Relationship between Cam Timing and Ignition Timing

[RRoller123]

Curious to research the relationship between Cam Timing and Ignition Timing. Not a lot of clear info on the web that I have been able to find. Anyone knowledgeable on this subject Driving the Computronics off the crank, with independently timed Cams on our engines, means there is some + and - relationship between the two, vis-à-vis performance, emissions, etc. It seems to me that when driven off the exhaust Cam, the ignition timing would move with the exhaust cam timing, in either a standard distributor or the Compu. But when driven off the crankshaft or aux shaft, they are entirely independent.

i.e. advance the cam timing by a few degrees from spec, and what does that so to ignition timing does that also have to be advanced a few degrees to keep the relationship intact.

Pete

fyi, this is a pretty good general article on ignition timing, but doesn't address the relationship to cam timing.

http://www.autospeed.com/cms/article.ht ... t&A=109132

This one is pretty good too, but doesn't exactly address the situation we have with 2 independently timed Cams:

http://www.badasscars.com/index.cfm/pag ... prd370.htm

[So Cal Mark]

timing is related to crankshaft position. It takes approximately 30 degrees of crankshaft rotation to complete the combustion process, so the ignition drive doesn't matter if it's mounted on the cam, aux shaft or crankshaft. Since the relationship is between the crank and distributor, it's more accurate to drive the ignition as close to the crank as possible

[RRoller123]

I guess I should rephrase my question, or inquiry actually: In all the American muscle engines of my pals that we see at the car shows, they have a single cam that has only TWO (2) DEGREES OF FREEDOM [DOF]. (If it is adjustable at all). Their cams can be advanced or retarded, but that is it. The LSA is fixed, etc. The V8 is driven off the single cam mounted just above the crankshaft.

But in a belt driven twin-cam engine like ours, with adjustable cam wheels, there are EIGHT (8) DEGREES OF FREEDOM. Picture the cams set up nominally to the particular Cam's specs. In further tuning, the Exhaust Cam can then be Advanced or Retarded while the Intake cam is held fixed (2 Degrees of Freedom), the same for the Intake while the Exhaust is held (2 more DOF), the Intake Cam can be Advanced while the Exhaust Cam is Retarded (1 DOF), the same in reverse (1 more DOF), they can both be advanced, or they can both be retarded. Total is 8 DOF. Basically the LSA is widely adjustable, and can be moved in relationship to TDC.

Obviously the set point of ignition timing is entirely independent of these cam angles, (unless of course, with an Exhaust Cam mounted distributor, one changes the exhaust cam timing and doesn't re-time the ignition, duh). But it does not seem intuitively correct to me that there would not be some effect on the optimum Ignition Firing point based upon changes in Cam Timing. Many articles I have read touch upon the topic, but only peripherally, so I am asking if anyone has a good advanced technical reference book, article, engineering design guide, etc that addresses this issue

Octane and CR are obviously also involved in this, but right now I am just researching the optimum Ignition Timing point vis-à-vis the Cams.

Pete

[klweimer]

Pete,
No doubt having two cams that can be adjusted independently gives us a lot of freedom to goof around with the dynamics of the combustion process. I don't think there is a single optimum solution to be had. Unless you have variable valve timing, you will always have to compromise on power vs. RPM. That's why people end up going on the dynamometer to fine tune what they want, given how they drive. Great question.
Kirk

[RRoller123]

Being the poor, and cheap bastard that I have been and remain, there is little chance that I will ever go to any dynamometer! But my interest remains academic, and I will start fiddling with it and see what the effects are.

I need to follow up on my earlier post about reading the O2 sensor with an analog meter versus a digital one. I got my analog meter working, now just have to figure out how to calibrate it at low voltages. I think I will use the digital one to read the voltage of a fresh 1.5 V battery, and then adjust the analog meter to match. Does this make sense The analog meter has a thumbwheel to zero the needle.

Will likely get at this tomorrow and update that post.

[So Cal Mark]

All actual internal combustion engines rely on KEEPING that explosion pressure for as long as possible! In Calculus terms, the total effect regarding rotating the crankshaft is the Integral of the instantaneous net force (or actually torque) actually applied to the crankshaft by that connecting rod for as long as there is explosive pressure inside the cylinder. In an engine that is operating properly, contributions to this Integral begin at the instant of ignition and end when the exhaust valve begins to open. The instantaneous force applied as torque in rotating the crankshaft continuously changes during this "Power Stroke". It actually begins with a slight negative contribution since ignition is timed to occur before TDC, but not much pressure yet develops since the flame is still spreading inside the cylinder. The contribution becomes exactly zero at TDC, and then quickly rises (positively) as the internal burning and pressure continues and the leverage angle at the crankshaft also improves. As the piston goes downward, its expanding volume constantly reduces the remaining pressure, and engine cooling also does this, and good design times the exhaust valve to begin opening about when productive torque is no longer available. Quite a bit of Calculus is required to design a new engine to do this optimally.

