reduced valve lift and early inlet valve closure

[pickler]

I have been messing around with my Subarus ECU for quite a while now. i have made some improvements in ignition timing and AFR which has made it both responsive and slightly more fuel efficient. For example I advanced part throttle ignition timing by about 3-8*.

recently i learned about its VVL system (valve lift) and i started tinkering with that. this engine does not have VVT or variable valve timing. I learned that from factory the VVL activates once idle conditions are not met. meaning as soon as throttle is opened high lift cams are engaged even at very low engine speeds. I changed the parameters so high lift cams are only switched to above 3000 rpm which is around where i near my highway top speed of 77mph, though i barely go above 70, but i wanted some buffer. Now according to service manual the VVL only controls lift of half of the intake valves, the other half are always in high lift mode to promote swirl. Also the low lift cams have a much much earlier closure intake timing of 10BBDC or -10ABDC. the duration of the intake cams in low lift is also very short. I also realized that operating with low lift cams requires much less ignition timing unless you approach higher RPMs (3200+) compared to high lift cams.

now here is my queston; will a reduced intake valve lift, early inlet valve closure and short duration improve fuel economy at part throttle?

 

[Lou Scannon]

Given CAFE requirements, not to mention bragging rights, it's unlikely that the OEM left much on the table in the way of fuel economy.
There may be some theoretical gain from moving the charge throttling from the intake manifold to the intake valve, but without knowing the overall OEM engine management strategy, if you dick around with one parameter, you might unknowingly adversely affect something else.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[Panther140]

The OEMs have to meet NOx emissions standards when calibrating their EMS. Garage tuners do not.

Bigger concern is the fact that you're running "low lift" mode past idle. I am not 100% sure of the mechanism they use to achieve low lift, but it may not be suitable for higher RPMs. You might get away with it, but might not be safe on a mass production basis.

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

 

[Lou Scannon]

The OEMs have to meet NOx emissions standards when calibrating their EMS. Garage tuners do not.

True enough, but part load is not a high NOx mode; however, it comprises the major part of the various test cycles over which official fuel economy numbers are established.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[JayMaechtlen]

Well - you said that you made those changes - have you been able to evaluate the results?

Jay Maechtlen

http://www.laserpubs.com/techcomm

 

[pickler]

yes and the results were poor. it faired better with more ignition timing and higher cam lift and longer duration. AFR target is 14.7 at cruise and that seems to give best economy at this CAM setting

 

[FeX32]

It (the lower INT lift to 3Krpm) will certainly kill some power. As for fuel economy; that will depend on more factors. For example, because of the loss in power will you be compensating with more throttle without really realizing it..

 

[Panther140]

Id imagine that low lift and early intake valve closure was done to increase the dynamic compression ratio at idle, and then recover pumping losses on the compression stroke. Do you know the cam specs for each mode? Is this naturally aspirated or turbocharged?



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

 

[gruntguru]

"AFR target is 14.7 at cruise and that seems to give best economy at this CAM setting "
That would be an unusual engine. Best economy is almost always somewhere between 17 and 19 (with lots of extra ignition advance).

je suis charlie

 

[Panther140]

Is there a universally agreed upon % of load that is implied when you say "cruise"? Cruising at 55 and 75 are different conditions. IIRC, my smaller turbo cars would actually build boost at 80 just cruising.

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

 

[BrianPetersen]

There can be no such thing. "Cruise" on my 12 horsepower Honda CBR125 is wide open throttle most of the time. At steady speed, my late model Chrysler van runs the engine in the highest gear possible with as much load as it can (consistent with driver's power demand). If you are at a steady speed in top gear, as you press the accelerator the first action is to increase load and it does that up to full load before downshifting. If it is already in top gear it will take the engine to near full load barely above idle speed if it can. For a given power output that is only a fraction of rated power, the most efficient condition is near full load at the lowest possible revs. I doubt if that engine is using enrichment under those conditions.

 

[Panther140]

If you aren't knock-limited, it makes sense to run it as lean as possible at high loads (less pumping loss). Does it have any problem with knocking? I wonder what the trade off is when meeting the power demand with a lean mix at high RPMS vs. an enriched charge at WOT and low speed.

 

[BrianPetersen]

I'm pretty sure the Pentastar has a knock sensor (probably one for each bank), and the compression ratio is pretty high, but it also has a well-designed pentroof combustion chamber and variable valve timing on both intake and exhaust, so the system has plenty of ammunition against knock.

For something less than full load, there probably is a theoretical tradeoff between slightly rich at lower revs (which theoretically can make a little more torque) and stoichiometric at higher revs, but emissions considerations likely dictate that it runs stoichiometric.

 

[Lou Scannon]

The thing to keep in mind regarding air-fuel ratio and thermal efficiency, is that the best results will only be obtained when the combustion system is optimized for the target air-fuel ratio. For the purpose of this discussion, "combustion system" should be understood to include compression ratio, mixture preparation, in-cylinder charge motion, spark energy/characteristics, etc.
Since, by dint of having to provide the correct feedstock to the 3-way catalyst at all times excluding AECD operation, gasoline passenger car combustion systems are optimized mainly for stoichiometric operation. Consequently, I wouldn't expect these engines to exhibit a lot of potential for improved efficiency at leaner than stoichiometric air-fuel ratios.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[gruntguru]

I think any SI engine and especially modern EFI ones, can be easily leaned at least 15% from stoichiometric and see a 5 - 10% efficiency improvement. The only change required would be some extra spark advance. 20 - 30+% leaning would still only require minor changes - hotter intake air, higher spark energy. Direct injection permits further leaning and a system like Mahle TJI extends the lean limit beyond 100% (lambda 2.0+). At mixtures this lean and with rapid combustion courtesy of TJI, NOx is controlled without need for a 3-way cat. The downside is, with 50% less fuel, the engine power is probably reduced by about 40% so increased displacement or forced induction is required.

je suis charlie

 

[Lou Scannon]

No argument with any of the above. I will point out that, in my book, everything after

would still only require minor changes

constitutes part of the combustion system that I mentioned.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[TrackRat]

The shorter duration cams and lower lift valve action on half of the intake valves is likely to increase pumping losses as the rpm increases. IME it's a balancing act to find the best combination for each application depending on the performance goals, emissions requirements and engine aspects that can be altered. I don't think anyone could give exact parameters other than from firsthand knowledge of a very specific engine and application which may have different objectives than the OPs. What works in a 2500 lb. pass car is not likely be the ideal engine calibration in a 3800 lb. SUV, etc.