Basic Principles
After multi-valve technology became standard in engine design, Variable Valve Timing becomes the next step to enhance engine output, no matter power or torque.As you know, valves activate the breathing of engine. The timing of breathing, that is, the timing of air intake and exhaust, is defined by the shape and phase angle of cams. To optimise the breathing, engine requires different valve timing at different speed. When rev increases, the duration of intake and exhaust stroke decreases so that fresh air becomes not fast enough to enter the combustion chamber, while exhaust gas becomes not fast enough to leave the combustion chamber. Therefore, the best solution is to open the inlet valves earlier and close the exhaust valves later. In other words, the Overlapping between intake period and exhaust period should be increased as rev increases.
Without Variable Valve Timing technology, engineers used to choose the best compromise timing. For example, a van may adopt less overlapping for the benefits of low speed output. A racing engine may adopt considerable overlapping for high speed power. An ordinary sedan may adopt valve timing optimised for mid-range so that both low-speed drivability and high-speed output will not be sacrificed too much. No matter which one, the result is just optimised for a particular engine speed.
With Variable Valve Timing, power and torque can be optimised across a wider rpm band. The most noticeable results are:
- The engine can rev higher, thus raises peak power. For example, Nissan's 2-litre Neo VVL engine produces 25% more peak power than its non-VVT version.
- Low-speed torque increases, thus improves drivability. For example, Fiat Barchetta's 1.8 VVT engine provides 90% peak torque between 2,000 and 6,000 rpm.
Moreover, all these benefits come without any drawbacks.
Variable LiftIn some designs, valve lift can also be varied according to engine speed. At high speed, higher lift quickens air intake and exhaust, thus further optimise the breathing. Of course, at lower speed such lift will generate counter effects like deteriorating the mixing process of fuel and air, thus decreases output or even leads to misfire. Therefore the valve lift should be variable according to engine speed.
VVT's benefit to fuel consumption and emission
EGR (Exhaust Gas Recirculation) is a commonly adopted technique to reduce emission and improve fuel efficiency. However, it is VVT that really exploits the full potential of EGR.
In theory, maximum overlap is needed between intake valves and exhaust valves’ opening whenever the engine is running at high rev. However, when the car is running at medium speed on highway, in other words, the engine is running at light load, maximum overlapping may be useful as a means to reduce fuel consumption and emission. Since the exhaust valves do not close until the intake valves have been open for a while, some of the exhaust gases are recirculated back into the cylinder at the same time as the new fuel / air mix is injected. As part of the fuel / air mix is replaced by exhaust gases, less fuel is needed. Because the exhaust gas comprise of mostly non-combustible gas, such as CO2, the engine can run at the leaner fuel / air mixture without the risk of misfire.
Cam-changing VVTHonda pioneered road car-used VVT in the late 1980s with its famous VTEC system (Valve Timing Electronic Control). First appeared in Civic, CRX and NSX, then became standard in most production models.
You can see the VTEC mechanism as 2 sets of cams having different shapes to enable different timing and lift. One set operates during normal speed, say, below 4,500 rpm. Another substitutes at higher speed. Obviously, such layout does not allow continuous change of timing and lift, therefore the engine performs modestly below 4,500 rpm but above that it will suddenly transform into a wild animal.
This system does improve peak power - it can raise red line to nearly 8,000 rpm (even 9,000 rpm in S2000), just like an engine with racing camshafts, and increases top end power by as much as 30 horsepower for a 1.6-litre engine !! However, to exploit such power gain, you need to keep the engine boiling at above the threshold rpm, therefore frequent gearchange is required. As low-speed torque gains too little (remember, the cams of a normal engine usually serves across 0 to 6000 rpm, while the "slow cams" of VTEC engine still needs to serve across 0 to 4500 rpm), tractability is unlikely to be too impressive. In short, cam-changing system is best suited to sports cars.
