Spark Plugs

Topics covered in this presentation: Spark Plug Construction Spark Plug Reach, Seating and Heat Range Spark Plug Servicing

Spark Plugs

The Spark Plug

The spark plug:

Typical spark plug components:

Ribbed insulator

Terminal

Insulator nose

Centre electrode

Ground electrode

Shell

Conductive seal

Ignites the air and fuel mixture.

Removes heat from the combustion chamber.

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Spark Plugs

The Steel Shell

The shell is threaded so it can be screwed into the cylinder head.

To minimize corrosion, its surface is electroplated with nickel.

Thread

It is machined with a hexagonal section, for plug removal and fitting.

Hex

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Spark Plugs

How many sides does a spark plug have on the part designed for plug removal and replacement?

Enter your answer and press SEND.

Question 1

Spark Plugs

Purpose:

The Ceramic Insulator

It is made from aluminium oxide and glazed to discourage dirt.

Ribs increase the surface area of the insulator to reduce flashover to ground.

Nose

Ribs

To prevent high voltages from shorting to ground.

To remove heat from the combustion chamber.

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Spark Plugs

2

A) Aluminium nitrate

B) Aluminium hydroxide

C) Aluminium oxide

D) Aluminium carbonate

What material is the spark plug's ceramic insulator made from?

Question

Spark Plugs

Electrodes are made from nickel alloy or platinum.

Ground and Centre Electrodes

The ground electrode is welded to the shell.

They must be resistant to high temperatures and corrosion.

The centre electrode has a machined tip.

Ground electrode

Centre electrode

Spark erosion

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Spark Plugs

3

A) Shell

B) Ceramic insulator

C) Centre electrode

D) Insulator nose

In a spark plug, what is the ground electrode connected to?

Question

Spark Plugs

Spark Plug Dimensions

Thread Diameter

Spark gap

Thread diameter

ReachSpark Gap

Spark Plugs are made in 10, 12, 14, and 18 mm thread diameters.

The gap must be correct.

If it is too small, a weak spark is produced.

If it is too large, the mixture will burn inefficiently.

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Spark Plugs

4

Spark plugs are made in four different thread diameters. Is this true or false?

Answer True or False.

Question

Spark Plugs

Spark Plug Dimensions

Correct reach

Cylinder head

TooshortToolong

Spark Plug ReachIncorrect reach can cause engine running problems or spark plug damage.

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Spark Plugs

Spark Plug Seating

Flat Seating Spark plugs must be fitted with a metal washer to seal combustion chamber and remove heat.

MetalwasherNo washer

Conical Seating No washer required as plug is sealed by its conical seat.

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Spark Plugs

Spark Plug Temperature

Ideal spark plug tip temperature = 500 to 850°C.

If temperature = low, carbon and combustion chamber deposits will not be burnt off.

If temperature = high, the plug may overheat, damaging the electrodes.

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Spark Plugs

What temperature, in degrees C, would be within the ideal range of operation?

Enter your answer and press SEND.

Question 5

Spark Plugs

100%Fresh air

Heat Dissipation

The combustion process produces heat.

20% is absorbed by intake stroke fresh air.

Dissipation occurs as follows:

58% is absorbed by the cylinder head walls.

20% is absorbed by the insulator and spark plug side walls.

2% is absorbed by spark plug wires.

2%

10%

10%

58%

20%

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Spark Plugs

6

A) 2%

B) 20%

C) 25%

D) 58%

Approximately what percentage of heat from the combustion process is absorbed by the cylinder head walls?

Question

Spark Plugs

Spark Plug Heat Range

Heat range = spark plug's ability to remove combustion chamber heat.

Determined by:

Insulator nose length = distance from tip of insulator to where it meets the metal shell.

Insulator nose

length

Insulator nose length.

Gas volume around the insulator nose.

Composition of the insulator and the centre electrode.

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Spark Plugs

Spark Plug Heat Range

Hot Plug:

Long insulator nose, exposing a large surface area to combustion gases.

Cold Plug:

Shorter insulator nose, minimizing surface area exposed to combustion gases.

Dissipates heat slowly.

Firing end heats up quickly.

Dissipates heat quickly.

Firing end heats up slower. Next >

Spark Plugs

Other Types of Spark Plugs

Some spark plugs have multiple ground electrodes.

They provide multiple spark paths which reduces spark erosion.

This surface discharge plug can be used with CDI systems.

It provides good sparking, even when fouled.

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Spark Plugs

Resistor Spark Plugs

Spark plug with carbon compound resistor in centre core.

The resistor suppresses radio frequency interference (RFI).

Plug with carbon resistor

Resistancematerial

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Spark Plugs

7

A) Ferrite beads

B) Wire wound resistor

C) Capacitor

D) Carbon resistor

What can be incorporated into a spark plug to suppress radio frequency interference?

Question

Spark Plugs

Spark Plug Removal

Typical Procedure:

Use an air line/brush to remove all debris from around each plug.

Remove all plugs and place on bench in cylinder order.

Inspect each spark plug.

Remove the lead from each plug, by using the rubber boot.

Slacken each spark plug.

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Spark Plugs

Spark Plug Conditions

Normal use

Overheated

Gap bridged

Cracked insulation

Pre-ignition

Carbon fouled

Deposit fouled

Splashed deposits

Oil fouled

Glazed deposits

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Spark Plugs

8

A) Carbon fouled

B) Glazed deposits

C) Gap bridges

D) Oil fouled

What is this spark plug suffering from?

Question

Spark Plugs

Tool toalter gap

Feelergauge

Spark Plug Gap Adjustment

Obtain gap information from shop manual.

If necessary, adjust gap using feeler gauge tool.

It should be a sliding fit.

Typical Procedure:

Check gap between electrodes with a feeler gauge.

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Spark Plugs

9

A) Micrometer

B) Ruler

C) Feeler gauge

D) Microscope

What device is normally used to measure the spark plug gap?

Question

Spark Plugs

Conical seat

Flat seat

Spark Plug Fitting

Apply grease to spark plug threads.

Tighten plugs as per shop manual.

Refit spark plug leads.

Typical Procedure:

Hand screw spark plugs into cylinder head.

90°

15°

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Spark Plugs

Summary

You should now be aware of:

Spark Plug Construction

Spark Plug Reach, Seating and Heat Range

Spark Plug Servicing

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