When does vvt kick in?

In internal combustion engines, variable valve timing (VVT) is the process of altering the timing of a valve lift event, and is often used to improve performance, fuel economy or emissions. It is increasingly being used in combination with variable valve lift systems.

Many of the vehicles, some even on the Indian road come with the engine running VVT, VVL or both the techs. But what exactly are VVT and VVL? How does it help? and How does it work? So, without further wasting any time, let’s get going.

But without going into the detail of VVT we first need to look at what is valve timing.

To understand valve timing, let us take you back to “what are camshafts?” here. In simple terms, camshafts are responsible for the opening and closing of the valves.

Not going into the detail, here is a brief about valve timing. But before doing so, here are some technical terms used in the automotive world. TDC (Top-Dead Centre) when the piston is at its topmost position. BDC (Bottom-Dead Centre) is when the piston is at its lowest position.

Now the question comes why isn’t the intake and exhaust valve opening and closing in sync with the pistons TDC and BDC? And why at TDC both the valves are open for some duration? The reason likes in the three crucial factors called Blow Down, Overlap, and Ram Effect.

Both the valves remain shut to perform the combustion process efficiently during the compression stroke all the way up to the power or expansion stroke. ‘Blow-down’ is the process where the exhaust valve opens before the piston reaches the BDC. This releases the excess pressure from the combustion chamber. This also confirms that there is no other pressure exerting on the piston during its motion to BDC. If it the exhaust valve were to remain close till the BDC, some engine power had to be wasted in order to assist the piston to move from BDC to the TDC.

In the diagram, it is very prominent that when the piston reaches TDC in the exhaust stroke, both the valves, intake and exhaust, are open. This is no way a manufacturing fault and the opening of the intake valve slightly before TDC and closing of the exhaust valve later after TDC is deliberately done. This is to assist in pulling the fresh charge from the intake manifold into the combustion cylinder just like the Siphon Effect. Not doing so can cause some burnt exhaust gasses to remain inside the combustion chamber and dilute the air-fuel mixture

Ram Effect

This is the situation where the intake valve closes at a few degrees after the BDC. Like the others, this is also intentionally done to let in more air into the combustion chamber. How is this possible? You may ask. This is a physical phenomenon wherein a large amount of air entering the cylinder rapidly cannot stop itself. In simple terms, the air is rammed inside the combustion chamber. This is why high revving engines tend to keep the intake valve open for a longer duration to let the air in. But this is not so prominent at low speeds and the piston will push some of the air out of the cylinder.

Interesting Fact: Well, the ram effect is the most important variable to keep in mind as this will definitely have a huge impact on the engine’s performance, definitely more than the other two.

So, with the basics of valve timing clear, let’s move to the variable valve timing. In order to understand the effects of VVT. So, let’s have a look at how the valve timing behaves without this tech.

Majority of the IC engine on the road have a fixed valve timing. The engine running on a constant valve timing has to be intelligently tuned. What does that mean? The motor has to be designed for a specific function. Of course, one has to compromise on the other aspects.

By now you might have an idea what the variable valve timing would be. If not, let’s explain.

As said before, VVT changes the valve timing when the engine is running. But how does the engine do the same? Long story short, VVT optimized the timing depending upon the RPMs, to get the most out of the engine for the desired RPM.

It’s been a while and the technology has advanced with leaps and bounds. With that, the VVT tech too reaches new heights.

Different companies use different mechanisms hence different names. Here are some!

In internal combustion engines, variable valve timing (VVT) is the process of altering the timing of a valve lift event, and is often used to improve performance, fuel economy or emissions. It is increasingly being used in combination with variable valve lift systems. There are many ways in which this can be achieved, ranging from mechanical devices to electro-hydraulic and camless systems. Increasingly strict emissions regulations are causing many automotive manufacturers to use VVT systems.

Two-stroke engines use a power valve system to get similar results to VVT.

