VARIABLE VALVE TIMING

The valves within an internal combustion engine are used to control the flow of the intake and exhaust gases into and out of the combustion chamber. The timing, duration and lift of these valve events has a significant impact on engine performance. Without variable valve timing or variable valve lift, the valve timing is the same for all engine speeds and conditions, therefore compromises are necessary. An engine equipped with a variable valve timing actuation system is freed from this constraint, allowing performance to be improved over the engine operating range.

Piston engines normally use valves which are driven by camshafts. The cams open (lift) the valves for a certain amount of time (duration) during each intake and exhaust cycle. The Timing of the valve opening and closing, relative to the position of the crankshaft, is important. The camshaft is driven by the crankshaft through timing belts, gears or chains.

An engine large amounts of air when operating at high speeds. However, the intake valves may close before enough air has entered each combustion chamber, reducing performance. On the other hand, if keeps the valves open for longer periods of time, as with a racing cam, problems start to occur at the lower engine speeds. Opening the intake valve while the exhaust valve is still open may cause unburnt fuel to exit the engine, leading to lower engine performance and increased emissions. Early variable valve timing systems used discrete (stepped) adjustment. For example, one timing would be used below 3500 rpm and another used above 3500 rpm. More advanced "continuous variable valve timing" systems offer continuous (infinite) adjustment of the valve timing. Therefore, the timing can be optimized to suit all engine speeds and conditions.

The simplest form of VVT is cam-phasing, whereby the phase angle of the camshaft is rotated forwards or backwards relative to the crankshaft. Thus the valves open and close earlier or later; however, the camshaft lift and duration cannot be altered solely with a cam-phasing system. Achieving variable duration on a VVT complex system, such as multiple cam profiles Late intake valve closing (LIVC) The first variation of continuous variable valve timing involves holding the intake valve open slightly longer than a traditional engine. This results in the piston actually pushing air out of the cylinder and back into the intake manifold during the compression stroke. The air which is expelled fills the manifold with higher pressure, and on subsequent intake strokes the air which is taken in is at a higher pressure. Late intake valve closing has been shown to reduce pumping losses by 40% during partial load conditions, and to decrease nitric oxide (NOx) emissions by 24%. Peak engine torque showed only a 1% decline, and hydrocarbon emissions were unchanged.

Background theory

Continuous versus discrete Early variable valve timing systems used discrete (stepped) adjustment. For example, one timing would be used below 3500 rpm and another used above 3500 rpm. More advanced "continuous variable valve timing" systems offer continuous (infinite) adjustment of the valve timing. Therefore, the timing can be optimized to suit all engine speeds and conditions

Cam phasing versus variable duration The simplest form of VVT is cam-phasing, whereby the phase angle of the camshaft is rotated forwards or backwards relative to the crankshaft. Thus the valves open and close earlier or later; however, the camshaft lift and duration cannot be altered solely with a cam-phasing system. Achieving variable duration on a VVT complex system, such as multiple cam profiles

Typical effect of timing adjustments

Late intake valve closing (LIVC) The first variation of continuous variable valve timing involves holding the intake valve open slightly longer than a traditional engine. This results in the piston actually pushing air out of the cylinder and back into the intake manifold during the compression stroke. The air which is expelled fills the manifold with higher pressure, and on subsequent intake strokes the air which is taken in is at a higher pressure. Late intake valve closing has been shown to reduce pumping losses by 40% during partial load conditions, and to decrease nitric oxide (NOx) emissions by 24%. Peak engine torque showed only a 1% decline, and hydrocarbon emissions were unchanged.

Early intake valve closing (EIVC) Another way to decrease the pumping losses associated with low engine speed, high vacuum conditions is by closing the intake valve earlier than normal. This involves closing the intake valve midway through the intake stroke. Air/fuel demands are so low at low-load conditions and the work required to fill the cylinder is relatively high, so Early intake valve closing greatly reduces pumping losses. Studies have shown early intake valve closing reduces pumping losses by 40%, and increases fuel economy by 7%. It also reduced nitric oxide emissions by 24% at partial load conditions. A possible downside to early intake valve closing is that it significantly lowers the temperature of the combustion chamber, which can increase hydrocarbon emissions.

