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Engine Ignition. An Overview of the Ignition Systems Utilized in the Early Internal Combustion Engines. Hot Tube Ignition Igniter (low tension) Ignition Spark Plug (high tension) Ignition. Types. Simplified Overview

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engine ignition

Engine Ignition

An Overview

of the

Ignition Systems Utilized

in the

Early Internal Combustion


Hot Tube Ignition

Igniter (low tension) Ignition

Spark Plug (high tension) Ignition

hot tube ignition
Simplified Overview

Hot tube ignition was used on the oldest of the internal combustion engines. Usually rather large engines. No electricity or “spark” is needed.

Hot Tube Ignition

Air / FuelMixture


Closed end tube


Figure 1

Heat source

As the air/fuel mixture is compressed, a portion of the compressed mixture is forced into the heated closed end tube. When the temperature and pressure within the tube reach the required point, the mixture ignites and ignites the mixture in the cylinder. (Figure 1)

Think of a deisel engine with a glow plug

An igniter is simply a set of contacts or “points”. It consists of one moveable and one fixed contact. The two contacts bodies are separated and insulated from each other by a mica tube and washer. (Figure 2)

Igniter body / mounting bracket

Mica insulation

Electrical contact

Fixed contact

Moveable contact

Trip mechanism


Figure 2

  • Igniters are used in low tension ignition systems, which include:
  • Battery and coil
  • Low tension magneto
battery and low tension coil
The battery and low tension coil ignition system consists of a battery, a single field inductive coil, the igniter and a switch.

When the engine is not running, the switch should be open to prevent the flow of current through the coil. The igniter contacts may or may not open base on the cycle of the engine. ( Figure 3A)

Current passing through the coil for an extended period of time will cause the coil to heat up and possibly burn out the coil.







Figure 3A

Battery and Low Tension Coil

Before starting the engine, rotate the flywheels until the igniter is in the “open” position. Close the switch and start the engine. (Figure 3B)







Figure 3B


As the engine passes through the compression cycle, the engine mechanism closes the igniter contacts just prior to the point of firing. This causes the electrical circuit to close and current to flow through the coil. The current flowing through the coil establishes an electrical magnetic field (EMF). (Figure 3C)









Figure 3C

When the engine reaches the ignition cycle, the igniter is released, the contacts “snap” open. With the contacts open, current ceases to flow through the coil. The established EMF collapses onto the core with the reverse voltage. This reverse voltage seeks the ground potential through the gap of the igniter contacts causing a spark. (Figure 3D)









Figure 3D


From Fairbanks-Morse - Principles Of Magnetos

If a piece of steel is bent into a "U", to make the ordinary "magnet," the space between the ends will be filled with invisible magnetic "lines of force." The magnetism will be stronger at this point than anywhere else about the magnet. If a coil of wire is moved in and out of the space between the poles, or is revolved in this space, an electric current will be generated in the wire, and if the ends of same are separated, a spark will be produced between them.

Figure 4


The MAGNETO is simply a magnet, with a coil of wire revolving between its poles (Armature), the coil being provided with a suitable means whereby the current generated is conducted off to the engine igniter, the points of which are alternately opened and closed at the proper time.A peculiar thing about the Magneto is that the current generated in the wire is strongest at only two points in each revolution of the wire, and to get the best spark the igniter points in the engine must snap when the wire is at one of these points.

Figure 5





oscillating magneto


Figure 6A

The magneto is usually attached directly to the Igniter. The trip mechanism closes the igniter points and pre-sets the magneto armature to the optimum position. (Figure 6B)

Figure 6B

As the trip mechanism releases, the armature (under spring tension) rotates through the optimum lines of force. At the same time the points open. The resulting halt in current flow collapses the field and reverse current flow causes the spark. (Figure 6C)

Figure 6C



The continuous rotating magneto works like the oscillating magneto. The armature rotating through the magnet field while the points are closed and opened at the proper time.

The magneto is usually located near and attached to the drive mechanism or timing gear. The igniter trip mechanism is a separate system. This requires that the armature rotation must be “in time” with the Igniter. (Timing marks, crank location, etc.)

Note: Many continuous rotation magnetos require a significant rotation speed to generate sufficient current to create a spark. Therefore, many models require starting the engine with a “battery & coil” and then switch over to the magneto. The Fuller and Johnson early engines had a two-way switch attached for just that purpose.


High Tension Ignition (Spark Plug)

Buzz coil

High Tension Magneto


As the trip contact closes, the circuit through the primary coil is complete and the current begins to flow. The current establishes an electrical field around the iron core causing it to become an electro-magnet. The electro-magnet pulls on one of the buzzing contacts. (Figure 8A)

Figure 8A

As the buzzing contacts separate, the current flow through the primary coil will cease and causing the field to collapse around the secondary coil and cause the current to spark across the spark plug gap. (Figure 8B)

Figure 8B

As the field collapses, the iron core will loose its magnetism. The buzzing contact will return to its normally closed position and start the process over again The buzz coil will continue to oscillate between these two states as long as the trip contacts remain closed.


High Tension Magneto

  • Single Trip
  • Continuous Rotation


Figure 9

Figure 10


The Wico “EK” magneto consists of the following components. (Figure 10)

Figure 10

Electrically, the magneto looks like the following. (Figure 11)

Figure 11


Theory of Operation

When the magneto is in the normal state (un-tripped), the keeper (armature) channels the EMF through the ferris cores and the magnets are in equalibrium. (Figure 12A)

Figure 12A

During the first part of the mechanical trip, the keeper (armature) is disconnected from the ferris cores and the EMF field is generated around the coils. (Figure 12B)

Figure 12B

During the second part of the mechanical trip, the points are opened and the field collapses around the secondary coil and causes a spark. (Figure 12C)

After the trip, the armature is returned to the normal state and the process repeats with the next mechanical trip.


Continuous Rotation Magneto (High Tension)

  • International Harvester H1
  • Wico “H”

The continuous rotation high tension magneto operates the same way as the single trip magneto only in a continuous manner. This requires “timing” to match up to the engine mechanics, just as with the continuous rotation low tension magnetos.

For more information check out: