So I discovered that the engine crank does indeed kick back strongly when I forgot to fully retard the spark advance. Fortunately I was using left hand no thumb and was not injured. Why does it do that?
What does the spark advance do? Why does it only seem to exist on Model T's and not modern cars?
"Why does it kick back when spark advance (lever) is not retarded?"
Because ignition of the fuel/air mixture occurs prior to the piston reaching top dead center.
Ignacio, Glad you were not injured. A "kick back" occurs because the spark lever is advanced when you are cranking the car. That means the spark plug fires a spark in ADVANCE of the piston reaching the fully up (top dead center) position while it is on the compression stroke (in the process of compressing the air/fuel mixture) and has not reached the fully up position (Top Dead Center). If spark fires now(igniting the air/fuel mixture) as the piston is traveling up on the compression stroke, the force of the explosion pushes the piston back in the opposite way it was moving and since the piston is connected to the crank shaft that you are turning (clock wise) the crank is abruptly turned the opposite way (counter clock wise) which poses a safety threat to your hand/arm if it is still on the crank or in the way of its travel.
When the spark lever is fully retarded, the piston is permitted to complete the compression of the air/fuel mixture and allowed to travel slightly past the fully up position (Top Dead Center) by about 15 degrees of rotation BEFORE the spark plug sparks to ignite the air/fuel mixture. At this point, the force of the explosion again forces the piston down, however, the piston is now positioned AFTER the fully up position (AFTER top dead center) where the downward force on the piston propels the crank shaft in the same direction you are cranking the engine (Clockwise). The crank pin in which the crank handle is connected is permitted to exit the crank mechanism because it begins moving faster than you are cranking and in the process pushes the crank mechanism out of the engagement with the crank pin so you don't experience a "kick back" when spark fires when the piston is positioned AFTER top dead center. Hopefully someone has a picture of the process that will aid in understanding this process.
Welcome to the hobby, good luck and be safe!
When you don't retard the spark, the engine kicks back because the spark plug is firing before top dead center, or before the piston reaches the top of the compression stroke.
In very simple terms: The spark advance controls the ignition timing or firing of the spark plugs. Fully retard will have the spark occur close to or possibly after top dead center, but as the engine speeds up, the speed of the fuel burning in the cylinder does not change, so the spark need to occur before top dead center so when the fuel fully ignites, the piston should be slightly past dead center. Faster the engine, more advance (before top dead center) the spark needs to occur. Others with more technical knowledge can more fully explain this, but this should be a simple way to get the idea across.
All early cars had this manual advance. Modern cars do the same thing, but is electronically controlled by computer now.
Spark timing is variable due to the fact that the fuel and air burn rate is more or less constant. That means that if the ignition timing were fixed and happened at exactly top dead center, then as the engine speed increased, peak combustion pressure would happen later and later in the expansion stroke. The spark is adjustable so that the pressure of combustion can be optimized for any RPM, theoretically at least.
When running, and the spark lever is advanced, your T ignition system fires the spark plugs several degrees before the piston reaches the top of the stroke. Thats fine when running, but at the very slow speed of hand cranking it means that the pressure of combustion will reverse the rotation of the engine or "kick back". So, when cranking, you have to retard the spark so that the spark arrives after the piston has come to the top of the stroke and has just begun to go down. A spark arriving then will just accelerate the crankshaft in the normal direction of rotation.
Spark timing on modern engines is controlled automatically. Automatic spark advance started to be incorporated into auto ignition systems in the late 20's and early 30's.
Modern cars have electronic advance/retard, before then the carburetor supplied vacuum to the vacuum diaphragm mounted on the distributor that advanced or retard the points plate to change timing of the spark depending on rpm and load.
Here is an explanation of what the spark advance does.
The Model T Ignition System and Spark Timing
After reading this article think about the two explanations Mike and Les provided above and it should be clear to you.
The piston hasn't reached the top of the compression stroke and the spark is igniting the mixture in "advance" of that happening.
The explosion causes the piston to retreat back in the direction that it came from (backwards) causing the engine to rotate backwards.
The crank handle, and any body parts near the crank handle, are along for the ride.
By retarding the spark, you're making that ignition event happen just slightly after the piston reaches the top and is headed back down.
At that point, the explosion makes the engine rotate in the direction that it's supposed to.
Why, you may ask, would you want the spark to go off before the piston reaches the top of the compression stroke?
As the engine speed increases, there's literally less time for the explosion and combustion of the mixture to occur.
So..to put it simply (and probably a bit incorrectly) you need to kind of speculate about the event occurring in the future and light that spark in advance.
Almost like a baseball batter starting his swing before the ball gets to him. The fast the pitch, the sooner he needs to start swinging before the ball gets to him. The slower the pitch, the more time he can wait to start swinging.
There was someone in the distant past who put a starter switch on the spark retard lever. You had to retard the spark to engage the starter. I haven't seen one but it would not be of any help when hand cranking anyway.
Sometimes an image is good. In this series if the spark occurs in the 2nd view during the compression phase the piston will be pushed down and the crank rotation will be reversed.
In more modern engines the spark timing is controlled automatically, there is no manual control like the Model T. The timing needs to be changed as the engine speed increases because the explosion of the air fuel mixture takes some time and needs to start before top dead center at speed.
Then, a driver that is not accustomed to driving a Model T may need to know how the spark control lever should be used under different conditions (they don't teach that in drivers training now days).
That's a great diagram for how to manage spark and throttle. I have been doing it less than correctly :-)
Good to read that you're alive and well Jim !!
There was a question or two recently....obviously we have missed your pictures !!
Ignacio, maybe I missed it but I didn't see any posts yet about NOT cranking a T or any other hand crank engine with your thumb wrapped around the handle. Place your thumb on the same side of the crank your other fingers are on!
If you forget that the spark is advanced the crank will have a really good chance of backing out of you hand when you try to crank it and not breaking or wrenching your shoulder or arm.
To bad when they made that chart of positions they forgot where the spark lever should be when you are stopping the engine!! If the spark lever is retarded at stopping you will not forget plus you get more free starts! I think this chart is for battery equipped Model T's?? Bud.
Instructions for hand cranked starting a Model T on the magneto only is a bit different.
Here is a copy of the 1917 Ford instructions for hand crank starting a Model T with no battery and a magneto only (i.e. no FA Starting and Lighting System)
Chocking a front wheel is also a good idea. My thick oil is often stronger than my parking brake.
@Kenneth I love 'free starts'. It is a fun surprise when it happens.
It was worth the 4 months to get Shirley running again. I haven't had this much fun in a long time.
Welcome back Jim, your pictures of the four stroke cylinder operation are wonderful illustrations of the whole process and make it more clear why the cylinder gets pushed back if spark occurs on the compression stroke.