Honda has already improved its 2-stage VTEC into 3 stages on some models. Of course, the more stage it has, the more refined it becomes. It still offers less broad spread of torque than other continuously variable systems. However, cam-changing system remains to be the most powerful VVT, since no other system can vary the Lift of valve as it does.
| Advantage | Powerful at top end delivery |
| Disadvantage | 2 or 3 stages only, non-linear; no much
improvement to torque; complex mechanism. |
| Who use it ? | Honda VTEC, Mitsubishi MIVEC, Nissan Neo VVL. |
Example: Honda 3-stage VTEC
Honda's latest 3-stage VTEC has been applied in Civic SOHC engine in Japan. The mechanism has 3 cams with different timing and lift profiles. Note that their dimensions are also different - the middle cam (fast timing, high lift), as shown in the above picture, is the largest; the right hand side cam (slow timing, medium lift) is medium sized; the left hand side cam (slow timing, low lift) is the smallest.
This mechanism works like this :
Stage 1 (low speed)
The 3 pieces of rocker arms move independently. Therefore the left rocker arm, which actuates the left inlet valve, is driven by the low-lift left cam. The right rocker arm, which actuates the right inlet valve, is driven by the medium-lift right cam. Both cams' timing are relatively slow compare with the middle cam, which actuates no valve now.
Stage 2 (medium speed)
Hydraulic pressure (painted orange in the picture) locks the left and right rocker arms together, leaving the middle rocker arm and cam to run on their own. Since the right cam is larger than the left cam, those connected rocker arms are actually driven by the right cam. As a result, both inlet valves obtain slow timing and medium lift.
Stage 3 (high speed)
Hydraulic pressure locks all 3 rocker arms together. Since the middle cam is the largest, both inlet valves are actually driven by that fast cam. Therefore, fast timing and high lift are obtained in both valves.
Another example - Nissan Neo VVLVery similar to the Honda system, but the right and left cams have the same profiles. At low speed, both rocker arms are driven independently by those slow-timing, low-lift right and left cams. At high speed, 3 rocker arms are locked together such that they are driven by the fast-timing, high-lift middle cam.
You might think it must be a 2-stage system. No, it is not. Since Nissan Neo VVL duplicates the same mechanism in the exhaust camshaft, 3 stages could be obtained in the following way:
Stage 1 (low speed): both intake and exhaust valves are in slow configuration.
Stage 2 (medium speed): fast intake configuration + slow exhaust configuration.
Stage 3 (high speed): both intake and exhaust valves are in fast configuration.
Cam-phasing VVTCam-phasing VVT is the simplest, cheapest and most commonly used mechanism at this moment. However, its performance gain is also the least, very fair indeed.
Basically, it varies the valve timing by shifting the phase angle of camshafts. For example, at high rev, the inlet camshaft will be rotated in advance by 30° so to enable earlier intake. This movement is controlled by engine management system according to need, and actuated by hydraulic valve gears.
Continuous or Discrete
Simpler cam-phasing VVT systems offer just 2 or 3 fixed phasing angles, such as either 0° or 30°. Better systems can vary phase angle continuously. Obviously, this provides the most suitable valve timing at any rev, thus greatly enhance engine flexiblility. Moreover, the transition is seamless and hardly noticeable, contributing to refinement. Today, continuous systems have put discrete systems in extinction.
Intake and Exhaust
Some designs, such as BMW's Double-Vanos system, has cam-phasing VVT at both intake and exhaust camshafts. This enables more overlapping, hence higher efficiency. This explain why BMW M3 3.2 (100hp/litre) is more efficient than its predecessor, M3 3.0 (95hp/litre) whose VVT is bounded at the inlet valves.
In the E46 3-series, the Double-Vanos shifts the intake and exhaust camshaft within a range of 40° and 25° respectively.
