Variable Valve Timing (VVT) Parte 2

Variable Valve Timing (VVT) Parte 2

Rover's unique VVC system
British car maker Rover introduced its own VVT system called VVC (Variable Valve Control) on MGF in 1995. Many experts regarded it as the best VVT system at the time. The VVC can continuously vary the duration of intake valve opening from a minimum 220 degree (crank angle) to a maximum 295 degree. This is unlike cam-phasing VVT systems, which can only shift the point of valve opening forward or backward but have nothing to do with the duration of opening. As a result, the VVC enables higher flow into the combustion chambers at high rev, benefiting high-end power output. On the other hand, unlike cam-changing systems, its adjustment of valve opening duration is continuous, thus mid-range torque is optimized. This make it a better compromise between power and flexiblity than either systems. The following diagram shows its valve timing:

To realize this continuous variation of valve opening duration is a big technical challenge. At high rev, the duration of intake valve opening shall be lengthened, while the duration of intake valve closing shall be shortened. Therefore, the intake camshaft has to rev slower just when the cam lobe is acting on the intake valve. Once the valve is closed, the camshaft has to speed up to shorten the valve closing duration. In the next cycle, the camshaft has to slow down again when the intake valves open, so forth. How to realize such a non-constant, pulsation-like camshaft rotation speed ?

The Rover VVC system uses a very complicated mechanism to implement that. It is difficult to understand, but in essence it utilizes the special property of eccentric drive wheel. Because an eccentric drive wheel rotates about an off-center shaft, if you turn its outer drive ring at constant speed, the shaft will rotate at non-constant, pulsation-like manner. The speed difference depends on the distance between the shaft and the wheel center, i.e. the longer the distance, the larger difference of rotational speed. The VVC uses a slidable shaft to vary this distance hence the speed difference.

The problem is, a camshaft serves multiple cylinders, which have contradicting requirements. For example, cylinder 1 is working at intake stage while other cylinders have their intake valves closed. Suppose the engine is running at high rev, cylinder 1 calls for a slower rotation of intake camshaft while other cylinders need quicker rotation. As a result, the VVC cannot adopt a single intake camshaft like conventional engines. In fact, it needs 4 camshafts for a 4-cylinder engine ! The right hand side picture above shows the 4 camshafts are arranged in 2 groups. Each group has a rigid camshaft (for the inner cylinder) running within a hollow camshaft (for the outer cylinder). Each group is driven by a double-VVC actuator which has 2 drive rings to actuate the 2 camshafts. Because the two groups of camshafts are not connected at all, an additional drive belt has to be introduced. To save space and weight, Rover simply uses the exhaust camshaft to drive the other intake camshaft group.

As you can see, the VVC is a very sophisticated kind of engineering. Compare the Rover 1.8 VVC engine with its non-VVC version, its output is lifted from 120 hp to 145 hp, while maximum torque is improved from 122 to 128 lbft. On the down side, its complexity means higher costs. A four-cylinder engine needs 2 VVC actuators. A V6 engine even needs 4 of them. And then there are the more complicated camshafts and drive belts. These disadvantages prevent it from becoming popular. Following the demise of Rover, the VVC also came to the end.
       

Advantage	Continuous variation of intake valve opening duration improves power and flexiblity.
Disadvantage	Complex mechanism thus expensive; lack of variable lift means not ultimately as powerful as cam-changing VVTs.
Who use it ?	Rover 1.8 VVC engine on MGF, Caterham and Lotus Elise 111S.

Continuous Variable Valve Lift (CVVL)

The earliest variable valve lift systems like Honda VTEC vary valve lift by switching between slow and fast cams at a threshold point. Such discrete mechanism not only creates a step in the power curve (which is perceived as “unrefined”) but its breathing is also a compromise. An ideal variable valve lift (VVL) system should be capable of varying valve lift continuously according to rev, i.e., the higher the rpm, the higher lift is required. Compare with a fixed valve lift compromised for mid-range rev, VVL enhances power at high rev by supplying the engine more air to breath. At low rpm, its reduced valve lift speeds up the air flow, improving air / fuel mixture thus translate to better fuel economy and cleaner emission. Moreover, car makers can make use of CVVL to regulate engine output, thus eliminate the need of throttle butterfly and reduce so called “pumping loss” (see more info in our Green Technology section).

