DucatiHoney said:
OK, so I'm getting the whole spark thing--which is great! I'm needing more about the compression part and how the transmission is related to that.
Rolling down hill, bike switch is on (and battery has just a little bit of life left in it...) I'm in 2nd gear and I let out the clutch quickly...what is happening inside? I've got electrical current because the turning of the rear wheel is in a long-hand way creating the spark. But how's the fuel getting to where it needs to be to get lit?
(by the way, I suuuuuck at bump-starting, and I think it's largely because I don't understand it. This is helping!)
When you roll down the hill and let the clutch out, the rear wheel pushes the engine through it's normal routine. The normal routine includes creating electricity, drawing fuel into the carburator (more later) and creating spark at the appropriate time (more later.)
I'm aiming for the big principles here, so if there are mistakes, those who can, please correct me.
The normal routine:
Four stroke motor:
1. Intake. The intake valve opens near the top of the piston stroke. As the crankshaft turns, the piston is drawn down in the cylinder creating a vacuum. On a carburated bike the air/fuel mixture coming from the carburator is drawn into the cylinder.
2. Compression. The crankshaft continues to turn (usually because the flywheel carries enough energy to keep the rotation going) and it then begins to push the piston back up into the cylinder. The intake valve is closed now. As the piston nears the top of the stroke, the air/fuel mixture is compressed, making it's ignition more like an explosion. The ignition system has ways of detecting where the bike is at this point, and high voltage is passed to the spark plug where it ignites the fuel/air mixture.
3. This begins the power stroke. The expanding "explosion" of the air/fuel mixture forces the piston back down in the cylinder. The power of this is controlled mostly by how much air/fuel is in there to be burnt. This is where the power on your bike comes from- the energy is passed to the transmission, chain, and wheel.
4. At the bottom of the power stroke, the exhaust valve opens, the piston begins to rise in the cylinder again (because we've just introduced a whole lot of rotational force into the system with power stroke) and as the cylinder is pushed up by the flywheel, the exhaust gasses flow out into the exhaust system. The exhaust vavle closes and you go back to step one.
When you push start your bike, you introduce the power to turn the crankshaft by using the energy created by your moving bike. (linked there by the chain to the transmission to the clutch to the crankshaft.)
By turning the crankshaft in this manner, the stator (or alternator) creates electricity because it is being pushed. In reality it is
always being pushed by the engine. It doesn't know the difference between the starter/engine/push start. It's designed to generate electricity when it turns, so when it does turn, it does generate. The faster you turn it, the more it can make. This could come into play when you push start your bike, since you may not be able to generate enough to get a good spark.
The carburator works on a venturi principle (very basic description follows.) The carbuator is a tube that is big on one end, has a restriction in the middle, then is big again on the other. There are many ways to accomplish this, so it may not be readily obvious when you put your eyeball to work. So, as the air passes through the restriction and then into the larger tube at the end of the carb, the air has lower pressure. The carbuator is designed to adjust the pressure by allowing a measured amount of fuel to be drawn into this lower pressure section. Thus, in essence equalizing the pressure using gas molocules. The gas comes in through jets that are designed to atomize the fuel (like the perfume atomizers- really!) so that it burns evenly when it's in the engine. The control of how much air/fuel comes in is a big disk that turns in the carburator (linked to the throttle) to open, close, or modulate the flow of air.
So, just like the alternator, the carburator is completely passive and is designed to work based on the amount of air being drawn through it. When you bump start your bike, the piston draws air through the carburator, the air/fuel mixture flows in.
Some considerations... Our bikes all have headlights that are on in the default. This creates a draw on the electrical system any time that the ignition key is on. With a dead battery, the lights will draw electricity away from the engine when you try to bump start it. You will be more successful if you disable them- easily done by pulling the fuse.
