Atkinson Cycle using VVT control unit 1

[pickler]

I would like to create an Atkinson cycle like behavior for my VVT (both intake and exhaust) enabled 6 cylinder 3.6L Subaru engine. Currently the VVT settings are as such under loads experienced cruising @65mph.

advertised Intake duration 244*
ivo 15* btdc
ivc 49* abdc
intake cam can be advanced or retarded 25* from this

advertised Exhaust duration 228*
evo 24* bbdc
evc 24* atdc
exhaust cam can be retarded or advanced 20* from this

The duration cannot be adjusted however the timings can. How can I change the timings here to induce Atkinson cycle behavior or to effectively reduce dynamic compression and engine displacement? This is obviously to achieve better fuel economy at cruise. I'm also assuming ignition timing needs to be advanced?

My guess would be to retard Intake CAM and Advance Exhaust CAM.

 

[BrianPetersen]

Delay intake cam a lot and delay exhaust cam a little.

To make full use of the expansion stroke, exhaust valve opening has to coincide with bottom-dead-centre. In practice the last few degrees before BDC accomplish little expansion and it's better to let the cylinder blow down excess pressure.

This will of course delay exhaust valve closure into the intake stroke, but that's ok, because you are also delaying intake valve opening (until after TDC, in your case). What this will do is create some "internal EGR" that helps to reduce the amount of intake charge (remember, you are running at part load, and you are TRYING to reduce the amount of intake charge while also trying to open the throttle and have less intake vacuum for lower pumping loss).

The delayed intake valve closure after BDC will pump some of the charge back out into the intake manifold, which is how these Atkinson systems all work.

Don't be surprised if Subaru already thought of this strategy.

 

[pickler]

this stock, not sure if it's accomplishing what you are suggesting:

This is what i was thinking of setting the VVT as:

am i getting this right? Is my exhaust too retarded/delayed?

 

[Tmoose]

Here are some incomplete Prius engine valve timing specs. I could not find valve event iming for ~ 2001 Prius, which had no variable valve timing

http://electrifyingtimes.com/priustechspecs.html

 

[gruntguru]

Be prepared to delay the intake cam even further. I assume you can only retard another 10* before you reach the 25* limit (still only 74* ABDC). I expect you would need to close the intake valve as much as 120* ABDC to achieve light loads with WOT.

je suis charlie

 

[svizoman]

Problems with modern aftertreatment exhaust systems is that they need at least 3 bar pressure. Cylinder pressure at BDC on attkinson cycle is almost that of ambient air.

 

[gruntguru]

"Problems with modern aftertreatment exhaust systems is that they need at least 3 bar pressure."
What?

je suis charlie

 

[Panther140]

Does that car's EFI system stay in closed loop for all load conditions? I'm trying to imagine being in a high-load open-loop condition with a car that is now rejecting much of the intake fuel/air charge back into the intake.

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

 

[pickler]

Ok so this is my final result. I ran this last night without issue on my drive home, about 30min of highway cruising. Car didn't feel any different but I am running 2* more ignition timing with this, though I feel as though there is less EGR compared to stock. The throttle response is more crisp. The intake can't be delayed any further. it's already at 50* retard, exhaust is only retarded 4*. It has room for 36* more if needed. It seems subaru's strategy was completely the opposite of Toyota's...it uses the early/late while prius uses the late/early VVT.

Closed or open loop it doesn't matter. even in closed loop the car can go as rich as 13.7:1 depending on load (I'm running straight non ethanol 91 octane gasoline). So long as I stay below 2500 I won't be running rich. But at cruise wideband shows 15.2:1 AFR. It seems to run leaner with this much reversion. I can tune the fuel ratios as well by the way.

 

[Panther140]

That's cool! Any change in observed gas mileage?

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

 

[gruntguru]

"The intake can't be delayed any further. it's already at 50* retard, exhaust is only retarded 4*. It has room for 36* more if needed."

I am confused by your diagram. It shows the exhaust cam advanced if I am not mistaken?

If you want to explore real Atkinson timing, you need a lot more intake delay. Did you look at the Prius numbers in Tmoose's post?

If you don't need all the intake advance available in your VVT system, you could retard the static setting of the intake cams (by one tooth perhaps?) The ideal would probably be an intake closing range from 50* to 100* approx.

Fascinating project BTW.

je suis charlie

 

[pickler]

If you don't need all the intake advance available in your VVT system, you could retard the static setting of the intake cams (by one tooth perhaps?) The ideal would probably be an intake closing range from 50* to 100* approx.

hey the intake is painted red above and it is closing at 74* which is in between what you suggested. so my current timing is:

IVO 10* ATDC
IVC 74 ABDC
EVO 40 BBDC
EVC 8 ATDC

any change in observed gas mileage?

well what is happening now is the wideband in the exhaust has started to read too little fuel...aka i'm running lean. This happens anytime i retard the intake to simulate this atkinson cycle. I think this is because some of the fuel mixture gets pushed back into intake tract (reversion) and not all of the intended fuel (as estimated by MAF sensor) makes its way to exhaust. Because of positive fuel trims caused by this i seem to be actually using more fuel. I'm thinking of removing closed loop so that it will not add additional fuel and allow me to run in a 'synthetic' lean burn mode.

