Spark at BTDC?

[Mannyb18b]

Alright, i got into a lil argument with gsr916 about when the spark fires. Now I said that the spark fires BTDC, but hes saying that it fires ATDC. So to clear this up, does the spark occur on the pistons way up (btdc) or on the way down (atdc)?

[b18ls]

At the top...
http://auto.howstuffworks.com/engine4.htm

[Mannyb18b]

um im pretty sure it does not spark at the very top, i think that is what the degrees of timing mean.

[b18ls]

The piston goes up to create compresion, the spark hits the fuel/air mixture and creates an explotion which pushes the piston back down. This also release exhaust.
Please correct me if I'm wrong.

[94tegRS]

it fires BTDC, so that as the piston gets to the top, the spark plug ignites the mixture. spark doesnt go from the coil through the rotor through the cap through the wires through the plugs instantaneously, real quick but not instant, thats why timing starts the process a little bit BTDC

[Mannyb18b]

Thank you 94tegrs! Thats all i needed to hear. Thanx for the info

[gsr916]

haha manny you bastard i should slap you! I didnt ARGUE with you about it and i never said i was 100% sure it was atdc. Besides before you even posted this i admitted you were right. Your goin down boy! And the time it takes for the spark to get from the coil to the plug would have nothing to do with it firing btdc. ignition btdc has to do with where the maximum cylinder pressure is going to occur, which is not necissarily at tdc but is a combination of compression and where the fuel is completely burned. If you start the burning btdc, then it will have burned enough to make max cylinder pressure atdc. Having max pressure atdc is intuitive, otherwise your engine would not be able to make power. So without ever really becomming knowledgable at timing, i made an assumption that the fuel was ignited atdc without taking into consideration burn rate. Im explaining all of this because manny has succeeded in making me look like a retard, making you all believe i was saying i was 100% sure it fired atdc (which is false, i never said i was sure and he knows that!!).

bad manny. i kno where u work!

[94tegRS]

[quote=gsr916]Having max pressure atdc is intuitive, otherwise your engine would not be able to make power.QUOTE]


well its always gonna have maximum cylinder pressure at @ TDC of the compression storke because @ TDC the piston is as close to the head as it will ever get and has compressed the air the most. not sure what you meant there, but if you mean having the mixture ignite ATDC then yeah otherwise it would be exerting force against the stroke of the engine instead of helping it after it had passed TDC. also Im not 100% sure every car is timed from factory BTDC, some igniiton systems are probably different and dont need to be fired as early, and maybe some manufacturers get best results from ignition set @ TDC or even a bit after. but on my vehicles its BTDC

[94tegRS]

Originally Posted by gsr916

Actually no your not always going to have maximum cylinder pressure exactly at TDC. .................................................. ....

I was only referring to the pressure of the unburnt air/fuel mixture. Im sure cylinder pressures are greater after the plug ignites the mixture, but if the plug didnt spark, then the mixture wouldnt burn and create more pressure. so it wouldnt be possible to ignite it at its peak cylinder pressure cuz it has to be ignited to acheive that pressure. so I was only referring to the compression the engine will build since we are discussing the ignition timing itself. if it fires early then it will be exerting force against piston trying to push it down as its making its way up. ignite it just a bit ATDC then you are igniting the mixture when its compressed the most and no chance of the combustion working against you, ignite too much after TDC and now the mixture isnt as compressed, not as combustible and you just lost precious time where the crank could be spinning from force and not just inertia.

[gsr916]

Originally Posted by 94tegRS
well its always gonna have maximum cylinder pressure at @ TDC of the compression storke because @ TDC the piston is as close to the head as it will ever get and has compressed the air the most. not sure what you meant there...

How sure are you that max pressure is always @ TDC?? I understand that the piston is as close to the head as possible, meaning the air is compressed as much as it possibly can be... But with the addition of the heat from the fuel being burned (which makes the gas expand inducing a rise in cylinder pressure), wouldnt it be that max cylinder pressure would actually be after TDC?? that is what i meant there. Since timing is not advanced in mine, seems like honda would try to make it so that max pressure occured after TDC to promote engine longevity. Seems to me advancing timing just makes the point at which the fuel has finished burning closer to TDC to make max cylinder pressure higher than it is when timing is not advanced at all, which gives the added power. Correct me if I'm wrong (again i guess). I havnt researched anything...

[gsr916]

Originally Posted by 94tegRS
if it fires early then it will be exerting force against piston trying to push it down as its making its way up. ignite it just a bit ATDC then you are igniting the mixture when its compressed the most and no chance of the combustion working against you, ignite too much after TDC and now the mixture isnt as compressed, not as combustible and you just lost precious time where the crank could be spinning from force and not just inertia

i realized that it doesnt fire ATDC last night when talking to manny, long before he ever started this thread. I was not sitting there going oh im sure it ignites atdc, i never was like promoting it did i just had never read or truly thought about it. I deleted the last post and reworded it by the way cuz it seemed like i was being an asshole lol

[94tegRS]

well, I cant really get it out of my brain into this thread what Im saying. ill try once more real quick though. after the plug ignites the mixture, pressure increases in the cylinder, just like you are saying, but I was just saying since we are talking about ignition timing, specifically when it ignites the mix. you would want it to ignite it when it is compressed as much as possible, which is at TDC. your not wrong your just thinking ahead of me in the engines process but you are correct about the mixture burning increasing pressure.

oh, and also, I dont get my PM's, email notification comes but I log in to an empty inbox everytime.

[gsr916]

I was just tellin u to AIM me: MaTtL04

[94tegRS]

at my sisters house, only msn and yahoo, and I have no yahoo ID.

if you got msn though.