Different Types of VVT

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 2.0-litre - continous inlet
Audi 3.0 V6 - continous inlet, 2-stage exhaust
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
Ford Falcon XR6's VCT - inlet, 2-stage discrete
Jaguar AJ-V6 and updated AJ-V8 - inlet, continuous
Lamborghini Diablo V12 since SV - inlet, 2-stage discrete
Mazda MX-5's S-VT - continous inlet
Mercedes V6 and V8 - inlet, 2-stage ?
Nissan QR four-pot and V8 - continuous inlet
Nissan VQ V6 - inlet, continuous ?
Nissan VQ V6 since Skyline V35 - inlet, electromagnetic
Porsche Variocam - inlet, 3-stage discrete
PSA / Renault 3.0 V6 - inlet, 2-stage
Renault 2.0-litre - inlet, 2-stage discrete
Subaru AVCS - inlet, 2-stage ?
Toyota VVT-i - continuous, mostly inlet but some also exhaust
Volvo 4 / 5 / 6-cylinder modular engines - inlet, continuous
 Volkswagen VR6 - inlet, continuous ?
Volkswagen (Audi) W8 and W12 - continuous inlet, 2-stage exhaust

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 VVTL-i

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

o Continuous cam-phasing variable valve timing

o 2-stage variable valve lift plus valve-opening duration
o 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 1.8-litre 190hp for Celica GT-S and hot Corolla

Example 2: Porsche Variocam Plus

 Variocam of the 911
Carrera
uses timing chain for
Variocam Plus uses hydraulic phasing actuator and
cam phasing.
variable tappets

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, 911 Carrera 3.6
 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:

o Continuous cam-phasing variable valve timing

o 2-stage variable valve lift plus valve-opening duration
o 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 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 ? 2.0 i-VTEC four for Stream, Civic, Integra and more to come.

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 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 ? Audi 2.8 V6

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.