Was thinking about this late last night. I believe my HEI does have a cap and rotor. Coil I understand is fired by the ECU(no points) and also timing is controlled by SA-MAP.

Old school points system is centrifugal and vacuum adjusting timing. So one could say from cam to dist to eventual spark fire it is all mechanical. Rotor position and eventual timing is mechanical as the rotor physically is moved.

Now on HEI/ECU when coil is discharged spark occurs when rotor approaches the contacts in dist cap. How could advance retard occur if rotor is not being affected mechanically? If ECU sends signal for 36dA spark will not occur till rotor hits contact in cap regardless. Seems the rotor needs to be moved to achieve 36dA in this example?

 

Ronny

It is a combination of mechanical and ECU control. If you look at HEI cap terminal they are very wide - about 0.250. The reason is when ECM fires say at 36da - it is actually 18 dA at camshaft rotation. The posts are spaced every 45 deg (camshaft rotation). If the HV post is positioned between 0 and 18 deg then spark does not have too far to jump from rotor tip to dizzy spark plug terminal.

//RF

 

So I presume flame speed is a factor that is addressed in the timing tables.

So one can achive adequate spark even though the initial onset os S event is well before dead center on alignment? Does the coil fully discharge well B4 the rotor is dead center?

 

Yes, a while back I have seen an article about flame front propagation and its effects on combustion chamber efficiency. I wish I could remember where I saw it.

As far as the coil magnetic field build up time (aka dwell time) and discharge - this varies with engine RPM , but according to one study a sweet spot between 3 to 5 mSec seems result in best engine performance. Since I am not a combustion chamber expert I can comment no further on this subject.
http://www.insipub.com/ajbas/2010/4691-4694.pdf

//RF

 

All I know of flame speed propagation is my tuning experience with 2 stroke engines. A big selling point on performance is multiple cylinders of same cc allowing shorter distances for flame to travel. Less susceptible to detonation. Thats it!

well maybe quench?

 

The timing is probably one of the most complex aspects of tuning IMO. The flame speed will be a function of the cylinder pressure, temperature, combustion chamber design, fuel in use, air fuel ratio, and probably lots of other factors. Chambers like the fast burn as well as quench induce turbulence into the chamber, which helps convey the flame front faster than it would in a more stagnant mixture. With quench, the remaining mixture is squeezed out and pushed towards the center of the chamber, where it meets up with the flame front, and combusts.

The idea behind timing is to ignite the fuel in advance so the mixture is fully burned by the time the piston has passed TDC (the actual optimal point is slightly after TDC). If the mixture is ignited to early, you risk detonation. You also loose power to compressing the already burned mixture. This can also cause the engine to run rough, as the sharp pressure increase slows the piston down. Too late, and the mixture is still burning after TDC, and cant transmit as much work to the piston on its power stroke.

Faster flame speeds do translate to more resistance to detonation as your igniting the mixture later in the compression stroke. This means that the remaining fuel in the chamber doesnt have to withstand the pressure and temperature as long. In an engine that needs lots of timing, there will be residual fuel ahead of the flame front that needs to resist decomposition until its ignited. The pressure rise from the mixture being ignited travels at the speed of sound, so it propigates much faster than the flame front, and will be felt by the unignited fuel. As the mixture burns and the piston continues to travel upwards, the pressure increases. When the pressure and temperature reach the point of autoignition ahead of the flame front, the remaining fuel goes off more or less all in one shot. That results in a sharp pressure pulse, which is heard as detonation.

As far as the coil, I think it collapses pretty fast. I cant see it taking more than a few hundred microseconds to generate a spark. Basically, the primary coil reisists sharp curren transients as its a current conserving component. Once the flow of current is cut, the coil developes a sharp voltage potential in order to try to keep the current flowing. The sharp voltage spike is stepped up many times by the secondary to generate the spark. If you had a scope, you could pretty much determine how long the coil needs to dwell, as well as how fast it collapses after the power is cut by the module. That would probably be the only way to really properly set the dwell and bias for the coil. The time the coil is activated is actually calculated by taking the position that the coil should fire at, and adding in the dwell time and coil offset. That then is translated from the time domain to an angular position BTDC at which the coil will be triggered. In later ignitions, this is all done by the PCM using the crank position sensor as a reference. It sends a waveform to an amplifier, which then drives the coil.