The valves within an internal combustion engine are used to control the flow of the intake and exhaust gases into and out of the combustion chamber. The timing, duration and lift of these valve events has a significant impact on engine performance. Without variable valve timing or variable valve lift, the valve timing is the same for all engine speeds and conditions, therefore compromises are necessary. An engine equipped with a variable valve timing actuation system is freed from this constraint, allowing performance to be improved over the engine operating range.

Piston engines normally use valves which are driven by camshafts. The cams open (lift) the valves for a certain amount of time (duration) during each intake and exhaust cycle. The timing of the valve opening and closing, relative to the position of the crankshaft, is important. The camshaft is driven by the crankshaft through timing belts, gears or chains.

An engine requires large amounts of air when operating at high speeds. However, the intake valves may close before enough air has entered each combustion chamber, reducing performance. On the other hand, if the camshaft keeps the valves open for longer periods of time, as with a racing cam, problems start to occur at the lower engine speeds. Opening the intake valve while the exhaust valve is still open may cause unburnt fuel to exit the engine, leading to lower engine performance and increased emissions. According to engineer David Vizard's book "Building Horsepower", when both intake & exhaust are open simultaneously, the much-higher-pressure exhaust pushes the intake-charge back, out from the cylinder, polluting the intake-manifold with exhaust, in worst cases.

Early variable valve timing systems used discrete (stepped) adjustment. For example, one timing would be used below 3500 rpm and another used above 3500 rpm.

More advanced "continuous variable valve timing" systems offer continuous (infinite) adjustment of the valve timing. Therefore, the timing can be optimized to suit all engine speeds and conditions.

The simplest form of VVT is cam-phasing, whereby the phase angle of the camshaft is rotated forwards or backwards relative to the crankshaft. Thus the valves open and close earlier or later; however, the camshaft lift and duration cannot be altered solely with a cam-phasing system.

Achieving variable duration on a VVT system requires a complex system, such as multiple cam profiles or oscillating cams.

Late intake valve closing (LIVC) The first variation of continuous variable valve timing involves holding the intake valve open slightly longer than a traditional engine. This results in the piston actually pushing air out of the cylinder and back into the intake manifold during the compression stroke. The air which is expelled fills the manifold with higher pressure, and on subsequent intake strokes the air which is taken in is at a higher pressure. Late intake valve closing has been shown to reduce pumping losses by 40% during partial load conditions, and to decrease nitric oxide (NOx) emissions by 24%. Peak engine torque showed only a 1% decline, and hydrocarbon emissions were unchanged.

Early intake valve closing (EIVC) Another way to decrease the pumping losses associated with low engine speed, high vacuum conditions is by closing the intake valve earlier than normal. This involves closing the intake valve midway through the intake stroke. Air/fuel demands are so low at low-load conditions and the work required to fill the cylinder is relatively high, so Early intake valve closing greatly reduces pumping losses. Studies have shown early intake valve closing reduces pumping losses by 40%, and increases fuel economy by 7%. It also reduced nitric oxide emissions by 24% at partial load conditions. A possible downside to early intake valve closing is that it significantly lowers the temperature of the combustion chamber, which can increase hydrocarbon emissions.

Early intake valve opening Early intake valve opening is another variation that has significant potential to reduce emissions. In a traditional engine, a process called valve overlap is used to aid in controlling the cylinder temperature. By opening the intake valve early, some of the inert/combusted exhaust gas will back flow out of the cylinder via the intake valve, where it cools momentarily in the intake manifold. This inert gas then fills the cylinder in the subsequent intake stroke, which aids in controlling the temperature of the cylinder and nitric oxide emissions. It also improves volumetric efficiency, because there is less exhaust gas to be expelled on the exhaust stroke.