Example: BMW Vanos / Double Vanos| Advantage | Cheap and simple, continuous VVT improves
torque delivery across the whole rev range. |
| Disadvantage | Lack of variable lift and variable valve
opening duration, thus less top end power than cam-changing VVT. |
| Who use it ? | Most car makers now. |
Whether push or pull is determined by the hydraulic pressure. There are 2 chambers right beside the cap and they are filled with liquid (these chambers are colored green and yellow respectively in the picture) A thin piston separates these 2 chambers, the former attaches rigidly to the cap. Liquid enter the chambers via electromagnetic valves which controls the hydraulic pressure acting on each chambers. For instance, if the engine management system signals the valve at the green chamber open, then hydraulic pressure acts on the thin piston and push the latter, accompany with the cap, towards the camshaft, thus shifts the phase angle forward.
Continuous variation in timing is easily implemented by positioning the cap at a suitable distance according to engine speed.
The Vanos system works at intake camshaft only. However, it can be dupicated at the exhaust camshaft to provide a wider range of adjustment. BMW calls this Double Vanos or Bi-Vanos.
Another example: Toyota VVT-i
However, the word "Intelligent" emphasizes the smart control program. Not only varies timing according to engine rev, it also considers other parameters such as acceleration, going up hill or down hill.
Cam-changing + Cam-phasing VVTCombining cam-changing VVT and cam-phasing VVT may satisfy the requirement of both top-end power and flexibility throughout the whole rev range, though it is inevitably more complex. At the time of writing, only Toyota and Porsche have such designs. However, I believe in the future more and more sports cars will adopt this kind of VVT.
Example: Toyota VVTL-i
- Continuous cam-phasing variable valve timing
- 2-stage variable valve lift plus valve-opening duration
- Applied to both intake and exhaust valves
The system could be seen as a combination of the existing VVT-i and Honda’s VTEC, although the mechanism for the variable lift is different from Honda.
Like VVT-i, the variable valve timing is implemented by shifting the phase angle of the whole camshaft forward or reverse by means of a hydraulic actuator attached to the end of the camshaft. The timing is calculated by the engine management system with engine rev, acceleration, going up hill or down hill etc. taken into consideration. Moreover, the variation is continuous across a wide range of up to 60°, therefore the variable timing alone is perhaps the most perfect design up to now.
What makes the VVTL-i superior to the ordinary VVT-i is the "L", which stands for Lift (valve lift) as everybody knows. Let’s see the following illustration:
Like VTEC, Toyota’s system uses a single rocker arm follower to actuate both intake valves. It also has 2 cam lobes acting on that rocker arm follower, the lobes have different profile - one with longer valve-opening duration profile (for high speed), another with shorter valve-opening duration profile (for low speed). At low speed, the slow cam actuates the rocker arm follower via a roller bearing (to reduce friction). The high-speed cam does not have any effect to the rocker follower because there is sufficient spacing underneath its hydraulic tappet.
A flat torque output (blue curve)
When the engine revs passed the threshold point, the sliding pin is pushed by hydraulic pressure to fill the space. The high-speed cam becomes effective. Note that the fast cam provides a longer valve-opening duration while the sliding pin adds valve lift. (For Honda VTEC, both the duration and lift are implemented by the cam lobes)
Obviously, the variable valve-opening duration is a 2-stage design, unlike Rover VVC’s continuous design. However, VVTL-i offers variable lift, which lifts its top end power output a lot. Compare with Honda VTEC and similar designs for Mitsubishi and Nissan, Toyota’s system has continuously variable cam phasing which helps it to achieve far better low to medium rev flexibility. Therefore it is easily the most versatile VVT as of the time of writing. However, it is also more complex and expensive to build.
Example 2: Porsche Variocam Plus
Variocam Plus uses hydraulic phasing actuator and variable tappets
Porsche’s Variocam Plus was said to be developed from the Variocam which serves the Carrera and Boxster. However, I found their mechanisms virtually share nothing. The Variocam was first introduced to the 968 in 1991. It used timing chain to vary the phase angle of camshaft, thus provided 3-stage variable valve timing. 996 Carrera and 986 Boxster also used the same system. This design is unique and patented, but it is actually inferior to the hydraulic cam phasers favoured by other car makers, especially as it doesn’t allow as much variation to phase angle.
