Maurice

I was looking at the site http://www.howstuffworks.com/engine1.htm and I have a question about setting your timing and where the piston is in relation. I am assuming that 0 degrees advance timing would be when the piston is exactly at the top of its stroke. [SMALL DISCLAIMER] Please correct me where I am wrong [/END SMALL DISCLAIMER] When we are setting are timing and its advance, I assume we are changing the point at which the spark is applied to the compressed air/gas mixture right? So we are now not igniting the the air/gas mixture at its most compressed state, but in a less compressed state and we getting more power this way? I am not a physics person, but will the explosion be the same magnitude regardless of how much it is compressed? It is still the same amount of gas and air either way. So my last question is, in a standard stroke 350 engine, if the timing is advanced 36 degrees, does that mean that the crankshaft has turned an additional 36 degrees from the top of the stroke to the ignition stroke? Or has the cam turned 36 degrees (I don't think they turn at the same rate do they?). Shouldn't there also be a mathmatical optimum advance that would be the best place to have the spark to keep the crankshaft turning? I hope this made sense to somebody. :crazy:

mrvette

Well, first off, it's a highly controlled burn, not an explosion, a diesel is more near an explosion, hense the knocking they make....but even then it is still controlled....
now imagine that it takes time for the fuel to burn, maybe not a whole lot of time, but still takes time for that fuel to ignite somewhere, and the burning to start in solidly, and consume all the gasoline....well firing the mix at TDC or piston on top of stroke will not allow max power from mix to push piston down....so we have to 'lead the duck' so to speak....

so depending on load, rpm, gas, compression, etc...timing can vary all over the place, from the minimum cranked in at idle...which is usually about 8-12 degrees measured at the crank....to about a max allowable on most engines of 36 degrees....under certain circumstances....

the camshaft turns at exactly 1/2 the rpm of the crankshaft...driven by gears and chain like a bicycle on front of engine...under the 'timing chain' cover...

Larry B.

Look at like leading a moving taret when shooting a rifle.

thejaf

Here's my simpleton understanding of ignition timing as it relates to crank degrees:

1. You want the air/fuel mixture to burn in a controlled manner at exactly TDC to get maximum power

2. The chemical reaction occuring requires a certain amount of time, and it is a constant. IE, it takes the same amount of time regardless of engine speed.

3. As your engine speed increases, you need to fire the spark before TDC in order for the reaction (which is a constant speed) to occur at TDC. hence you have a timing curve, with spark advance increasing as RPM increases.

Maurice

So advancing the timing is moving the spark farther before TDC?

john's '81 mouse

Quote: " So advancing the timing is moving the spark farther before TDC?"


Yep, it sure does, that's why too much timing advance will in some cases cause engine to knock, or early detonation. This "pinging" of the engine, is the explosion working against the force of the piston and rod which are still going up, but the early explosion is trying to drive it down.

Maurice

AHHHH, it is starting to make sense now!!! So the spark is being sent earlier in the cycle the higher you rev with the timing curve. I had always made the assumption that by advancing your timing you were moving the spark farther past TDC not before TDC. Thank you, now I just need to do a little trig and figure out how much the 36 degrees relates to in the amount of space the piston travels upwards after spark.

clem zahrobsky

plug location is also a big factor as is dome on the piston,type of gasoline,hi test burns slower then regular.

Chris B

No the "explosion" is not the same, the magnitude is much greater with increased compression. As has been already described, the advance is intended to cause the most intense point of the burn to coincide with TDC, thus wringing the most force from the event. In fact (as my understanding goes) the mixture is not even volatile until a certain compression level is reached, with volatility increasing as compression increases.

Maurice

This stuff just keeps making more sense all the time. Thank you all for the explanations! Next topic - How does a carb work. :D