Waste Spark?
- Thread starter CobraSix
- Start date
It's a way to simplfy ignition by using a single coil for two cylinders.
Except for the odd 5-cylinder engine, on most engines, whenever one piston reaches TDC on the compression stroke, there is another at TDC on the exhaust stroke. A single coil fires both, but only the one on compression will do anything. The other spark fires without effect in the cylinder on exhaust stroke.
So half the number of coils to control.
Except for the odd 5-cylinder engine, on most engines, whenever one piston reaches TDC on the compression stroke, there is another at TDC on the exhaust stroke. A single coil fires both, but only the one on compression will do anything. The other spark fires without effect in the cylinder on exhaust stroke.
So half the number of coils to control.
A
Anonymous
Guest
Remember that it takes more voltage to make a spark at high pressure?
filter that through E=I*R (Electromotive force/voltage equals current times resistance). When the coil fires through both plugs to complete the circuit, the higher resistance of the compressed fuel/air charge requires more voltage to bridge the gap. Voltage*current=power. so, same current, more voltage, most of the power goes through the fuel/air charge to ignite it. On the other plug, the exhaust gases are very hot and under low pressure, and it doesn't take much juice to cross that gap. less voltage, same current, less power. Very simple electromechanical system.
hope that helps.
Ern
filter that through E=I*R (Electromotive force/voltage equals current times resistance). When the coil fires through both plugs to complete the circuit, the higher resistance of the compressed fuel/air charge requires more voltage to bridge the gap. Voltage*current=power. so, same current, more voltage, most of the power goes through the fuel/air charge to ignite it. On the other plug, the exhaust gases are very hot and under low pressure, and it doesn't take much juice to cross that gap. less voltage, same current, less power. Very simple electromechanical system.
hope that helps.
Ern
The waste spark also reduces unburned HC in the exhaust.
StrangeRanger":3rcghg9c said:
I reckon it can't hurt, but just how much good that little tiny spark does in the middle of that firestorm of hot exhaust gases ....
Most small engines such as Briggs & Stratton, etc. have used a waste spark system for years as it is much simpler to build.
Joe
A
Anonymous
Guest
The 'waste spark' approach is useful for high RPM, too - like 4000 and up. 4-cylinder bikes use it to help reduce the effects of their large overlap cams like this: the exhaust opens late (10 degrees after BDC), which gives inertia to the exhaust gases in the manifold because it creates a slight vacuum pulse near the exhaust valve at certain RPM ranges (high RPM ranges). The intake opens early (BTDC, about 5 degrees) and closes late (10 degrees BBDC). The exiting gases have burnable fuel, and igniting them a little (it's a bigger spark, because of the lower cylinder pressure) causes a slight pressure rise in the cylinder to "push back" slightly against the incoming intake charge until about 4000 RPM or so. This has the effect of "shortening" the cam duration at lower engine speeds while leaving high overlap for the all-important intake charge at higher RPM - like 12,000, my favorite area to work (and roadrace). This "shortening" gives better low-end torque for the around-town and stopsign-to-stopsign driving that gets you to work (to pay for the racing weekends). Then, as the RPM rises, the pressures in the "open" cylinder also rises, which diminishes the spark a little, which diminishes the intake backpressure effect a little...which is eventually overcome entirely by the speed at which the incoming air inertia rams itself down the open valve's throat -- at about 6000 RPM.
If you haven't done hi-RPM tuning like this, my advice is: print this out, get a beer, put your feet up, and it will come to you as you think thru the physics of air inertia at speed....
It all actually works better as the intake tract becomes smaller, because the velocity of the air (and hence, it's inertia) is higher. Honest, that's why you can hit 14,000 - 16,000 RPM with a Honda 500 or 750 from the 1970s! Been doing it ever since then.....
If you haven't done hi-RPM tuning like this, my advice is: print this out, get a beer, put your feet up, and it will come to you as you think thru the physics of air inertia at speed....