Therefore, the Variocam Plus used in the new 996 Turbo finally follows the industrial trend to use hydraulic cam phasers instead of chain. However, the most influential changes of the "Plus" is the addition of variable valve lift. It is implemented by using variable hydraulic tappets. As shown in the picture, each valve is served by 3 cam lobes - the center one has obviously less lift (3 mm only) and shorter duration for valve opening. In other words, it is the "slow" cam. The outer two cam lobes are exactly the same, with fast timing and high lift (10 mm). Selection of cam lobes is made by the variable tappet, which actually consists of an inner tappet and an outer (ring-shape) tappet. They could by locked together by a hydraulic-operated pin passing through them. In this way, the "fast" cam lobes actuate the valve, providing high lift and long duration opening. If the tappets are not locked together, the valve will be actuated by the "slow" cam lobe via the inner tappet. The outer tappet will move independent of the valve lifter.
As seen, the variable lift mechanism is unusually simple and space-saving. The variable tappets are just marginally heavier than ordinary tappets and engage nearly no more space.
| Advantage | Variable cam phasing improves torque
delivery at low / medium rev; Variable lift and duration improves high
rev power. |
| Disadvantage | Slightly more complex and expensive |
| Who use it ? | Most Porsche engines since 996 Turbo |
Example 3: Honda i-VTECIf you know how VTEC and VVT-i works, you can easily imagine how to combine them into a more powerful VVT mechanism. Honda calls it i-VTEC. Like Toyota's VVTL-i, it provides:
- Continuous cam-phasing variable valve timing
- 2-stage variable valve lift plus valve-opening duration
- Can be applied to both intake and exhaust valves
Basically, the camshaft is purely VTEC - with different cam lobes for implementing 2-stage variable lift and timing. On the other hand, the camshaft can be phase-shifted by a hydraulic actuator at the end of the camshaft, so valve timing can be varied continuously according to need.
The i-VTEC was first introduced in Stream MPV, in which only the intake side applies i-VTEC. Theoretically, it can be applied to both intake and exhaust camshafts, but Honda seemed less generous than Toyota - even the Integra Type R uses only i-VTEC at intake side plus the regular VTEC at exhaust side.
| Advantage | Continuous
variable cam phasing improves torque delivery across a wide rev range;
Variable lift and duration improves high rev power. |
| Disadvantage | More complex and expensive |
| Who use it ? | Honda 2.0 i-VTEC engine for Stream, Civic,
Integra and more. |
Example 4: Audi ValveliftAudi's Valvelift system made its debut in the company's 2.8-liter direct injection V6 and is expected to be expanded for use in many other members of the 90-degree V6 / V8 family. The Valvelift system itself is a cam-changing type VVT, but as Audi's V6 / V8 engines are already equipped with cam-phasing VVT, I classify it as the combination type VVT here.
Compare with Honda's or Toyota's mechanism, Audi's seems to be simpler and more efficient. It does the variable lift without using complex intermediate parts (e.g. hydraulic-operated lockable rocker arms), so it saves space and weight while reduces frictional loss and, theoretically, improves revvability. How can Audi do that? the answer is: in Valvelift system, the cam pieces can slide in longitudinal direction to change the actuating cams.
Each intake valve can be actuated by a fast cam (11mm lift) or a slow cam (5.7mm in one intake valve and 2mm in another in order to create swirl in the air flow for better fuel mixing at low speed). The two cams are mounted on a single cam piece. Which cam acts on the roller cam follower depends on the longitudinal position of cam piece. This is controlled by a pair of metal pins incorporated at the cam cover. There is a spiral groove rolled into the camshaft. When one metal pin is lowered, it engages the spiral groove on the camshaft and pushes the cam piece by 7mm in longitudinal direction. A spring-loaded locker will lock the cam piece in the new position. In this way, the operating cams are changed from one set to another set.
To revert to another cam, another metal pin presses against a reverse spiral groove and moves the cam piece back to the original position. The cam piece is locked by the spring-loaded locker again. The change from one cam set to another takes one combustion cycle, or two engine revolutions. As Audi reprogrammed the ignition and electronic throttle to smoothen the transition between the two cam sets, it can be hardly detectable.
