Engine load

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1930

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I understand that things are just generally accepted and sometimes life would be easier if we didnt ask the why but I have an issue with this so ergo my next question............

I am having a hard time understanding engine load as it has been referenced to my distributor schooling. Below are posts that touch on/reference this.......................

Quote: In a truck I'm not sure that a fast advance curve is ideal. If it is worked hard a fast curve is a recipe for detonation w/o backing off the initial to less than ideal. Given that the application is a bit different than a street car I think that if you want to optimize the advance curve that it will take some experimentation.

Also: If it truly is a truck,in my opinion,there is no need for a quick advance.
Leave the quicker advance for the lightweight cars.

The tuned dist. is the part that will take some iteration. Since it is a truck that presumably will be used as a truck the usual spring kit(s) may not be the best plan. Their advance curve could easily be too aggressive for the application. I'm reminded of Smokey Yunick's words of wisdom about ignition timing "two degrees too little doesn't matter, two degrees too much is a disaster."

What you WANT: Lots of initial advance, but not enough to cause low throttle ping (spark knock) or "kick" on starting,

i would be willing to bet even your truck is going to want 12-16* initial timing......................End Quote:

Also ( and tied to my initial question Im sure ) I am having a hard time understanding what the problem is with having too fast of an initial advance.......I understand the total timing thing now to see why we needed the long slot governors....( to make up for an initial lack of base timing due to pollution standards that had to be met ) But why ( as long as we do not exceed the total time ) would having too much initial timing be an issue.

Is it because initially the engine is not given enough of a chance to draw in enough air/fuel within the cylinder and so with too quick of an advance we would be igniting an un-balanced/inadequate mixture?

Engine load has me stumped, when I stomp on the fuel pedal it takes more power to move a heavier object ( like my truck ) from a standstill position than it would take to move a lighter vehicle, I get this but how does an engine sense this load ?

I understand how an engine adjust for this load = Timing, but air is drawn in, fuel is drawn with it, there is spark, poof power is made.

If the engine runs best when it is advanced enough to fire the compressed charge at a certain # of degrees BTDC than why is it a problem if that advance come in too quickly?

I am doing on-line search to better understand engine load. Not alot out there that dosent get too technical.

I hope some of this makes sense to some people so that they can help me make sense of this.


 
"Is it because initially the engine is not given enough of a chance to draw in enough air/fuel within the cylinder and so with too quick of an advance we would be igniting an un-balanced/inadequate mixture? "

I can see no connection between spark setting ( advance) and air / fuel flow to the cylinders. Cam timing and valve size can influence air / fuel flow to the cylinders, but not spark timing.

Concerning initial timing and vehicle weight,, there can be two vehicles with very simular weight and engine parameters that each need different ignition advance curves.
And that would be because the vehicles have different gear set ups. A low 1st gear in a manual tranny lets a heavy vehicle power away from a stop with less effort (load) on the engine.

To suggest an answer on why a heavy vehicle would want a fast advance curve,, actually this applys to any IC engine. What you want is the spark curve that is optimized for that engine-vehicle-gear setup. The right advance curve is what separates a responsive engine from a lazy one.

this video may help you understand the basics
[ame="https://www.youtube.com/watch?v=HE-J-xhh1hs"]World War II Training Film #497: Mechanics, Champion Ignition and Spark Plug. c. 1941-1945 - YouTube[/ame]
 
The 'load signal' used for this era of engines is the vacuum, either ported or manifold, which gives you some indication of load. The vacuum level is determined by throttle opening combined with airflow and where the vacuum signal is taken from.

The combustion process is supposed to be a smooth burn, not an explosion. If you get the explosion or any sudden pressure increase, you get the knocking.

When you have the combustion process going on, there is an increase in pressure in the burning cylinder gases, and this pressure rises and falls varies over the burn cycle. There is an ideal pressure increase and decrease in the burn cycle that we would love to achieve that would maximize power transfer from the expanding gases to the piston. But all we can do is approximate that ideal burn cycle with widely varying RPM's and the burn rate characterisitics of practical fuels.

The timing of the spark (advance or retard) is the one thing we can control in the timing of the burn cycle to optimize this rise and fall of cylinder pressure. The classic case of too much advance means that the pressure rises from burning AND also rises due to the piston still compressing, and the result is an explosion, not a continuation of the smooth burn process.

To simplifiy this so you migth easily visualize this: You have to put in your mind that the burn process takes a certain amount of time (it is not instantaneous), and that during the time, the piston is moving too. Proper timing is such that the peak burn pressure occurs after the piston has reach the TDC position and is already moving down again. If the peak burn pressure occurs too early, it will coincide with the piston reaching TDC, and the combination of the peak burn pressure with the piston pressurizing the mixture will cause too high a total pressure and the remaining mixture will explode rather than continue to burn smoothly.

