Variable Valve Timing

Variable Valve Timing (VVT)Basic TheoryAfter multi-valve technology became standard in engine design, Variable Valve Timing becomes the next step to enhance engine output, no matter power or torque.As you know, valves activate the breathing of engine. The timing of breathing, that is, the timing of air intake and exhaust, is controlled by the shape and phase angle of cams. To optimise the breathing, engine requires different valve timing at different speed. When the rev increases, the duration of intake and exhaust stroke decreases so that fresh air becomes not fast enough to enter the combustion chamber, while the exhaust becomes not fast enough to leave the combustion chamber. Therefore, the best solution is to open the inlet valves earlier and close the exhaust valves later. In other words, the Overlapping between intake period and exhaust period should be increased as rev increases.  

  Without Variable Valve Timing technology, engineers used to choose the best compromise timing. For example, a van may adopt less overlapping for the benefits of low speed output. A racing engine may adopt considerable overlapping for high speed power. An ordinary sedan may adopt valve timing optimise for mid-rev so that both the low speed drivability and high speed output will not be sacrificed too much. No matter which one, the result is just optimised for a particular speed.With Variable Valve Timing, power and torque can be optimised across a wide rpm band. The most noticeable results are:  

o The engine can rev higher, thus raises peak power. For example, Nissan's 2-litre Neo VVL engine output 25% more peak power than its non-VVT version.

o Low-speed torque increases, thus improves drivability. For example, Fiat Barchetta's 1.8 VVT engine provides 90% peak torque between 2,000 and 6,000 rpm.

  Moreover, all these benefits come without any drawback.Variable LiftIn some designs, valve lift can also be varied according to engine speed. At high speed, higher lift quickens air intake and exhaust, thus further optimise the breathing. Of course, at lower speed such lift will generate counter effects like deteriorating the mixing process of fuel and air, thus decrease output or even leads to misfire. Therefore the lift should be variable according to engine speed.

1) Cam-Changing VVTHonda pioneered road car-used VVT in the late 80s by launching its famous VTEC system (Valve Timing Electronic Control). First appeared in Civic, CRX and NS-X, then became standard in most models.You can see it as 2 sets of cams having different shapes to enable different timing and lift. One set operates during normal speed, say, below 4,500 rpm. Another substitutes at higher speed. Obviously, such layout does not allow continuous change of timing, therefore the engine performs modestly below 4,500 rpm but above that it will suddenly transform into a wild animal.This system does improve peak power - it can raise red line to nearly 8,000 rpm (even 9,000 rpm in S2000), just like an engine with racing camshafts, and increase top end power by as much as 30 hp for a 1.6-litre engine !! However, to exploit such power gain, you need to keep the engine boiling at above the threshold rpm, therefore frequent gear change is required. As low-speed torque gains too little (remember, the cams of a normal engine usually serves across 0-6,000 rpm, while the "slow cams" of VTEC engine still need to serve across 0-4,500 rpm), drivability won't be too impressive. In short, cam-changing system is best suited to sports cars.Honda has already improved its 2-stage VTEC into 3 stages for some models. Of course, the more stage it has, the more refined it becomes. It still offers less broad spread of torque as other continuously variable systems. However, cam-changing system remains to be the most powerful VVT, since no other system can vary the Lift of valve as it does.  

Advantage:Powerful at top end

Disadvantage:2 or 3 stages only, non-continuous; no much improvement to torque; complex

Who use it ?Honda VTEC, Mitsubishi MIVEC, Nissan Neo VVL.

Example - Honda's 3-stage VTEC

Honda's latest 3-stage VTEC has been applied in Civic sohc engine in Japan. The mechanism has 3 cams with different timing and lift profile. Note that their dimensions are also different - the middle cam (fast timing, high lift), as shown in the above diagram, is the largest; the right hand side cam (slow timing, medium lift) is medium sized ; the left hand side cam (slow timing, low lift) is the smallest.This mechanism operate like this :

Stage 1 ( low speed ) : the 3 pieces of rocker arms moves independently. Therefore the left rocker arm, which actuates the left inlet valve, is driven by the low-lift left cam. The right rocker arm, which actuates the right inlet valve, is driven by the medium-lift right cam. Both cams' timing is relatively slow compare with the middle cam, which actuates no valve now.

Stage 2 ( medium speed ) : hydraulic pressure (painted orange in the picture) connects the left and right rocker arms together, leaving the middle rocker arm and cam to run on their own. Since the right cam is larger than the left cam, those connected rocker arms are actually driven by the right cam. As a result, both inlet valves obtain slow timing but medium lift.

