Perfect Power Application Notes.


Application Note - AN 6


The XMS4 systems can only drive coils directly if they have built in amplifiers, if the coils trigger with a negative pulse an external driver is needed. The dwell time can be adjusted accordingly. The following units have the indicated amount of drives :
  XMS4A-4 XMS4B-4 XMS4B-8 XMS4C-4 XMS4C-8
Ignition Drives : 4 4 8 4 8
Injector Drives : 4 4 8 4 8
Set Point Drives : 4 4 4 8 8

Therefore, if it is required to use a XMS4A-4 or the XMS4B-4 to drive an 8 cylinder wasted spark application, then some external components must be used. This application note describes this.

Likewise, some modern engines use "coil packs" for wasted spark applications, and these packs are very nice and convenient to use. This application note also describes the use of these "packs".



Wasted Spark Background

A wasted spark describes the method where a cylinder pair is fired simultaneously, one spark ignites the fuel, and the other is WASTED into the exhaust stroke. It does nothing! This method is very popular because it eliminates the distributor, and it allows greater dwell times. Also the spark intensity is higher, which someone may argue, can be compensated for.


Most important is the elimination of the distributor :

  • Mechanical Space
  • Gear (it runs at half the crank speed)
  • Mechanical Inaccuracies
  • Pickup Arrangements (Optical, Magnetic, Other)
  • High Tension Distribution


To put it simply : A distributor is out, multiple coils are in! Multiple coils (one per cylinder) can be fired sequentially. One reason for a sequential ignition would be the doubled dwell time, and subsequent halved current.



Dwell Time Background

The dwell time is the time a coil is "charged up" before it discharges into the spark plug. This is what happens :
Once a voltage is put onto the coil, a current starts flowing. It rises slowly, (well, slowly in electrical terms) until the can't anymore due to some limitations. This is the saturation point. Nothing is gained by letting the current flow any longer. The coil is charged up. Switching the current off results in a spark. The spark has almost the same energy as the "charged up" energy.
This can be demonstrated as follows :


Coil Charge : 0.005 sec, 5 Amps, 12 Volts = 0.3 Avs - 0.3 Ws
Coil Discharge : 0.0004 sec, 0.015 Amps, 50 000V = 0.3 Ws


Of course, there are losses, and the above is not quite true. But important is the fact that the voltage will rise until a current flows for the time until the charged up energy is dissipated. If you reduce the voltage, then the current and time will increase proportionally. This explains why the spark "colour" will change from blue (high voltage) to yellow (low voltage).


Modern coils can be charged up in 2 to 5 milliseconds (0.005 sec). Due to the use of ferrite material, they can discharge in 0.0001 seconds, and deliver any voltage possible.


Things that influence the spark : spark leads, spark plugs, coil construction and driving electronics.



External Amplifiers

It is a device, which converts a relatively low voltage and current to a high current suitable for a coil. These devices are available as a standalone unit to be mounted to the chassis or engine, or as a device INTEGRATED in the coil. This is acceptable, would another variable not confuse the issue :



Inverting or Non-Inverting Drive

This drive term applies to the voltage, and it is best explained as follows :


NON-INVERTING : LOW voltage in, current flows, the coil voltage is LOW.
INVERTING : HIGH voltage in, current flows, the coil voltage is LOW.


Most external amplifiers use an INVERTING signal, because it is very convenient. The XMS4 output, because it cannot drive a coil directly, requires an INVERTING external amplifier. The various combinations are explained in the following diagrams, which indicate signal levels as well.



Pull Ups

It is a resistor, connected to +12V, and the signal, to "PULL" it up to a voltage. The value of a pull up is not critical, 1 KOhm to 2K2 are universally acceptable.



XMS4 driving a single coil using an Inverting Amplifier


XMS4 Stand Alone Unit - XMS4 Driving a Single Coil and Inverting Amplifier





XMS4 driving multiple coils directly in Sequential (built-in Amplifiers)



XMS4 Stand Alone Unit - XMS4 Driving Multiple Coils Directly in Sequential





XMS4 driving multiple coils directly in batch (built-in Amplifiers)



XMS4 Stand Alone Unit - XMS4 Driving Multiple Coils Directly in Batch





XMS4 driving 8 coils directly in Sequential (built-in Amplifiers)



XMS4 Stand Alone Unit - XMS4 Driving 8 Coils Directly in Sequential





XMS4 driving 8 coils directly in Batch using an inverting amplifier



XMS4 Stand Alone Unit - XMS4 Driving 8 Coils Directly in Bathc using an Inverting Amplifier