• The site migration is complete! Hopefully everything transferred properly from the multiple decades old software we were using before. If you notice any issues please let me know, thanks! Also, I'm still working on things like chatbox, etc so hopefully those will be working in the next week or two.

Engine Guru's, Help Me Understand Ignition Timing

SyntheticShield

SyntheticShield

New member
Okay, so Ive been trying to delve into motor operation a bit more and go a bit deeper in my understanding. One thing I have been looking at is ignition timing. I dont promise this will be short, so bear with me.

We run what, 16, 18, 20* timing at WOT depending on tune and knock and all that stuff. This timing is BTDC correct? That is the spark is fired at a point before the piston reaches TDC, right?

Okay, I understand why that is. It takes time to for the flame front to propagate and to build pressure. The idea being that peak pressure occurs somewhere at TDC so that you have maximum force pushing down on the piston to develop even more torque.

That being true (and Im kinda thinking out loud here) why does everyone what more timing, i.e., they want to run 22* timing instead of 20*? It would seem to me the more in advance you have to the spark occurring that less efficient your system really is. That is to say that if the air/fuel mixture were more turbulent (better mixing and atomization) then ignition timing should be a smaller number since that would cause the flame front to advance faster and build pressure quicker. So would not higher ignition timing numbers indicate a issue with a/f mixture and burn? Or at least not at its optimum?

As well, if I were able to mix and atomize better and therefore use much smaller timing advance numbers, would not that help reduce temps some since the heat/explosion if you will is in the cylinder a smaller amount of time?

My understanding is that ideally you would want timing at 0*, the pressure built immediately and more pressure was available to push down on the piston, correct? Are you not wasting some of the energy that could go to moving the piston in the downward stroke?

So if I could come up with a way to cause the flame front to propagate faster, build pressure faster, and move ignition timing more towards 0*, would that not be a good thing?
 


ok but it'll be lkong and drawn out since rotating assembly geometry plays a huge part in how and where peak cylinder pressure occurs (which ideally would be at around 30* ATDC)
 
turbocharged406sbc said:
ok but it'll be lkong and drawn out since rotating assembly geometry plays a huge part in how and where peak cylinder pressure occurs (which ideally would be at around 30* ATDC)
Click to expand...

Okay, Im kinda glad you dropped in because I was unsure if the ideal timing was at 0* or after, but that helps.

So we would want spark to occur say at 0* and maximum pressure build at 30* ATDC for example, correct?
 
you also have to figure in the time it takes for the flame to ignite the mixture which I think (going back to my mechanics classes 15 years ago) is about 2*
 
archemedes said:
you also have to figure in the time it takes for the flame to ignite the mixture which I think (going back to my mechanics classes 15 years ago) is about 2*
Click to expand...

Cool, another great piece of information. Thank you for that. So if maximum pressure ideally occurs at 30* ATDC, then you would want ignition timing around 28* ATDC, am I understanding correctly?
 


SyntheticShield said:
So we would want spark to occur say at 0* and maximum pressure build at 30* ATDC for example, correct?
Click to expand...
in an ideal IC engine yes... but in almost every case, combustion chamber design, piston crown shape, fuels, detonation, rotating geometry all make for compromises for peak power

archemedes said:
you also have to figure in the time it takes for the flame to ignite the mixture which I think (going back to my mechanics classes 15 years ago) is about 2*
Click to expand...
by flame i assume your refering to the spark kernel (wheres that in cylinder vid of combustion at?)
that varies significantly with RPM/load
 
archemedes said:
yes, but it's still not a ton of variance (mechanical advance was only about 5* wasn't it?)
Click to expand...
when the engine is at 6000 rpm its a ton of variance remember the speed at which the mixture ignites is dep on the fuel quality (VOC's, rapid vaporization, R or M octane rating etc.
then you have to take into account the mixture...is it a homogenous mixture near the plug, has the cylinder fill event occured where fuel is actually near the cylinder walls or concentrated near the spark plug.

the big biys have been playing with wet flow benches for a while now...look at the huge jumps in cylinder head/port/chamber design in just the last 5 years
 
Thank you James for your information and that link. Im gonna be soaking that up for a bit. May I ask how you came to have such an understanding of the inner workings of an engine?
 


turbocharged406sbc said:
i shave, sh!7, shower and sleep thinking and reading about this stuff...

remember i'm a psychotic gearhead :th_king-evil:
Click to expand...

I think I'm heading that way...

I'll see you at the next meeting.

Good stuff in this thread. I've loved reading this kind of info ever since I picked up "Maximum Boost" by Corky Bell. Very informative.
 
yeah, I would love to come your way too with a laptop, notebook, voice recorder and perhaps even a video recorder and just absorb all I could.
 


This is why I always wondered if we had Top mounted spark plugs, would we make more power, cause the flame front moves and hits the piston relatively flat, while in our motor it seems that the plug side of the piston would see the pressure first, than it would spread.......... but I liked reading that 12.6-13 was the optimum rich ratio, everyone on the other forum had been saying something in the 11's and that always seemed way too rich to me.....

If all of had access to a dyno, It would be time consuming, but rather easy to find the optimum advance for our specific setups.... In a perfect world I geuss :cool:
 
If nothing else it would give a good starting point.

Im trying to educate myself a bit because I want to kinda delve into what can be done to move the timing to a more ideal set up. It seems to me if you could move the timing of the spark to a point closer to peak pressure, not only would you be making more power but also better fuel economy.
 
true, but you need a faster burning mixture to move the timing closer, and a little bit before TDC the flame and expansion is still begining, this would not put any pressure on the piston BTDC, the flame front doesn't expand at a consistant rate either, it starts small, than as it gets bigger than it spreads even faster... this being said, a few deg. BTDC may be the ideal time to initiate spark, but remember, when the spark is ignited and when the flame starts expanding may be a few degrees apart... nothing is instant

If anyone wants to correct me feel free, cause everyone is wrong sometimes :)
 
As I see it, and using what little I know, there are a couple ways to get a faster burning mixture. You have to use a more volatile fuel which would cause stability, safety and consistency issues or you could vaporize the fuel infinitely better which is the direction I would take. Liquid fuel doesnt burn, vapors do. There is little available that I could use in the way of higher fuel pressure which would cause better atomization of the fuel from the injector. I suppose you could use a different injector that has a better nozzle designed to promote more atomization but I have never seen anything like that available for the GP.

I have, however, seen some very promising ideas with regards to inducing extreme turbulence that works towards atomization and some vaporization. I might end up experimenting a bit on my Isuzu just to see if I can prove out any of the ideas. But Im a ways from that and I need to do more reading and educating myself to make sure everything is on a solid ground.
 
You must log in or register to reply here.
Back
Top