# Ignition Systems

**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, a battery being held at full

float charge needs around 13 volts, and a running charging automotive system is

typically in the low to mid 14-volt range.

This low voltage must be

increased to

over 15,000 volts to arc across the spark plug’s gap. The resulting spark must have

enough plasma (heat) energy to ignite the mixture. Any extra energy, beyond

reliably lighting the mixture and starting the flame front, has little or no useful

effect. To be sure, excess energy simply eats away at gaps faster. The

igntion voltage requires stepping the 12-volt electrical system up a

few thousand times. The most efficient way to accomplish this is with some form

of energy storage

with a sudden energy or stored charge release. A little bit of energy is stored

over a long time, and then released all at once to generate a high-energy

high-temperature spark. This is where the phrase “spark

discharge” comes from.

Sparks or arcs behave in a unique way.

Initially, there is a

rather high hold-off voltage before any gaps (in the distributor and plug) break

down. Once the gap

“fires” the gap fills with a plasma, causing the arc sustaining voltage

to become quite low. At this point the arc requires much more current and much

less voltage to sustain the plasma. The ideal voltage is very high at first with

a great deal of current surge to back it up. The ideal waveform hits the series gaps

with a very fast application of extreme voltages, then sustains the series arcs at a

much lower voltage and higher current. The arc has to sustain until the combustion

process fully starts, all taking place in a small fraction of a second.

The ignition system operates under the same basic principles as

common low voltage storage

battery charging and starting systems. The ignition system just has much higher

voltage and, because of the short discharge time and much lower power, a whole lot less

overall energy.

In systems like this we have to consider load time and

power levels. To do this, engineers measure the stored and used

energy in ** joules,** which is the number of watts over a

certain time period. One joule is one watt-second. A car battery is a few million joules, while an ignition system

might be down around 1/10th of one joule!

Both systems

accumulate energy in some storage device over a long time period to satisfy a short duration

much higher peak load.

With the car electrical system, the alternator charges a storage battery over minutes and hours to

both meet the

starter’s high peak energy cranking demand and to “fill in” missing power whenever

the vehicle’s electrical load exceeds

the alternator’s output. The ignition system stores much less energy, with a

charging period measured in thousandths of a second and an extremely short spark discharge

time.

The standard measurement of ignition energy is the joule. A joule is

defined as one

watt-second of energy (USA home electrical service bills in kilowatt-hours). One joule can be one

watt for one second, 100 watts for 1/100th of a second, or any other time

multiplied by

power combination that

comes out as one watt-second. Typical ignitions are in the 50 millijoule (50 mJ) to 200

millijoule (200 mJ) range, or .05 joules to .2 joules. The Ford factory TFI, for

example, was around the lower 100 mJ range. You’ll see why these numbers are

important when comparing ignition systems.

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 repeating standard ignition

system sparks. This is 2.5 milliseconds between sparks, or 1/.0025 = 400 sparks

per second.

The crankshaft of a 6000 RPM engine rotates 360*RPM/60 =

36,000 crank rotational degrees per second. We can simplify this to

seconds per degree = 1/36,000 or .0000278 seconds per degree at 6k RPM. At 6,000 RPM every one degree crank position

change takes about 28 millionths of a second (28uS). 1 degree spark timing change is just 28 uS

at 6,000 RPM. We can see how recovery time might be a problem.

Even firing fouled plugs, poorly

burning mixtures, or difficult to light mixtures spark only needs to last

several 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. If

ignition time goes beyond a 5-10 degrees there can be substantial power loss.

10 degrees at 6k RPM is 10*28uS = 280 uS. Spark lasting beyond 0.2 mS

(.0002 seconds) is pretty much wasted. After 0.2 mS, the likelihood of obtaining

any meaningful 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.

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