It all actually works better as the intake tract becomes smaller, because the velocity of the air (and hence, it's inertia) is higher. Honest, that's why you can hit 14,000 - 16,000 RPM with a Honda 500 or 750 from the 1970s! Been doing it ever since then.....MarkP":2dnm0ah9 said:
That's the way I had it figured for spark energy. There is obviously much more to this subject than meets the eye though, it seemed to me that this tiny spark wouldn't have much effect amongst all those still-burning gases, sorta like holding a lighted match up to a burning blowtorch. But then, I have never designed a 15,000 rpm Honda, either
I am a bit confused about the overlap/duration numbers.Joe
A
Anonymous
Guest
Well, JW, it's gets a little "backwards" when the RPM starts getting real high. The waste spark on the "unloaded" side of the coil gets the lion's share of the coil's energy, and it sparks a little longer, which improves that thru-burn "propulsion" effect. The real fly in the ointment, though, is the overlap. While cars engines (5000 RPM or less) like having early intake valve openings, which helps those big inlet ports get moving, in a bike with little ports, early opening causes anti-pressure against the incoming charge. If you try to compensate to have more "breathing time" by widening the overlap, the darn thing won't run at low speeds - or it's hard to start.
So, the overlap is still there, but it gets shifted toward the exhaust side, closing both valves very late, like 25 degrees ABDC/ATDC. Then it becomes a "race" between how fast the fuel will burn (and for how long) versus the pulse-time in the exhaust pipe. If the pipe's length is just right, it generates a slight header-like suction at about 6000-7000 RPM to get the engine past this "flat spot" that typically shows up. All of the spark advance is up front, like 40-45 degrees by 2500 RPM, because the slow-engine model is running on about 60% of normal cylinder charge. The carbs are always rich in this range, too, to try to help get some torque.
Then comes the magic moment, at about 8000 RPM. The inlet velocity reaches the point where it can push through the open valves during overlap and the extra spark "kicks" the exhaust down the pipe a bit, with the late exhaust valve closure improving the exit velocities. At about 10,000 RPM, it helps if the spark can retard (or be set back) about 3-5 degrees to get past the next "hump" where the fuel is still too rich, due to the mechanics of the simple carbs. This is where the fine-wire sparkplugs help: they do not yet ionize. At about 11,000 they start ionizing and begin to add extra timing advance of their own. When 12,000 RPM is reached,they start adding about 1 degree of "advance" per 500 RPM until the valves begin to float and pressures drop. This point is real noticeable, because when the float occurs and the ionization stops suddenly, it feels like someone popped a parachute out behind you.
So, in the end, the power "starts" at about 10,000 and runs out after about 14,000 RPM. The engines will easily hit 16,000 when they are fresh, in lower gears. They sort of sound like loud honeybees....
So, the overlap is still there, but it gets shifted toward the exhaust side, closing both valves very late, like 25 degrees ABDC/ATDC. Then it becomes a "race" between how fast the fuel will burn (and for how long) versus the pulse-time in the exhaust pipe. If the pipe's length is just right, it generates a slight header-like suction at about 6000-7000 RPM to get the engine past this "flat spot" that typically shows up. All of the spark advance is up front, like 40-45 degrees by 2500 RPM, because the slow-engine model is running on about 60% of normal cylinder charge. The carbs are always rich in this range, too, to try to help get some torque.
Then comes the magic moment, at about 8000 RPM. The inlet velocity reaches the point where it can push through the open valves during overlap and the extra spark "kicks" the exhaust down the pipe a bit, with the late exhaust valve closure improving the exit velocities. At about 10,000 RPM, it helps if the spark can retard (or be set back) about 3-5 degrees to get past the next "hump" where the fuel is still too rich, due to the mechanics of the simple carbs. This is where the fine-wire sparkplugs help: they do not yet ionize. At about 11,000 they start ionizing and begin to add extra timing advance of their own. When 12,000 RPM is reached,they start adding about 1 degree of "advance" per 500 RPM until the valves begin to float and pressures drop. This point is real noticeable, because when the float occurs and the ionization stops suddenly, it feels like someone popped a parachute out behind you.
So, in the end, the power "starts" at about 10,000 and runs out after about 14,000 RPM. The engines will easily hit 16,000 when they are fresh, in lower gears. They sort of sound like loud honeybees....
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