Spark Plugs
Chapter 1 Outline
06
The ignition system applies several thousand volts across the gap between the
electrodes of the spark plug to generate spark to discharge and ignites and combusts
the air-fuel mixture in the cylinder with this spark energy.
Igniting at the optimum time for the varying engine speed, load, etc. is also an
important condition.
Three conditions for attaining the performance of gasoline engines
Good Air-Fuel Mixture
Good Compression
Good Spark
Diesel engines directly inject fuel into high-temperature, high-pressure air and have the fuel ignite on its own, so they do not
require ignition system.
Diesel engine ignition system
REFERENCE
The mixture that makes the gasoline combust most readily
1 Gasified and well mixed with air 2 Always the correct air-fuel ratio (mixture proportions)
By compressing the air-fuel mixture, it is
possible to obtain large power when the
mixture is combusted.
The degree of compression of the air-fuel
mixture is expressed by the following
compression ratio.
Generally, raising the compression ratio
provides larger combustion pressure, but if
the pressure is too high, knocking occurs.
General engines are designed for
compression ratios of 9-10.
The proportion by weight of gasoline and air in the air-fuel mixture; these proportions must be varied with the engine state.
•
Stoichiometric air-fuel ratio (about 15:1) Stoichiometric value for complete combustion
•
Output air-fuel ratio (about 12:1) The mixture ratio that provides the maximum torque
•
Economy air-fuel ratio (about 16:1) The mixture ratio that provides the best fuel economy
•
In lean burn engines, this can be as high as about 23:1.
Good Air-Fuel mixture means
Air-Fuel ratio (A/F)
REFERENCE
Compression ratio
REFERENCE
The ignition timing is adjusted to match the running state of the
engine (speed and load) and obtain efficient combustion power.
High voltage of several thousands of volts are applied between
the electrodes of the spark plug for the spark to discharge to
ignite and combust the gas mixture.
"Good spark" is the field for which the ignition system is responsible.
Strong spark
Correct ignition timing
REFERENCE
Role of the Ignition System
Role of the Ignition System
1
2
3
2
Compression
ratio
Intake air volume
Compressed air
volume
=
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