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. 

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. taken into consideration. 

Moreover, the variation is continuous across a wide range of up to 60°, therefore the variable timing alone is perhaps the best design up to now.

What makes the VVTL-i superior to the ordinary VVT-i is the "L", which stands for Lift (valve lift). 

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. Then the high speed cam becomes effective. Note that the fast cam provides a longer valve-opening duration while the sliding pin adds the 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. 

Compared with Hondas VTEC and similar designs from 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 possibly the best VVT available today. However, it is also more complex and probably more expensive to build.

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, their mechanisms virtually share nothing. The Variocam was first introduced to the 968 in 1991. It used the timing chain to vary the phase angle of camshaft, and  provided 3-stage variable valve timing. The 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 as it doesn’t allow as much variation to phase angle.

Therefore, the Variocam Plus used in the new 911 Turbo finally uses the popular hydraulic actuator instead of chain. 

The most influential change of the "Plus" is the addition of variable valve lift. It is implemented by using variable hydraulic tappets.  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 only marginally heavier than ordinary tappets and engage little more space.

At the moment the Variocam Plus is just offered for the intake valves.