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Computer Controlled Ignition

One of the first mass produced attempts at controlling spark timing accurately
without the use of advance mechanisms was the Chrysler Spark
Control Computer.

In this circuit, spark timing is controlled by the Spark Control Computer - which
contains timer circuits to determine when to collapse the Primary magnetic field, and
the large transistor used to control the field switching. The distributor pickup is fixed,
and determines engine speed and crankshaft position. Engine load is measured using
a vacuum diaphraghm mounted on the computer. Additional inputs on this system
that will help determine spark timing include:

A throttle position transducer - to determine throttle opening.
A coolant switch - to measure engine temperature.
An idle stop switch - to measure when the engine is in idle position.

Later models included more inputs, and fuel and emission controls were also added to
the computer output functions.

GM and Ford

GM and Ford brought out computer controlled spark timing systems that were
designed to help maintain more accurate timing over a wide range of operating
conditions, and included such inputs as knock sensors to help prevent engine
problems due to over-advanced spark timing.

These systems still contain a number of moving parts that can wear
or fail, causing reduction in performance, loss of fuel economy, and
increased emissions. To overcome these problems, Direct Ignition
Systems were developed.

Direct Ignition Systems

On this General Motors Direct Ignition System (also known as Distributorless
Ignition System) the pickup coil and distributor shaft have been replaced by an iron
wheel and electromagnetic crankshaft position sensor to measure engine speed and
position. There are three coils for a six cylinder engine. They are connected to the
ignition control module underneath the coils. The module is controlled by the ECM
(Engine Control Module - the Computer). Each coil fires two spark plugs at the same
time - with one cylinder on compression and the opposite on exhaust.

Note that each Ignition Driver will control a coil that in turn
will fire two spark plugs simultaneously.

What's Next?

There are two problems with this DIS system:

There are still spark plug wires that can fail and result in no spark reaching the

cylinder.

Half the plugs will fire in the normal direction (center electrode to ground)

and the other half will fire from the ground to the center electrode (which

is less efficient).

Coil on Plug Ignition Systems

To overcome this problem, most of today's manufacturers are moving

towards "Coil on Plug" ignition coils, where each individual spark plug

has its own ignition coil controlled from the Engine Control Computer.

Note that a DC convertor is used to change the 12 volts of the
battery to 400 volts to ensure that the primary winding
will build up a strong magnetic field.

This system:

Eliminates spark plug wires.
Ensures that all plugs fire in the same direction.
Ensures that coil failure will affect only one cylinder.

Direct Ignition Inputs

Because a distributor is not used, sensors must be located where they can
measure crankshaft or camshaft position and speed. If the plugs are fired in pairs,
a crankshaft position sensor is adequate. If the plugs are fired individually (such
as in the "coil on plug" system) - a camshaft position sensor must be used.

Electromagnetic Sensor

In this instance, an iron wheel with notches is machined in as part
of the crankshaft. A sensor, consisting of a permanent magnet
surrounded by a coil of wire, is located close to the reluctor wheel.
As the wheel turns, the notches will cause a deflection of the
magnetic field in the sensor, and result in a small voltage signal
being sent to the computer. To determine which cylinder is at
Top Dead Center at any given time - the #1 cylinder position has
a double notch on the reluctor wheel.

Hall Effect Sensors

A Hall Effect sensor switch consists of a special Hall Effect
transistor and a permanent magnet. When the magnetic field
crosses the transistor at right angles, the transistor is "switched
on". If a piece of iron (shutter) is passed between the magnet
and the transistor - the magnetic field is blocked and the
transistor is "switched off". A shutter wheel is driven by the crankshaft,
camshaft - or on older systems - a distributor shaft.This will control
voltage signal outputs to the computer to indicate crankshaft or
camshaft position and engine speed.

There are a number of inputs that will determine correct spark timing - and these will
vary with vehicle manufacturers. These inputs will also be used to help control
other systems - and will be covered in a little more depth later on in the
Fuel Injection and Emission sections of this course.

Next - Unit Four Assignment

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