AVCS - Intake and Exhaust Timing

[Kodename47]

Been looking into this a bit at the moment.

Any ideas what the base cam angles are? Any idea what the min and max values are for the intake and exhaust tables? This would be helpful to calculate overlap etc.

Useful info on dual AVCS is thin on the ground but there's 2 good threads that I've read:
Tuning dual AVCS
Single/Dual AVCS Comparison

[mad_sb]

Originally Posted by Kodename47

Been looking into this a bit at the moment.

Any ideas what the base cam angles are? Any idea what the min and max values are for the intake and exhaust tables? This would be helpful to calculate overlap etc.

Useful info on dual AVCS is thin on the ground but there's 2 good threads that I've read:
Tuning dual AVCS
Single/Dual AVCS Comparison

Thanks for the links, reading now. This pdf has a lot of good info in summary format, cam system included.

I wrote a thread a while back on cam tuning for these cars. It was done using brzedit but the info still applies.

[Kodename47]

That's all really handy info. I had forgotten about that post and it didn't appear on any Google searches either.


Typically I'm a little confused. You stated that the intake cam is adjustable from -10 to 40 which would give it a 50 degree range, but the pdf shows that the control operation range is 68 degrees!?


The exhaust system isn't too bad as I figure you have control from 0 to 50 deg retard and the diagram shows 54 degrees control range.


Do I assume for calculating overlap etc that 0 exhaust retard starts at 27 deg BBDC and the intake is 0 (on the map) intake advance starts at 14 deg ATDC?

[arghx7]

Originally Posted by Kodename47

That's all really handy info. I had forgotten about that post and it didn't appear on any Google searches either.


Typically I'm a little confused. You stated that the intake cam is adjustable from -10 to 40 which would give it a 50 degree range, but the pdf shows that the control operation range is 68 degrees!?


The exhaust system isn't too bad as I figure you have control from 0 to 50 deg retard and the diagram shows 54 degrees control range.

Reference for above statement, from the pdf, assuming it is accurate on some level:



regarding duration, compare to reported duration for the stock cams from Crower:

Originally Posted by DAMotorsports

The info we have for the stock cams is only the lift and duration as tested by BC.

Duration@.05: Intake 206 Exhaust 200
Lift@.05: Intake 268 Exhaust 265

0.05 is inches, I presume the .268 is inches but that would be about 6.8 mm lift on the intake side, which seems way too low. I don't know how there could be less than 10mm lift. So maybe those measurements are suspect. Now to answer your question:

Do I assume for calculating overlap etc that 0 exhaust retard starts at 27 deg BBDC and the intake is 0 (on the map) intake advance starts at 14 deg ATDC?

I would say "yes" with the caveat that we don't know what lift this diagram is at, assuming it reflects this engine at all. Doing quick duration calculations shows over 250 degrees duration at intake. So that might include the whole width of the intake cam at very very low lifts.

See the tricky thing is, we don't know what the actual ramp of the cam grind is. I've posted this diagram before, showing the full cam grind for a BMW 2.0 direct injection turbo engine at full valve lift:



this is the kind of thing you ideally need. You can calculate an overlap duration, but it's hard to get the full story from that because you don't know the grind of the cam. You really need to calculate the overlap volume at a given set of cam positions. The overlap volume is the shaded area when the two cam grinds overlap due to phasing. Also, for reference, here are the stock cam phasing maps:

[mad_sb]

Originally Posted by Kodename47

That's all really handy info. I had forgotten about that post and it didn't appear on any Google searches either.


Typically I'm a little confused. You stated that the intake cam is adjustable from -10 to 40 which would give it a 50 degree range, but the pdf shows that the control operation range is 68 degrees!?

Yeah, i was reviewing everything last night and it does seem the tech docs specify a greater range for the cams than what the rom is showing with a couple of tools.

However,... the 2d and 3d tables in the rom include the data scaling in the table definition so the units and range presented are correct, they just may not be scaled the way we think i guess you could say.

