This is an interesting read regarding power and injection timing.
https://www.hindawi.com/journals/jc/2016/6501462/
This is an interesting read regarding power and injection timing.
https://www.hindawi.com/journals/jc/2016/6501462/
I struggle to explain this, but here it is. Air temp difference between runs was within a few degrees and spark timing was less than 0.2 deg difference. Everything else was the same.
eoi torque.JPG
Hell from the graph, it kinda looks like any EOI after BDC (180) hurts torque....
Last edited by engineermike; 07-06-2021 at 10:50 AM.
Originally Posted by engineermike
Nice graph, my take away would be that anything past IVC hurts torque based on that graph. I don't think the EOI is later than BDC early in the RPM range? The piston speed should still be slow enough still to get enough fuel in there before and during peak torque.
I was out testing yesterday on a Z06 and for whatever reason the beta VCM Scanner didn't capture KPA properly but Airmass was the same between E60 and E70. However, E60 had better acceleration rates. EOI was no less than 170 on E60 but was down to 150 on E70. I'm pretty much convinced that running higher E content but letting EOI drop too far actually makes less power than E50-E60(or whatever blend for your setup) which keeps EOI closer to 180(BDC). This is something that many have said before on here so it isn't new. However, no one has ever shared much info on it rather than to just say "this is X and you should listen".
This is a pretty good article: https://core.ac.uk/download/pdf/4433861.pdf
These are a few notable quotes from it:
"The challenge for DI engines is making sure fuel evaporation takes place on time and there is adequate time for mixing so that the fuel doesn't hit the walls or the piston and the mixture in the end is truly homogeneous." - And here I was trying to give it <1 millisecond for evaporation between EOI and spark...
"Higher rotating speed means there is less time available for the fuel to evaporate." - Implies it's based on time, not angle. Time being a function of angle and speed. Could help explain why the difference on mine was much more dramatic at 7000 rpm than 6000.
"Injecting early during the intake stroke increases the chances of fuel wall impingement as the piston is close to the injector. Later injection generally results in higher charge cooling realized. However if injection is so late that part of the fuel evaporation takes place after Intake Valve Close (IVC), the charge cooling is still realized and helps with avoiding knock but doesn't contribute as much to increasing airflow rate and thus power output" - A bit counter to my results since injection timing can't help or hurt air flow if PD supercharged and it just killed my power which isn't mentioned.
"Higher pressure leads to better atomization and thus reduces droplet lifetimes." - Worried about time for evaporation again. It's also worth noting and I forgot to mention earlier that I raised GDI pressure from 2600 psi in the late EOI graph to 3200 in the early EOI graph. So not only am I stopping EOI earlier, but also should be getting faster evaporation due to the higher pressure.
"...in-cylinder charge and fuel spray motion in an engine will enhance heat and mass transfer conditions significantly." - Referring to the time for evaporation again. But in this case they imply that turbulence is a factor.
"the [fuel droplet] lifetime is 1-2 ms for 400* K air. ... Scaling these numbers by 4 to account for the higher heat of vaporization and stoichiometric fuel air ratio of ethanol" - They seem to be stating that higher Ethanol content requires MUCH longer evaporation and thus, earlier EOI. This supports your observation that the lower Ethanol content combined with the earlier EOI result in more power due to improved vaporization.
"Valve timing can affect the in-cylinder flow patterns and thus improve or worsen evaporation." - Pointing back to IVC as being related.
Kinda sounds like time is the primary concern but other things, such as GDI fuel pressure and relationship to IVC, can help or hurt the situation. I think it's going to take some dyno time to nail it down for me, but man the car sure is running so much better now I'm just tickled.
Great read there!
Do you have access to a 5 gas where you dyno the Mustang?
Sure don’t…
Ok, just thought I would ask, I want to see what CO and Hydrocarbons do as injection timing moves around. Looks like I need to buy a 5 gas!
That thing costs more than the next two mods I’m saving for!
Ok one last link for the day....
https://www.mdpi.com/2076-3417/9/3/449/pdf
Interesting quotes:
"air velocity also needs to be considered. Delaying the [injection] process moves it in a region with lower average fluid motion and turbulence intensity, thus hindering mixture formation"
"DI systems generate fuel impingement on the walls and piston that provokes the well-known pool fire regions [38,39]. ... This phenomenon can create diffusion-controlled flames that can persist well after the first combustion phase [40]. Just before the opening of the exhaust valves..... It is possible to observe that the delay of the injection timing increased the size and intensity of the pool fire regions." - What have I done???!!!
One thing I'm learning is that SOI should really fall somewhere around 30 deg ATDCintake (=330 BTDCfiring) +/- a few degrees with not much to be gained or lost either way. In pushing EOI back, you gain in potential GDI % blend due to extra time for injection, but stand to lose due to any number of things found in and quoted from the technical papers.
I suppose the best tuning path is:
1. Settle on a fuel, boost level, lambda, and cam timing.
2. Set WOT SOI at 330 BTDC or a touch earlier.
3. Run a GDI pressure on the high end of what is known to work.
4. Start increasing GDI blend until a) your pump hits 100% duty cycle or b) you start losing torque or power (especially at 6500+ rpm).
5. If you hit 100% pump DC first, start lowering GDI pressure. This will extract more pump flow but also push EOI later. Do this until you start losing torque or power.