So, from a truly accurate (Physics) perspective, a VERY complicated graph of resultant instantaneous torque would first need to be determined, and then that graph would be Integrated to determine actual total engine torque generated, and that quantity is divided by the angle range involved to calculate the AVERAGE torque, at that engine speed and under those conditions of spark advance and the rest. Such analysis is rarely actually done, and nearly always, simply experimental measurements of real engines are found by experiment to learn these things. (Not quite the way Physicists like to do things, but it has worked well for a hundred years!)

You might note that the pressure must be maintained within the cylinder throughout the entire power stroke for decent performance. This explains why an engine loses much of its power once the piston rings are worn (and therefore leaking pressure) or once the valve seats become worn or the valves distorted (and therefore leaking pressure). If an engine actually just relied on the instantaneous effects of the explosion, worn rings or valves would be of minimum importance, but the fact that the basic design of all ICEs relies on HOLDING the pressure while GRADUALLY actually using it, make those components extremely important.

It turns out to be sort of fortunate that the "flame-speed" of the explosion of the gasoline-air mixture is relatively slow! Under the conditions that generally exist inside a cylinder (during highway cruising), the flame front velocity is on the order of around 90 feet per second, or 60 mph. Mark's Standard Handbook for Mechanical Engineers, Section 9, Internal Combustion Engines, Flame Speed. Depending on exactly where the spark plug is located, that flame front must travel two to four inches in order to ignite all the gases in the cylinder. At 90 ft/sec, this then requires around 0.002 to 0.004 second for the combustion to complete. This might not sound like much, but engines spin amazingly fast, and these brief time durations of combustion always take many degrees of crankshaft rotation.

So even though the ignition occurred BEFORE TDC, and the very start of the combustion actually acts to try to make the engine run backwards, the ignition timing is carefully scheduled so that MOST of the combustion (and therefore combustion pressure on the piston head) occurs AFTER TDC. By the time that a maximum amount of the gas-air mixture is burning, the crankshaft has rotated a slight distance past TDC. This situation, and its consistency (due to consistency of the quality and burning characteristics of the gasoline), enables a modern engine to avoid seriously trying to spin backwards! The mathematics below shows that, for an engine speed around 1500 rpm (a normal driving situation) this is commonly around 10° AFTER TDC, when the greatest explosion pressure is present in the combustion chamber. Let's look at some preliminary calculations.

[divace73]

Many thanks Mark, enjoyed reading that...that should be a sticky page for tech info

[RRoller123]

That is excellent, adds a lot of knowledge to the discussion!

Here is a lot of this info, presented in visual form:

https://www.youtube.com/watch?v=_KEbXwvTxkg

This fellow is actually using closed loop control of ignition timing versus cylinder pressure. Poorly texted, but the idea comes across.

https://www.youtube.com/watch?v=_0K5HUFTS3s

This one starts a little slowly, but gets really interesting at about 5:0. It begins to directly address my inquiry somewhat (cam timing vis-à-vis ignition timing) at about 15:00.

https://www.youtube.com/watch?v=bmIEbnglVj0

Another: Well worth watching. This may be the best of the bunch so far

https://www.youtube.com/watch?v=Kwy72aaAFvo

But again, none of them really fully address the initial question I asked, regarding the relationship of all of this to cam timing. So I will just assume that use of the dynamometer is the answer, until I find something else of value.

Pete

[fiatfactory]

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[RRoller123]

Thanks, that is very helpful!

[ORFORD2004]

If you have high performance cam, why don't you time them from the spec of the cam?

[RRoller123]

Ignition time from the cam spec I am not sure I understand your question.

[RRoller123]

Optimal ignition timing is based on the shape/volume of the combustion chamber and the engines cycling speed.

Optimal cam timing (both adv/retard and LSA between the in/ex valves) is more a function of managing the pressure found in the cylinder (both plus and minus values) at the moment of the valves opening / closing, and thereby optimising the mass of air/fuel that can be inhaled during the engines intake phase, and limiting the addition of residuals (exhaust gasses being drawn back into or not being expelled from the cylinder)

So Ignition timing isn't affected by changes in cam timing per se, as the flame propagation isn't affected by the charges density/mass (which is what cam timing changes will achieve) as the flame front speed is a constant (only really affected by the type/quality of the fuel being used)

SteveC

That is the best explanation yet, so if the flame propagation speed isn't effected by the cam timing (via mass density variations), then it seems intuitive that the ignition timing point would be independent of this as well, and remain geometrically and octane dependent . Partially explains why this relationship is rarely brought up. Thanks for the comments! Time to move on to something else.

[stuartrubin]

Holy, shit, Mark! Many years ago, I had a college professor make an offhanded comment that has stuck in my head for 20+ years. He said, "You don't really need much math to design an [car] engine, but you need calculus to optimize it."

I'm not a Mechanical Engineer, but for some reason that sentence has been rolling around in my brain for my entire adult life. FINALLY someone actually used calculus to discuss engine optimization!

Well done, Mark!

[fiatfactory]

You can read the whole article here...

http://mb-soft.com/public2/engine.html

if you liked Mark's cut and paste.

SteveC