Ignition Systems

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What Makes the Spark?

Our cars have what we call "12 volt" electrical systems. A fully charged car battery in resting state is around 12.6 volts, and a battery being held at full change needs just over 13 volts. Somehow, this low voltage must be stepped up to well over 15,000 volts to fire across the spark plug gap. That spark has to have enough voltage and current to ignite the mixture. Any extra energy, beyond reliably lighting the mixture and starting the flame front, has little useful effect. Creating the spark requires stepping the 12-volt electrical system up a few thousand times. This requires some type of energy storage or charging time, with a sudden energy or charge release. This is where the phrase "spark discharge" comes from.

The spark system operates under a similar principle to a battery charging system. Both systems store energy to satisfy a sudden peak load. With the car electrical system, the alternator charges a storage battery over minutes and hours to meet the starter's high peak energy demand, or to fill in when electrical load exceeds alternator output. The ignition system stores much less energy, with a charging period measured in thousandths of a second and an even shorter spark discharge time.

The standard measurement of energy is joules. A joule is one watt-second of energy (our houses are measured in kilowatt-hours). It can be one watt for one second, or 100 watts for 1/100th of a second, or anything else that comes out as one watt-second. Typical ignitions are in the 20 millijoule to 200 millijoule range, or .02 joules to .2 joules.

Spark Power and Time

The standard four-cycle engine spark repetition time rate formula is t=120/(N*RPM) where t is seconds and N is number of cylinders. A V8 at 6000 RPM has 120/48000 = .0025 seconds between sparks. This is 2.5 milliseconds between sparks, or 1/.0025 = 400 Hz.

The crankshaft of a 6000 RPM engine rotates 360*RPM/60 = 36,000 degrees per second.  We can simplify this to seconds per degree = 1/(.006*RPM), or 28 milliseconds per degree at 6k RPM.

Even if we are trying to fire a misfiring plug or poorly burning mixture, spark only needs to last a few degrees of crank rotation. A spark duration of 20-30 degrees will pretty much cover the time to obtain any useful mechanical power from a poorly firing or misfired cylinder. At 6k RPM, spark has to last than 0.1 mS (.0001 seconds). After 0.1 mS, the likelihood of obtaining useful power from a misfire or incomplete ignition rapidly decreases. At  0.8 mS, the piston would be 29 degrees ATC. The piston will have moved so far below top dead center, firing the cylinder would provide virtually no power. The piston would be 30-50 degrees past the position of peak power.

There is little reason to worry about a long spark or multiple sparks at speeds much above idle, and a long spark will actually reduce intensity at the optimum spark point. We really want all spark energy concentrated at the optimum ignition time. Spark duration beyond a few degrees of crank rotation is largely a waste, except for air pollution reduction during misfires. Based on a spark duration of 30 degrees and a battery drain of 8 amperes in an ideal induction ignition breaker point system:

 

Cylinders Strokes Amps Avg Deg Dwell  
8 4 8 30  
RPM btwn spk time mS Spk Duratn mS Dwell t Max mS Max millijoules
1000 15.00 5.00 10.00 80.00
1500 10.00 3.33 6.67 53.33
2000 7.50 2.50 5.00 40.00
2500 6.00 2.00 4.00 32.00
3000 5.00 1.67 3.33 26.67
3500 4.29 1.43 2.86 22.86
4000 3.75 1.25 2.50 20.00
4500 3.33 1.11 2.22 17.78
5000 3.00 1.00 2.00 16.00
5500 2.73 0.91 1.82 14.55
6000 2.50 0.83 1.67 13.33
6500 2.31 0.77 1.54 12.31
7000 2.14 0.71 1.43 11.43
7500 2.00 0.67 1.33 10.67
8000 1.88 0.63 1.25 10.00
8500 1.76 0.59 1.18 9.41
9000 1.67 0.56 1.11 8.89
9500 1.58 0.53 1.05 8.42
10000 1.50 0.50 1.00 8.00

 

A standard CD system handily beats a theoretically perfect induction system at high RPM, because energy millijoules in a CD system are almost constant across the RPM range. For example, this small Summit Racing CD system (made by MSD) has over 100 millijoules of energy storage, and draws only 1 amp per 1000 RPM. It has more spark at 8000 RPM than a conventional inductive system running at 8 amperes can supply at 1000 RPM!