  Example: BMW Valvetronic

Debuted in BMW 316ti Compact in 2001, Valvetronic was the first continuous variable valve lift mechanism made into production. Instead of enhancing power, the goal of Valvetronic was to reduce fuel consumption. According to the position of throttle pedal, it regulates engine output by varying the depth of valve lift. This mean conventional throttle butterfly can be disabled thus reduces pumping loss. Overall, BMW achieved 10% reduction in fuel consumption with Valvetronic.

Compare with a conventional engine, Valvetronic adds an electric motor, an eccentric shaft and at each intake valve an intermediate rocker arm. The intake camshaft acts on the intermediate rocker arms through roller bearings. When the driver calls for more power, the electric motor turns the eccentric shaft, which pushes the intermediate rocker arms and in turn pushes the valve to open deeper. You can understand its theory easily by reading the illustrations below.

 Although Valvetronic is effective to reduce fuel consumption at part-load, it does not benefit top end power at all, because its additional components result in additional friction and inertia, thus limit the engine’s revvability. This is why BMW has never applied Valvetronic to its high-performance M-power engines. Another disadvantage is its size, which occupies a lot of space above the cylinder head.

Advantage	Reduce fuel consumption
Disadvantage	Large size, additional friction and inertia thus not suitable to high-revving engines
Who use it ?	BMW inline-4, inline-6, V8 and V12

Example: Nissan VVEL

Nissan introduced its Variable Valve Event and Lift (VVEL) in 2007 as the world's second CVVL system. The first application was on the VQ37VHR V6 engine of Skyline Coupe (Infiniti G37). Compare with BMW's Valvetronic, Nissan's system is more compact, involve less parts and less energy loss, therefore it is suitable to high-performance engines.

Though saying VVEL employs less parts, it is still a complicated design and not easy to understand. The above diagrams show its internal construction, which doesn't look like conventional valve gears at all. The VVEL does not use conventional intake camshaft. Each valve is actuated by a cam which is pivoted on - but not fixed to - the camshaft. While conventional cams rotate about the camshaft, the cam in VVEL swings up and down reciprocatingly, this is why it does not need a symmetric profile. Its movement is driven by the camshaft via a series of components, i.e. eccentric cam (which is fixed at the camshaft), link A, rocker arm and link B. Isn't it very complicated ? The following animation will help you understand how it operate:

 High lift

Low lift

How does VVEL vary valve lift ? This is implemented by the eccentric control shaft inside the rocker arm. By rotating the eccentric control shaft, the position of rocker arm is shifted, changing the geometry of Link A and B, then the swing angle of cam. The swing angle of cam determines the degree of valve lift, as you can see from the above diagrams.

Nissan said VVEL saves 10% fuel at light load due to the reduced role of throttle butterfly (it does not eliminate throttle completely), but it did not specify how much gain in horsepower. The VQ37VHR produces 8 percent more horsepower than its predecessor, the non-VVEL VQ35HR. Taking its increased displacement and compression ratio into account, VVEL seems to contribute little to top end power. This is because its benefit in breathing efficiency is largely cancelled out by the additional friction of VVEL components. However, the VQ37VHR engine can rev up to 7500 rpm, proving that VVEL does not compromise top end performance like BMW Valvetronic.

Advantage	Enhanced power at high rev. Save fuel by eliminating throttle butterfly.
Disadvantage	Mechanism still complicated, bulky and expensive.
Who use it ?	Nissan VQ37VHR V6

  Example: Toyota Valvematic

Toyota joined the CVVL club in 2008 with its Valvematic technology. Compare with BMW Valvetronic and Nissan VVEL, Valvematic seems better in many aspects: its construction is relatively simple; It is compact and does not increase the height of cylinder head; Most importantly, it adds little inertia and friction, thus does not compromise top end power. Toyota claims it improves 10% in power output while reduces 5-10% fuel consumption in regular driving.

Valvematic employs an intermediate shaft (blue part in top left picture) to achieve continuous variable valve lift. The intermediate shaft has an actuating member for each cylinder. Each actuating member is made of two finger followers laminating a roller bearing member (top right picture). The finger followers can rotate in relation to the roller member by means of internal gear threads and an electric motor attached to the end of the intermediate shaft. Note that the gear threads of roller member and finger followers are in opposite direction. This mean when the shaft swivels, the roller member and finger followers will move in opposite direction, moving either apart or closer together. In this way, the axle angle between them can be varied infinitely by the electric motor.