Stage 3 ( high speed ) : hydraulic pressure connects all 3 rocker arms together. Since the middle cam is the largest, both inlet valves are actually driven by that fast cam. Therefore, fast timing and high lift are obtained in both valves.

Another example - Nissan Neo VVL

Very similar to Honda's system, but the right and left cams are with the same profile. At low speed, both rocker arms are driven independently by those slow-timing, low-lift right and left cams. At high speed, 3 rocker arms are connected together such that they are driven by the fast-timing, high-lift middle cam.You might think it must be a 2-stage system. No, it is not. Since Nissan Neo VVL duplicates the same mechanism in the exhaust camshaft, 3 stages could be obtained in the following way:Stage 1 (low speed) : both intake and exhaust valves are in slow configuration. Stage 2 (medium speed) : fast intake configuration + slow exhaust configuration. Stage 3 (high speed) : both intake and exhaust valves are in fast configuration.

2) Cam-Phasing VVTCam-phasing VVT is the simplest, cheapest and most commonly used mechanism at this moment. However, its performance gain is also the least, very fair indeed.Basically, it varies the valve timing by shifting the phase angle of camshafts. For example, at high speed, the inlet camshaft will be rotated in advance by 30° so to enable earlier intake. This movement is controlled by engine management system according to need, and actuated by hydraulic valve gears.  

Note that cam-phasing VVT cannot vary the duration of valve opening. It just allows earlier or later valve opening. Earlier open results in earlier close, of course. It also cannot vary the valve lift, unlike cam-changing VVT. However, cam-phasing VVT is the simplest and cheapest form of VVT because each camshaft needs only one hydraulic phasing actuator, unlike other systems that employ individual mechanism for every cylinder.Continuous or DiscreteSimpler cam-phasing VVT has just 2 or 3 fixed shift angle settings to choose from, such as either 0° or 30°. Better system has continuous variable shifting, say, any arbitary value between 0° and 30°, depends on rpm. Obviously this provide the most suitable valve timing at any speed, thus greatly enhance engine flexiblility. Moreover, the transition is so smooth that hardly noticeable.Intake and ExhaustSome design, such as BMW's Double Vanos system, has cam-phasing VVT at both intake and exhaust camshafts, this enable more overlapping, hence higher efficiency. This explain why BMW M3 3.2 (100hp/litre) is more efficient than its predecessor, M3 3.0 (95hp/litre) whose VVT is bounded at the inlet valves.In the E46 3-series, the Double Vanos shift the intake camshaft within a maximum range of 40° .The exhaust camshaft is 25°.

Advantage:Cheap and simple, continuous VVT improves torque delivery across the whole rev range.

Disadvantage:Lack of variable lift and variable valve opening duration, thus less top end power than cam-changing VVT.

Who use it ?Most car makers, such as:  Audi V8 - inlet, 2-stage discrete  BMW Double Vanos - inlet and exhaust, continuous  Ferrari 360 Modena - exhaust, 2-stage discrete  Fiat (Alfa) SUPER FIRE - inlet, 2-stage discrete  Ford Puma 1.7 Zetec SE - inlet, 2-stage discrete  Jaguar AJ-V6 and updated AJ-V8 - inlet, continuous  Lamborghini Diablo SV engine - inlet, 2-stage discrete  Porsche Variocam - inlet, 3-stage discrete  Renault 2.0-litre - inlet, 2-stage discrete  Toyota VVT-i - inlet, continuous  Volvo 4 / 5 / 6-cylinder modular engines - inlet, continuous

Example : BMW's VanosFrom the picture, it is easy to understand its operation. The end of camshaft incorporates a gear thread. The thread is coupled by a cap which can move towards and away from the camshaft. Because the gear thread is not in parallel to the axis of camshaft, phase angle will shift forward if the cap is pushed towards the camshaft. Similarly, pulling the cap away from the camshaft results in shifting the phase angle backward.Whether push or pull is determined by the hydraulic pressure. There are 2 chambers right beside the cap and they are filled with liquid (these chambers are colored green and yellow respectively in the picture) A thin piston separates these 2 chambers, the former attaches rigidly to the cap. Liquid enter the chambers via electromagnetic valves which controls the hydraulic pressure acting on which chambers. For instance, if the engine management system signals the valve at the green chamber open, then hydraulic pressure acts on the thin piston and push the latter, accompany with the cap, towards the camshaft, thus shift the phase angle forward.Continuous variation in timing is easily implemented by positioning the cap at a suitable distance according to engine speed.  