Early/late exhaust valve closing Early and late exhaust valve closing timing can be manipulated to reduce emissions. Traditionally, the exhaust valve opens, and exhaust gas is pushed out of the cylinder and into the exhaust manifold by the piston as it travels upward. By manipulating the timing of the exhaust valve, engineers can control how much exhaust gas is left in the cylinder. By holding the exhaust valve open slightly longer, the cylinder is emptied more and ready to be filled with a bigger air/fuel charge on the intake stroke. By closing the valve slightly early, more exhaust gas remains in the cylinder which increases fuel efficiency. This allows for more efficient operation under all conditions.

The main factor preventing this technology from wide use in production automobiles is the ability to produce a cost-effective means of controlling the valve timing under the conditions internal to an engine. An engine operating at 3000 revolutions per minute will rotate the camshaft 25 times per second, so the valve timing events have to occur at precise times to offer performance benefits. Electromagnetic and pneumatic camless valve actuators offer the greatest control of precise valve timing, but, in 2016, are not cost-effective for production vehicles.

The history of the search for a method of variable valve opening duration goes back to the age of steam engines when the valve opening duration was referred to as "steam cut-off”. The Stephenson valve gear, as used on early steam locomotives, supported variable cutoff, that is, changes to the time at which the admission of steam to the cylinders is cut off during the power stroke.

Early approaches to variable cutoff coupled variations in admission cutoff with variations in exhaust cutoff. Admission and exhaust cutoff were decoupled with the development of the Corliss valve. These were widely used in constant speed variable load stationary engines, with admission cutoff, and therefore torque, mechanically controlled by a centrifugal governor and trip valves.

As poppet valves came into use, a simplified valve gear using a camshaft came into use. With such engines, variable cutoff could be achieved with variable profile cams that were shifted along the camshaft by the governor. The Serpollet steamcars produced very hot high pressure steam, requiring poppet valves, and these used a patented sliding camshaft mechanism, which not only varied the inlet valve cut-off but allowed the engine to be reversed.

An early experimental 200 hp Clerget V-8 from the 1910s used a sliding camshaft to change the valve timing. Some versions of the Bristol Jupiter radial engine of the early 1920s incorporated variable valve timing gear, mainly to vary the inlet valve timing in connection with higher compression ratios. The Lycoming R-7755 engine had a Variable Valve Timing system consisting of two cams that can be selected by the pilot. One for take off, pursuit and escape, the other for economical cruising.

The desirability of being able to vary the valve opening duration to match an engine's rotational speed first became apparent in the 1920s when maximum allowable RPM limits were generally starting to rise. Until about this time an engine's idle RPM and its operating RPM were very similar, meaning that there was little need for variable valve duration. The first use of variable valve timing was on the 1903 Cadillac Runabout and Tonneau created by Alanson Partridge Brush Patent 767,794 “INLET VALVE GEAR FOR INTERNAL COMBUSTION ENGINES” filed August 3, 1903, and granted August 16, 1904. Some time prior to 1919 Lawrence Pomeroy, Vauxhall's Chief Designer, had designed a 4.4 L engine for a proposed replacement for the existing 30-98 model to be called the H-Type. In this engine the single overhead camshaft was to move longitudinally to allow different camshaft lobes to be engaged. It was in the 1920s that the first patents for variable duration valve opening started appearing – for example United States patent U.S. Patent 1,527,456.

In 1958 Porsche made application for a German Patent, also applied for and published as British Patent GB861369 in 1959. The Porsche patent used an oscillating cam to increase the valve lift and duration. The desmodromic cam driven via a push/pull rod from an eccentric shaft or swashplate. It is unknown if any working prototype was ever made.

Fiat was the first auto manufacturer to patent a functional automotive variable valve timing system which included variable lift. Developed by Giovanni Torazza in the late 1960s, the system used hydraulic pressure to vary the fulcrum of the cam followers (US Patent 3,641,988). The hydraulic pressure changed according to engine speed and intake pressure. The typical opening variation was 37%.

