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MENG422 AUTOMOTIVE
SPARK & COMPRESSION
IGNITION ENGINES
. . .
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There are two kinds of piston engines:
1. Spark-Ignition Engine
. -
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SPARK IGNITION ENGINE
INTRODUCTION
The actions in the spark-ignition engine can be dividedinto four arts. Each art consists of a piston stroke.
This is the movement of the piston from BDC to TDC,
or from TDC to BDC. The complete cycle of events inthe engine cylinder requires four piston strokes. These
are intake, compression, power and exhaust. The
the four piston strokes. This makes the engine a four-
stroke-c cle en ine. It is also called a our-stroke or
four-cycle engine.
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Intake Stroke:
During the intake stroke of a spark-
gn on eng ne, e p s on s mov ng own.
The intake valve is open. Air-fuel mixtureows roug e n a e por an n o e
cylinder. The fuel system supplies the
m x ure. s e p s on passes rougBDC, the intake valve closes. This seals off
e upper en o e cy n er.
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SPARK IGNITION ENGINE OPERATION Compression Stroke:
After the piston passes BDC, it startsmoving up. Both valves are closed. The
upward moving piston compresses the air-
fuel mixture into a smaller space, betweenthe top of the piston and the cylinder head.
This space is combustion chamber. The
mixture is compressed 1/8 or less of itsoriginal volume. The amount that the
mixture is compressed is compression
ratio.
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SPARK IGNITION ENGINE OPERATION
Power Stroke:
As the piston nears TDC at the end of the
,
gap at the spark plug. The heat from the spark
i nites the com ressed air-fuel mixture. The air-
fuel mixture then burns rapidly. These high
temperatures cause very high pressure which
pushes down the piston. The connecting rod carriesthis force to the crankshaft, which turns to move
.
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Exhaust Stroke:
As the piston approaches BDC on the
power stroke, the exhaust valve opens.
After passing through BDC, the pistonmoves up again. The burned gases escape
through the open exhaust port. As the
piston nears TDC, the intake valve opens.When the piston passes through TDC and
starts down again, the exhaust valve
closes.
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DIESEL ENGINE
INTRODUCTION
Diesel engines are similar to sparkignition engines in construction. Both have
pistons, with piston rings, moving up and
down in cylinders. Both burn fuel incombustion chambers in the upper part of
the cylinders. The high pressure produced
by the burning fuel pushes the pistonsdown. This rotates the crankshaft and the
rotary motion is carried through shafts and
gears to the drive wheels.
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Common Rail
System
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DIESEL ENGINE OPERATION
Intake Stroke:
The diesel engine takes in air alone.
No throttle valve impedes the airflow.
-mixture of air and fuel enters the
stroke. The throttle valve controls the
amoun a en ers.
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ompress on tro e:
In the diesel engine, the upward-
alone. On the other hand, in the
spark ignition engine, the piston- .
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Power Stroke:
diesel fuelis injected into thecompresse an o a r. e ea
of com ression i nites the fuel. In
the spark-ignition engine, a spark
compressed air-fuel mixture.
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Exhaust Stroke:
The exhaust stroke is the same for
both engines. The exhaust valveo ens and the burned ases flow
out as the piston moves up the.
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SPARK & COMPRESSION
IGNITION ENGINES
.
The way the fuel gets into the
cyclinders.
.
Compression Ratios
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