Another Example : Toyota VVT-i

  Macro illustration of the phasing actuator

Toyota's VVT-i (Variable Valve Timing - Intelligent) has been spreading to more and more of its models, from the tiny Yaris (Vitz) to the Supra. Its mechanism is more or less the same as BMW’s Vanos, it is also a continuously variable design.However, the word "Integillent" emphasis the clever control program. Not only varies timing according to engine speed, it also consider other conditions such as acceleration, going up hill or down hill.

3) Cam-Changing + Cam-Phasing VVTCombining 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 VTL-i

 Toyota’s VVTL-i is the most sophisticated VVT design yet. Its powerful functions

include:  

o Continuous cam-phasing variable valve timingo 2-stage variable valve lift plus valve-opening durationo 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 Celica GT-S

Example 2: Porsche Variocam Plus

Variocam Plus uses hydraulic phasing actuator and variable tappets

Variocam of the 911 Carrerauses 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

4) Rover's unique VVC systemRover introduced its own system calls VVC (Variable Valve Control) in MGF in 1995. Many experts regard it as the best VVT considering its all-round ability - unlike cam-changing VVT, it provides continuously variable timing, thus improve low to medium rev torque delivery; and unlike cam-phasing VVT, it can lengthen the duration of valves opening (and continuously), thus boost power.Basically, VVC employs an eccentric rotating disc to drive the inlet valves of every two cylinder. Since eccentric shape creates non-linear rotation, valves opening period can be varied. Still don't understand ? well, any clever mechanism must be difficult to understand. Otherwise, Rover won't be the only car maker using it.VVC has one draw back: since every individual mechanism serves 2 adjacent cylinders, a V6 engine needs 4 such mechanisms, and that's not cheap. V8 also needs 4 such mechanism. V12 is impossible to be fitted, since there is insufficient space to fit the eccentric disc and drive gears between cylinders.  

Advantage:Continuously variable timing and duration of opening achieve both drivability and high speed power.

Disadvantage:Not ultimately as powerful as cam-changing VVT, because of the lack of variable lift; Expensive for V6 and V8; impossible for V12.

Who use it ?Rover 1.8 VVC engine serving MGF, Caterham and Lotus Elise 111S.

VVT's benefit to fuel consumption and emission

EGR (Exhaust gas recirculation) is a commonly adopted technique to reduce emission and improve fuel efficiency. However, it is VVT that really exploit the full potential of EGR.In theory, maximum overlap is needed between intake valves and exhaust valves’ opening whenever the engine is running at high speed. However, when the car is running at medium speed in highway, in other words, the engine is running at light load, maximum overlapping may be useful as a mean to reduce fuel consumption and emission. Since the exhaust valves do not close until the intake valves have been open for a while, some of the exhaust gases are recirculated back into the cylinder at the same time as the new fuel / air mix is injected. As part of the fuel / air mix is replaced by exhaust gases, less fuel is needed. Because the exhaust gas comprise of mostly non-combustible gas, such as CO2, the engine runs properly at the leaner fuel / air mixture without failing to combust.

 Cam-Torque Actuated Variable Valve Timing System

Cam Phasers, Activate! BorgWarner's new approach to variable valve timing is cam-torque actuated.

AUGUST 2010

BY MICHAEL AUSTIN 

ILLUSTRATION BY PETER SUCHESKI

 7 SHARES 

 TWEET 

Most modern variable valve-timing (VVT) systems use a cam phaser that rotates the position of each camshaft relative

to the timing chain. Think of making a record turntable go faster or slower by spinning it with your hands. The cam

phaser has two basic components: an outer sprocket connected to the timing chain and an inner rotor (connected to

the camshaft) that varies the valve timing by adjusting the rotation angle of the cam.

This inner rotor consists of a set of lobes, and oil fills the space between the outer housing and the lobes. Left alone,

the rotor will simply spin at the same rate as the outer housing. If you add oil to one side of the lobe and remove it

from the other, the rotor moves, and—voilà!—there’s your variable valve timing.

The majority of these VVT systems use oil pressure to push the rotor back and forth, but BorgWarner thinks its cam-

torque-actuated (CTA) system marks an important step forward. Oil-pressure-actuated (OPA) systems require an

upsize oil pump to produce the extra pressure that’s required to work the cam phasers, which saps some of the fuel-

economy gains of VVT. With a mechanical oil pump, OPA systems don’t work well at low engine speeds because the

pump doesn’t build pressure and volume until the revs get higher.

The CTA system avoids those pitfalls by using Newton’s Third Law of Motion—for every action there is an equal and

opposite reaction—to move the oil in the cam phasers. When a cam lobe pushes a valve open, the valve spring resists

that force and pushes back. Similarly, when the valve spring pushes a valve closed, it also pushes on the cam lobe in

the opposite direction from the valve opening. When multiplied over an entire camshaft, there is enough energy from

these back-and-forth forces to make cam phasing work.

