shown in the picture above. When the engine is in the low speed range, the hydraulically controlled thimble is in a contracted state, there is no contact between the thimble and the camshaft, the camshaft is in a free state, and the working cam is a low-speed cam. When the engine switches to high-speed operating conditions, the hydraulic ejector pin is filled with oil, the ejector pin extends downward, and the stainless steel ejector pin head gets stuck in the spiral groove on the camshaft. Since the ejector pin is fixed to the cylinder block, as the camshaft continues to rotate, the camshaft will be pushed to the left by the ejector pin and the spiral groove *** (direction of the green arrow in the picture). The working cam is switched to the high-speed cam. When the thimble is retracted, the camshaft will return to its free state, which means that the low-speed cam will be used to drive the valve.
Variable valve timing technology is by far the simplest, lowest cost and most common variable valve mechanism. However, its performance and effect are also minimal. It is just a fuel-saving and emission reduction technology, not performance-oriented.
Basically, this technique changes the timing of the camshaft. For example, at high speeds, the intake camshaft will rotate forward by 30° to advance the opening time of the intake valve. This action is issued by the engine management system (ECU and sensors) based on engine load and speed and other working conditions, and then the hydraulic drive element executes the action.
What I want to emphasize here is that variable valve timing technology cannot change the duration of valve opening. It can only achieve the effect of opening the valve early or late. But early opening and early closing means that the duration of valve opening remains unchanged. Furthermore, it cannot change the valve lift, unlike variable cam valve technology. However, variable valve timing technology is the simplest and lowest-cost valve technology because each camshaft only requires a stepper hydraulic actuator, unlike other systems that require an independent mechanism in each cylinder to complete the action.
The angle variable process is divided into continuous and discontinuous:
The simpler version only has 2-3 fixed preset angles to choose from, such as 0° or 30°. More advanced systems will provide continuous angle selection, which means that any angle between 0° and 30° can be selected, depending on the current engine speed. Obviously, this can provide the most appropriate valve timing at any engine speed, giving the engine greater flexibility. Plus, the transition between the two angles is so smooth that it’s almost imperceptible.
Intake and exhaust:
Some designs, such as BMW's Double Vanos system, have variable positive camshafts on both the intake and exhaust camshafts. timing system, so that a deeper overlap of intake and exhaust can be achieved, bringing a stronger ventilation effect. This can explain why the BMW E46 M3 3.2 (more than 100 horsepower per liter) is better than its predecessor E36M3 3.0 (95 horsepower per liter), which is only equipped with a variable timing mechanism on the intake side. There is greater efficiency.
On the E46-generation 3 Series, the maximum advance angle of the intake end of Double Vanos can reach 40°, while the maximum advance angle of the exhaust end is 25°.
Advantages
Simple structure and low cost; rich and smooth torque at full speed;
Disadvantages
No increase in valve opening duration and Lift function
Technology with similar principles
Among the variable valve timing technologies of many car manufacturers, BMW's Vanos? It has excellent fuel consumption and can achieve strong output. It cooperates with BMW's design and tuning technology and complements each other. It is indeed a model of success.
Similar variable valve timing technologies from other car manufacturers use similar principles. A hydraulic actuator is installed between the camshaft and the timing gear, and the ECU and sensor signals control the hydraulic servo mechanism to execute the action. , so that the camshaft can rotate through a certain angle relative to the timing gear to advance or delay the valve timing, thereby improving the ventilation efficiency of the engine in each speed range.
Picture: The gear on the left in the picture is the timing gear, with a meshing helical gear inside; the one on the right is the hydraulic actuator of Vanos. The internal spline of the hollow shaft is used to cooperate with the camshaft, and the external bevel The gear meshes with the timing gear.
From the pictures in this article, we can easily understand the operating principle of Vanos: the timing gear end of the camshaft is fixedly connected to a section of gear rack (note that the gear is not straight teeth but helical teeth). This The gears of the gear rack mesh with the inside of a cover that can move back and forth along the camshaft axis. Since the gear rack has helical teeth, moving the cover forward will drive the camshaft to rotate clockwise relative to the cover, which will advance the valve timing. Similarly, when the cover moves backward, the camshaft will rotate counterclockwise. , causing the valve timing to be delayed.
