Variable Timing - how does it work?

[Dickensheets]

I've heard of this in past years but paid little attention because Soob didn't offer it. It looks as though that is changing, so a question...

 

how does it work?

[RallyKeith]

Essentialy there is a mechanism between the cam sprocket and the cam shaft that allows the cam to be changed a certain number of degrees in relation to the normal position. This is basically built into the cam sprocket. Oil pressure is used in the same way a power steering rack works to add pressure to one side or another to move the cam in relation to the normal position. The mechanism that controls the pressure lives ontop of the cylinder head. So, you literally spin the cam backwards a few degrees while the valves are open to infact leave them open longer than if it was just following the cam lobe. Then move it back forwards to normal to open the valves at the corect time again. I'm probably not doing it justice, but I'm trying to explain what I know.

 

Keith

[cookie]

In real life it is a very nice feature. My BMW M Roadster has what BMW calls Vanos variable valve timing. Although it is one more thing to have problems with, only a few folks I know have had trouble with it yet.

My 65 GTO idels with a rumpety rumpety rumpety and does not come on the cam untill about 4000 RPM. It is really a bit ugly to drive in traffic and don't ask about gas mileage.

The BMW idles like a tractor and spins up like an electric motor, The only time I notice the vanos is when I come into the basement garage at work. I am trying not to lay rubber so I go pretty easy. When you let off the throttle you can feel the vanos shift to tractor mode, and then the car will idle smoothly along.

Subie may be able to add this feature to later models, but it does seem to take up a bit of room in the head.

[RallyKeith]

This above post made me think about something and I thought I should point out that the Subaru system is truely variable valve timing. The honda Vtec isn't. It merely engages a differnt cam lob to change the valve timing.

 

Keith

[Commuter]

There are different ways they go about this. I'm not familiar with how Subaru is doing it, but I can make an educated guess from the pics in the brochure.

 

I was up on Honda's VTEC when introduced. They had a second cam with 'half' a rocker on it that could be locked to the regular rockers at higher rpm. The valves would then follow the action of the second cam, which had higher lift and/or duration. The iVTEC (intelligent) I think was simply programming. The original versions kicked in at one set rpm. The iVTEC would kick in at various rpm depending upon conditions (sensors).

 

I'm looking at the pic of the cams in the Subaru brochure. The cam appears segmented. The inner part is one profile, and the outer 2 parts another profile. The cam is directly acting on the valve via some sort of tappet (no rocker here). It appears that the tappet body follows the outer part of the cam, while (I'm guessing) a smaller diameter pin/cylinder in the center can be engaged and made to extend such that it would follow the inner cam profile. This is most likely done by allowing oil pressure into the unit. The tappet (as I'm calling it) would be much like the hydraulic lifters of old in the pushrod cars I imagine.

 

Both the systems I've just talked about make use of 2 different cam profiles. Neither one varies the camshaft position itself in relation to the crankshaft. This is another approach as RallyKeith described. You need a mechanism between the camshaft sprocket and the camshaft itself to allow several degrees of motion. This affects valve timing. The cam itself affects valve lift and duration. Obviously, both can be combined. I have no idea if Subaru is using this feature. (See below.)

 

The ultimate is to just get rid of the camshaft and just stick some sort of actuator (think solenoid) directly on the valve. Then it can open and close in a flash (no cam profile to be concerned with) and the timing and duration (and even the lift) can be infinitely controlled / varied by programming. I know some manufacturers were toying with this decades ago, but cost and reliability were an issue. I'm not sure where such efforts are at today.

 

Edit - I was just reading the brochure a little further (it's a 2005) and it indicates that the 3.0 H6 engine has AVCS (Active Valve Control System) and AVLS (Active Valve Lift System). So, judging from the names, Subaru is using both methods in this engine. The 2.5L turbo mentions AVCS only, which makes sense. Valve lift doesn't matter as much when you have a turbo pushing the air in.

 

Commuter

[RallyKeith]

 

I'm looking at the pic of the cams in the Subaru brochure. The cam appears segmented. The inner part is one profile, and the outer 2 parts another profile. The cam is directly acting on the valve via some sort of tappet (no rocker here). It appears that the tappet body follows the outer part of the cam, while (I'm guessing) a smaller diameter pin/cylinder in the center can be engaged and made to extend such that it would follow the inner cam profile. This is most likely done by allowing oil pressure into the unit. The tappet (as I'm calling it) would be much like the hydraulic lifters of old in the pushrod cars I imagine.

 

 

What brochure are you looking at? This is not at all how it works based on the Factory Service Manual I have for my USDM 2004 STi, nor is it from what I can see on my engine. The system on the USDM STi works as I have described. I've never seen anythin like you have described. I'm curious...