Early intake valve opening Early intake valve opening is another variation that has significant potential to reduce emissions. In a traditional engine, a process called valve overlap is used to aid in controlling the cylinder temperature. By opening the intake valve early, some of the inert/combusted exhaust gas will back flow out of the cylinder, via the intake valve, where it cools momentarily in the intake manifold. This inert gas then fills the cylinder in the subsequent intake stroke, which aids in controlling the temperature of the cylinder and nitric oxide emissions. It also improves volumetric efficiency, because there is less exhaust gas to be expelled on the exhaust stroke.

Early/late exhaust valve closing Early and late exhaust valve closing timing can be manipulated to reduce emissions. Traditionally, the exhaust valve opens, and exhaust gas is pushed out of the cylinder and into the exhaust manifold by the piston as it travels upward. By manipulating the timing of the exhaust valve, engineers can control how much exhaust gas is left in the cylinder. By holding the exhaust valve open slightly longer, the cylinder is emptied more and ready to be filled with a bigger air/fuel charge on the intake stroke. By closing the valve slightly early, more exhaust gas remains in the cylinder which increases fuel efficiency. This allows for more efficient operation under all conditions.

The main factor preventing this technology from wide use in production automobiles is the ability to produce a cost-effective means of controlling the valve timing under the conditions internal to an engine. An engine operating at 3000 revolutions per minute will rotate the camshaft 25 times per second, so the valve timing events have to occur at precise times to offer performance benefits. Electromagnetic and pneumatic camless valve actuators offer the greatest control of precise valve timing, but, in 2016, are not cost-effective for production vehicles.

Automotive nomenclature

Manufacturers use many different names to describe their implementation of the various types of variable valve timing systems. These names include:

• AVCS (Subaru)
• AVLS (Subaru)
• CPS (Proton) but proton use vvt engine for their new model of 2016
• CVTCS (Nissan, Infiniti)
• CVVT ( developed by Hyundai motor Co., Kia, but it can also be founded on Geely, Iran Khodro, Volvo)
• DCVCP - dual continuous variable cam phasing (General Motors)
• DVT (Desmodromic variable timing, Ducati)
• DVVT (Daihatsu, Perodua, Wuling)
• MIVEC (Mitsubishi)
• MultiAir (FCA)
• VCT (Ford)
• N-VCT (Nissan)
• S-VT (Mazda)
• Ti-VCT (Ford)
• VANOS - VAriable NOckenwellenSteuerung 'camshaft timing' without and with added Valvetronic (BMW)
• Variatore di fase Alfa Romeo(VCT)Phase variator Alfa Romeo is a valve timing variation system designed by Alfa Romeo, the first used in a series production car(ALFA ROMEO spider duetto 1980)
• VarioCam (Porsche)
• VTEC, i-VTEC (Honda, Acura)
• VTi, (Citroen, Peugeot, BMW group)
• VVC (MG Rover)
• VVL (Nissan)
• Valvelift (Audi)
• VVA (Yamaha)
• VVEL (Nissan, Infiniti)
• VVT (Chrysler, General Motors, Proton, Suzuki, Maruti, Isuzu, Volkswagen Group, Toyota)
• VVT-i, VVTL-i (Toyota, Lexus)
• VTVT (Hyundai)

Methods for implementing variable valve control (VVC)

Cam switching

This method uses two cam profiles, with an actuator to swap between the profiles (usually at a specific engine speed). Cam switching can also provide variable valve lift and variable duration, however the adjustment is discrete rather than continuous.

The first production use of this system was Honda's VTEC system. VTEC changes hydraulic pressure to actuate a pin that locks the high lift, high duration rocker arm to an adjacent low lift, low duration rocker arm(s).

Cam phasing Variator (variable valve timing)

Many production VVT systems are the cam phasing type, using a device known as a variator. This allows continuous adjustment of the cam timing (although many early systems only used discrete adjustment), however the duration and lift cannot be adjusted.

Oscillating cam

These designs use an oscillating or rocking motion in a part cam lobe, which acts on a follower. This follower then opens and closes the valve. Some oscillating cam systems use a conventional cam lobe, while others use an eccentric cam lobe and a connecting rod. The principle is similar to steam engines, where the amount of steam entering the cylinder was regulated by the steam "cut-off" point.