Picture: Schematic diagram of Vanos structure and working principle
The forward and backward movement of the cover completely depends on the hydraulic actuator located behind it. In the large "bulge" behind it are two hydraulic chambers filled with hydraulic oil separated by lamella pistons - the lamella piston is rigidly connected to the cover mentioned above. The forward and backward movement of the piston depends on the amount of oil and oil pressure in the front and rear hydraulic chambers. The liquid in and out of the two working chambers are completely controlled by solenoid valves, and the pressure difference in the two chambers is used to push the lamella piston forward or backward. It can be seen that the variable valve timing mechanism can adjust the valve timing by controlling the position of the hydraulic piston. The principle is simple and easy to implement.
Since variable cam variable valve technology and continuously variable valve timing technology have their own special features, each with its own advantages and disadvantages, many car manufacturers have already thought of combining these two technologies with different personalities. The variable valve technology is organically combined to take into account high-speed power performance and smoothness throughout the entire speed range, but at the cost of complex structure and high cost.
Picture: The picture shows Audi's "Valvelift" dual variable technology structure
With the development of electronic technology and mechatronics technology, these two variable valve technologies are combined The possibility of use has become a reality, and all major car manufacturers have selected this "lift and timing dual variable" valve variable technology in their models. For example, Toyota's VVTL-i, Porsche's Variocam Plus, Honda's i-VTEC and Audi's Valvelift technology, etc., can achieve excellent results of superimposing advantages and complementing disadvantages based on book data and actual use effects. . It is not difficult to predict that as future cars move towards low emissions, high energy efficiency and the pursuit of power density, this "dual variable lift and timing" valve variable technology will definitely become popular due to its excellent performance.
As an upgraded and improved version of VVT, the secret weapon of Toyota's variable valve timing technology, VVTL-i (Variable? Valve? Timing? and? Lifting? with? Intelligence) has the basic function of VVT-i, in addition to different speed ranges, which can also adapt to the vehicle's stepless adjustment of valve timing under different load conditions. It also has the function of changing the valve lift in two stages, which further strengthens the Regarding the power performance of the engine, for example, the 2ZZ-GE engine used in the last-generation Celica is equipped with a VVTL-i system and squeezes out 190 horsepower with a displacement of 1.8.
VVTL-i is Toyota’s most advanced variable valve technology currently in use. It can be seen as a combination of Toyota VVT-i technology and technology similar to Honda’s VTEC.
Figure: Structural diagram and working principle diagram of VVTL-i
In terms of controlling valve timing, the principle is the same as VVT-. The end of the camshaft is connected to the hydraulic actuator, and the camshaft is rotated forward or backward by a certain angle through the hydraulic system. The appropriate valve timing is determined by inputting signals such as engine speed, engine load rate, uphill or downhill road conditions, etc. The maximum difference in timing change angle can reach 60°.
The "upgrade" of VVTL-i compared to VVT-i lies in its "L", which means valve lift "lifting". Similar to VTEC technology, VVTL-i also uses a rocker arm to control two intake valves (the same applies to the exhaust valve, the same below). When the large engine is at low speed, the low-speed cam directly acts on the valve rocker arm, and the low-speed cam controls the valve opening and closing; the rocker arm under the high-speed cam has a tappet inside the rocker arm, and the movement of the tappet will not interfere with the rocker arm action. . At high speed, after the gap under the lifter in the rocker arm under the high-speed cam (large persistence angle, high lift) is filled by the hydraulically controlled slider, the action of the high-speed cam directly acts on the rocker arm to drive the valve. Opening and closing, thereby realizing a two-stage conversion between low-speed valve lift and high-speed valve lift.
Compared with ordinary VVT-i and even VVT technology, VVTL-i not only retains the average and smooth characteristics of torque distribution in the low to medium speed range of the engine, but also has variable valve lift. The introduction of technology directly improves the high-rev performance of the engine, allowing the engine's power to extend to the high-rev area. Therefore, regardless of whether it is considered in terms of ride comfort, power or fuel economy, VVTL-i is Toyota's most superior variable valve system today.
Advantages
The VVT ??system retains the torque flat characteristics in the entire speed range, and the variable valve lift technology enhances the engine's high-speed output characteristics.
