HOW A ENGINE OF A CAR WORKS
There are two types of engines that are commonly used by the
car industries and are proved to be efficient enough for marketing.
1. Four-Stroke
Petrol Engine or Spark Ignition Engine
2. Four-Stroke
Diesel Engine or Compression Ignition Engine
1. FOUR-STROKE
PETROL ENGINE OR ‘SPARK IGNITION ENGINE’
In a four stroke petrol engine the cycle of operation is
completed in four strokes of the piston or two revolution of crankshaft. It involves
following steps
i.
Suction Stroke
ii.
Compression Stroke
iii.
Expansion or Power Stroke
iv.
Exhaust Stroke
i.
SUCTION
STROKE
Fig: SUCTION
In
the following figure when the piston is about to move downwards from top dead
center, the inlet valve opens instantaneously and the exhaust valve is a closed
position. So a suction pressure is created due to the downward motion of the
piston and the charge consisting of air-fuel mixture is drawn into the
cylinder. When the piston reaches bottom dead center the suction stroke ends
and the inlet valve closes instantaneously.
ii.
COMPRESSION
STROKE
Fig: COMPRESSION
In
the following figure the charge that fills the entire cylinder is compressed by
the return stroke of the piston i.e. the upward motion of the piston to clearance
volume. During this stroke both the valves inlet and exhaust are in closed condition.
At the end of the compression stroke , the compressed charge is ignited with
the help of Spark Plug located on cylinder head.
iii.
EXPANSION
OR POWER STROKE
Fig:
EXPANSION
The
ignited charge began to expand within the cylinder resulting in sudden push to
the piston in downward direction. During this stroke both the valves are in
closed condition. Of all four strokes, this stroke produces power. Both
pressure and temperature decreased during expansion.
iv.
EXHAUST
STROKE
Fig: EXHAUST
At the end
of expansion stroke the exhaust valve opens instantaneously and the inlet valve
remains closed. The burnt gases the produced in the expansion stoke is pushed away
by the piston when it moves upward to the top dead center. As soon as it
reached top dead center the exhaust valve closed instantaneously and the fresh
charge is ready to drawn to repeat the suction stroke.
2. FOUR-STROKE
COMPRESSION IGNITION ENGINE OR ‘DIESEL ENGINE’
These types of engines run by diesel, so it is often called
as diesel engine. It also involves 4 strokes but here instead of spark plug we
are using fuel injector at the end of compression stroke for ignition. The compression
ratio {clearance volume/cylinder volume } is higher as compared to SI engines
and it ranges from 16 to 20 whereas SI engine compression ratio ranges from 6 to 10. Due to
high compression ratio of the engine the temperature inside the cylinder after
the compression stroke is high enough to self-ignite the fuel injected into the
combustion chamber.
i.
SUCTION
STROKE
Due
to the downward motion of the piston from TDC to BDC air alone is drawn into
the cylinder during the suction stroke. During this stroke the inlet valve
opens and the exhaust valve is closed.
ii.
COMPRESSION
STROKE
The
inducted air gets compressed by the return stroke of the piston to clearance volume.
During this stoke both the valves are in closed condition.
iii.
EXPANSION
OR POWER STROKE
At
the end of the compression stroke fuel injection starts. Due to the high
temperature inside the combustion chamber the fuel get self-ignited and
expanded inside the cylinder. Due to the sudden expansion the piston is pushed
downwards from TDC to BDC and we get a power stroke. In this stroke both the
valves are in closed condition.
iv.
EXHAUST
STROKE
After
the expansion stroke the exhaust valve opens instantaneously and the inlet
valve remain closed. The piston moves from TDC to BDC pushing away the product
of combustion {burnt gases}.
FIG: CYCLE
OF OPERATION OF 4-STROKE DIESEL ENGINE
by: Radhe