Another trick to BorgWarner’s system is the way it moves oil. A center spool valve, controlled by a solenoid inside the cam-

phasing rotor, directs the flow. With the valve open in one direction, oil flows into only one side of the oil pockets and can’t

leave. By sliding the valve back and forth, the system can mete out the precise amount of oil flow on either side of the rotor

lobes.

The key advantages of the CTA system are that it responds quickly even at idle and can operate using a standard engine’s

oil pump. But there are downsides. As engine speeds increase, the CTA system becomes less effective. This happens

because the valve events occur more frequently, reducing the time available to move the oil. Conversely, OPA systems

work better as oil pressure increases and are better at high rpm. So there’s not much of a peak power gain from a CTA

system; it improves performance and efficiency in other areas of the rev range. Also, CTA cam phasing is at the mercy of

the natural oscillations of those forces on the camshaft. Valve openings and closings in an inline-six are spaced too closely

for the system to work well. But a V-6 (or inline-three) is perfectly suited because there isn’t as much overlap between each

valve event. The system also works on V-8 engines.

CTA variable valve timing debuted on Ford’s 3.0-liter Duratec V-6, beginning with the 2009 Escape and the 2010 Fusion.

The 3.7-liter V-6 in the Mustang uses BorgWarner’s system, too, as do the 2011 Edge and Lincoln MKX. You can also find

it on the Mustang’s 5.0-liter V-8 as well as the V-8 engines used in Jaguar and Land Rover vehicles. These engines’

efficiencies show the virtues of the CTA system. View Photo Gallery

The Benefits of Variable Valve Timing

February 17, 2012

Learn about the fuel economy and performance benefits to variable valve timing, and the various acronyms used for the

technology: VTEC, MIVEC, VVL, etc.

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Many modern engines are now equipped with variable valve timing systems to improve the performance of the engine.

Variable valve timing increases an engine's flexibility under different conditions, which can result in increased fuel economy

or better performance. Many people are familiar with terms like VTEC, VVT-i, VVL and VANOS but most don't know what

they mean. Here is a quick breakdown of what some of the different terms mean, and which manufacturers use them.

VTEC:Variable Valve Timing and Lift Electronic Control (used by Honda)

VVT-i or VVTL-i:Variable Valve Timing and Lift with Intelligence (used by Toyota)

MIVEC:Mitsubishi Innovative Valve timing and lift Electronic Control (used by Mitsubishi)

VVL:Variable Valve Lift (used by Nissan)

VANOS: Variable Onckenwellen Steuerung (German-designed system used by BMW, Ford, Ferrari and Lamborghini)

There are several other variable valve timing systems available from different manufacturers.

How Is Variable Valve Timing Accomplished?

Variable valve timing involves complex mechanical and hydraulic processes inside the vehicle's engine. Each

manufacturer's variable valve timing system differs slightly, but most function on the same basic rules. To get a basic

understanding of the principals at work we can take a look and Honda's VTEC system, which is one of the longest standing

and most common systems. Honda's system operates using three basic steps to regulate the functioning of the vehicle's

engine.

Low RPM Drivability.When running at lower RPM, the VTEC engine uses a camshaft with a profile designed to provide a

smooth idle, good fuel economy and better low end power and torque

Electronic monitoring and switch.The engine's computer monitor's the conditions under which the engine is operating,

including the position of the throttle pedal and the engine's speed or RPM, to decide when to switch to a different camshaft

profile

High RPM performance. If the engine detects a condition under which the high-performance camshaft is necessary, an

electronic switch is activated and hydraulic pressure is used to switch the valve operation to a different camshaft. The high-

performance camshaft provides the vehicle with considerably more power at high RPM's. In fact, Honda drivers whose

vehicles are equipped with the VTEC system can often hear and feel when the VTEC system is activated

Variable valve timing systems aren't only used to increase performance. Several manufacturers now offer variable valve

timing systems designed specifically to maximize the fuel economy of engines in vehicles that are less performance-

oriented.

Other Benefits of Variable Valve Timing

Internal exhaust gas recirculation.By allowing for more direction for internal gases, the variable valve timing system can cut

down on emissions, which is critical for auto makers working to get their cars and trucks in compliance with federal or state

emissions controls

Increased torque.Variable valve timing systems can provide better torque for an engine

Better fuel economy. with more precise handling of engine valves, some auto makers have shown that VVT can produce

better fuel economy for vehicles

Why Use Variable Valve Timing?

