Variable Valve Timing (VVT)

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:
 

    • The engine can rev higher, thus raises peak power. For example, Nissan's 2-litre Neo VVL engine output 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 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.

1) Cam-Changing VVT

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.
 

Advantage:

Powerful at top end

Disadvantage:

2 or 3 stages only, non-continuous; no much improvement to torque; complex

Who use it ?

Honda VTEC, Mitsubishi MIVEC, Nissan Neo VVL.


 

Example - Honda's 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 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.


Another example - Nissan Neo VVL

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.

 

2) 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 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.

 

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, such as: 

Audi V8 - inlet, 2-stage discrete

BMW Double Vanos - inlet and exhaust, continuous

Ferrari 360 Modena - exhaust, 2-stage discrete

Fiat (Alfa) SUPER FIRE - inlet, 2-stage discrete

Ford Puma 1.7 Zetec SE - inlet, 2-stage discrete

Jaguar AJ-V6 and updated AJ-V8 - inlet, continuous

Lamborghini Diablo SV engine - inlet, 2-stage discrete

Porsche Variocam - inlet, 3-stage discrete

Renault 2.0-litre - inlet, 2-stage discrete

Toyota VVT-i - inlet, continuous

Volvo 4 / 5 / 6-cylinder modular engines - inlet, continuous


Example : BMW's Vanos

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.
 


Another Example : Toyota VVT-i

 

 
Macro illustration of the phasing actuator 

 

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 BMWs 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.

 

3) Cam-Changing + Cam-Phasing VVT

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.

 

 

 

 

 

 

 

 

Example: Toyota VTL-i

 

 

Toyotas 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 Hondas 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. Lets see the following illustration :

Like VTEC, Toyotas 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 VVCs 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, Toyotas 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.

 

Advantage:

Continuous VVT improves torque delivery across the whole rev range; Variable lift and duration lift high rev power.

Disadvantage:

More complex and expensive

Who use it ?

Toyota Celica GT-S

 


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.

 
Porsches 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 doesnt 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.

 

Advantage:

VVT improves torque delivery at low / medium speed; Variable lift and duration lift high rev power.

Disadvantage:

More complex and expensive

Who use it ?

Porsche 911 Turbo

 

4) Rover's unique VVC system

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.
 

 

 

 

Advantage:

Continuously variable timing and duration of opening achieve both drivability and high speed power.

Disadvantage:

Not ultimately as powerful as cam-changing VVT, because of the lack of variable lift; Expensive for V6 and V8; impossible for V12.

Who use it ?

Rover 1.8 VVC engine serving MGF, Caterham and Lotus Elise 111S.

 


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 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.