Originally Posted by Kodename47

The exhaust system isn't too bad as I figure you have control from 0 to 50 deg retard and the diagram shows 54 degrees control range.


Do I assume for calculating overlap etc that 0 exhaust retard starts at 27 deg BBDC and the intake is 0 (on the map) intake advance starts at 14 deg ATDC?

Yes, i think so, but you can look at the stock maps in combination with the phase diagrams to figure it out for the most part. I think i will put to together a collage mapping the diagrams from the pdf onto the stock map this am.

[arghx7]

There's a difference between the specification of the phaser, i.e. its adjustment range as advertised by the supplier, and the amount of cam phasing used in the final production calibration.

It's certainly possible that the diagram from the service manual is based on the hardware's capability and not what was chosen as the final values. It's not uncommon for the supplier to provide a phaser with a wider range of authority than the calibration engineers decide to use. That's not unusual.

Think of it this way:

One guy provided the requirements for the phasers, most likely by some simulation or guesstimate before the engine ever existed in a physical form.
The engineer responsible for the phaser design release worked with purchasing and the supplier to get something that meets the minimum requirements provided to him.
Some guy came up with initial cam phasing maps in early development.
Some other guy(s) came up with the final cam phasing values after a gazillion hours of dyno work.
A completely different guy put that service manual together.

[mad_sb]

Here is a starter image that combines the pdf data with the stock cam maps. I know they say "safe" but logging has confirmed over and over these are the maps used 99% of the time, lots to fill in with max overlap areas etc. I have some basic zone overlays but i want to study the info some more.

[arghx7]

Some rules of thumb to understand about cam timing:

Overlap

1)When the overlap occurs before top dead center intake, with the intake valve opening early, air/mixture pushes back into the intake port. This is because the piston is still rising in the exhaust stroke when the intake valve opens. This will throw hot exhaust gases back into the intake port. It can reduce pumping losses, improve vaporization of fuel on the back of the valves (port injection), and could potentially increase the chances of forming carbon deposits.

2)When the overlap occurs after top dead center (intake valve opens after top dead center intake, exhaust valve is still open), the piston will tend to draw exhaust gases back into the cylinder. This is common to reduce NOx emissions and improve fuel economy.

3) If intake pressure exceeds instantaneous in-cylinder and exhaust pressures, scavenging will occur. Extra air will blow through the cylinder, in the process evacuating residual gases from the chamber. This will also increase the mass flow to a turbocharger wheel.

Intake Valve Timing

4) Geometrically speaking, max effective volume and compression ratio occur when the intake valve closes right around bottom dead center (give or take say 10 degrees).

5) Closing after bottom dead center reduces effective volume, effective compression ratio, and mixture temperature at TDC firing. This is because the piston has pushed air back out the intake valve. This will reduce the tendency to knock and in some instances could be called atkinson cycle or miller cycle.

6) Closing before bottom dead center has some similar effects as late closing, with lower effective compression, but mixture temperature can increase and so can knocking. Early intake valve closing is a form of dethrottling.

7) The air flowing into the cylinder has inertia. By closing the intake valve later at high speeds, more time is allowed to fill the cylinder up. Filling time vs effective compression ratio are the two major and in some ways competing aspects of intake closing timing.

8) Intake valve opening timing is a little less important for high speed power and cylinder filling. Opening later in the intake stroke, when the piston is at higher speeds, improves motion in the combustion chamber. However there can also be an increase in pumping work. Late intake valve opening is generally not good for power or torque, but it is good for in-cylinder motion and stable combustion.

Exhaust Valve Timing

9) The exhaust valve event is divided into two phases: the blowdown phase and the scavenge or evacuation phase--whatever you want to call the second one. Basically, at exhaust valve open there is a big high pressure pulse released, and that pressure pulse is bigger the earlier you open the exhaust valve.

