Atkinson Cycle using VVT control unit
Atkinson Cycle using VVT control unit
(OP)
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.
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.





RE: Atkinson Cycle using VVT control unit
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.
RE: Atkinson Cycle using VVT control unit
This is what i was thinking of setting the VVT as:
am i getting this right? Is my exhaust too retarded/delayed?
RE: Atkinson Cycle using VVT control unit
http://electrifyingtimes.com/priustechspecs.html
RE: Atkinson Cycle using VVT control unit
je suis charlie
RE: Atkinson Cycle using VVT control unit
RE: Atkinson Cycle using VVT control unit
What?
je suis charlie
RE: Atkinson Cycle using VVT control unit
"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin
RE: Atkinson Cycle using VVT control unit
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.
RE: Atkinson Cycle using VVT control unit
"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin
RE: Atkinson Cycle using VVT control unit
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
RE: Atkinson Cycle using VVT control unit
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
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.
RE: Atkinson Cycle using VVT control unit
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..)
RE: Atkinson Cycle using VVT control unit
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
RE: Atkinson Cycle using VVT control unit
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.
RE: Atkinson Cycle using VVT control unit
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.
RE: Atkinson Cycle using VVT control unit
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
RE: Atkinson Cycle using VVT control unit
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).
RE: Atkinson Cycle using VVT control unit
je suis charlie
RE: Atkinson Cycle using VVT control unit
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.
RE: Atkinson Cycle using VVT control unit
- it has a 3.5in diameter aluminum mandrel bent single air intake with heatshield with 2 path ways one through above radiator and another through the brake duct for cold air support. The intake is separated from rest of engine bay with steel shield. This tube is about 30in long. There is a dual 62mm throttle body that it hooks up to. The intake manifold has a relatively small plenum at about ~15% of engine size i would estimate. The intake runners are about 10in long each. The exhaust manifold is a log type cast iron with about 5 to 10in long primaries and 15in collectors which include the catalyst. There are 2 wideband sensors before cat and 2 narrowband after. so 4 in total 1 in each bank. Then it merges in a Y pipe, the rest is not important.
Anyway I tried the stock retarded exhaust timing as suggested above. I'm still running lean but now the throttle response is poor. I don't see a difference in fuel consumption but my average fuel consumption has gone down since i started retarding the intake few days ago.
RE: Atkinson Cycle using VVT control unit
And I'm not surprised that you're getting poor throttle response now. The whole deal with this sort of cam timing is to try to make the engine run at part load with as little throttling as possible (very low intake vacuum) ... and an inherent consequence will be that when it's in that operating mode, changing the throttle angle won't do much. The OEM system mapping will change both the throttle position and the target cam phasing as you vary torque request (accelerator position). Ideally the cam angles and the throttle position are mapped together so that the whole system simulates how normal accelerator pedal response feels. I gather that since you are talking about fixed cam angles (not varying them with requested torque a.k.a accelerator position) you are not there yet.
RE: Atkinson Cycle using VVT control unit
RE: Atkinson Cycle using VVT control unit
je suis charlie
RE: Atkinson Cycle using VVT control unit
For cars with vacuum-operated power brakes and other systems, vacuum pumps are becoming commonplace.