Now see the picture below. The intake valve is actuated by camshaft via intermediate shaft. More precisely speaking, the camshaft acts on the roller member of intermediate shaft, transferring the movement to both finger followers, then towards the roller rocker arms and eventually to the intake valves.

As you can see from the picture above, when the finger follower is set at narrow angle in relation to the roller member, it results in low valve lift. When the angle of finger follower is increased (picture below), the valve lift is also increased. In this way, Valvematic can vary valve lift by adjusting the angle of finger followers. In the first 2.0-liter Valvematic engine, lift can vary from 0.97mm to 11mm. The former saves the need of throttle butterfly thus reduce fuel consumption in part load. The high lift enables stronger top end power. Take the 2.0-liter Valvematic engine as example again, it produces a maximum 158 horsepower, up from 143 hp of the regular dual-VVT-i version.

Advantage Increased power, reduced fuel consumption, compact size. Disadvantage - Who use it ? Toyota 1.6 / 1.8 / 2.0 Valvematic engines

Electrohydraulic valve control

  Example: FIAT Multiair
Some variable valve control systems, such as BMW Valvetronic, Nissan VVEL and Toyota Valvematic, are capable to vary valve lift infinitely according to needs. In addition to continuous variable cam phasing, they seemed to be very capable already. However, these systems are still far from perfect. An ideal valve control system should allow unlimited flexibility of valve strategy - in other words, any valve lift at any time - to achieve the desired combustion effect. No mechanical systems can ever achieve that function. Therefore some consultants and suppliers are working on fully electromagnetic valvegears. However, until now such technology still faces technical challenges difficult to overcome, such as size / weight, costs, reliability and energy consumption. Instead of that, the powertrain engineers at FIAT group developed a more practical, but still very flexible enough, type of variable valve control mechanism. They call it "Multiair". The first application is to be introduced to the 1.4 FIRE engine in 2009, then follow by a new 900cc twin-cylinder engine.

Unquestionably, Multiair is the most flexible kind of VVT system until now. From the graphs below you can see it enables at least 5 different types of intake valve strategies to suit different running conditions. Apart from the usual transition between long duration and short duration, high lift and low lift, it also allows late valve opening, early valve closing and, wow, multiple valve lifts during an intake stroke !

Now let us see its mechanism. The Multiair system initially works with SOHC 4-valve construction because its additional electrohydraulic components occupy the space originally left for intake camshaft. While the single camshaft operates the exhaust valves directly in conventional way, it operates intake valves via a series of components: roller rockers ----> hydraulic pistons ---> hydraulic chambers (which incorporate electronic-controlled solenoid valves) ---> hydraulic valve actuators. This mean the actuation is implemented by a combination of mechanical and hydraulic means.

Normally, when the solenoid valve is de-energised and closed, the oil cannot enter the hydraulic chamber, thus it flows directly from hydraulic piston to valve actuator. You can see this hydraulic link as a solid body because the oil has no where to escape. Therefore the intake valve movement follows exactly the intake cam lobe profile. As the intake cam profile is designed to favour high power (i.e. high lift and long opening duration), this strategy is suitable for high rev running. (Fig 1)



When the solenoid valve is energized, it opens and allows the oil to flow into the hydraulic chamber. As a result, no oil will flow to the valve actuator, thus the intake valve will close under the force of its rebound spring. In this way, Multiair can shut down the intake valves at any desired instant. (Fig 3 and Fig 4)

Suppose the intake valve has closed for a while, then the solenoid valve of hydraulic chamber is closed again. What will happen ? In this case, oil will flow directly to valve actuator again, thus the intake valve will follow the cam profile and open again. However, as some time and oil volume has already "lost" (at the hydraulic chamber) during the solenoid valve opening, the valve lift will be reduced. The degree of reduction depends on the instant of solenoid valve closure. The later the solenoid valve close, the lower valve lift will be obtained. In this way, Multiair can vary the lift and opening duration of intake valves. (Fig 2)

Now let us see the above valve lift graphs again:

Fig 1 is suitable for high rpm running.

Fig 2 is sutiable for low-load operation. Its late valve opening leads to a partial vacuum in the combustion chamber. In addition to the low valve lift, the intake air stream is greatly speeded up, generating turbulence thus improve air and fuel mixture. This benefits fuel economy and emission.