Theoretically, the Valvelift system should deliver better power than Toyota's VVTL-i and Honda's i-VTEC, but in the 2.8-liter V6 its priority is put on fuel economy. We shall see whether Audi will use its advantage in its performance engines in the future.
Advantage Continuous variable cam phasing improves torque delivery across a wide rev range; Variable lift and duration improves high rev power.
Disadvantage More complex and expensive
Who use it ? Audi 2.8 and 3.2 V6, Volkswagen EA888 4-cylinder.
Example 5: Mercedes CamtronicMercedes introduced its own variable valve lift system on the new M270 series four-cylinder engine in 2012. Called Camtronic, its main objective is not to enhance power but to reduce fuel consumption. At light or part-load, the Camtronic switches to low-lift cams to limit the amount of air intake, thus the throttle butterfly can remain wide open and reduce pumping loss. This principle is similar to BMW's Valvetronic system, but the Camtronic is a 2-stage system rather than continuously variable. Mercedes argues that it achieves 80 percent of the benefit of a continuous system while costing only a fraction, as it involves fewer parts. The Camtronic saves 4 percent of fuel in European combined cycle testing.
The mechanism of Camtronic is pretty simple. The intake camshaft is served with a conventional variable cam-phasing actuator at its end as well as the Camtronic variable valve lift components. The camshaft itself consists of an inner carrier shaft and 2 hollow cam-pieces, each serves 2 adjacent cylinders. Each cam has 2 profiles (low lift and high lift), which of them is engaged depends on the longitudinal position of the cam-pieces. When the engine needs to switch cam profiles, a centrally-mounted actuator applies steel pins to the grooves on the cam-pieces, thus the rotation of camshaft causes the cam-pieces to slide in longitudinal direction and engage the alternative cam profiles within one revolution.
The principle of Camtronic is very similar to Audi's valvelift, but it uses fewer cam-pieces and actuator thus is less costly to build.
| Advantage | Cut fuel consumption; Less costly to build. |
| Disadvantage | Not high-revving and power-enhancing. |
| Who use it ? | Mercedes M270 1.6 turbo (A and B-class),
M274 1.6 turbo (C-class). |
Example 6: GM iVLCGeneral Motors introduced its first variable valve lift system on its direct injection 2.5-liter four-cylinder enigne in late 2012. Its first applications were Chevrolet Impala and Malibu. The iVLC (Intake Valve Lift Control) applies to the intake camshaft and is compatible with variable cam phasing. It utilizes a special roller finger follower to implement the variable lift function. This finger follower consists of 2 parts - an inner roller finger follower which acts on the intake valve directly, and an outer roller finger follower. They can be detached or locked together by a lash adjuster, which is driven by oil pressure and controlled by ECU.
As in most other VVL designs, each of its intake valve is served with 3 cam profiles, i.e. 2 identical high-lift / long-duration "fast cams" sandwiching a low-lift / short-duration "slow cam". They activate the intake valve via the roller finger follower. The outer fast cams press on the outer finger follower. At low rpm, the lash is unlocked, thus the outer finger follower moves up and down freely without actually pressing on the valve. Meanwhile, the inner slow cam acts on the inner roller finger follower and activates the valve, therefore the engine runs with low valve lift.
At high rpm, where more air flow is demanded, the lash locks the outer and inner finger followers together, thus the fast cams can activate the valve via the locked finger followers.
Because of the 2-piece finger followers, I suppose iVLC could introduce more frictional loss than most other VVL systems, especially in low-lift mode. The additional moving mass could also limit its revvability a little. Judging from its output figures alone, the first 2.5-liter iVLC engine does not show any obvious advantages over the old engine.
| Advantage | Improved power and torque across a wider
band. |
| Disadvantage | Additional friction and moving mass could
limit output and efficiency. |
| Who use it ? | GM 2.5-liter 4-cylinder |
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ENTRADAS RELACIONADAS
Variable Valve Timing (VVT) Part 2
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