Increased Volumetric Efficiency

Reduction of Pumping Losses

Lower Parts Count

More efficient

Variable Valve Timing or VVT is common on most engines and is responsible for the elimination of many EGR valves as

well as increased performance and fuel economy on many engines. Most systems are lube oil activated and use a control

solenoid as well as the camshaft sensor, crankshaft sensor, and PCM for control. Newer systems operate off rotational

torque from the engine. Some DTCs set by these systems can be confusing to a technician and the OE diagnostics often

leave much to be desired. This article will focus on the different type of VVT systems and there operation.

Description And Operation

The PCM calculates and determines the desired camshaft position. It will continually update the VCT solenoid duty cycle

until desired positioning is achieved. When the VCT solenoid is energized, engine oil is allowed to flow to the VCT actuator

assembly which advances or retards the cam timing. One half of the VCT actuator is coupled to the camshaft and tlie other

half is connected to the timing chain. Oil chambers between the two halves couple the camshaft to the timing chain. When

the flow of oil is shifted from one side of the chamber to the other, the differential change in oil pressure forces the camshaft

to rotate in either an advance or retard position depending on the oil flow. A difference between the desired and actual

camshaft position represents a position error in the PCM’s VCT control loop. For the VVT system to operate properly, the

engine oil must be clean and at the proper level. The correct viscosity and type of oil must be used and the oil filter must

have a drain back valve if the vehicle was equipped with one originally. Adequate oil pressure is required to operate the

VVT system.

Fixed Timing Limitations

With a fixed camshaft engineers had to balance between idle quality and performance on the one side and low emission

and fuel economy on the other. As a result, none of these goals was achieved completely. Variable valve timing allows the

engine to obtain a smooth idle while achieving the rest of the goals. Modern VVT systems combined with technologies like

electronic throttle control and direct fuel injection allow smaller engines to produce high horsepower and torque at lower

RPM.

Idle Valve Timing

In the (idle valve timing) camshaft timing chart at the end of the article the left hand trace represents exhaust valve opening

while intake valve opening is on the right. For smooth idle operation the VVT system reduces overlap (where both the

exhaust and intake valves are open.) Smooth idle requires that the exhaust and intake valves are not both open due to the

low air velocity. At low engine speeds the time the valves remain open is longer. If the exhaust valve is open it will cause

pulsing into intake.

Performance Valve Timing

For increased performance the exhaust cam is retarded a small amount to promote engine breathing. Higher engine

speeds mean shorter valve open times and higher air velocity. The increased velocity pushes more exhaust out of the

cylinder. Increased Volumetric Efficiency: The retarded exhaust valve timing increases Volumetric Efficiency. The exhaust

valve is still open when the intake opens. Outgoing exhaust pulse creates a low pressure zone behind the valve. This

increases the pressure differential between the intake port and the combustion chamber resulting in better cylinder filling.

Remember, this can’t be done at idle due to low air speeds.

EGR Function Valve Timing

To provide an EGR function, the exhaust cam is fully retarded adding lots of overlap. This causes exhaust gas to remain

trapped in the cylinder. This ability allows a reduction in hardware and service issues from carbon. Exhaust Gas Retention:

Opening the exhaust valve later retains more exhaust pressure in the cylinder. This causes pushback to the intake charge

and exhaust Gas RETENTION. Trapped Exhaust Charge: This provides EGR without hardware, even distribution, no

problems with clogged ports, and more precise EGR control.

Types Of Variable Valve Timing

Exhaust Only: The exhaust camshaft is retarded at part throttle. This delays the exhaust valve closing which allows some

exhaust gas into the intake stroke, which has an EGR effect. Th is also delays exhaust valve opening which increases the

expansion stroke.

Intake Only:

The intake camshaft is advanced at part throttle and WOT. This opens the intake valve sooner and allows some exhaust

gas into the intake stroke, which has an EGR effect. This also closes the intake valve sooner which increases the

compression stroke. On a cold engine opening the intake valve sooner will also warm the intake charge and assist in

reducing start up emissions.

Dual Equal:

Both the intake and exhaust camshafts are retarded at various points in the throttle range. This will increase the EGR effect

and improves fuel economy by reducing pumping losses.

Dual Independent:

The exhaust camshaft is retarded and the intake valve is advanced independent of each other. This maximizes the EGR

effect, and further reduces pumping losses for maximum efficiency.

Cylinder view of a trapped exhaust gas charge.

Cylinder view for stable idle control.

Graph of valve timing in performance mode.

Cylinder view of increased volumetric efficiency.

