Different Types of VVT
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
Toyota’s
VVTL-i is the most sophisticated VVT design yet. Its powerful functions
include:
- Continuous
cam-phasing variable
valve timing
- 2-stage variable
valve lift
plus valve-opening duration
- Applied to both
intake and exhaust
valves
The system could be seen
as a combination of the existing VVT-i and Honda’s VTEC, although the
mechanism
for the variable lift is different from Honda.
Like VVT-i, the
variable
valve timing is implemented by shifting the phase angle of the whole
camshaft
forward or reverse by means of a hydraulic actuator attached to the end
of the camshaft. The timing is calculated by the engine management
system
with engine 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.
| 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
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, 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:
- Continuous
cam-phasing variable
valve timing
- 2-stage variable
valve lift
plus valve-opening duration
- Can be applied
to
both intake
and exhaust valves
Basically, the camshaft
is purely VTEC - with different cam lobes for implementing 2-stage
variable
lift and timing. On the other hand, the camshaft can be phase-shifted
by
a hydraulic actuator at the end of the camshaft, so valve timing can be
varied continuously according to need.
The i-VTEC was first
introduced
in Stream MPV, in which only the intake side applies i-VTEC.
Theoretically,
it can be applied to both intake and exhaust camshafts, but Honda
seemed
less generous than Toyota - even the Integra Type R uses only i-VTEC at
intake side plus the regular VTEC at exhaust side.
| Advantage: |
Continuous
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
|
Copyright©
1998-2006 by Mark Wan
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