Variable Valve Timing (VVT)
VVT stands for Variable Valve Timing, which is a technology used in many modern car engines to improve their performance, efficiency, and emissions. VVT is a system that allows the engine to vary the timing of the opening and closing of the engine's intake and exhaust valves.
After multi-valve technology became standard in engine design, variable valve timing became the next step in improving engine output, regardless of horsepower or torque.
As you know, the valves activate the engine breather. The timing of the breath, i.e. the timing of the intake and exhaust of the air, is controlled by the shape and phase angle of the cams. To optimize breathing, the engine requires different valve timing at a different speed. As rpm increases, the duration of the intake and exhaust stroke decreases, so fresh air doesn't get fast enough to enter the combustion chamber, while the exhaust doesn't get fast enough to exit the combustion chamber. Therefore, the best solution is to open the intake valves first and then close the exhaust valves. In other words, the overlap between the intake period and the exhaust period should increase as the rpm increase.
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| VVT System 1-Rotor 2-Vane 3-Stator |
Without variable valve timing technology, engineers chose the best compromise timing. For example, a van may adopt fewer overlaps due to the benefits of low-speed starting. A racing engine can take on significant overrides for high-speed power. A stock sedan may adopt mid-rpm optimized valve timing so that both low-speed drivability and high-speed output are not sacrificed too much. No matter what, the result is only optimized for a particular speed.
With the variable valve timing, power and torque can be optimized over a wide rev range. The most notable results are:
- The engine can rev up, so the maximum power is increased. For example, Nissan's 2-litre Neo VVL engine produces 25% more peak horsepower than its non-VVT version.
- Low-speed torque is increased, thereby improving driveability. For example, the 1.8 VVT engine of the Fiat Barchetta delivers a maximum torque of 90% between 2,000 and 6,000 rpm. Also, all these benefits come without any drawbacks.
variable elevation
On some models, valve lift can also be varied based on engine speed. At high speeds, the higher elevation draws air in and out, further optimizing your breathing. Naturally, at a lower speed, such lifting will have opposite effects, such as deterioration of the air-fuel mixing process, which will reduce horsepower or even cause misfires. Therefore, the lift must be variable according to the engine speed.
By varying the timing of the valve opening and closing, the engine can better control the amount of air and fuel that enters the combustion chamber, and the amount of exhaust gas that exits. This allows the engine to operate more efficiently, with improved fuel economy and lower emissions.
Type of VVT
There are several types of VVT systems used in modern engines, including:
Cam phasing VVT
This system uses a hydraulic actuator to change the position of the camshaft, which in turn changes the timing of the valves.
Cam-phasing VVT is the simplest, cheapest and most used mechanism at the moment. However, its performance gain is also minimal, very discreet. Basically, you vary the valve timing by changing the phase angle of the camshafts. For example, at high speed, the intake camshaft will rotate 30° earlier to allow for earlier intake. This movement is controlled by the engine management system as needed and actuated by hydraulic valve gears.
Note that the VVT cam phase cannot vary the duration of the valve opening. It only allows an earlier or later opening of the valve. An early opening translates into an early closing, of course. Also, you can not vary the valve lift, unlike the VVT cam shift. However, cam VVT is the simplest and cheapest form of VVT because each camshaft needs only one hydraulic phaser actuator, unlike other systems which employ an individual mechanism for each cylinder.
Continuous or discrete
The simplest phase shift VVT has only 2 or 3 fixed shift angle settings to choose from, such as 0° or 30°. The best system has a continuously variable displacement, say any arbitrary value between 0° and 30°, depending on the number of revolutions. This of course provides the most suitable valve timing at any engine speed, greatly improving engine flexibility. Plus, the transition is so smooth it's barely noticeable.
intake and exhaust
Some designs, such as BMW's Double Vanos system, have cam VVTs on the intake and exhaust camshafts, allowing for more overlap and therefore greater efficiency. This explains why the BMW M3 3.2 (100 bhp/litre) is more efficient than its predecessor, the M3 3.0 (95 bhp/litre), whose VVT is limited by the intake valves.
