The mixture of fuel and air in an
engine cylinder cannot produce power, unless it is ignited in every cycle at
the proper instant. The basic methods for igniting the fuel charge are:--
· Ignition by an electric spark.
· Ignition by compression.
In a carburetor engine a mixture of
fuel and air is compressed in the cylinder. Then a high voltage spark jumps
across the spark plug electrode gap to fire the charge. The spark must be
strong enough to jump across the gap and it must do so at predetermined time. If
the spark to early, before the piston has reached TDC then the engine tends to
run in the reversed direction, or kicks. At full engine speed ,however the
spark must occurs earlier so that proper pressure from the burning fuel charge
is built up ,just as piston starts moving down from TDC to BDC .the ignition
system is so design that the spark is retarded for starting the engine and as
the engine picks the speed the spark is advanced .
In diesel engine the fuel ignites when it is
sprayed into the cylinder which contains highly compressed hot air. To start
the burning the introduction of spark is not necessary.
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