Disadvantages
More expensive and more complex in structure than conventional variable air intake systems
Applications
Toyota? 1.8L? 2ZZ -GE?190hp: The last generation of Celica, high-performance version of Corolla
Porsche's Variocam?Plus technology is said to have evolved from the Variocam technology used in Carrara and Boxter models. But by comparison, there are no special parts between the two. Variocam made its debut on the 968 in 1991. It used a timing chain to change the angle of the camshaft, providing 3-stage intake timing changes. 996 and Boxter also use this old system. Although this design is unique and novel, it is a bit limited compared to the hydraulically controlled variable timing systems of other car manufacturers, especially since it does not allow large, multi-stage changes.
Porsche Variocam system
Picture: Working principle of Variocam Plus system
Therefore, the new 911 Turbo is replaced with a new Variocam system. The Plus system introduces a hydraulically controlled variable timing mechanism, which greatly improves the flexibility and adaptability of the variable valve timing system. Not only that, the more important change of Variocam Plus is the addition of a variable valve lift system. This new set of valve lift variations is achieved by variable hydraulic valve lifters. As you can see in the picture, each valve is controlled alternately by 3 cams. The middle one is the low-speed cam, with the shortest valve lift (3mm) and duration; the cams on both sides are actually the same, with larger valve lift (10mm) and longer duration. The realization of variable lift actually relies on variable tappets. As you can see from the picture above, the tappet is actually divided into two parts, the central part and the peripheral part. There is a hydraulic locking pin between the two parts to connect them. When the lock pin locks the two parts, the valve lifter is actually controlled by the high-speed cam; similarly, when the lock pin is unlocked, the peripheral part has no effect and the valve is driven by the central part lifter.
Advantages
The structure is compact and simple, requiring fewer parts; greatly improving the performance of the engine in the early, middle and final stages.
Disadvantages
It is expensive, needs to ensure strength, and the hydraulic drive mechanism is complex
Application
Porsche?911?Turbo,?911? Carrera?3.6 (997th generation)
The full name of AVTEC is Advanced?VTEC, which is the improved VTEC. Unlike the previous VTEC system which can only have 2 or 3 stages of variable valve lift, the AVTEC system can achieve continuous variable valve lift.
However, after careful exploration, the author found that this "improvement" can be said to be not only an improvement, but also a subversion and breakthrough in the working principle of the entire VTEC system.
Since the advent of the VTEC system, it has always used its unique two-stage (the later improved second-generation VTEC has a three-stage) camshaft to hydraulically control whether the rocker arm group corresponding to the cam can Control the valve lifter action to achieve variable valve lift. However, nearly 20 years after the advent of this principle, it has been constantly imitated and improved by subsequent competitors, and its performance has gradually become less outstanding. Although the newly released AVTEC system is still VTEC technology with the improvement of continuously variable valve lift, the basic working principle is an earth-shaking change.
Picture: This "Advanced" is so small that it can be ignored, but it is a symbol of epoch-making changes in VTEC technology.
In order to achieve continuous variation of engine valve lift and valve timing, the variable valve actuator (1) includes the following parts: the camshaft ( 8)?, can rotate like a conventional camshaft, and only contains one shape of cam (7); the main valve rocker arm (9)? is also fixed through the rocker arm shaft (32); the valve side of the main rocker arm There is a slider (12) that controls the valve stem (6). A semi-open camshaft housing (11) is fixed on the cylinder head, wrapping part of the camshaft, and can be controlled by hobbing (25) and control rod (26). ) to rotate a certain angle under the control of The upper rocker arm pulley (22) can make the auxiliary rocker arm and the main rocker arm mesh smoothly; the camshaft rotates in the camshaft housing to push the auxiliary rocker arm, and then the auxiliary rocker arm pushes the main rocker arm to drive the camshaft.
Figure: The structure of the camshaft, camshaft sleeve and auxiliary rocker arm. (14) is the bearing fixed to the cylinder head.
Picture: The picture on the left shows the engine at low speed, and the picture on the right shows the engine at high speed. Pay attention to the position of the auxiliary rocker arm. This is the essence of AVTEC.
AVTEC technology was released last year, and starting from this year it will first be equipped on the K24A engine. However, the official data does not clearly introduce the power figure of this engine, but it is quite flaunted. Technology performance in terms of fuel economy and environmental protection: AVTEC's fuel consumption is 13% lower than i-VTEC, while emissions are 25% lower than Japan's domestic 2005 emission standards.
This article comes from the author of Autohome Chejiahao and does not represent the views and positions of Autohome.