On the E46 3 Series, the Double Vanos moves the intake camshaft within a maximum range of 40°. The exhaust camshaft is 25°.
Note:-
Advantage:
Economical and simple, Continuous VVT improves torque delivery throughout the rev range.
Downside:
Lack of variable lift and variable valve opening duration, so less top-end power than cam-shifted VVT. Who uses it?
Most car manufacturers, such as: Audi V8 - aspirated, discreet 2-stage
BMW Double Vanos - intake and exhaust, continuous
Fiat (Alfa) SUPER FIRE - input, 2 discrete stages
Ford Puma 1.7 Zetec SE - aspirated, discreet 2-stage
Updated Jaguar AJ-V6 and AJ-V8 - input, continuous
Renault 2.0 liter - suction, 2 discrete stages
Toyota VVT-i - input, continuous
Volvo 4/5/6 cylinder modular engines - continuous suction
Cam changing VVT
This system uses two different cam profiles, one for low RPMs and another for high RPMs. The engine switches between these profiles depending on the engine speed.
Honda pioneered the use of VVT road vehicles in the late 1980s by launching its popular VTEC (Valve Timing Electronic Control) system. It first appeared on the Civic, CRX and NS-X, then became standard on most models. You can think of it as 2 sets of differently shaped cams to allow for different times and lifts. One set runs at normal speed, such as under 4500rpm. Another replaces at higher speed. Obviously such a design does not allow for continuous change of timing, so the engine revs modestly below 4,500rpm, but above it suddenly turns into a wild animal. This system improves peak horsepower: It can raise redline to nearly 8,000 rpm (even 9,000 rpm on the S2000), like a racing overhead cam engine, and boost peak horsepower up to 30hp per a 1.6 liter engine. ! However, to take advantage of that horsepower gain, you need to keep the engine simmering above the rev threshold, so frequent gear changes are required. Because low-speed torque gains very little (remember, a regular engine's cams generally spin between 0 and 6000rpm, while the VTEC engine's "slow cams" still need to operate between 0 and 4500rpm), the driving skill won. Don't be too impressive. In short, the cam shift system is more suited to sports cars.
Honda has already upgraded their VTEC from 2-stage to 3-stage for some models. Naturally, the more scenery you have, the more refined it becomes. It still offers a narrower torque spread than other continuously variable systems. The cam shift system is still the most powerful VVT, however, as no other system can vary valve lift as well as it can.
Note:-
Advantage:
Powerful at the top end
Downside:
Only 2 or 3 stages, discontinuous; not much torque improvement; complex
Who uses it?
Honda VTEC, Mitsubishi MIVEC, Nissan Neo VVL.
Cam-Changing + Cam-Phasing VVT
The combination of cam shift VVT and cam phased VVT might satisfy the need for maximum power and flexibility throughout the rev range, but it's inevitably more complex. At the time of writing, only Toyota and Porsche have such designs. However, I believe that more and more sports cars will adopt this type of VVT in the future.
Ex- Toyota VTL-i
Toyota's VVTL-i is the most sophisticated VVT design ever. Its powerful features include:
- Variable valve timing with continuous cam timing
- 2-stage variable valve lift plus valve opening duration
- Applied to the 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's. Like VVT-i, variable valve timing is implemented by changing the phase angle of the entire camshaft forward or reverse by means of a hydraulic actuator attached to the end of the camshaft. The engine management system calculates synchronization with engine speed, acceleration, hill/descent, etc. keep in consideration. Furthermore, the variation is continuous over a wide range up to 60°, so variable valve timing alone is perhaps the most perfect design ever. What makes the VVTL-i superior to the regular VVT-i is the "L", which stands for Lift, as everyone knows. Let's see the following illustration
Like VTEC, the Toyota system uses a single rocker arm to actuate both intake valves (or exhaust valves). It also has 2 cam lobes that act on that rocker arm, the lobes have a different profile: one with a longer valve opening duration profile (for high speeds), another with a longer valve opening duration profile (for low speed) . At low speed, the slow cam operates the balance wheel using a roller bearing (to reduce friction). The high-speed cam has no effect on the rocker arm because there is enough room under its hydraulic lifter.