The advantage of this design is that adjustment of lift and duration is continuous. However, in these systems, lift is proportional to duration, so lift and duration cannot be separately adjusted.

The BMW (valvetronic), Nissan (VVEL), and Toyota (valvematic) oscillating cam systems act on the intake valves only.

Eccentric cam drive

Eccentric cam drive systems operates through an eccentric disc mechanism which slows and speeds up the angular speed of the cam lobe during its rotation. Arranging the lobe to slow during its open period is equivalent to lengthening its duration.

The advantage of this system is that duration can be varied independent of lift (however this system does not vary lift). The drawback is two eccentric drives and controllers are needed for each cylinder (one for the intake valves and one for the exhaust valves), which increases complexity and cost.

MG Rover is the only manufacturer that has released engines using this system.

Three-dimensional cam lobe

This system consists of a cam lobe that varies along its length (similar to a cone shape). One end of the cam lobe has a short duration/reduced lift profile, and the other end has a longer duration/greater lift profile. In between, the lobe provides a smooth transition between these two profiles. By shifting area of the cam lobe which is in contact with the follower, the lift and duration can be continuously altered. This is achieved by moving the camshaft axially (sliding it across the engine) so a stationary follower is exposed to a varying lobe profile to produce different amounts of lift and duration. The downside to this arrangement is that the cam and follower profiles must be carefully designed to minimise contact stress (due to the varying profile).

Ferrari is commonly associated with this system, however it is unknown whether any production models to date have used this system.

Two shaft combined cam lobe profile

It consists of two (closely spaced) parallel camshafts, with a pivoting follower that spans both camshafts and is acted on by two lobes simultaneously. Each camshaft has a phasing mechanism which allows its angular position relative to the engine's crankshaft to be adjusted. One lobe controls the opening of a valve and the other controls the closing of the same valve, therefore variable duration is achieved through the spacing of these two events.

The drawbacks to this design include:

• At long duration settings, one lobe may be starting to reduce its lift as the other is still increasing. This has the effect of lessening the overall lift and possibly causing dynamic problems. One company claims to have solved the uneven rate of opening of the valve problem to some extent thus allowing long duration at full lift.
• Size of the system, due to the parallel shafts, the larger followers etc.

Coaxial two shaft combined cam lobe profile

The operating principle is that the one follower spans the pair of closely spaced lobes. Up to the angular limit of the nose radius the follower 'sees' the combined surface of the two lobes as a continuous, smooth surface. When the lobes are exactly aligned the duration is at a minimum (and equal to that of each lobe alone) and when at the extreme extent of their misalignment the duration is at a maximum. The basic limitation of the scheme is that only a duration variation equal to that of the lobe nose true radius (in camshaft degrees or double this value in crankshaft degrees) is possible. In practice this type of variable cam has a maximum range of duration variation of about forty crankshaft degrees.

Helical camshaft

It has a similar principle to the previous type, and can use the same base duration lobe profile. However instead of rotation in a single plane, the adjustment is both axial and rotational giving a helical or three-dimensional aspect to its movement. This movement overcomes the restricted duration range in the previous type. The duration range is theoretically unlimited but typically would be of the order of one hundred crankshaft degrees, which is sufficient to cover most situations.

The cam is reportedly difficult and expensive to produce, requiring very accurate helical machining and careful assembly.

Camless engines

Engine designs which do not rely on a camshaft to operate the valves have greater flexibility in achieving variable valve timing and variable valve lift. However, there has not been a production camless engine released for road vehicles as yet.

Hydraulic system

This system utilizes the engine lube oil to control the closure of inlet valve. The intake valve opening mechanism incorporates a valve tappet and a piston inside a chamber. There is a solenoid valve controlled by the engine control system which gets energized and supplies oil through a non-return valve during the time of cam lift and the oil gets filled in the chamber and the return channel to the sump is blocked by the valve tappet. During the downward movement of the cam, at a particular instant, the return passage opens and the oil pressure gets released to the engine sump.

Variable valve timing explained

Variable Valve Timing Explained

In this video it explains why you have a variable valve timing system on your car. It also explains how it exactly works and operates. Some of the more common known variable valve timing systems are Vtec, VVT, VTVT and VVTi. All these systems work very similar to one another.