Graph of the idle valve timing function.

Cylinder view of exhaust gas retention.

Graph of the valve timing function for exhaust gas recirculation.

Lexus Case

January 15, 2013   Example Cases

2001 Lexus GS300- 2JZ-GE Inline 6

VIN: JT8BD69S51

Vehicle came into the shop with codes P0300 (random misfire) and the full rainbow of individual

cylinder misfire codes (P0301, P0302, P0303, P0304, P0305, P0306). P1349 was also present for VVT

system malfunction. Since the history of the vehicle was unknown, a tuneup was performed by another technician who

noted severe open insulation of the coil control wiring from the main engine wiring harness. The

technician also noted that the connectors to the coils were brittle/breaking; he subsequently repaired the

wiring insulation and replaced the connectors. He reported that the car drove out of the shop fine for

customer pickup. He erased the codes.

Two days later the car returned presenting with the same codes identified previously but this time

would stall out while driving and with a rough idle. It would also intermittently crank but not start.

I was called into the shop to look at the vehicle. On first cold start-up the car would start and drive fine

immediately; however as it was driven and warmed up it would idle rough and stall out making the car

completely undriveable. This appeared to happen more often and predictably as the engine warmed up

and entered closed loop; in fact the only time the car could be counted on to drive correctly was when

the vehicle was cold and it was in open loop on initial startup. Since the problem appeared to present

itself only during closed loop, I checked live data PIDs on my scan tool to see if a sensor was giving

false information to the PCM.

I noticed that fuel trims trended from +8-12% on both banks at idle to +2% at fast idle which made me

suspicious of a vacuum leak. I smoke tested the vehicle for vacuum leaks with my Redline SmokePro

and noticed multiple large vacuum leaks present; some were in the PCV system. I sealed the vacuum

leaks and replaced both PCVs on the engine. However the vehicle still idled rough and fuel trims now

showed -6% at idle and +8-10% at fast idle. Again, suspicious that a part of the closed loop feedback

system was lying I decided to labscope the MAF sensor (the vehicle also pinged under snap-throttle).

The MAF sensor tested bad not even achieving a peak voltage of 2.8V on snap-throttle (known good

Lexus MAF sensor waveforms indicate that the MAF should achieve at least ~4V at snap-throttle). This

is the bad MAF sensor waveform captured below:

The MAF sensor was removed and noted to be covered in dirt/oil. Subsequent cleaning did not bring

much of an improvement at only 2.9V at snap-throttle; the MAF sensor was condemned and replaced.

The new MAF sensor tested correctly going up past 4.1V at snap-throttle and eliminating the ping.

The car now ran well enough that it could be taken for a road-test (it still ran extremely rough and had

an intermittent no-start). Fuel trims revealed that under load, oxygen sensors pegged lean on both banks

and fuel trims trended high.

Since increasing fuel trims, lack of power, and oxygen sensor pegging lean under load are usually

indicative of fuel starvation, we checked fuel pressure. Fuel pressure was found to be 42 psi (deadhead,

at idle, and under load). Lexus specifies that fuel pressure should be 44-50 psi but at 42 psi we felt this

was not a low enough fuel pressure to explain the random misfire, lean cylinders, and lack of power.

One of the things an automotive technician is concerned about, when seeing random misfires and lack

of power under load, is the engine being out of time, especially on a vehicle with an uncertain

maintenance history. The timing cover was removed and the timing marks were found to be lined up.

However, visual identification of timing marks is not the most precise way to check timing (the gears

only have to be off slightly to create driveability problems). I pulled out my Picoscope 4423 (a 4-

channel high resolution labscope) and backpinned camshaft position and crankshaft position signals at

the ECU.

This is what I found:

A normal good crank/cam timing relationship. The red graph is the camshaft position signal; the blue is

the crankshaft position signal. This is what I like to see as the vehicle was idling well.

I then brought the vehicle up to fast idle and, since we had the VVT code, I activated the VVT solenoid

manually. The waveform capture below was obtained:

Notice the difference between the cam/crank relationship with VVT engaged versus the previous

waveform with VVT not engaged.

Now the interesting part is that as I was manually triggering the VVT solenoid, I noticed that it did not

always release and remained stuck (you can feel/hear the solenoid engage/release as you activate it). I

brought the car back down to idle and it immediately started to idle rough and die as it normally would.

Now look at the cam/crank waveform at idle as it was acting up:

Notice the similarity between this waveform and the one above it?

The relationship is identical! This means that the VVT is in fact stuck in the engaged position! The

VVT is NEVER supposed to be activated while at idle because it forces the vehicle to behave exactly

as this one was in the shop. This explains our P1349 as well as our P0300 random misfires!