6. Now that the GDI system is optimized for charge cooling and evaporation time, start advancing spark to find the new knock limit.
Last edited by engineermike; 07-06-2021 at 07:49 PM.
Ok, I lied....just one more: http://oa.las.ac.cn/oainone/service/...703250000039LZ
It appears as though the GDI trough/bowl/cup/dish isn't really there for the reasons I thought. It's a cold-start feature to allow stratefied charge to take place to improve combustion stability, I guess since when cold the fuel can't consistently evaporate quick enough for stable combustion. Now that I think of it, the Gen2 Ecoboost with dual-injection has a very minimal ball-milled GDI trough as compared to the GDI-only Gen1 Ecoboost that has a very elaborate 2-axis CNC'd trough.
Notable quote:
"Piston optimization is done mainly at CSER operation, the 5-15 second catalyst heating phase after a cold start. The 3D CFD CSER simulation is accomplished by using multiple cycles. The goal of piston optimization is to generate a rich fuel cloud around the spark plug at retarded spark timings, 15-20 deg aTDC. The 2.3L GTDI DOE piston top design is shown in Figure 19. During cold start operation a split injection strategy is used with the first injection during the intake stroke and the second injection during the compression stroke. The piston bowl is designed to capture the spray jets during the late compression stroke and generate a rich fuel cloud at spark timing. Figure 19 shows that the spray pattern and piston bowl work well together, and the bowl captures the spray jets injected late in the compression stroke....The spray jets are targeted in the bowl and generate a fuel rich cloud around the spark plug for good combustion stability.
Great read! Thanks for sharing. I believe when it comes to boosted GDI applications you can move SOI closer to TDC without an issue which I suspect is due to having help with higher air velocity and volumes into the combustion chamber. Stock LT4's are tuned from the factory to reach up an SOI of 356. Then there is a 10 degree allowance for variation which I suspect is due to the cam moving towards retarding. I've seen 363 SOI on a bone stock, stock tuned LT4 before. Now the NA LT1 is a different story. Those are tuned closer to the 330 SOI you've mentioned. I wonder if any of these papers were considering forced induction or only naturally aspirated engines.
Either way this has turned out to be a great thread with great contributions to the knowledge base.
This is kind of like the stratified charge engines Honda has developed in the past, a rich pocket to help ignite a cold or super lean charge. Good to information!
Thanks for all the great reads!
The Wikipedia article on gdi seems to indicate all the major players have basically given up on stratefied charge at anything beyond cold-start.
https://asmedigitalcollection.asme.o...dFrom=fulltext
Interesting article, but more they had to heat the charge to get consistent combustion using a late EOI. Seems a bit counterproductive.
If you spray to far into the compression cycle you will start to misfire due to poor mixing. Its like spitting into the wind. Also note this misfire, in its lightest form, does not become noticeable until you catch it on a five gas....or know what your doing. Once you back it off power is usually picked up and you avoid destroying the engine. Also during these misfire events your lambda reading is incorrect so you can't really use it with credibility. If you have a good ear for engines your can hear a crisper engine when the combustion is on point. DI motors have a low tolerance for combustion instability due to poor mixing.
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Do you see the CO start to creep up on the 5 gas or do both the CO and HC increase when the poorly mixed charge starts to burn in this situation?
Howard, would you mind sharing a ballpark value of what you consider too far? Anything lower than 150 seemed to start having negative effects for me on an LT4 setup.
For starters I created two pids that inverse GM's SOI and EOI values relative to TDC. The GM cal expresses this BTDC and as an engine builder it has to invert the numbers. I look at 0 degrees as TDC of the intake stroke and 180 as BDC of the intake stroke. "If" you can spray from 0-180 you know your always in the sweet spot of the intake cycle. Once the piston comes off BDC you are starting the compression stroke. Spraying another 10 degrees from BDC is fine bit after that you start trying to spray into a cylinder that is rising in pressure at a fast rate...ever try spitting into the wind? Does work out... If the injector tries spraying into a combustion chamber that is very turbulent it won't mix. This is more an issue on high boost engines with camshafts (which is everything today). If you continue to let the controller move the EOI to far into the compression cycle your also heading towards spark impingement which is when the spark goes off. Not cool.
If you have a engine that is spraying at say 20 degrees ATDC and the spark is ending at 240.... start the injection earlier and pull the EOI back to 190-180 and you will typically clean up power and more importantly not misfire and crack a piston. There is no debate on if your out of fuel. You only have so much time which equates to so much fuel mass and it has to be delivered at the right time. period.
The 5 gas shows this clear as day as the hydrocarbons are up dramatically which is indicating you are not burning the fuel. As I had started prior if you have an ear for it you can hear the difference of a motor that is crisp vs a motor with poor cylinder mix..
Hope this makes sense...
Last edited by Redline MS; 07-25-2021 at 07:46 PM.
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Makes perfect sense, I appreciate the feedback.
Rookie question. If I've been fighting blowing black smoke on 93 with my L86 with 218/226. When I log Primary SOI it shows high 340's to low 350's right at the begining of the pull. Which is right when it blows the most smoke. Then it drops to 330 or lower which by then it clears up.
Should I just go to that area in the Base SOI map and lower it 5 degrees or so?
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