A flat torque output (blue curve)
When speed is increased to the threshold point, hydraulic pressure pushes the slide pin to fill the gap. The high-speed cam is activated. Note that the quick cam provides longer valve open duration, while the sliding pin increases valve lift. (for Honda VTEC both duration and lift are implemented by the cam lobes)
Obviously the variable valve opening duration is a 2-stage design, as opposed to the continuous Rover VVC design. However, VVTL-i offers variable lift, dramatically increasing its high-speed power. Compared to Honda VTEC and similar designs for Mitsubishi and Nissan, Toyota's system has continuously variable valve timing that helps it achieve much better low- to mid-speed flexibility. Therefore, it is undoubtedly the best VVT today. However, it's also more complex and probably more expensive to build.
Note:-
Advantage:
Continuous VVT improves torque delivery throughout the rev range; Variable lift and lasting lifting power at high engine speeds.
Downside:
more complex and expensive
Who uses it?
Toyota Celica GT-S
Rover's unique VVC system
Rover introduced their own system called VVC (Variable Valve Control) at MGF in 1995. It is considered by many experts to be the best VVT considering its overall capabilities: unlike the cam-shifted VVT, it provides continuously variable valve timing, which improves the lows and midrange. torque delivery; and, unlike cam VVT, it can extend the duration of the valve opening (and continuously) and thus increase horsepower. Basically, VVC uses an eccentric rotating disc to operate the intake valves of every other cylinder. Since the eccentric shape creates non-linear rotation, the opening period of the valves can be varied. Still don't understand? well, any clever mechanism must be hard to figure out. If not, Rover won't be the only automaker using it.
VVC has one drawback: since each single mechanism serves 2 adjacent cylinders, a V6 engine needs 4 of those mechanisms, and it's not cheap. V8 also needs 4 of these mechanisms. V12 is impossible to install, as there is not enough space to place the eccentric disc and drive gears between the cylinders.
Note:-
Advantage:
Continuously variable timing and open duration provide both drivability and high-speed power.
Downside:
Ultimately, it is not as powerful as the cam-shifted VVT, due to the lack of variable lift; Expensive for V6 and V8; impossible for V12.
Who uses it?
Rover 1.8 VVC engine for MGF, Caterham and Lotus Elise 111S.
Camless VVT
This system eliminates the camshaft altogether and uses electro hydraulic or electromagnetic actuators to open and close the valves.
VVT advantage for fuel consumption and emissions
EGR (exhaust gas recirculation) is a commonly used technique to reduce emissions and improve fuel efficiency. However, it is the VVTs that really unleash the full potential of EGR. In theory, you need maximum overlap between the opening of the intake and exhaust valves whenever the engine is running at high speed. However, when the car is traveling at average highway speeds, i.e. the engine is running under light load, the maximum overlap can be useful as a means of reducing fuel consumption and emissions. Since the exhaust valves do not close until the intake valves have been open for some time, part of the exhaust gases are recirculated back into the cylinder at the same time as the new air-fuel mixture is injected. Since part of the air-fuel mixture is replaced by exhaust gases, less fuel is required. Since exhaust gases consist mainly of non-combustible gases, such as CO2, the engine can operate properly on a leaner air/fuel mixture without combustion problems.
Conclusion
VVT systems provide significant benefits to modern car engines, improving their performance, efficiency, and emissions. VVT technology has become increasingly common in recent years and is now used in many different types of engines, from small four-cylinder engines to large V8s.
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