2 ignition coils

[coop1er]

Hey there.

I am wondering for an engine that has two spark plugs per cylinder, and I wanted to fire two individual coils at the same time, which method would provide the most spark. Wire the coils in parallel or in series?

Thanks for any replies.

 

[Tony Stewart]

Breakdown voltage rises in series and under 8:1 compression, so parallel

 

[coop1er]

Ok thankyou.

Just to provide extra detail, this is for a turbocharged engine with the spark being controlled by an ignition module and a hall effect sensor in the distributor.

Also is there any difference in the spark output if I was to use just one coil (ie. delete one coil and not fire the second spark plug)?

Are there any potential problems if the coils are wired in parallel but not identical?

 

[Tony Stewart]

Naturally available current will be divided to the plugs or doubled to the coil as per the design details.

 

[MikeMl]

Aircraft engines have two plugs per cylinder, each fired by two independent magneto systems, primarily for redundancy. I can turn off either magneto at will in flight. Shutting down either mag causes a rpm drop, and a loss of power. Magnetos are timed to fire the plugs simultaneously. Mixture burns in a flame front that propagates across the cylinder, with the two flame fronts meeting at the center. If only one plug fires, the burn profile inside the cylinder changes because only one flame front has to propagate all the way across the cylinder, and less power is developed.

 

[crutschow]

I had a 1984 Datsun that had dual plugs per cylinder (apparently to slightly improve power and gas mileage).
As I recall it would fire both plugs under normal load but only one of the plugs under heavy throttle to minimize pre-ignition (the slower flame-front from only one plug firing reduced the peak cylinder pressure, and thus the chance of pre-ignition.

 

[coop1er]

Interesting information. Thanks for all responses.

 

[tcmtech]

I have a antique two cylinder 3 KW Fairbanks Morse gen set that runs a separate coil for each cylinder but only has one set of points so the both coils are fired at the same time.

With that one it uses two 6 volt coils that are wired in series on its 12 volt supply. When I tried switching it over to use two 12 volt coils in parallel the points burned up in a matter of minutes but when they were put in series it had really weak spark on both coils. Given that apparently the correct way to run it is with the two 6 volt coils in series.

So given experience I vote for the coils to be connected in series.

 

[Tony Stewart]

With two plugs in parallel with different Breakdown Voltages, they need separate magnetos's or coils as the arc is a like an SCR with a negative resistance.

So if the Arc voltage is sufficiently high, from one coil only, series makes more sense to share current. It's not about the fastest or synchronous spark that gives power.

 

[Tony Stewart]

From what I have read and understand better now, is that there are several different designs of twin plugs. This is my understanding.

For more reliability, twin combustion plugs must have twin ignition coils but are triggered at the same time. Sometimes one for starting and two for running or manually controlled.

For lower emissions, twin plugs in different cylinders of different thermal sizes are in series using a single coil with the plug tips in series. Thus one conducts from body to tip and the other from tip to body. One is for the exhaust stroke of unspent fuel on an opposite cylinder while the other fires during compression stroke.

For more power or combustion efficiency twin plugs in the same cyl. using a differential coil with series connected plugs Note that when in series, one plug arcs from tip to ground, while the other arcs from ground to tip. But they can no longer be both centered in the cylinder, so I suspect equidistant.

The current is broken in the primary coil with a switch (points or transistor) which creates the large voltage spike and is stepped up to the secondary. It triggers before TDC since ionization has a delay before detonation that varies with amount of fuel in the chamber, this is why timing is before TDC but delayed with loss of vacuum pressure during acceleration as ionization time reduces.

Today, ECU's look at inputs from the throttle position sensor, airflow sensor, coolant sensor, MAP sensor and even the transmission to determine how much timing advance to give each plug.

The gap between the spark plug tip and piston is very small and I once destroyed an old boat engine with a newer prop that ran at slightly higher RPM. It started pinging due to thermal pre-ignition and one day the over-travel in the pre-ignition piston reflex caused the piston to crush the plug at TDC and made a crater in the cylinder wall from the debris.