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.

 

[pickler]

ok here is some info on car:
- 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.

 

[BrianPetersen]

So you have no way of knowing whether you have a cylinder-to-cylinder distribution problem.

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.

 

[pickler]

Sorry I’m very confused by your post but the throttle angle based on accelerator pedal angle can be changed/tuned. I have already done this to simulate a linear but sensitive power delivery (like drive by cable). Anyway I had a road trip this weekend and averaged 7.9L/100km or 30mpg which is I think very good for a 3.6L fulltime AWD car with 3800 lbs total weight. I’m not sure how I would have performed with stock ECU and VVT settings…but this car has EPA ratings of 18 city and 25mpg highway from factory. Average speed was 53mph which is very high as I did quite a bit of the way at 75+ mph. One thing I noticed however was water temperature was higher than usual by about 3* Celsius.

 

[gruntguru]

Brian is suggesting that if you rely on throttle angle alone, while leaving the cams at the "economy" setting, you should expect poor response. When you push the accelerator down, the cams should move to a setting that produces good torque.

je suis charlie

 

[BrianPetersen]

And the OEM systems that use this strategy will do so in a progressive manner so that the response to the accelerator pedal feels "normal", even though the (drive-by-wire) throttle itself may be doing strange things, such as spending a lot of the time open far enough that it presents little restriction to flow and there may be little intake manifold vacuum.

For cars with vacuum-operated power brakes and other systems, vacuum pumps are becoming commonplace.