 

[FeX32]

Looks like you need to change your camshaft phasers to electric. That way the range would increase.

You never mentioned what load condition this if for? You have to change the map for all load conditions and interpolate in between. It should be easy to notice pumping loss changes at 0% load. When you take your foot off the throttle it will take a bit longer to go back to idle (depending on transmission..)

 

[gruntguru]

Hi pickler.
Stock EVO 29 BBDC EVC 19 ATDC
Now EVO 40 BBDC EVC 8 ATDC
So Exhaust is advanced?

When I said The ideal would probably be an intake closing "range" from 50* to 100*, I meant "electronically adjustable range". At the moment your "range" is 24* to 74*.

je suis charlie

 

[BrianPetersen]

Exhaust opening is happening sooner now than stock ... that means what you have now has the exhaust "advanced". To make use of the expansion stroke at lower revs, the exhaust has to be "retarded".

You may find that the increased overlap that this results in, helps your lean condition.

And, don't be surprised if Subaru knew what they were doing when they set up the stock cam timing the way they did.

 

[pickler]

yes I had to advance the exhaust to ensure it closes before the intake valve is opened. This is what Toyota is employing as well on the Prius. No overlap on the intake stroke is a characteristic of Atkinson cycle. However 40* evo is still comparatively 'retarded'. On the small more fuel efficient BRZ/Impreza 2.0 DI engine the EVO is much more advanced at 70-50*. EGR is not the best way to control fuel consumption because the same amount of fuel is still consumed by the engine. In an atkinson cycle without intake stroke overlap the fuel is ejected back into intake. Now my only issue is dealing with the cylinders that are 'exhibiting' lean burnoff as detected by front wideband sensors. The main reason i think is because the maf sensor detects say 40 grams of air and injects say 3 grams of fuel, however due to late IVC some of the fuel doesn't get used...this causes the rear o2 sensors to freak out...where did the rest of the fuel go? The computer just calculates that 3grams of fuel in should equal 3grams of co2 out.

as for the loads this is between 0 to 0.9 g/rev cylinder filling. so cruising loads and engine speeds. As soon as the throttle is opened intake timing is advanced.

 

[gruntguru]

The air and fuel ejected during the compression stroke still gets consumed - overall mixture shouldn't change with LIVC.

Are you saying the mixture distribution between cylinders is unequal during LIVC operation? This is most likely caused by unequal sharing of the ejected fuel. Have a look at the intake layout and firing order and try to judge which cylinder is most likely to get the ejected fuel from the previous cylinder's charge. If it is not the next cylinder in the firing order, you have a problem. You won't be able to run significant LIVC without changes to the manifold/plenum. You might also need to consider end to end airflow within the plenum as a cause of unequal distribution of ejected fuel.

This is all a little bit surprising with the long runners typical of Subaru. The cylinder needs to eject enough mix to find its way all the way back to the plenum to cause any problems. Of course wave action will carry the fuel that far too - at certain rpm.

je suis charlie

 

[BrianPetersen]

I have my doubts that with the long-runner intake and the injector sitting right next to the head, that the amount of fuel shared between cylinders is any more than negligible.

Try retarding the exhaust cam timing. I don't care what your theory is ... try it. I'd shoot for EVO around 25 degrees BBDC and EVC 23 degrees ATDC (in other words, about 15 degrees retarded from what your last diagram showed).

 

[gruntguru]

Brian. If the OP does have a distribution problem during LIVC, do you have an alternate theory?

je suis charlie

 

[BrianPetersen]

Do we know that there is in fact a distribution problem?

Is there a separate wide-band sensor per cylinder?

When I see the words "front" and "rear" O2 sensors I understand it as the "front" sensor being pre-catalyst (but shared for all cylinders) and "rear" being post-catalyst (obviously shared for all cylinders). If the engine has two banks of three with a separate catalyst for each (common on V6) then the proper terminology is "bank 1 front", "bank 1 rear", "bank 2 front", "bank 2 rear".

Original poster needs to tell us, accurately, what instrumentation he has, and if there are specific cylinders acting up, how he knows that.

And it's quite possible for the pulses in the intake system to be wreaking havoc even if they are not carrying fuel or recirculated exhaust with them. If it uses an oem "log" exhaust manifold, or if it has an integrated exhaust manifold cast into the cylinder head (meaning no exhaust header pipes from individual cylinders) then there could be cross-talk between cylinders on the exhaust side, too.