Variable Valve Lift V/s Variable Valve Timing

What is VVL? What is VVT? How does VVL/VVT work? VW VR6 Engine Explained

VARIABLE VALVE TIMING

Here's how variable valve timing works on your car's engine. In this video a variable timing gear, actuator and engine head is taken apart to demonstrate how the system works.

Here's how the Variable Valve Timing System Works on a car's engine.The four-stroke internal combustion engine consists of gasoline-rich air entering the cylinder in the intake stroke, compressed, then ignited to push the piston down to rotate the crankshaft in the power stroke. The piston then returns to the top pushing the exhaust gases out the valves, only for the cycle to be repeated over again. Variable valve timing optimizes fuel economy, thermal efficiency and emissions. An oil activated cam-phaser actuator is controlled by the ECU through an oil control valve. Oil pressure is sent through the VVT gear to fill up cavities to advance or retard timing. A closed loop control system utilizing the cam and crank position sensors is formed to ensure optimal timing by the ECU for either more power, or less emissions.In this video, the engine from an Infiniti G35 (VQ35DE) is taken apart to explain the components of the variable valve timing system, including the oil control valve, camshaft bearing caps, timing tensioner, engine head, valves, camshaft and pistons.

Hyundai's brilliant engine technology continuously varies valve duration! Variable Valve Timing vs Lift (VVL vs VVT)

The world’s first production CVVD engine! CVVD stands for continuously variable valve duration. This technology is currently used in the 1.6L turbo engine of the 2020 Hyundai Sonata. In order to understand it we need to understand how valves work on an engine. Of course, you have the four engine strokes: intake, compression, power, and exhaust. Intake you have the intake valve open, obviously, and your exhaust stroke has the exhaust valve open. On the most basic of engines, everything about how these valves operate is fixed, because those valves follow the cam profile of a fixed camshaft.

On modern car engines, there are variables we can change; three of them, as it relates to valves. First, there’s variable valve lift. This means you can change how far down the valve travels allowing for more or less airflow into the engine. Second, you have variable valve timing, this means you can change when you actually open this valve, you can open it sooner, or open it later, relative to its standard timing. And third, and this is what Hyundai is adding into the mix, is variable valve duration. This is controlling how long you actually leave the valve open. You could have it open and close very quickly, you could have it remain open for a longer duration, or anything between.

Up until this point, no mass produced engines have actually been able to vary how long a valve remains open, relative to the engine speed. If you’re wondering about Koenigsegg’s freevalve, and we’ll discuss that in the video as well. So, how does Hyundai do it? Check it out!

VIDEO ON Electronic Variable Valve Timing Actuators

HOW VVT SYSTEM WORK IN PETROL ENGINE

VARIABLE VALVE TIMING

Want to know how to make more power with your vehicle's VVT? In this Tech Gerrot Jacobson takes a look at variable valve timing, the old way vs. new way of camshaft timing, and what it all can do for your tune. Check out latest Dodge material

CONTINUOUS VARIABLE VALVE DURATION TECHNOLOGY

Hyundai Motor Group has developed the world’s first Continuously Variable Valve Duration (CVVD) technology to feature in future Hyundai and Kia vehicles.

Besides, different manufacturers use custom acronyms for their VVT systems which are as follows:

Company

1 CVVT Continuous Variable Valve Timing Renault

2 CVVT Continuous Variable Valve Timing Volvo

3 VCT Variable Cam Timing Ford

4 VVT Variable Valve Timing Suzuki

5 VVT Variable Valve Timing Volkswagen

6 DCVCP Dual Continuous Variable Cam Phasing GM

7 VVTi Variable Valve Timing (intelligent) Toyota

8 VTVT Variable Timing and Valve Train Hyundai

9 N-VCT Nissan-Variable Cam Timing Nissan

10 S-VT Sequential Valve Timing Mazda

11 MIVEC Mitsubishi Innovative Valve timing Electronic Control Mitsubishi

12 i-VTECIntelligent Variable Valve Timing and Lift Electronic Control Honda, Acura

13 Camtronic--- Mercedes Benz

14 VANOS Variable Nockenwellensteuerung BMW

15 Valvelift--- Audi

16 VarioCam--- Porshe