Why does bump-starting work?

[DucatiHoney]

...and why is it harder to do on a fuel-injected bike? I'm still trying to get my brain around how an engine works, to be perfectly honest, especially a carb'ed one. (hey, I ride 'em...I don't work on 'em if I don't have to.)

I'm not asking the technique for starting one's bike if the battery is dead. I'm actually asking for a step-by-step of what happens inside all those shiny metal casings with those little turning bits when I push a bike and it doesn't have a battery.

Curious minds want to know...

 

[NorCalBusa]

Fuel injection requires a pump to pressurize the fuel so it atomizes (turns to fuel vapor) in the combustion chamber. These fuel pumps are electric, so with a low/dead battery- it doesn't run to pump the fuel (or in a fashion it will ignite).

 

[zeefrenchspy]

maybe try thinking about it the other way around. Why isn't your bike (carbed) starting when the battery is dead?

Because it can't make a spark to ignite the gases. The carbs and the engine itself on a non-FI bike are pretty much purely mechanical/vaccum driven so are not affected by the battery at all.

So when you go to bump start your bike, pushing it along happily and then dropping the clutch, what happens is you get a burst of juice out of your stator, which in turn will allow you plugs to spark and start your engine.

Very simplified explanation


Footnote: for that reason it is easier to bump start a bike if the headlights are unplugged (less drain on any juice produced by the stator), don't ask me how I know.

 

[ChuckBecker]

Absolutely true about an FI bike needing enough battery power to boot the ecu, run the fuel pump, and provide spark. Same thing for modern carb'd bike, except (usually but not always) no fuel pump to pressurize. What with all the exceptions, it can tend to get complex.

To simply run the, engine needs fuel/air, compression, and ignition. Starting a bike, in addition, requires cranking speed. If you're in a configuration where the only missing part is cranking speed, then bump starting can get your engine fired.

On very simple bikes with gravity fed carbs and non-ecu ignitions, a bump start can get the engine turning fast enough to produce enough spark to fire the mixture, then you're off to the races.

Or something like that...

 

[DucatiHoney]

OK, so I'm rolling down a hill, the backwheel's motion moves the chain which moves the sprockets which gets the engine turning. The turning of the engine must rotate some belts involved with the stator???

OK, so I looked up stator. It's only one part of a generator apparently. You need a rotor turning around it to create the current. And this is all part of an alternator--at least in a car. Motorcycles don't have alternators, do they? If not, how does the battery get recharged?

So I don't completely get the whole stator thing (maybe that'll get explained later...) but that's where the spark comes from? So how is that affected by popping the clutch? Why is it nearly impossible to bump start my bike when it's cold?

 

[auntiebling]

some fuel injected Cars require a minimum voltage for the computers to tell the engine to run. the computer might be on, but it won't let the engine come to life. i suspect there are examples of this same mechanism on motorcycles.

there is a difference between alternators and generators. generators produce electricity if you spin them. simple as that. the basic idea and, i think consruction, is the same. however an alternator requires electricity to produce electricity. i don't know if motorcycles use alternators or generators, i suspect alternators. if your battery is completely dead dead dead you can push it till you have a heart attack it won't start. the stator is what has the wire windings that are actually producing electricity. the quantitiy of wire wound around a post, number of posts.... i'm not an electrical engineer but they all affect the voltage and aperage capability of a given stator.

i don't know how the motion of the motor is transferred to the alternator. belts, chains, gear drives are all possibilities. i'm sure there are examples of all three in the motorcycle world.

the spark comes from a coil. a coil is something like a capacitor, which accumulates charge into the tens of thousands of volts. the ignition system takes electricity from the alternator, stores it in the coil, and releases it near TDC across the gap in a spark plug to create the power stroke of the engine. there are various methods of controlling the timing of the spark, some 100% mechanical some electronic.

4 stroke engine basics

 

[ALANRIDER7]

The stator is a series of wire coils (held in place in the crankcase cover) that a magnet (on the end of the spinning crankshaft) rotates around creating alternating current. When the magnetic field lines cut across a coil of wire, alternating current is produced. (It can be called an AC generator as well, but technically, the magnet in a generator is stationary). That AC current goes to the rectifier where it is turned in to DC current that the bike uses. This electricity is then used to power the various electric devices on the bike- the ECU, the fuel pump, the ignition coils, etc. The regulator will send DC juice to the battery if it's low for recharging purposes. If the battery is charged, the excess juice is shed as heat through the fins of the regulator/rectifier. When it comes to bump starting, you need some electrical power to run things- even if it's low, you may still be able to bump start the bike. As soon as the crankshaft starts turning, AC juice is produced. It may not be enough to get the thing running though, especially a fuel injected bike.

If your bike is having electrical gremlins, sort them out- push starting bikes sucks the big one. If your carburetors are out of whack, it will be nearly impossible to bump start.

http://www.rowand.net/Shop/Tech/AlternatorGeneratorTheory.htm

 

[DucatiHoney]

If your bike is having electrical gremlins, sort them out- push starting bikes sucks the big one. If your carburetors are out of whack, it will be nearly impossible to bump start.

The bike's working perfectly fine. The freakin' "how sh*t works" sites don't allow me this type of q and a. I don't have a book on engines either, and the manual just shows static pictures. ...I'm curious about these things.

 

[ALANRIDER7]

The bike's working perfectly fine.

Then why bump start it???

 

[LS1Bandit]

Then why bump start it???

I think her question was merely theoretical in nature.

In any case, in case it helps, here is a picture to help understand the stator a little better. This picture is from an SV650 with the generator cover taken off. You can safely ignore some of the superfluous comments in the picture - it's taken from a tutorial on installing an advance ignition key.

