Variable valve timing (VVT) in-depth

[Nikolaiski]

Power Boosting Technology

 

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Variable Valve Timing (VVT)

Basic Theory

After 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:

 

- 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.

 

- 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 Lift

 

In 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.

 

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[Nikolaiski]

1) Cam-Changing VVT

 

Honda 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.

 

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Examples

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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.

 

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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.

 

 

[Nikolaiski]

Cam-phasing VVT

 

Cam-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

Edited February 5, 2009 by Nikolaiski

[Nikolaiski]

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. However, I believe in the future more and more sports cars will adopt this kind of VVT.

 

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Examples

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Toyota's VVTL-i

 

Toyota

Edited February 5, 2009 by Nikolaiski

[Nikolaiski]

Rover's unique VVC system

 

Rover 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.

 

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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