Alfa Romeo was the first manufacturer to use a variable valve timing system in production cars (US Patent 4,231,330). The fuel injected models of the 1980 Alfa Romeo Spider 2000 had a mechanical VVT system. The system was engineered by Ing Giampaolo Garcea in the 1970s. All Alfa Romeo Spider models from 1983 onward used electronic VVT.

In 1989, Honda released the VTEC system. While the earlier Nissan NVCS alters the phasing of the camshaft, VTEC switches to a separate cam profile at high engine speeds to improve peak power. The first VTEC engine Honda produced was the B16A which was installed in the Integra, CRX, and Civic hatchback available in Japan and Europe.

In 1992, Porsche first introduced VarioCam, which was the first system to provide continuous adjustment (all previous systems used discrete adjustment). The system was released in the Porsche 968 and operated on the intake valves only.

Variable valve timing has been applied to motorcycle engines but was considered a non-useful "technological showpiece" as late as 2004 due to the system's weight penalty. Since then, motorcycles including VVT have included the Kawasaki 1400GTR/Concours 14 (2007), the Ducati Multistrada 1200 (2015), the BMW R1250GS (2019) and the Yamaha YZF-R15 V3.0 (2017), the Suzuki GSX-R1000R 2017 L7.

Variable valve timing has begun to trickle down to marine engines. Volvo Penta's VVT marine engine uses a cam phaser, controlled by the ECM, which continuously varies advancement or retardation of the camshaft timing.

In 2007, Caterpillar developed the C13 and C15 Acert engines which used VVT technology to reduce NOx emissions, to avoid the use of EGR after 2002 EPA requirements.

In 2010, Mitsubishi developed and started mass production of its 4N13 1.8 L DOHC I4, the world's first passenger car diesel engine that features a variable valve timing system.

Manufacturers use many different names to describe their implementation of the various types of variable valve timing systems. These names include:

This method uses two cam profiles, with an actuator to swap between the profiles (usually at a specific engine speed). Cam switching can also provide variable valve lift and variable duration, however the adjustment is discrete rather than continuous.

The first production use of this system was Honda's VTEC system. VTEC changes hydraulic pressure to actuate a pin that locks the high lift, high duration rocker arm to an adjacent low lift, low duration rocker arm(s).

Many production VVT systems are the cam phasing type, using a device known as a variator. This allows continuous adjustment of the cam timing (although many early systems only used discrete adjustment), however the duration and lift cannot be adjusted.

These designs use an oscillating or rocking motion in a part cam lobe, which acts on a follower. This follower then opens and closes the valve. Some oscillating cam systems use a conventional cam lobe, while others use an eccentric cam lobe and a connecting rod. The principle is similar to steam engines, where the amount of steam entering the cylinder was regulated by the steam "cut-off" point.

The advantage of this design is that adjustment of lift and duration is continuous. However, in these systems, lift is proportional to duration, so lift and duration cannot be separately adjusted.

The BMW (valvetronic), Nissan (VVEL), and Toyota (valvematic) oscillating cam systems act on the intake valves only.

Eccentric cam drive systems operates through an eccentric disc mechanism which slows and speeds up the angular speed of the cam lobe during its rotation. Arranging the lobe to slow during its open period is equivalent to lengthening its duration.

The advantage of this system is that duration can be varied independent of lift (however this system does not vary lift). The drawback is two eccentric drives and controllers are needed for each cylinder (one for the intake valves and one for the exhaust valves), which increases complexity and cost.

MG Rover is the only manufacturer that has released engines using this system.

This system consists of a cam lobe that varies along its length (similar to a cone shape). One end of the cam lobe has a short duration/reduced lift profile, and the other end has a longer duration/greater lift profile. In between, the lobe provides a smooth transition between these two profiles. By shifting area of the cam lobe which is in contact with the follower, the lift and duration can be continuously altered. This is achieved by moving the camshaft axially (sliding it across the engine) so a stationary follower is exposed to a varying lobe profile to produce different amounts of lift and duration. The downside to this arrangement is that the cam and follower profiles must be carefully designed to minimise contact stress (due to the varying profile).