10) Early blowdown can help relieve pressure in the cylinder, but depending on the exhaust manifold design blowdown can cause interference with other cylinders' exhaust pulses. It's one of the reasons why you have 4-2-1 headers and twin scroll turbos on I4 engines.

11) The earlier you open the exhaust valve, the more expansion work is wasted, but in many cases the less pumping work to move exhaust gases out. Late exhaust valve opening is better for increasing expansion work, at the expense of additional pumping work. It's one of the principles of variable exhaust lift systems.

11) Later exhaust valve closing timing can give more time to allow exhaust gases to escape, but too late could potentially draw gases back in (besides possible unwanted overlap).

Keep in mind this sort of "textbook" PV diagram:



Now when you're actually tuning the thing in the vehicle, you basically are doing what @mad_sb did in his very fine thread on cam phaser sweeps. With a turbo though it gets more complicated, because you want a lot of overlap down low to spool the turbo through scavenging. But keep those effects in mind when you are looking at valve timing diagrams and cam phasing maps.

[Kodename47]

I thought I'd add this. I did a table to calculate the overlap of the stock map for those interested. This is based on 0 setting in the map as the base cam position which was part derived from the diagram above and shown in the pic. If anyone reckons that needs changing then let me know and I'll update:

[arghx7]

Good chart. As I have mentioned before though, it's hard to get an exact picture of how much effective overlap there is without knowing the grind of the cam. Many of those cells are listed as 17 degrees overlap. Well if the last 9 degrees of exhaust closing and the first 8 degrees of intake opening is at .01mm lift, well that may be effectively 0. Ideally we could plot up the cam grinds with the stock centerlines in MS Excel, and then add or subtract the degrees of phasing at given speed & load point. Then you can visualize what's really going on: what's the overlap volume (area under the curve where the two cams meet, like a Venn diagram), and how much of it falls in the exhaust stroke and how much is in the intake stroke.

[mad_sb]

Originally Posted by Kodename47

I thought I'd add this. I did a table to calculate the overlap of the stock map for those interested. This is based on 0 setting in the map as the base cam position which was part derived from the diagram above and shown in the pic. If anyone reckons that needs changing then let me know and I'll update:

intake cam range in software is -10 to 40, based on the info i assume -10 is full retard on *intake or about 10* after tdc.. can't tell if you used -10 or 0 for the intake.

*corrected from exhaust

[arghx7]

Wait. Looking at the diagram from the service manual again, doesn't the default most advanced position of the intake cam have intake opening at 24 ATDC intake, not 14 degrees?

Ok I see what you're saying now. You're referring to the -10 thing in the intake phasing map. Maybe -10 is 24 ATDC, and 0 is 14 ATDC.

[Kodename47]

Originally Posted by arghx7

Good chart. As I have mentioned before though, it's hard to get an exact picture of how much effective overlap there is without knowing the grind of the cam. Many of those cells are listed as 17 degrees overlap. Well if the last 9 degrees of exhaust closing and the first 8 degrees of intake opening is at .01mm lift, well that may be effectively 0. Ideally we could plot up the cam grinds with the stock centerlines in MS Excel, and then add or subtract the degrees of phasing at given speed & load point. Then you can visualize what's really going on: what's the overlap volume (area under the curve where the two cams meet, like a Venn diagram), and how much of it falls in the exhaust stroke and how much is in the intake stroke.

This is a really great idea, the issue is we need this info. Even a firm operating range of the cams would be a good start.


My basis is that full intake retard is 24 deg ATDC, so 0 on the map is 14 deg ATDC and therefore full advance opens at 26 deg BTDC.

[Kodename47]

Out of interest, how close are your VVT systems to the mapped values? Either I've got some poor data or my exhaust cam isn't playing ball. The intake cam is spot on, it's within a degree or 2, but my exhaust goes about 5 degrees out (low) and stays around that between 4-6k.



EDIT - Found answer below