This will happen very easily at low engine speeds and heavy load, like when you have it in too high a gear with the throttle wide open. The low vacuum 'load signal' will cause maximum vaccum advance in the distributor: with too much total advance, the burn process will start but the piston is still moving slowly (due to the low RPM's), and the burn pressure will increase too much, too fast relative to the position of the piston since the slow moving piston still is increasing cyliner pressure as it reaches the TDC point.

The way to handle that low RPM advance problem is to have the mechnical advance set so that it's portion of the total advance is small to none when you are at low RPM's. Which I what you are thinking, if I read your post right.
 
my68barracuda said:
"Is it because initially the engine is not given enough of a chance to draw in enough air/fuel within the cylinder and so with too quick of an advance we would be igniting an un-balanced/inadequate mixture? "

I can see no connection between spark setting ( advance) and air / fuel flow to the cylinders. Cam timing and valve size can influence air / fuel flow to the cylinders, but not spark timing.

Concerning initial timing and vehicle weight,, there can be two vehicles with very simular weight and engine parameters that each need different ignition advance curves.
And that would be because the vehicles have different gear set ups. A low 1st gear in a manual tranny lets a heavy vehicle power away from a stop with less effort (load) on the engine.

To suggest an answer on why a heavy vehicle would want a fast advance curve,, actually this applys to any IC engine. What you want is the spark curve that is optimized for that engine-vehicle-gear setup. The right advance curve is what separates a responsive engine from a lazy one.

this video may help you understand the basics
https://www.youtube.com/watch?v=HE-J-xhh1hs
Click to expand...
Although I have not fully reviewed it looks like an excellent video. ( with the exception of watermark)

Thanks for posting the link, I will check it out and I am sure I will pull things from it that will help with my questions.
 
nm9stheham said:
The 'load signal' used for this era of engines is the vacuum, either ported or manifold, which gives you some indication of load. The vacuum level is determined by throttle opening combined with airflow and where the vacuum signal is taken from.

The combustion process is supposed to be a smooth burn, not an explosion. If you get the explosion or any sudden pressure increase, you get the knocking.

When you have the combustion process going on, there is an increase in pressure in the burning cylinder gases, and this pressure rises and falls varies over the burn cycle. There is an ideal pressure increase and decrease in the burn cycle that we would love to achieve that would maximize power transfer from the expanding gases to the piston. But all we can do is approximate that ideal burn cycle with widely varying RPM's and the burn rate characterisitics of practical fuels.

The timing of the spark (advance or retard) is the one thing we can control in the timing of the burn cycle to optimize this rise and fall of cylinder pressure. The classic case of too much advance means that the pressure rises from burning AND also rises due to the piston still compressing, and the result is an explosion, not a continuation of the smooth burn process.

To simplifiy this so you migth easily visualize this: You have to put in your mind that the burn process takes a certain amount of time (it is not instantaneous), and that during the time, the piston is moving too. Proper timing is such that the peak burn pressure occurs after the piston has reach the TDC position and is already moving down again. If the peak burn pressure occurs too early, it will coincide with the piston reaching TDC, and the combination of the peak burn pressure with the piston pressurizing the mixture will cause too high a total pressure and the remaining mixture will explode rather than continue to burn smoothly.

This will happen very easily at low engine speeds and heavy load, like when you have it in too high a gear with the throttle wide open. The low vacuum 'load signal' will cause maximum vaccum advance in the distributor: with too much total advance, the burn process will start but the piston is still moving slowly (due to the low RPM's), and the burn pressure will increase too much, too fast relative to the position of the piston since the slow moving piston still is increasing cyliner pressure as it reaches the TDC point.

The way to handle that low RPM advance problem is to have the mechnical advance set so that it's portion of the total advance is small to none when you are at low RPM's. Which I what you are thinking, if I read your post right.
Click to expand...
I appreciate your effort, let me chew on this for a day or two and report back.

Enjoy your Holiday weekend!
 
and here are a couple on ignition timing,, but I do recommend watching the 1st film, first.

[ame="https://www.youtube.com/watch?v=xm4OmiVH2J0"]Vintage VW Training Film - Ignition Timing Part 1 - YouTube[/ame]


[ame="https://www.youtube.com/watch?v=BbX-oC1WSbs"]Vintage VW Training Film - Ignition Timing II - YouTube[/ame]
 
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