Ignition Fundamentals..... what's in a spark???

[RAd28]

Hey all...

been thinking about my ignition system lately, and i'm struggling to understand why there's a need for vacuum advance, and how inlet charge temps effect timing...

oh... i'm talking about carby engines btw... not that it makes a huge difference...

my understanding of static (base) timing is that you simply set the car up where the spark is initiated BTDC just enough that the spark has ignited the charge by the time the piston is ATDC so that the full effect of the charge is used to push the piston down.

centripetal advance is due to the increased engine speed moving bob-weights in the distributor to induce a mechanical advance, which is needed to initiate the spark slightly more BTDC then previously because the piston speed has increased, but since you can't increase the speed of the flame front, you need to set it off earlier.

now... vacuum advance advances the ignition further when your on cruise/decel, but will retard the ignition when you give it a boot full... but why? what's changed? we now have a richer fuel/air mix, is that harder or easier to ignite? if it's easier to ignite then i can understand that, but i thought it would have been more difficult :confuse:? is there another reason we need retarded igniton on heavy load?

the last couple of morning on the way to work my car has been an absolute dream, just feels so damn responsive, like it did before i started stuffing around with the timing, swapping dizzy's etc... my first thought would be that the colder air has allowed more fuel to mix in the charge, hence a better igniton blast. but i haven't touched my carbies since i've had the car, only played with the timing... so i figure that at some point i've had the timing set where it is ideal for normal ambient temp days, but now it's set so it's only ideal on cold mornings?? is the colder air making it easier to ignite the charge? hence slightly retarded timing would be better?

another only slightly related questions is this.... when a spark ignites in the chamber, it creates a change in pressure... right? now since P=F/A... does that mean that if you could increase the area of the piston top you would get alot more force pushing down on the piston for a given combustion pressure? say for arguments sake, you could make the piston top the same shape as a meat tenderiser??? just an idea

[BeRad]

Me speaka no english

EDITED.

[MWP]

Its good to think about these things now and then...

centripetal advance is due to the increased engine speed moving bob-weights in the distributor to induce a mechanical advance, which is needed to initiate the spark slightly more BTDC then previously because the piston speed has increased, but since you can't increase the speed of the flame front, you need to set it off earlier.

Yup, correct.

now... vacuum advance advances the ignition further when your on cruise/decel, but will retard the ignition when you give it a boot full... but why? what's changed? we now have a richer fuel/air mix, is that harder or easier to ignite? if it's easier to ignite then i can understand that, but i thought it would have been more difficult :confuse:? is there another reason we need retarded igniton on heavy load?

Itll be something like... when under load there is a denser mixture in the chamber so the ign front moves quicker.
Im not sure exactly what the explination is, but itll be something along those lines.

the last couple of morning on the way to work my car has been an absolute dream, just feels so damn responsive, like it did before i started stuffing around with the timing, swapping dizzy's etc... my first thought would be that the colder air has allowed more fuel to mix in the charge, hence a better igniton blast. but i haven't touched my carbies since i've had the car, only played with the timing... so i figure that at some point i've had the timing set where it is ideal for normal ambient temp days, but now it's set so it's only ideal on cold mornings?? is the colder air making it easier to ignite the charge? hence slightly retarded timing would be better?

Maybe your carbs are running a little rich, so the colder air if effectivley bringing the AFR back towards stioc?

another only slightly related questions is this.... when a spark ignites in the chamber, it creates a change in pressure... right? now since P=F/A... does that mean that if you could increase the area of the piston top you would get alot more force pushing down on the piston for a given combustion pressure? say for arguments sake, you could make the piston top the same shape as a meat tenderiser??? just an idea

Nope, wont help.
Its not about surface area, its about what can move.
The piston is the only object in the cyl that can be moved, so thats where all (well close enough) the force goes.

[allencr]

vacuum advance advances the ignition further when your on cruise/decel, but will retard the ignition when you give it a boot full... but why?

Detonation protection over-kill.

[RAd28]

Maybe your carbs are running a little rich, so the colder air if effectivley bringing the AFR back towards stioc?

that's what i was thinking... but the car was running just as good a few weeks back and air temp wasn't that cold... so i figured there's gotta be a way to "trick" the timing so that it performs just as well... if your right about the denser air/fuel charge burning faster, then perhaps with a warmer (less dense) air/fuel charge i need to advance the timing slightly more again? does that make sense at all??

Nope, wont help.
Its not about surface area, its about what can move.
The piston is the only object in the cyl that can be moved, so thats where all (well close enough) the force goes.

lol, i knew it wouldn't help, otherwise all pistons would have a high surface area crown... just trying to figure out why it doesn't work like i imagine

[MWP]

then perhaps with a warmer (less dense) air/fuel charge i need to advance the timing slightly more again? does that make sense at all??
:

Other way around.
Cooler air needs more advance.
Thats why on hot days (or overly hot engine), you hear lots of other cars pinging.

[Mos]

because the piston speed has increased, but since you can't increase the speed of the flame front, you need to set it off earlier.

The flame speed does increase with increased engine revolutions but not at the same rate, hence needing advancing.

so i figured there's gotta be a way to "trick" the timing so that it performs just as well...

