Spark Plugs

Chapter 2 Study Tips The spark plugs are critical parts that dominate the engine combustion and bear a major responsibility for higher engine performance.

Spark Plugs

Chapter 2 Spark Plugs

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The required voltage changes drasticallywith various conditions.The three factors withparticularly large impact arethe spark plug gap, thecompression pressure, and theair-fuel mixture temperature.

The required voltage rises in proportionto the spark plug gap.The spark plug gap widens bit by bit asthe electrode wears, so high requiredvoltage becomes necessary and misfireoccurs more easily.

Spark Plug Gap1

● Conditions that affectthe required voltage

G: Spark plug gap V: Required voltage

Even for the same spark plug gap, ifthere are edges on the electrode,discharge occurs more easily.Older spark plugs have electrodes thathave rounded, so discharge becomesmore difficult and misfire more likely.

Electrode shape2

Easy discharge Difficult spark

Electrode shape33

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The required voltage rises in proportion to thecompression pressure.The compression pressure is higher for low speedand high load. Sudden acceleration for startingout fits these conditions, so higher voltage isrequired then and misfire occurs more easily.

Compression pressure3

P: Compression pressure T: Air-Fuel mixturetemperature V: Required voltage

The required voltage drops as the air-fuel mixture temperature rises.The lower the engine temperature, thehigher the required voltage, so misfireoccurs more easily at low temperature.

Air-Fuel mixture temperature4

P: Compression pressure T: Air-Fuel mixturetemperature V: Required voltage

As the humidity rises, the electrodetemperature decreases, so the requiredvoltage becomes slightly higher.

Humidity7

H: Relative humidity V: Required voltage

The required voltage drops as theelectrode temperature rises.The electrode temperature rises inproportion to the engine speed, somisfire occurs more easily at low speed.

Electrode temperature5

T: Electrode temperature V: Required voltage

There is a tendency for the requiredvoltage to be higher the leaner the air-fuel mixture (the larger the air-fuel ratio).If the air-fuel mixture becomes leanerdue to fuel system trouble misfire occursmore easily.

Air-Fuel ratio6

A/F: Air-fuel ratio V: Required voltage