Ferrari is commonly associated with this system, however it is unknown whether any production models to date have used this system.

This system is not known to be used in any production engines.

It consists of two (closely spaced) parallel camshafts, with a pivoting follower that spans both camshafts and is acted on by two lobes simultaneously. Each camshaft has a phasing mechanism which allows its angular position relative to the engine's crankshaft to be adjusted. One lobe controls the opening of a valve and the other controls the closing of the same valve, therefore variable duration is achieved through the spacing of these two events.

The drawbacks to this design include:

This system is not known to be used in any production engines.

The operating principle is that the one follower spans the pair of closely spaced lobes. Up to the angular limit of the nose radius the follower 'sees' the combined surface of the two lobes as a continuous, smooth surface. When the lobes are exactly aligned the duration is at a minimum (and equal to that of each lobe alone) and when at the extreme extent of their misalignment the duration is at a maximum. The basic limitation of the scheme is that only a duration variation equal to that of the lobe nose true radius (in camshaft degrees or double this value in crankshaft degrees) is possible. In practice this type of variable cam has a maximum range of duration variation of about forty crankshaft degrees.

This is the principle behind what seems to be the very first variable cam suggestion appearing in the USPTO patent files in 1925 (1527456). The "Clemson camshaft" is of this type.

Also known as "combined two shaft coaxial combined profile with helical movement", this system is not known to be used in any production engines.

It has a similar principle to the previous type, and can use the same base duration lobe profile. However instead of rotation in a single plane, the adjustment is both axial and rotational giving a helical or three-dimensional aspect to its movement. This movement overcomes the restricted duration range in the previous type. The duration range is theoretically unlimited but typically would be of the order of one hundred crankshaft degrees, which is sufficient to cover most situations.

The cam is reportedly difficult and expensive to produce, requiring very accurate helical machining and careful assembly.

Engine designs which do not rely on a camshaft to operate the valves have greater flexibility in achieving variable valve timing and variable valve lift. The only production car that uses the camless design so far is the Koenigsegg Gemera.

This system utilizes the engine lube oil to control the closure of inlet valve. The intake valve opening mechanism incorporates a valve tappet and a piston inside a chamber. There is a solenoid valve controlled by the engine control system which gets energized and supplies oil through a non-return valve during the time of cam lift and the oil gets filled in the chamber and the return channel to the sump is blocked by the valve tappet. During the downward movement of the cam, at a particular instant, the return passage opens and the oil pressure gets released to the engine sump.

Basic Theory

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 controlled by the shape and phase angle of cams. To optimise the breathing, engine requires different valve timing at different speed. When the rev increases, the duration of intake and exhaust stroke decreases so that fresh air becomes not fast enough to enter the combustion chamber, while the exhaust 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 optimise for mid-rev so that both the 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 speed.

With Variable Valve Timing, power and torque can be optimised across a wide rpm band. The most noticeable results are:

Moreover, all these benefits come without any drawback.

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 decrease output or even leads to misfire. Therefore the lift should be variable according to engine speed.

Honda pioneered road car-used VVT in the late 80s by launching its famous VTEC system (Valve Timing Electronic Control). First appeared in Civic, CRX and NS-X, then became standard in most models.

You can see it 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, 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 increase top end power by as much as 30 hp 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 gear change is required. As low-speed torque gains too little (remember, the cams of a normal engine usually serves across 0-6,000 rpm, while the "slow cams" of VTEC engine still need to serve across 0-4,500 rpm), drivability won't 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 for some models. Of course, the more stage it has, the more refined it becomes. It still offers less broad spread of torque as 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.

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 profile. Note that their dimensions are also different - the middle cam (fast timing, high lift), as shown in the above diagram, 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 operate like this :

Stage 1 ( low speed ) : the 3 pieces of rocker arms moves 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 is relatively slow compare with the middle cam, which actuates no valve now.