There is, it's called EFI (or programmable ignition for the purists )

lol, i knew it wouldn't help, otherwise all pistons would have a high surface area crown... just trying to figure out why it doesn't work like i imagine

It doesn't work the way you think because only the surface area that's horizontal will be pushing down. Any surface area that's vertical will push sideways, cancelling out with the surface area pushing sideways from the other side.
If you had a piston with a vertical step in it (and two horizontal, semi-circular crowns at different heights), then whatever surface area forms the vertical portion will be pushing the piston sideways against the cylinder wall.
Also, any crown raised at an angle to the horizontal means the vertical component is pushing sideways, and cancelling out with the vertical component pushing sideways from the other side.

Mos.

[RAd28]

Other way around.
Cooler air needs more advance.
Thats why on hot days (or overly hot engine), you hear lots of other cars pinging.

if the flame front moves faster in a colder/more dense mixture, wouldn't you want the spark ignited closer to TDC (retarded) since it won't take as long for the flame to reach maximum energy? if you advanced the timing, then maximum energy would be reached BTDC, and not slightly ATDC where you want it... hence with a warmer slower burning mix, you advance the timing to give the flame time to reach maximum energy...

There is, it's called EFI (or programmable ignition for the purists )

i would consider EFI id i could keep an ITB setup on a budget, but that's not gonna happen just yet... but i'm planning on a fully programmable ignition system soon.. that's half the reason i wanna understand why/how the ignition system works, so i can get the best results...

[RAd28]

still pondering this topic... can anyone offer suggestions??

[timbosaurus]

I haven't had a chance to read the entire thread... so I apologise if others have already answered.

These are my opinions anyhoos.

my understanding of static (base) timing is that you simply set the car up where the spark is initiated BTDC just enough that the spark has ignited the charge by the time the piston is ATDC so that the full effect of the charge is used to push the piston down.

centripetal advance is due to the increased engine speed moving bob-weights in the distributor to induce a mechanical advance, which is needed to initiate the spark slightly more BTDC then previously because the piston speed has increased, but since you can't increase the speed of the flame front, you need to set it off earlier.

Close enough

now... vacuum advance advances the ignition further when your on cruise/decel, but will retard the ignition when you give it a boot full... but why? what's changed? we now have a richer fuel/air mix, is that harder or easier to ignite? if it's easier to ignite then i can understand that, but i thought it would have been more difficult :confuse:? is there another reason we need retarded igniton on heavy load?

It's not that richer mixtures are harder/easier to ignite, it's that they burn faster.

the last couple of morning on the way to work my car has been an absolute dream, just feels so damn responsive, like it did before i started stuffing around with the timing, swapping dizzy's etc... my first thought would be that the colder air has allowed more fuel to mix in the charge, hence a better igniton blast. but i haven't touched my carbies since i've had the car, only played with the timing... so i figure that at some point i've had the timing set where it is ideal for normal ambient temp days, but now it's set so it's only ideal on cold mornings?? is the colder air making it easier to ignite the charge? hence slightly retarded timing would be better?

Yeah, unfortunately dizzy's don't have temp advance to go with the vacuum advance, centripetal advance, and maintenance schedule advance

N/A motors also benifit from dense, cold charge air, that will also affect the performance you notice somewhat.

another only slightly related questions is this.... when a spark ignites in the chamber, it creates a change in pressure... right? now since P=F/A... does that mean that if you could increase the area of the piston top you would get alot more force pushing down on the piston for a given combustion pressure? say for arguments sake, you could make the piston top the same shape as a meat tenderiser??? just an idea

And a silly one I thought you were an engineer

you learnt about vectors in statics?

Timbo

[essaysixtythree]

Just having a squiz at some old posts, found this interesting

another only slightly related questions is this.... when a spark ignites in the chamber, it creates a change in pressure... right? now since P=F/A... does that mean that if you could increase the area of the piston top you would get alot more force pushing down on the piston for a given combustion pressure? say for arguments sake, you could make the piston top the same shape as a meat tenderiser??? just an idea

I like this idea of increasing the piston tops area. It has a few little physics problems..
Change in energy = work done
Work Done = Integral(F.dR)
F=force (a vector in i, j, k direction) => F = F1i + F2j +F3k
This is in the direction perpendicular to the surface as F = p . A

http://wpcontent.answers.com/wikiped...Vector.svg.png A diagram of unit vectors.

dR=the direction of motion (in our case the piston can only move down so dR is in the -k direction if you like.) dR= 0i + 0j - (dz)k
The dot product means that you multiply each i j k component to its respective i j or k component and the answer is a scalar.

Since dR is only in the -k (downwards direction) the i and j components of force are multiplied by 0. This makes them = 0. So your left with work =Integral(-F3dz)

So its pointless making the piston top like a meat tenderiser because although it does increase surface area the forces acting on those new surfaces are mostly pushing the piston sideways not up and down. May I suggest that this is why some race motors are interference motors. ie they dont have cut outs in the tops of the pistons to save your motor when the timing belt goes. The full surface area is flat and perpendicular to the direction of motion to translate more of the force into mechanical energy. So increasing piston surface area is a good idea just make the diameter bigger.

http://www.aa1car.com/library/engine_interference.jpg

As for why diesel engines have a bowl in their pistons im not sure but its to do with being able to inject the diesel into to chamber whilst having massive compression. The bowl is the chamber.

http://www1.eere.energy.gov/vehicles...sel_engine.pdf

end rant