Ignition SystemsIgnition Systems

Spark Plugs page 10Spark Plugs page 10

The requirement of the spark The requirement of the spark plug is to ……………… the plug is to ……………… the

petrol fuel mixture. petrol fuel mixture.

Ignite

It does this when a high voltage It does this when a high voltage surge of electricity is forced into surge of electricity is forced into the plug from the coil via the HT the plug from the coil via the HT lead. In its desire to get to earth lead. In its desire to get to earth

the high voltage spark must the high voltage spark must jump the ………………………… jump the ………………………… in order to complete the circuit. in order to complete the circuit.

Spark plug air gap

Have a go at labelling the spark Have a go at labelling the spark plug – these are the answersplug – these are the answers

Terminal

Thread reach

Air gap

Insulator Centre electrode

Sillment seal

Gasket

Earth electrode

Insulator – made from ceramic. Designed to resist up to 30,000V at high temperatures.

Centre electrode – this conducts the high voltage spark through the centre of the plug.

Sillment seal – it is impervious to heat, corrosion, and current flow.

Gasket – fixed to the plug to keep it in place

Earth electrode – welded to the body to get the spark to jump across.

Air gap – accurately set for optimum spark strength

Thread reach – too long a reach could result in piston damage

Terminal – connects to the HT lead

State the importance of the State the importance of the following variations in spark plug following variations in spark plug

design:design:

• Plug Reach:

• Plug reach too long can result in piston damage

• Plug reach too short, mixture wont burn well

Different reach spark plugs

Methods of sealing plug seat Methods of sealing plug seat

• Plugs are usually sealed with a washer, but sometimes use a tapered face seat – these should not be over tightened. This type can improve heat dissipation.

Label the drawings below to show Label the drawings below to show the hot plug and the cold plug the hot plug and the cold plug

Cold plug –short heat path

Hot plug – longer heat path

Questions Questions

1. Why does the spark plug have to be insulated?

2. What is the function of the earth electrode on the spark plug?

3. What would be a symptom of a spark plug gap being closed up?

4. What material is commonly used to insulate spark plugs?

5. Why is it important to use the correct reach spark plug?

Answers Answers

1. To stop the spark from tracking to earth.

2. To make the spark jump the gap, to ignite the mixture.

3. If gap closed completely, the spark cant jump and ignite the mixture.

4. Ceramic

5. Too long piston damage, too short mixture won’t burn correctly.