Ignition Spark Intensity

Spark intensity or quality varies with the storage and release of energy. We have to fill the storage reservoir of some type (like charging a battery), and release that energy at the appropriate time. There are two factors being traded when storing energy:

  • Charging time
  • Charging current

Energy storage is a direct product of current level over time. The less time we have to charge the system, the more current required. Electrical energy is measured in watt-seconds or joules. A charge or discharge rate of one ampere for one second is one joule. j = t*I where t is time in seconds and I is current in amperes. Car ignitions are in fractions of a joule.

Ignition Types

There are two common types of ignition systems, induction or "fly-back" coils and capacitor discharge or CD ignitions. Both systems work by energy storage, and the sudden release of that stored energy.

Typically, this is how both types compare in spark energy (based on a demand of 8 amperes maximum current):

 

Induction style ignition coils do not act as transformers. Consequentially, increasing primary voltage does not directly increase induction coil spark voltage. Increasing coil current or dwell time up to the point of core magnetic flux saturation, and reducing loading across the coil, increases spark voltage. Increasing voltage may or may not increase spark, the result being dependent on dwell time, dwell current, plug resistance, and coil characteristics.

Multiple Coil Systems

Multiple coil systems have the advantage of increasing dwell time. For every doubling of coil numbers, dwell time also doubles. In the ideal case doubling stored energy at any given RPM, up to a limit of core saturation.

In 1985, we manufactured a multiple coil marine system. Each coil was capable of storing 40 millijoules at saturation. To prevent excessive current at slow speeds, the coil was current limited by external electronics.  That system produced a flat spark energy curve up to about 5000 RPM. Above 5000, energy gradually tapered off.

This system produced around the same spark energy with any battery voltage over 8 volts. This is because the system was current limited, or ballasted. If the system was not current limited or ballasted, it would have drawn excessive current at low speeds.

Dwell

Dwell (coil charge time) is best explained by looking at an old point style ignition. The dwell for a 1964 327 Chevy engine is shown below:

 

Dwell is normally given in distributor degrees. With eight cylinders, there are 360 distributor degrees. Spark occurs at point opening, with each spark point 45-degrees apart. Allowing for wear, opening and closing times, and time to fully discharge coil, dwell must be set some reasonable number below 45 degrees.  Typically, this is around 30 degrees in a point-type ignition.

Electronic ignitions respond faster while avoiding mechanical friction-wear and contact arc problems. Significant reduction of wear and more consistent operation allows electronic systems to use higher dwell than breaker point systems, with dwells as high as 40 degrees possible. 

Coil Inductance (Stopped ignore all after this point)

Coil inductance is normally in millihenries, or thousandths of a Henry. Coil inductance, along with voltage and external resistance or current limiting, is what controls the charge time (dwell) required to fully charge an induction coil. There is a time constant, which is the time required to reach and there is a transient time. The formulas are:

Time constant = L/R

Transient time, or the time to reach what is normally considered full charge, is five times the time constant

The ballast resistor or ballast current should be set to fully saturate the coil at minimum operating voltage. Let's assume that current, in a heavy duty coil, is 8 amperes and the battery is always above 12 volts. This would be 12/8 = 1.5 ohms total ballast resistance (including coil R), or a current of 8 amps in a solid state current limited ignition.

An 8 mH coil impedance with a 1.5 ohm ballast has a time constant of .008/1.5 = 5.3 milliseconds. The transient time is five times that, or 26.7 mS.

An engine rotates .006*RPM = degrees per second. A 5000 RPM engine would rotate 30 degrees per second.

This is 1/.006*RPM = seconds per RPM degree. The 5000 RPM example would take .033 seconds (33 milliseconds) to move 1 degree.

At 5000 RPM an 8 mH coil with 1.5 ohm total ballast and coil resistance would fully saturate.

Spark would occur every 45 degrees in a V8. That would take 45*(1/.006*RPM ) =  The dwell time would be