Ideal AFR theory (modern 4cyl engines)
Ideal AFR theory (modern 4cyl engines)
(OP)
In tuning modern compact high compression NA engines, i have always had trouble dealing with knock/detonation under wide open thorttle. it seems these cars can exceed Minimum Best Timing for torque without knock under partial throttle but nearing full throttle they start to knock severely far from MBT. The cars I tune come fairly lean from factory ranging from 14.5:1 to 12.8:1 AFR based on engine speed under WOT. Enriching the mixture has helped me reduce knock at mid-high RPMs considerably and this also allows me to run more timing for more power (ideal can be more power?). The reason why I went richer rather than touch timing is because timing is retarded enough as it is. Most cars I see are usually pulling 15% timing due to knock. Most of these cars can't handle more than 22-25* timing by peak power.
I know the ideal Air Fuel Ratio varies with many factors. But assuming I retarded some timing due to higher flame front speed and knock, what negative effects are produced going from say a lean 13.5:1 fuel mix to a 12.3:1 rich mix under WOT? less power, reliability, efficiency? Is this mixture even ideal if i make minor gains (brake specific fuel consumption comes to mind)? I read conflicting report about fastest flame front speed AFR and some articles claim that best BSFC comes at ~0.9 lambda which only produces knock for me unless i retard timing another 3-5*. Yet less timing and leaner will surely cost lots of power no? There is also quite a bit of knock remaining at low RPMs, specially in higher gears. Adding fuel and taking away timing does not seem to help lower RPM knock though.
I know the ideal Air Fuel Ratio varies with many factors. But assuming I retarded some timing due to higher flame front speed and knock, what negative effects are produced going from say a lean 13.5:1 fuel mix to a 12.3:1 rich mix under WOT? less power, reliability, efficiency? Is this mixture even ideal if i make minor gains (brake specific fuel consumption comes to mind)? I read conflicting report about fastest flame front speed AFR and some articles claim that best BSFC comes at ~0.9 lambda which only produces knock for me unless i retard timing another 3-5*. Yet less timing and leaner will surely cost lots of power no? There is also quite a bit of knock remaining at low RPMs, specially in higher gears. Adding fuel and taking away timing does not seem to help lower RPM knock though.





RE: Ideal AFR theory (modern 4cyl engines)
Cheers
Greg Locock
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RE: Ideal AFR theory (modern 4cyl engines)
"Schiefgehen will, was schiefgehen kann" - das Murphygesetz
RE: Ideal AFR theory (modern 4cyl engines)
RE: Ideal AFR theory (modern 4cyl engines)
I agree with bTH about a cushion - I'd rather leave a few ponies on the table, than have melted piston tops when things go a little off target.
"Schiefgehen will, was schiefgehen kann" - das Murphygesetz
RE: Ideal AFR theory (modern 4cyl engines)
What throttle setting/BMEP are you talking about?
Are you tailoring your advance curve in specific regions along with throttle opening/manifold vacuum, or just moving the basic "static" timing point.
Are you measuring BSFC, or was that a report of your results tweaking timing at the 0.9 lambda?
RE: Ideal AFR theory (modern 4cyl engines)
RE: Ideal AFR theory (modern 4cyl engines)
Yes, it will be leaving horsepower on the table, but that allows the 3-way catalyst to do its thing practically all the time.
RE: Ideal AFR theory (modern 4cyl engines)
I'm just talking strictly WOT here. I have noticed that at part throttle MBT can be exceeded without knock anyway. so stoich with high timing is fine to keep EGT in check.
this particular engine is a 11.5 compression Subaru flat 4 sohc with high lift cams. so far an 11.8:1 AFR has finally allowed us to dial in just 20* of timing at peak power on 94 octane gas. stock it can handle 24* on 91 octane gas with 12.5:1 AFR. I usually deal with Mitsubishi, Honda, Nissan and some Mercedes NA powerplants.
RE: Ideal AFR theory (modern 4cyl engines)
"Schiefgehen will, was schiefgehen kann" - das Murphygesetz
RE: Ideal AFR theory (modern 4cyl engines)
Two thoughts.
If the engine is smallish for the vehicle that it is installed in, it's quite conceivable that the various emission testing protocols will require operating the engine near peak torque at lower revs during some part of the test cycle. If that is the case, then the manufacturer has essentially no choice but to operate in closed-loop under those conditions in order for the vehicle to pass its emission certification. I'm pretty sure that's the case with my car, because it will pretty easily see 100% load and full atmospheric pressure on MAP according to scangauge, and it also reports closed loop in those conditions.
And even if that's not entirely the case ... One of the ways the manufacturers eke out better fuel consumption figures is to run the engine at as low revs as possible and under high load, as opposed to letting the engine rev but with lower load. In order to do that, it has to simultaneously pass the emission certification tests (the test for emissions and fuel consumption is the same). In order to do THAT, it has to run in closed loop under those conditions.
RE: Ideal AFR theory (modern 4cyl engines)
however there is also question of flamefront propagation and speed. SAE claims ~12.2:1 AFR causes highest flame front speed but others claim richer 11:1 ratio is the fastest. Either way I would think one need to retard timing at fastest flame front speed no? I wish there was a definite answer to what ratio gives the fastest flame front speed. This could help distinguish between rich knock or high timing knock. Also this can mean that going rich at certain points will not allow you to run more timing so either staying lean or going super rich (like on turbo engines) is the ideal ratio.
RE: Ideal AFR theory (modern 4cyl engines)