Phaser-Style Variable Valve Timing System Controls And Operation

Figure 1: Camshaft phasers on a 2.4L Chrysler engine. The phasers use directed oil pressure to manipulate camshaft position relative to

the sprocket and therefore vary valve timing. Notice there is a spring on the exhaust cam phaser.

Vehicles with variable valve timing (VVT) have become commonplace over the last decade.

Different versions of VVT technology exist, including the ability to switch camshaft lobes within the rpm

band to offer valve lift, duration and timing numbers to suit varying operational needs.

Even more commonplace are engines that use a phaser to manipulate camshaft position and, hence, valve

timing.

(See Figure 1.)

The phaser style of VVT is the focus of this article.

Figure 2: The oil control valve is the traffic control device of oil pressure. In this hold position, neither chamber receives pressure nor is

drained.

Phasers commonly can be found on just the exhaust cam or on both the intake and exhaust cams.

Alteration of camshaft position changes the cam centerline and the lobe separation angle between intake

and exhaust cams.

This gives engineers flexibility in improving fuel economy and power while continuing to meet emissions

standards.

VVT presents additional diagnostic challenges and repair opportunities to the service industry including

new trouble codes.

If you are not familiar with these units, it’s time to advance your diagnostic readiness by examining the VVT

system, its controls and operation.

Mechanical, hydraulic and electrical controls have been

added to VVT engines.

Motor oil is the hydraulic medium that makes VVT

work.

That means it is imperative that engines are filled to the

correct level with clean motor oil of the proper viscosity.

Low oil level or the wrong viscosity can result in system

slow response codes such as P000A or P000B and

possible drive complaints including an illuminated MIL.

Oil pressure is critical, and as bearings wear and

develop clearance, pressure will be affected.

Engines are machined with additional oil galleys for

VVT and are equipped with one or more fine mesh

screens to prevent debris from entering components.

Replacing these screens often requires major engine

disassembly.

Sensors that monitor oil pressure and oil temperature

are common on VVT engines and are a part of system

control strategy.

The major control component in camshaft phasing is

the oil control valve (OCV). The OCV is a spool valve

much like those found in automatic transmissions. The

PCM (powertrain control module) duty-cycles a solenoid that alters valve position.

The OCV is an oil traffic control device of sorts. It determines which ports receive pressurized oil and which

are vented.

(See Figures 2, 3, 4 and 5.)

Pressurized oil travels through the OCV to one of the

camshaft bearing journals. Oil flows though

passageways inside and toward the front of the

camshaft.

(See Figure 6.)

At the nose of the camshaft, oil enters ports of the camshaft phaser. The phaser is a mechanism with two

major pieces, the rotor and the phaser body.

The phaser body is physically bolted to the camshaft

sprocket. The rotor is connected to the camshaft using

a dowel pin.

(See Figure 7.)

The two pieces are able to move about 20° (40

crankshaft degrees) independently of each other. Ports

inside the phaser direct oil in or out of eight chambers.

Four chambers are considered side “A” and the other four are side “B.”

As one group of chambers receives pressurized oil, the others are vented to provide the force necessary to

move or hold the rotor relative to the phaser body.

Oil seals fit into machined grooves of the rotor to

provide a tight seal between the chambers.

Vented oil from the phaser ports travels back through

the camshaft, the cam bearing ports, through the oil

control valve and then drains into the front timing cover.

There is a mechanical device inside the phaser known

as a lock pin.

The spring-loaded lock pin on the rotor engages into

the phaser body to lock the two pieces together.

(See Figure 8.)

The lock pin prevents noise and potential wear upon engine start. Oil pressure is required to disengage the

lock pin.

The 2.4L Chrysler engine that I disassembled also featured a spring on the exhaust camshaft.

The locked phaser positions on this engine are full

retard on the intake and full advance on the exhaust.

Because of the clockwise rotation when viewed from

the front of the engine, the exhaust rotor requires

additional assistance in reaching the full advance

position.

In the default position, there is no valve overlap. It

should be noted that service information does not

recommend phaser disassembly and individual parts

are unavailable.

As for service parts, phasers are sold as an assembly.

Electrically, the OCV solenoid has two terminals. I

measured the resistance of several solenoids from

various manufacturers.

They ranged between 7 and 12 ohms of resistance.

Both circuits connect to the PCM, which provides duty-

cycle control either on ground or the insulated (power)

side.

I found versions of both on our laboratory vehicles.

OCV solenoids are typically cycled upon ignition run

mode as part of a cleaning and diagnostic strategy.

