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.
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. |
Honda's
latest 3-stage VTEC has been applied in Civic sohc
engine in
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°.
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 · · Volvo 4 / 5 / 6-cylinder
modular engines - inlet, continuous |
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.
|
|
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
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,
< 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,
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 ? |
|
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 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.
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 |
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. |
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.