Ignition System

Ignition System

The ignition system is required to supply an electric spark in each cylinder of the engine at the precise time that the fuel vapour is corn pressed ready for combustion. This spark ignites the fuel vapour, thus starting the combustion process which forces the piston downward on the power stroke.

Elementary principles

The theory of electricity, magnetism, induction and current generation as applied to the motor vehicle is covered in a later chapter. In this phase ignition principles are briefly covered and many statements must be accepted without

explanation. To cause a current of electricity to jump across a gap in the form of a spark a high electrical pressure of between 10,000 and 20,000 volts is required. The current available battery is at the low pressure of ether 6 or 12 volts, and it is the function of the ignition coil to transform this low voltage to the necessary high voltage (called high tension).

There are therefore two electrical circuits in the ignition system:

(a)

The primary or low-tension circuit which connects the battery to the ignition coil.

(b)

The secondary or high-tension circuit which carries the high voltage produced by the coil through the distributor to the spark plug. (Study Fig. 10.1.)

Ignition Coil

The coil consists of two windings of insulated copper wire on a soft-iron core.

(a)

Primary winding — relatively few turns of medium-gauge copper wire, connected to the battery through the ignition switch.

(b)

Secondary winding — a large number of turns of very fine wire. In this winding the high voltage for spark production is induced by magnetic influence from the primary winding (Fig.

Distributor

There is a primary system and a secondary or HT system built into the distributor, as will be seen from Fig. 10.3. The cap with its high-tension cables carries the HT output of the coil to each of the spark plugs in their correct firing order. The primary section of the distributor is in its main housing, which supports the cam and breaker points which control the primary coil circuit and the time at which ignition will occur. Study the main components under the following headings:

(a)

Housing — a machined casting containing the distributor mechanism and the bearings which support the distributor drive shaft..

(b)

Breaker plate — the base on which the distributor contact points (also called breaker points) are mounted. These contacts are opened and closed, thus interrupting the primary circuit, by the action of a cam on the distributor shaft.

(c)

Rotor — a rotating arm which carries the HT current from the coil and distributes it to each spark plug lead as required. The arm is insulated by a moulded plastic material.

(d)

Cap — a moulded plastic cap attached to the top of the distributor housing, and retained by spring clips. High tension terminals are set into the cap which carry HT current, first from the coil to the rotor and thence to the spark-’ plug leads.

(e)

Condenser — a small metal canister containing layers of tinfoil insulated from each other by sheets of thin waxed paper. A small wire lead called a pigtail connects the condenser to the distributor points, and the outer case ‘is fixed by a screw to the housing. (See Fig. 10.4.)

Distributor Drive

As shown in Fig. 10.5, the distributor is driven by the camshaft and at the same speed, i.e., half crankshaft speed. In many designs, the : shaft is extended downwards to drive the oil pump for engine lubrication.

(See Fig 10.2).

This winding is also referred high tension or HT winding and may up to 20,000 turns of wire of .001-in.

This mechanism is similar in principle to a centrifugal governor, and is mounted in the housing beneath the breaker plate. As engine speed is increased, a set of weights driven by the distributor shaft tends to fly out- wards and by means of linkage move the dis tributer cam slightly forward. This causes the distributor contact points to open earlier, which results in earlier ignition in the cylinder, and is called “automatic spark advance” (Fig.

10.6).

Vacuum Control

An Spark small additional “advance” diaphragm control is usually fitted for or “retard” in the form of a operated by vacuum (or

“Suction”) from the intake manifold (Fig.10.7). This ensures that the spark timing incorrect, not only for high engine speeds (advanced) but also for low speeds under heavy load (retarded). The above controls supplant the hand spark-adjustment levers on early models.

Spark Plugs

These are the actual spark gaps screwed into the cylinders. The outer steel shell carries

porcelain or mica core through which r the centre electrode. This electrode, and side electrode to which the spark jumps, are made of heat- and corrosion-resistant alloy. The gap between the electrodes is set to the engine maker’s recommendation, many engines using a gap of .025 in. and others .035 in. to .040 in. A copper washer is used as a gasket between the threaded portion of the plug body and the

cylinder head (Fig. 10.8). Another type has cone seat without a gasket.

Action

(a)

Low-tension Circuit When the ignition switch is turned on rent flows from the battery through the

winding of the ignition coil to the distributor. Contact points (Fig. 10.9). If these points are

closed, the current completes the circuit by passing through the distributor housing to the

Frame of the vehicle (called “earth” or “ground”) and thence back to the battery.

Whenever a current of electricity flows through a wire it is surrounded by an area of magnetic

influence, or “lines of force”, called a “magnetic field”. When the wire is formed into a closely

wound coil with an iron core, this magnetic field is concentrated and relatively strong. Thus

when the primary circuit is complete a strong magnetic field surrounds the primary windings

and permeates the secondary windings. As the contact points close and open, with the rotation

of the cam, the current is turned on and off, accompanied by the alternate build-up and

collapse of the magnetic field. The condenser, also called a capacitor, is

connected across the breaker points, and has the capacity to temporarily absorb a “charge

of current when the points are opened, and this quenches the “arcing” or spark which would otherwise occur across the points. Further, the condenser then “discharges” its stored energy back into the coil primary in reverse direction to the original current flow, thus quickly “damping” the primary current and hastening the collapse of the magnetic field. The more rapid the collapse of this field, the more effective is the high-tension output from the secondary windings.

(b)

High-tension Circuit

It is the collapse of the magnetic field produced by the primary coil which induces the high-tension current in the secondary windings. The movement of this field as it collapses passes through or “cuts” the secondary windings, the resultant HT current being said to be generated by electro-magnetic induction. The spark therefore occurs when the distributor points open. One end of the high-tension winding is “earthed” inside the coil itself, while the other end is connected to the HT terminal of the

coil. A heavily insulated wire (called HT wire) connects the coil to the distributor cap. From here the HT current flows via the rotor to whichever spark plug it is required to “fire”.

Timing

When the distributor assembly is attached :: the engine, the driving-gear teeth are meshed: so that the contact points open at the exact time a certain spark plug is required to fire (Fig. 10.10). It is usual to time the distribute so that the points are just commencing to open when No. 1 piston is at the top dead centre on its compression stroke. In this position the rotor arm will be opposite the distributor cap terminal of the HT lead to No. 1 spark plus Each succeeding cap terminal (in the direction of rotation of the rotor arm) is connected to the plugs in the firing order of the engine Final adjustment is then made to makers Recommendations and timing marks are provided on the flywheel or crankshaft pulley at the front of the engine (Fig 10.11). A clamp retains most distributor housings, and when loosened allows them to be turned slightly clockwise or anti-clockwise until the correct timing position is achieved. Some units have a knurled adjusting screw. When the points commence to open well before TDC the ignition is said to be advanced. If the spark occurs later than it should, it is retarded.

SELF-TESTING QUESTIONS

CHAPTER 9

What is the principal function of the carburettor?

Which part of the carburettor is concerned with?

(a)

Fuel supply

(b)

Starting

(c)

Slow running

(d)

Rapid “pick-up”

Draw an “idle” or slow speed circuit.

What is the purpose of the compensating circuit?

What are the advantages of the fuel injection system over the carburettor system?

CHAPTER 10

Give the location and function of the fol- lowing ignition components: Vacuum control Condenser

High tension leads Pigtail Rotor 0

Describe the action of the automatic advance.

Name and describe the two windings in the ignition coil.

Describe the action of the low tension circuit.

What happens when the ignition spark occurs at the wrong time as, for example, on the intake stroke?

MGB SD1

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F100 MONARO GTS 350

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