 

Keith

[Commuter]

What brochure are you looking at? This is not at all how it works based on the Factory Service Manual I have for my USDM 2004 STi, nor is it from what I can see on my engine. The system on the USDM STi works as I have described. I've never seen anything like you have described. I'm curious...

Keith

See my edited note above. The AVLS is on the H6 only (in 2005) it appears.

 

Commuter

[RallyKeith]

See my edited note above. The AVLS is on the H6 only (in 2005) it appears.

 

Commuter

 

Ok, that's what I found while doing a little research. this is what I found :

 

AVCS and Variable Valve Lift

 

The Active Valve Control System (AVCS) in the Outback 3.0R now includes Variable Valve Lift, to increase engine power and torque output while at the same time gaining improvements in fuel consumption, exhaust emissions and idling stability.

 

By controlling the intake valve timing and lift to suit the engine load and speed conditions, AVCS optimises the engine volumetric efficiency and combustion process. Active variable valve timing is achieved by rotating the intake camshaft relative to the camshaft sprocket, within a maximum range of 50 crankshaft degrees.

 

Variable lift is achieved through the use of two different sets of cam profiles - low and high lift cams.

 

The Engine Control Module (ECM) calculates the optimum valve timing and lift, based on input signals from the airflow sensor, engine coolant temperature sensor, throttle position sensor and camshaft position sensors.

 

The ECM then generates a signal to an Oil Control Valve (OCV) positioned at each intake camshaft sprocket. This controls engine oil pressure, which is supplied to advance and retard chambers within the AVCS actuator.

 

Similarly, the ECM controls Variable Valve Lift. This is via a solenoid valve, which supplies oil pressure to a switchable tappet that alternates between the low and high cam lobes, depending on engine load and speed conditions.

 

In simple terms, the engine runs at low lift below 2,000 rpm, then switches between low and high between 2,000 and 4,000 rpm, depending upon load.

 

Above 4,000 rpm, valve lift is high, irrespective of engine load.

 

Valve timing is continuously and infinitely variable within the 50 crank degree range. It is controlled according to engine speed and load conditions.

 

Computer mapping is programmed for optimum valve timing, producing stable idling, improved fuel consumption and low emissions in the medium speed range, and maximum power at high engine speed and load.

 

Sounds pretty cool, and way better than what honda does. (And I used to own one of those Vtec hondas and thought it was so cool )

 

Keith

[1 Lucky Texan]

There are 2 different systems. There is a dual lift system which does involve a pin being engaged. There is the variable valve system whic is in the cam sprockets.

 

 

Now, is the variable valve system truly analog or is it simply switching from one timing to another?

 

Intersting stuff

 

carl

[ferret]

The oil solenoids which control the pressure to the cam sprocket area are Open or Closed. So Valve timing is Advanced......or Standard. Only inbetween during energizing or retracting. Think of it as a hydraulic advance.

 

Toyota has used this on some of thier V6's also. I haven't seen one of those leaks or cause problems. Maybe this is why Subaru went with it?????? (just a guess). And yes..I remember High Lift, advance degree cams on the 60's and 70's. Spent a lot of time and money on those which would not idle smoothly, or at all under 1000 rpm.

[1 Lucky Texan]

found this;>>>Overlapping the timing of the closing of the exhaust valves and the opening of the intake valves can help the engine perform better under heavy loads, but not under light loads. AVCS continuously varies this overlap through an infinite number of positions. Overlap ranges between a slight overlap (“retard” position) through as much as 35 degrees of the crankshaft rotation (“advance” position).<<< from here;

 

http://www.drive.subaru.com/Win05_WhatsInside.htm

 

and

 

>>>More Power and Enhanced Automatic Transmission

The naturally aspirated 2.5-liter boxer engine in Impreza 2.5 i and Outback Sport models gains

the new i-Active Valve Lift system that boosts horsepower to 173 from 165 in the 2005 models.

Just as important, the new technology improves real world drivability and performance. Peak

torque output remains at 166 lb.-ft. at 4,400 rpm – still one of the highest torque ratings in the

segment.

 

Using oil pressure generated by engine speed and load, the i-Active system varies intake valve lift

by locking the tappet into a higher position by means of a transverse pin. The effect is that of

having two separate tappets to act on the camshaft lobe operating the intake valve. Variable lift

works on one of the two intake valves. At low engine speeds the two valves open at different

levels, causing a swirl of air and fuel and boosting torque. At higher speeds, both valves are

opened in the “high” position to increase horsepower.<<<

 

 

Carl

[Setright]

Here come my contribution:

 

Valve opening, a glossary by Setright:

 

Valve Timing: Degrees related to the crankshaft rotation and therefore piston height in the cylinder. Indirectly related to ignition timing.

 

Valve Lift: How far does the valve lift off the seat at the highest point.