Stage 2 ( medium speed ) : hydraulic pressure (painted orange in the picture) connects 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 but medium lift.

Stage 3 ( high speed ) : hydraulic pressure connects 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.

Very similar to Honda's system, but the right and left cams are with the same profile. 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 connected 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 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 speed, 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 open results in earlier close, of course. It also cannot vary the 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 has just 2 or 3 fixed shift angle settings to choose from, such as either 0° or 30°. Better system has continuous variable shifting, say, any arbitary value between 0° and 30°, depends on rpm. Obviously this provide the most suitable valve timing at any speed, thus greatly enhance engine flexiblility. Moreover, the transition is so smooth that hardly noticeable.

Intake and Exhaust

Some design, such as BMW's Double Vanos system, has cam-phasing VVT at both intake and exhaust camshafts, this enable 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 shift the intake camshaft within a maximum range of 40° .The exhaust camshaft is 25°.

From the picture, it is easy to understand its operation. The end of 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 which 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 shift the phase angle forward.

Continuous variation in timing is easily implemented by positioning the cap at a suitable distance according to engine speed.

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

However, the word "Integillent" emphasis the clever control program. Not only varies timing according to engine speed, it also consider other conditions such as acceleration, going up hill or down hill.

Combining cam-changing VVT and cam-phasing VVT could satisfy the requirement of both top-end power and flexibility throughout the whole rev range, but 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.

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

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 speed, acceleration, going up hill or down hill etc. taking 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 (or exhaust 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 speed has increased to the threshold point, the sliding pin is pushed by hydraulic pressure to fill the spacing. 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 high speed power output a lot. Compare with Honda VTEC and similar designs for Mitsubishi and Nissan, Toyota’s system has continuously variable valve timing which helps it to achieve far better low to medium speed flexibility. Therefore it is undoubtedly the best VVT today. However, it is also more complex and probably more expensive to build.

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 Boxster also use the same system. This design is unique and patented, but it is actually inferior to the hydraulic actuator favoured by other car makers, especially it doesn’t allow as much variation to phase angle.

Therefore, the Variocam Plus used in the new 911 Turbo finally follow uses the popular hydraulic actuator instead of chain. One well-known Porsche expert described the variable valve timing as continuous, but it seems conflicting with the official statement made earlier, which revealed the system has 2-stage valve timing.

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.

Nevertheless, at the moment the Variocam Plus is just offered for the intake valves.

Rover introduced its own system calls VVC (Variable Valve Control) in MGF in 1995. Many experts regard it as the best VVT considering its all-round ability - unlike cam-changing VVT, it provides continuously variable timing, thus improve low to medium rev torque delivery; and unlike cam-phasing VVT, it can lengthen the duration of valves opening (and continuously), thus boost power.

Basically, VVC employs an eccentric rotating disc to drive the inlet valves of every two cylinder. Since eccentric shape creates non-linear rotation, valves opening period can be varied. Still don't understand ? well, any clever mechanism must be difficult to understand. Otherwise, Rover won't be the only car maker using it.

VVC has one draw back: since every individual mechanism serves 2 adjacent cylinders, a V6 engine needs 4 such mechanisms, and that's not cheap. V8 also needs 4 such mechanism. V12 is impossible to be fitted, since there is insufficient space to fit the eccentric disc and drive gears between cylinders.

EGR (Exhaust gas recirculation) is a commonly adopted technique to reduce emission and improve fuel efficiency. However, it is VVT that really exploit 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 speed. However, when the car is running at medium speed in highway, in other words, the engine is running at light load, maximum overlapping may be useful as a mean 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 runs properly at the leaner fuel / air mixture without failing to combust.

There may be other actions that occur during this process. The system might rotate the camshaft either forward or by slowing it. Making adjustments in the overlap time between when the intake valve opens and the exhaust valve closes can help improve your engine efficiency. The whole system seems complex at first, but it’s not really that difficult to understand.