Regardless of control specifics, the PCM monitors

solenoid circuits for faults including opens, shorts to

ground or shorts to voltage.

(See Figure 9.)

OCV solenoid circuit faults include P0010 and P0013.

We recently had a late-model V6 minivan in our lab with one of these codes.

The solenoid was internally leaking a small

amount of oil, just enough to soak the electrical

connector.

Electrical contact cleaner and a new OCV

solenoid repaired the vehicle.

These faults are on board diagnostic

comprehensive components rather than once-

per-trip monitors.

Besides retrieving trouble codes, scan tools are

useful to monitor desired versus actual

camshaft position and may also be equipped

with helpful actuator tests and cleaning routines.

(See Figure 10.)

Crankshaft position sensors (CKP) and camshaft position sensors (CMP) are used by the PCM to

determine camshaft phasing functionality. CMP tone rings or trigger wheels are usually connected to the

cam itself rather than the cam sprocket.

When the PCM commands the OCV

solenoid to advance or retard, CMP

patterns are compared to CKP patterns

to determine if the command is carried

out.

A variance or error value is calculated.

Once the variance reaches a certain

point, a fault is declared. These include

DTCs P0011 and P0014, which are

target performance errors.

This also makes it more important than

ever that camshaft timing be set

correctly during timing chain or belt

service.

A CKP or CMP sensor fault can also cause the

PCM to disable or limit VVT operation.

I tested a 2006 Pontiac Solstice equipped with

the dual VVT EcoTech engine using a duty-

cycle test box to command the OCV solenoids.

By comparing CKP and CMP patterns with the solenoids fully on then fully off, I determined about 48

crankshaft degrees of possible cam travel.

(See Figures 11-13.)

When the exhaust was commanded to the full retard position at idle, the engine ran rough and manifold

vacuum dropped below 15” of mercury.

When I manually commanded the intake to full advance, manifold vacuum dropped only 2” and the engine

started to sound like a diesel.

The vehicle was restored to proper operation and driven on a dynamometer to get a glimpse into VVT

control strategy.

At idle and during deceleration, the OCV solenoids remained off, resulting in the default position of fully

retarded intake and fully advanced exhaust.

The wide lobe separation angle with no overlap is

ideal for smooth idle. I also performed on-road data

recordings on a 2012 Dodge Avengerequipped

with a 2.4L dual VVT engine similar like that which I

had disassembled.

Like the Pontiac, the Dodge also maintained a 0%

duty cycle at idle and deceleration.

While testing these vehicles, the exhaust OCV

solenoid is quick to activate while the intake OCV

solenoid seems more sensitive to engine load.

I observed the most movement on both cams during

cruise. The solenoids on both vehicles were duty-

cycled moving the intake near full advance and the

exhaust near full retard.

(See Figure 14.)

This reduces lobe separation angle and results in a

large amount of valve overlap. This appears to be done to eliminate the EGR (exhaust gas recirculation)

valve.

None of our many lab vehicles with phaser-style VVT have an

EGR valve. Overlap at cruising speed is used to dilute the

intake charge.

Spent exhaust takes up space in the cylinder and results in

less oxygen content per charge, meaning less fuel is

necessary.

Combustion chamber temperature is reduced to lower oxides

of nitrogen (NOx) emissions.

Fuel economy is also improved.

Recordings were also performed at wide open throttle (WOT).

With manifold pressure near atmospheric pressure or vacuum held close to zero, I wanted to see the

influence of rpm build on the cams.

On the Avenger, the intake cam is the one with more correlation with rpm.

(See Figure 15.)

The intake cam starts out with large amounts of

advance for increased low-end torque, but it

gradually moves toward the full retard position

once rpms build to about 5,000.

This seems to make theoretical sense with

increased inertia of the intake charge able to

keep air flowing into the cylinders even as the

pistons start moving up on the compression

stroke.

The exhaust cam seems to hold steady at about

one-third of the way toward full retard.

This provides a balance between performance

and emissions.

There are most likely other control strategies on

these and other manufacturers’ vehicles.

With vehicle manufacturers having to use every tool at their disposal to meet sharply increasing CAFE

(corporate average fuel economy)

requirements, it is likely we will see more

VVT-equipped engines.

These systems are well engineered and

have proven to be reliable.

That being said, heat, age, wear and

lack of maintenance are likely reasons to

cause eventual failures.

Looking at these components, including

the fine mesh screens, provides great

incentive to keep oil changed regularly.

Hopefully this look at the system

components, controls and operation

leaves you ready in time to service the

growing number of VVT-equipped

vehicles.