 

Valve Duration: How long does the valve spend away from it's seat. What RallyKeith explained the first time has been used in MG/Rover engines and is more a question of duration than timing.

 

Valve Overlap: What length of time is spent with both intake and exhaust valves open. Large overlap gives the crappy idle talked about already, but excellent purging of the cumbustion chamber at higher revs. And also plenty of raw fuel into the exhaust - either killing catalysts or making the bangs that makes a highly strung race engine sound cool!

[RallyKeith]

Here come my contribution:

 

Valve opening, a glossary by Setright:

 

Valve Timing: Degrees related to the crankshaft rotation and therefore piston height in the cylinder. Indirectly related to ignition timing.

 

Valve Lift: How far does the valve lift off the seat at the highest point.

 

Valve Duration: How long does the valve spend away from it's seat. What RallyKeith explained the first time has been used in MG/Rover engines and is more a question of duration than timing.

 

Valve Overlap: What length of time is spent with both intake and exhaust valves open. Large overlap gives the crappy idle talked about already, but excellent purging of the cumbustion chamber at higher revs. And also plenty of raw fuel into the exhaust - either killing catalysts or making the bangs that makes a highly strung race engine sound cool!

 

Great descriptions!

 

It is, or is it correct to say that with the subaru system they can effect both the timing and duration? That is the way I understand that it can work.

 

Keith

[Commuter]

Here come my contribution:

 

Valve Duration: How long does the valve spend away from it's seat. What RallyKeith explained the first time has been used in MG/Rover engines and is more a question of duration than timing.

Well done Setright, but one little quibble...

 

I agree with the definition, but what RallyKeith explained the first time was a system for affecting valve timing (AVCS in Subaru-speak). That is, when the valve opens relative to the pistons/crankshafts position. It is the cam profile that will affect duration (AVLS in Subaru-speak). Correct?

 

I guess I should have just looked up some info as others have done. However, it seems that I surmised the operation of their AVLS pretty closely just from what I could see in the pic.

 

Commuter

[Setright]

Commuter, read his post again, I don't think I am wrong.

 

The cams are apparently shifted during opening, meaning increased duration.

[1 Lucky Texan]

I don't see how the cam's actual rotation speed is affected quickly enough to alter valve DURATION during a fraction of a stroke. To me it DOES look as if the cam is being 'shifted' to begin it's opening of the valve at different degrees (based on several ECU parameters as outlined at that link) to positively affect performance.

 

 

The i-Active thing MAY have an effect of increasing valve lift AND slightly increasing duration - but only because the 'pin' may allow the follwer to stay in contact with the lobe for a greater percentage of its profile - I'm less confi\dent about this - just a guess.

 

I wonder if the 2 systems can be combined someday in one engine?

 

Carl

[Commuter]

Commuter, read his post again, I don't think I am wrong.

 

The cams are apparently shifted during opening, meaning increased duration.

Yes, the cams are shifted (relative to the crank). However, that affects both the timing of when the valve opens and when it closes. The difference between those two times (which equals duration) is not affected. (It really would be better to express this in terms of degrees.) The opening and closing "difference" is purely a result of the cam profile. The mechanism in the cam sprocket does cause the valve to remain open later in the cycle (at higher rpms, as per the control logic), however, it means that the valve opens later as well.

 

Duration to me means the time between open and closed, regardless of when. Or as you well put it, the time the valve spends off the seat.

 

Timing means, when does it (it meaning valve opening) take place (relative to the 4 stroke cycle).

 

AVCS can infinitely vary the valve timing over a prescribed range. (50 degrees of crank rotation as posted by others.)

 

AVLS has only 2 positions, that of one cam profile, or the other. (Low lift below 2000 rpm, one or the other between 2000 and 4000 rpm, high lift over 4000 rpm, as posted by others.)

 

Fun stuff.

 

Commuter

[Commuter]

I wonder if the 2 systems can be combined someday in one engine?

 

Carl

They are... in the H6 engine.

 

Commuter

[1 Lucky Texan]

They are... in the H6 engine.

 

Commuter

 

WOW! didn't know that! beginning what year?

 

Carl

[Commuter]

WOW! didn't know that! beginning what year?

 

Carl

This came out in 2005. The hp got bumped up to 250. The original engine had 212 hp. AFAIK, the 3.0L H6 from 2001 to 2004 did not have these systems.

 

Commuter

[1 Lucky Texan]

This came out in 2005. The hp got bumped up to 250. The original engine had 212 hp. AFAIK, the 3.0L H6 from 2001 to 2004 did not have these systems.

 

Commuter

 

Yeah I don't think our '03 OBW H6 has any of that stuff. So THAT'S where the xtra ponies came from!

 

thanx

 

Carl