First, let’s take a look at this graphic from Car and Driver that shows what a VVT engine looks like.

Variable Valve Timing, Car and Driver, 2021

Variable valve timing is responsible for controlling three elements of the intake and exhaust valves. It’s responsible for valve timing, or when the valves open and close based on the points in the piston’s movement. Then, there’s the duration, which refers to how long the valves remain open (or closed). Finally, VVT also affects the valve lift, which is how far the valves actually open.

In order to monitor all of this, various sensors are responsible for feeding information to the ECU, or the computers onboard. In addition to sensors, there are physical mechanisms that help control the characteristics and behavior of the valves. The engine needs to monitor timing in order to handle all the variables of combustion, and without a conventional time scale, you can see just how quickly things actually move when you fire up the engine.

When you add in modern electronics, controls to optimize valve events, and premium fuel injection, it all comes down to precision that can’t be more than a millisecond off or it could ruin the entire engine’s operation.

The system is comprised of gears that are placed one inside the other, an inner gear that connects to the camshaft, and an outer gear that connects to the chain or drive belt. When normal driving conditions are engaged, the cogs are meshed together and turn at the same rate. However, oil pressure makes it easy to separate the gears, allowing them to change their relative speeds and help change the speed of the camshaft.

When you adjust the valve gears and controls, the adjustment will change the duration of valve lifts, which are responsible for controlling the intake and exhaust. Below, you’ll see the two main types of VVT engines and what each entails.

Essentially, the dual VVT system creates a solution that helps the engine breathe more efficiently. By adjusting the timing of the valves, you can generate more power, better fuel efficiency, and even reduced emissions. It also offers greater torque at low speeds without the risk of engine knocking. At high speeds, you’ll enjoy premium horsepower without the excess noise and vibrations that some older vehicles have.

Dual VVT also offers reduced emissions at the same time it’s providing you with excess power, so it gives you the best of both worlds. Even a single VVT, though, can be an economical upgrade to help save on fuel emissions and help the engine “breathe” more effectively.

The VVT has an economy profile (below 6,000 rpm) and a performance profile (over 6,000 rpm). When the VVT activates, oil pressure will be exerted on an actuator that presses the camshaft slightly, helping engage the “performance” setting. With dual or single timing, the same thing happens. In a dual system, the exhaust valves are activated in addition to the intake, which helps minimize stop/start pressures and ensures premium performance.

Adjusting the overlap sequences between your intake and exhaust valves also allows you to enjoy maximum scavenging of the intra-cylinder charge. High RPM and tremendous power combine to deliver impressive low-end torque that’s just as impressive as the performance that you’re getting.

Ignition timing is another part of this equation. This usually advances during the lighter load operations so that lean air-fuel mixtures don’t create knocks in the engine. This timing all happens in such short order that you can’t actually see it in action unless you’ve got the vehicle torn apart and know what you’re looking for. The spark occurs at a point between 0.002 seconds and 0.0002 seconds, which is 10 to 100 times faster than a single hummingbird wing flap.

When the engine is cold, timing can be delayed and late fuel injection is designed to be combined with earlier valve openings so that the catalytic converter can reach operating temperature more effectively.

Now that you have a better idea of how this feature works on your vehicle, let’s talk about the advantages that come from having this system in place:

This offers a much better solution than former methods, helping deliver premium power and efficiency all at once. Essentially, with a well-designed VVT system, you will get better fuel economy and lower emissions while still getting higher RMP and better power. Plus, when it’s all done properly, it can increase the lifespan of your engine compared to older models that used other methods for valve timing.

As with any of your vehicle’s systems, there are certain error codes and issues that are known to occur when it comes to the VVT system. The most common engine codes are:

Generally, these two error codes will let you know that something is wrong with your variable valve timing system and that it needs to be replaced. Aside from the valve timing being off, other problem areas include:

There are a number of things that can lead to problems with your VVT system. Dirty oil and sludge buildup can result in poor operation of the system. If left unchecked, it can eventually lead to cam failure. That’s why maintenance is such an important part of having this system.