The reddish coils are the stator and they basically fit inside what's labeled as the rotor. The rotor has magnets all around it and spins with with the engine. It's the spinning of these magnets around the stator that produces the AC electricity. Then as Alan explained, the rectifier turns that into DC for charging the battery and powering the fuel pump, ECU, etc. As for the spark, antibling explained that reasonably well in the last paragraph.

 

[auntiebling]

The bike's working perfectly fine. The freakin' "how sh*t works" sites don't allow me this type of q and a. I don't have a book on engines either, and the manual just shows static pictures. ...I'm curious about these things.

the best thing on the "how stuff works" page is the animation explaining the cycle.

 

[Bikerx260]

OK, so I'm rolling down a hill, the backwheel's motion moves the chain which moves the sprockets which gets the engine turning. The turning of the engine must rotate some belts involved with the stator???
...

You missed one part. The sprockets turn the transmission. You shift to engage 2nd or 3rd gear (or higher). The transmission is disengaged because you are holding the clutch lever in. Pop the clutch, and you now have the engine turning, which provides the compression.

The other explanations for the generator/alternator would parallel this action.

Thus completes the cycle of life- compression, spark, and fuel! YEA!

 

[Spec-ECU]

I think her question was merely theoretical in nature.

In any case, in case it helps, here is a picture to help understand the stator a little better. This picture is from an SV650 with the generator cover taken off. You can safely ignore some of the superfluous comments in the picture - it's taken from a tutorial on installing an advance ignition key.

The reddish coils are the stator and they basically fit inside what's labeled as the rotor. The rotor has magnets all around it and spins with with the engine. It's the spinning of these magnets around the stator that produces the AC electricity. Then as Alan explained, the rectifier turns that into DC for charging the battery and powering the fuel pump, ECU, etc. As for the spark, antibling explained that reasonably well in the last paragraph.

Very cool to see the stator and the magnets.

 

[Bronto]

More difference between Automotive Generators and Alternators.
Generators use brushes, Alternators are hardwired.
Generators are DC voltage, Alternators are AC voltage.
Generators use Voltage regulators, Alternators use Rectifiers and voltage regulators.

 

[nedro]

Think of it this way.
Suck
squeeze
blow your load
exhale

 

[ST Guy]

Not all alternators are created equal. Some utilize permanent magnets like the SV in the pix above. But many now used in bikes (usually air cooled alternators), create the magnetic field using electriciy rather than permanent magnets. Thus, in order for the alternator to fire up and start producing electricity, they require some amount of electricity from some other source. Namely the battery. If you have a really dead battery, it's almost impossible to get an alternator of this type to "fire up". Sometimes, if it gets spinning fast enough, it will spontaneously start to generate. But usually not. You need at least some current from the battery to kick it off. Once it's kicked off, it can run on it's own.

So, even if your bike is carburated and doesn't require a fuel pump, if it's got the more modern type of alternator, it won't generate any electricity if the battery is dead.

Oh, and I can't think of a bike made that doesn't have an alternator. Even kick start bikes have them. Gernerators are old school.

Generators utilize windings and permanent magnets and brushes to generate DC volts.

Alternators utilize windings and either permanent magnets OR more windings and brushes to generate AC volts.

A generator only needs a regulator to regulate the output voltage to the proper level.

An alternator needs a rectifier first to convert the AC voltage to DC. Then a regulator to regulate the output voltage to the proper level.

Virtually all motorcycle engines have their alternator gear driven directly off the motor.

Most automotive alternators are belt driven.

Hope at least some of that is helpful.

 

[ALANRIDER7]

Honda used to have the field coil type of charging system, like on the aircooled Nighthawks. It's pretty lame to put a charging system on a bike that requires electricity in order to produce electricity. Duh.

 

[Don Tuite]

It's always interesting when somebody asks a question and I think I know the answer and it turns out I don't.

So some follow-up questions:

Why aren't we calling the thing with the magnets a magneto?

Roughly, what's the ratio of bikes with wound rotors to bikes with permanent magnets?

I'm curious because I've been following a parallel trend away from universal motors and toward synchronous permanent magnet motors in appliances.

Don

 

[DucatiHoney]

I think her question was merely theoretical in nature.

In any case, in case it helps, here is a picture to help understand the stator a little better. This picture is from an SV650 with the generator cover taken off. You can safely ignore some of the superfluous comments in the picture - it's taken from a tutorial on installing an advance ignition key.

The reddish coils are the stator and they basically fit inside what's labeled as the rotor. The rotor has magnets all around it and spins with with the engine. It's the spinning of these magnets around the stator that produces the AC electricity. Then as Alan explained, the rectifier turns that into DC for charging the battery and powering the fuel pump, ECU, etc. As for the spark, antibling explained that reasonably well in the last paragraph.

Cool picture! So this entire assembly would be called the alternator then? Do all bikes look pretty much the same? Why does the current have to be changed from AC to DC? Is there oil in there and if so, why?

 

[ChuckBecker]

Cool picture! So this entire assembly would be called the alternator then? Do all bikes look pretty much the same? Why does the current have to be changed from AC to DC? Is there oil in there and if so, why?

1) Yah, the whole assembly is called the alternator.

2) Pretty muchthe same , but there are some wildly different configurations:
(http://www.morrismagneto.com/)

3) You'll be needing DC to charge your battery. Batteries provide DC. All the apparatus on your bike run off the battery before the engine is running. You could run lights, horn, etc on AC, but then you'd need an inverter. Just simpler, I guess, to make it all a DC circuit.

4) Judging from the picture, on this particular bike, yes