TOYOTA NEW TECHNOLOGY
INTRODUCTION
VVT-i, or Variable Valve Timing with
intelligence, is an automobile variable valve timing technology developed by
Toyota. The Toyota VVT-i system replaces the Toyota VVT offered starting in
1991 on the 5-valve per cylinder 4A-GE engine. The VVT system is a 2-stage hydraulically
controlled cam phasing system.
TYPE OF ENGINE USED IN TOYOTA CARS
I. VVT-i
Cutaway view of Variable ValveTiming with ignition engine
VVT-i, or Variable Valve Timing with intelligence, is an
automobile variable valve timing technology developed by Toyota. The Toyota
VVT-i system replaces the Toyota VVT offered starting in 1991 on the 5-valve
per cylinder 4A-GE engine. The VVT system is a 2-stage hydraulically controlled
cam phasing system.
VVT-i, introduced in 1996, varies the timing of the intake
valves by adjusting the relationship between the camshaft drive (belt,
scissor-gear or chain) and intake camshaft. Engine oil pressure is applied to
an actuator to adjust the camshaft position. Adjustments in the overlap time
between the exhaust valve closing and intake valve opening result in improved
engine efficiency.[1] Variants of the system, including VVTL-i, Dual VVT-i,
VVT-iE, and Valvematic, have followed.
II. VVTL-i
The cutaway of VVTL-i
engine
VVTL-i (Variable Valve Timing and Lift intelligent system)
(also sometimes denoted as VVT-iL or Variable Valve Timing and Intelligence
with Lift) is an enhanced version of VVT-i that can alter valve lift (and
duration) as well as valve timing. In the case of the 16 valve 2ZZ-GE, the
engine head resembles a typical DOHC design, featuring separate cams for intake
and exhaust and featuring two intake and two exhaust valves (four total) per
cylinder. Unlike a conventional design, each camshaft has two lobes per
cylinder, one optimized for lower rpm operation and one optimized for high rpm
operation, with higher lift and longer duration. Each valve pair is controlled
by one rocker arm, which is operated by the camshaft. Each rocker arm has a
slipper follower mounted to the rocker arm with a spring, allowing the slipper
follower to freely move up and down with the high lobe without affecting the
rocker arm. When the engine is operating below 6000-7000 rpm (dependent on
year, car, and ECU installed), the lower lobe is operating the rocker arm and
thus the valves, and the slipper-follower is freewheeling next to the rocker
arm. When the engine is operating above the lift engagement point, the ECU
activates an oil pressure switch which pushes a sliding pin under the slipper
follower on each rocker arm. The rocker arm is now locked into
slipper-follower's movements and thus follows the movement of the high rpm cam
lobe, and will operate with the high rpm cam profile until the pin is
disengaged by the ECU. The lift system is similar in principle to Honda VTEC
operation.
The system was first used in 2000 Toyota Celica with 2ZZ-GE.
Toyota has now ceased production of its VVTL-i engines for most markets,
because the engine does not meet Euro IV specifications for emissions. As a
result, this engine has been discontinued on some Toyota models, including that
of the Corolla T-Sport (Europe), Corolla Sportivo (Australia), Celica, Corolla
XRS, Toyota Matrix XRS, and the Pontiac Vibe GT, all of which had the 2ZZ-GE
engine fitted. The Lotus Elise continues to offer the 2ZZ-GE and the 1ZZ-FE
engine, while the Exige offers the engine with a supercharger. The Toyota Yaris
uses VVT-i on its gasoline engines.
III. Dual
VVT-i
The cutaway of Dual VVT-i engine
The Dual VVT-i system adjusts timing on both intake and
exhaust camshafts. It was first introduced in 1998 on the RS200 Altezza's 3S-GE
engine.
Dual VVT-i is also found in Toyota's new generation V6
engine, the 3.5-liter 2GR-FE first appearing on the 2005 Avalon. This engine
can now be found on numerous Toyota and Lexus models. By adjusting the valve
timing, engine start and stop occurs almost unnoticeably at minimum
compression. Fast heating of the catalytic converter to its light-off
temperature is possible, thereby reducing hydrocarbon emissions considerably.
Most Toyota engines including the LR engines (V10, used in
the Lexus LFA), UR engines (V8), GR engines (V6), AR engines (Large I4), and ZR
engines (Small I4) now use this technology.
TOYOTA VVTI SYSTEM EXPLAINATION
Any
mechanic or automotive enthusiast can tell you that an engine is essentially a
large air pump. The more an engine can suck in air to mix with fuel, the more
it can create power through combustion. Thus, the more efficiently an engine
removes exhaust gases from the cylinders, the better it can manage that power. The
key to a strong, healthy engine is adequate air from one end to another.
Air flow is affected by many different components in the motor, but the valves
in the cylinder head are what directly control the amount of air entering a
cylinder, and the volume of exhaust gases leaving it. The intake valves open up
just prior to combustion in order to allow air to flow in and mix with fuel,
and the exhaust valves open after the ignition of this mixture in order to suck
out the resulting gases. The timing of the valves is controlled by a rotating
shaft called the camshaft. The camshaft has lobes which push up on the valves
in order to open them and drop them back closed again.
How long these valves remain open, and at what point in the combustion cycle,
can have a big impact on the drivability and power generated by an engine. For
instance, if you want to have a really fast car, like a race car, you'll want
the engine to produce a lot of power at high RPMs. You can adjust the camshaft
to perform well at higher RPMs. This will result in poor performance at low
RPMs, but that's OK with a race car. Conversely, if you want a lot of low-end
torque - which is great for towing - you need to adjust the camshaft to perform
well at low RPMs. This, of course, will hurt high RPM performance.
Unfortunately, street vehicles are a compromise between reliability, fuel
efficiency and power. While race vehicles have engines with camshaft designs
that generate large amounts of power while being used only at specific, high
revolutions, your daily driver sees a wide range of RPMs that make a broader
power band necessary. While it is ok for a race car to have a lumpy idle that
barely runs below 1000 rpm, it would do you no good if your street car stalled
out at every stoplight. Regular vehicles usually have to make do with a
camshaft that provides a good amount of power in the most often used range of
engine RPMs, but runs out of steam at high speeds.
The
problem with compromise camshafts is that they're not all that efficient. Since
everyday vehicles operate at a variety of different RPMs, the engine needs to
be just as capable of accelerating from a dead stop as it is of zooming along
at highway speeds, and everything in between. The result is that your engine
often ends up burning too much fuel while underperforming.
Automakers have addressed this concern with something called "variable
valve timing" (VVT). The Toyota Tundra's i-Force 5.7L V8, Toyota's newest
VVT-i engine, has the ability to vary the timing of the valves in relation to engine
speed. It does this by using engine oil pressure to move the camshaft slightly,
so that more aggressive lobe designs are used when the engine is running at a
higher rpm. By doing this, the i-Force V8 is able to run a camshaft profile
that provides good fuel efficiency in every day driving, but is still able to
churn out gobs of power when the pedal is pressed to the floor.
Conclusion:
There
is no overlooking the TOYOTA VVTI hefty price. However, most VVTI owners are
extremely satisfied with their purchase. The car’s high-tech features and
stunning design make it a dream vehicle. Every driver will get addicted to the
VVTIhigh-revving engine.