If you do not keep up with oil changes, you could see anything from simple timing issues to a total system failure, depending on how long you let things go. A drop in oil pressure or a damaged sensor could also cause issues, as they prevent the system from operating at its peak performance.

The biggest thing to note with VVT is that there are virtually no drawbacks—you get all the perks without the concessions and you’re never going to have to compromise on efficiency to get the power that you need.

With variable valve timing, the engine no longer has an EGR valve. The Exhaust Gas Recirculation valve has become obsolete in vehicles with VVT, which controls the timing of the gasses and their release, including saving the inert gas for the next combustion cycle. It also helps by controlling the temperature of combustion and the production of those nitrous oxides in the first place.

A lack of regular oil changes can lead to a poorly lubricated system that doesn’t work properly. The VVT needs clean oil more than anything, so a lack of regular oil changes could quickly lead to serious damage if you’re not careful. You could cause issues with the VVT chain, the solenoid, or even the gear drive. Check your oil regularly, because low oil levels can also create problems with your variable valve timing system.

Newer systems also use continuous variable valve timing, or CVVT, which is digitally controlled by your ECU in the engine. This optimizes the valve timing at all speeds and for all engine conditions. There are different mechanisms and setups in every engine, but generally, CVVT occurs using solenoid valves and a variable timing camshaft, along with a flexible hydraulic connection to the sprocket.

An important note here: unless you’re a car enthusiast that works on your own vehicles and has extensive knowledge of engine operation, VVT issues are something that is generally best left to the professionals. Fixing these issues is a combination of tinkering with timing and checking the computer system for proper operation, and it can be tricky to diagnose and repair the issue if you’re not familiar with the systems.

If you do know how to handle your own engine timing, that’s great. However, be sure that you don’t get in over your head, and feel free to reach out to the pros if you want a little extra assistance. You don’t necessarily have to go to a dealership, but find a repair shop or service center that can help you with all of the timing issues on your engine.

While VVT systems are fairly similar from one vehicle to the next, they aren’t identical. Furthermore, some manufacturers are coming up with their own trademarked VVT systems, so you’ll need to take the time to research your specific make and model, as well as the timing system that is in place. The way that a system works in a BMW is going to be quite different than in a Toyota, in many cases.

VVT is used by most car makers, including the likes of:

Understanding your vehicle’s specific needs when it comes to the VVT system is a large part of vehicle ownership. Anytime you sense that something doesn’t feel right or that maybe performance is lacking, consider whether your VVT system is up to snuff. If you haven’t lately, get an oil change and have the engine oil pressure checked to make sure everything is operating as it should.

If you want to ensure that your VVT is properly cared for, make sure that you find an independent shop or mechanic that can help you with all of your needs. Consider whether the mechanics have experience with your type of VVT system or your make and model of vehicle, too, because each system is a little different.

Typically, however, today’s mechanics are well-versed in these systems and they know how to keep them functioning properly. If you’re in the market for something more than an oil change, you’ll be able to trust that they will take care of you, even when you can’t figure out what’s wrong on your own. VVT is a topic that’s often talked about in performance and racing circles, but it’s something that every driver can benefit from if they know what it is and how to use it.

It’s the 21st century and computers are changing everything we do. While all the new upgrades may make it seem like there’s some kind of “magic” happening when you run your engine, it’s not magic. It’s just technology, and it’s a technology that is helping people in several ways.

VVT has reached a compromise between power and performance and the risk of higher emissions, by doing its part to reduce the emissions that come out of the engine while also helping improve the engine’s operation. It’s a complex system, but once you understand how it works and what it does, you’ll have a much better appreciation for it and you’ll be sure to keep it in the best condition possible.

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