Fig 3 is suitable for a wide range of part-load operation. Depending on the requirement of power, the amount of air can be controlled by the early closing of intake valves. This eliminates the need of throttle butterfly (like BMW Valvetronic) and reduce pumping loss by up to 10%.

Fig 4 is designed for enhanced low-rpm acceleration. While it enables more intake air volume compare with Fig 2 & 3, its early valve closure ensures no air flow back into the intake manifolds near the end of the intake stroke. (Remark: the combination of fast cam timing and low rpm operation could lead to backflow, that's why Multiair needs to close the valves earlier. Other engines do not have this issue because they either use variable cam phasing or compromised cam timing)

Fig 5 is so-called "Multilift" mode and designed for very low rpm operation. It combines the strategy of Fig 2 & 3 and their benefits - regulated consumption and improved quality of air-fuel mixture.

Combining these modes, FIAT claims Multiair improves maximum power by 10%, low-rpm torque by 15% and fuel economy by 10%. Moreover, cold-start emission of HC/CO and NOx are reduced by 40% and 60% respectively due to its ability of exhaust gas recirculation. This technology is also compatitble with diesel engines, which means substantial cost reduction.

However, I can see a few weaknesses of Multiair: Firstly, at the moment it is compatible with SOHC engines only because of the bulky mechanism. This mean while it enables variable timing and lift for intake valves, it offers neither for exhaust valves. The addition of variable exhaust cam phasing may require a complex cam-in-cam mechanism like that used by Dodge Viper 8.4. Secondly, the SOHC design and the complicated electrohydraulic mechanism could generate extra friction, thus it is not suitable to high-revving high performance engines, which is a common problem shared with BMW Valvetronic. It is more suitable to mass production engines and low-revving turbocharged engines. Lastly, the electrohydraulic mechanism might complicate servicing and raise reliability issues.

Advantage Very flexible - ability to vary valve lift, early intake opening / closing and even multiple openings during an intake stroke, hence enhanced output, reduced consumption and emission. Disadvantage Mechanism quite bulky such that difficult to apply to DOHC engines; Not perfectly compatible with variable cam phasing. Who use it ? FIAT group 1.4 FIRE engine, 900cc SGE twin-cylinder engine...

Copyright© 1997-2011 by Mark Wan @ AutoZine
 

___________________________________________________________________________________

ENTRADAS RELACIONADAS

Variable Valve Timing (VVT) Part 1

___________________________________________________________________________________

FUENTE:

http://www.autozine.org/technical_school/engine/vvt_1.htm

Variable Valve Timing (VVT) Parte 1

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 Lift
In 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 VVT
Honda 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 VVL

Very 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 VVT
Cam-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.

Note that cam-phasing VVT cannot vary the duration of valve opening. It just allows earlier or later valve opening. Earlier opening results in earlier closing, of course. It also cannot vary valve lift, unlike cam-changing VVT. However, cam-phasing VVT is the simplest and cheapest form of VVT because each camshaft needs only one hydraulic phasing actuator, unlike other systems that employ individual mechanism for every cylinder.

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.

From this picture, it is easy to understand its operation. The end of intake camshaft incorporates a gear thread. The thread is coupled by a cap which can move towards and away from the camshaft. Because the gear thread is not in parallel to the axis of camshaft, phase angle will shift forward if the cap is pushed towards the camshaft. Similarly, pulling the cap away from the camshaft results in shifting the phase angle backward.

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

Toyota's VVT-i (Variable Valve Timing - Intelligent) has been expanding to more and more Toyota models, from the tiny Yaris (Vitz) to the Supra. Its mechanism is more or less the same as BMW Vanos. It is also a continuously variable design.

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 VVT
Combining 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

Toyota’s VVTL-i is the most sophisticated VVT design yet. Its powerful functions include:

- 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

Variocam of the 911 Carrera uses timing chain for cam phasing.

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-VTEC
If 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 Valvelift

Audi'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 Camtronic

Mercedes 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 iVLC

General 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

Copyright© 1997-2011 by Mark Wan @ AutoZine
 

___________________________________________________________________________________

ENTRADAS RELACIONADAS

Variable Valve Timing (VVT) Part 2

___________________________________________________________________________________

FUENTE:

http://www.autozine.org/technical_school/engine/vvt_1.htm