THE FOUR STROKE CYCLE
BUT HOW DOES IT WORK EXACTLY?
WE KNOW ABOUT:-
WHICH WE KNOW AS:-
LET’S LOOK IN MORE DETAIL
1. INDUCTION
SUCK
2. COMPRESSION
SQUEEZE
3. COMBUSTION
BANG
4. EXHAUST
BLOW
Lag
AND RELATE IT TO THIS
TDC
DIAGRAM
Inlet Lead
verlap
valve
Valve o
opens
Lag
Exhaust
valve
closes
Charge ignited
BANG BLOW
SQUEEZE
Exhaust
4
1
Inlet
valve
closes
PROPULSION 33-11
Lag
2
BDC Lead
Exhaust
valve
opens
1
Induction
Power
3
2
compression
30
SUCK
WE’LL SEE A
STEP BY STEP
EXPLAINATION
OF THIS
DIAGRAM
3
Operating Cycle
4
AND THIS DIAGRAM
1
Down
INDUCTION (first downstroke)
- Inlet valve open
- Piston moves down the cylinder (from TDC to BDC)
- Fuel air mixture (the charge) is drawn into the cylinder
Up
EXHAUST (second upstroke)
COMPRESSION(first upstroke)
- Exhaust valve open
- Both valves are closed
- Piston moves back up the
- Piston moves back up the
cylinder (from BDC to TDC)
cylinder (from BDC to TDC)
-The burnt (exhaust) gases, having
- Fuel air mixture is compressed
now performed their useful work on
into the top of the cylinder (the
the Power Stroke, escape into the
combustion chamber).
atmosphere via the exhaust pipe.
COMBUSTION (second downstroke)
- Both valves are closed
- Spark occurs, igniting the compressed fuel/air mixture
- Rapid expansion of the burning mixture forces the piston back down
the cylinder (from TDC to BDC).
2
4
Up
Down
3
1 ‘Stroke’ = the piston sliding either up or
down the cylinder
Therefore a ‘Four’ Stroke engine is 2 revs
of the crankshaft
Operating Cycle
PISTONDRAWN
MOVES
DOWN- SUCKED)
CYLINDER
AIR/FUEL
(INDUCED
INTO CYLINDER
TDC
CRANK ROTATES
STROKE 1
BDC
Operating Cycle
AIR/FUEL
- SQUEEZED) IN CYLINDER
PISTONTRAPPED
MOVES(COMPRESSED
BACK UP CYLINDER
TDC
CRANK
CONTINUES TO
ROTATE
BDC
STROKE 2
Operating Cycle
PISTON FORCED DOWN CYLINDER
TDC
AIR/FUEL IGNITED AND BURNS (COMBUSTION - BLOW) IN CYLINDER
CRANK STARTS
2ND ROTATION
STROKE 3
BDC
Operating Cycle
PISTON
MOVES
BACK
UP
AGAIN– BLOW)
BURNT AIR/FUEL
PUSHED
OUT
OFCYLINDER
CYLINDER (EXHAUST
TDC
CRANK CONTINUES
2ND ROTATION
STROKE 4
BDC
Operating Cycle
It is of course the start of
the induction stroke
And the whole
process starts again
When the end of the
exhaust stroke is
reached
Operating Cycle
But :Because valves don’t open and close
instantly, and
the air/fuel mixture doesn’t explode instantly
(it’s a rapid burning process)
valve operation and air/fuel ignition isn’t set at
the TDC and BDC positions.
These events are set to occur at the following
positions, designated as angular positions of
the crank shaft.
Operating Cycle
The start of Induction – Stroke 1
We’ll start the process at
30 degrees before TDC
TDC
At which point, the Inlet valve
begins to open
BDC
Operating Cycle
The start of Compression – Stroke 2
The induction stroke ends at
15 degrees after BDC.
TDC
Inlet valve
When the Inlet valve closes
opens
1. Induction
Trapping the air/fuel
mixture in the cylinder
BDC
Inlet valve
closes
Called Valve Lag because it is after
BDC
Operating Cycle
The start of Power – Stroke 3
Ignition occurs at 30 degrees
before TDC.
TDC
Inlet valve
At this point, both
valves are closed
opens
1. Induction
2. Compression
Ignition
Inlet valve
closes
BDC
And is the start of the
Power stroke
Operating Cycle
The start of Exhaust – Stroke 4
15 degrees before BDC,
Burning air/fuel mixture
the exhaust valve opens
reaches maximum expansion
TDC
And is the end of the
Power stroke and
start of the Exhaust
stroke
Inlet valve
opens
1. Induction
3. Power
2. Compression
Ignition
Called Valve Lead
because it is before BDC
Inlet valve
closes
BDC
Exhaust
valve opens
Operating Cycle
The end of Exhaust – Stroke 4
Inlet valve
opens
This position is towards the
end of the Exhaust stroke
TDC
the inlet valve
starts to open
4. Exhaust
1. Induction
3. Power
2. Compression
Ignition
And is ALSO the
beginning of the
Induction stroke
Inlet valve
closes
BDC
Exhaust
valve opens
Operating Cycle
So as the exhaust
valve starts to
close,
Valve Overlap - Stroke 4 to 1
This is called ‘Valve Overlap’
TDC
Inlet valve
opens
Both the inlet and exhaust
valves are partially open
Exhaust
valve closes
4. Exhaust
Inlet valve
closes
BDC
1. Induction
3. Power
2. Compression
Ignition
Therefore the Induction
stroke starts 45 degrees
before the end of the
Exhaust
Exhaust stroke
valve opens
Operating Cycle
The start of Induction – Stroke 1
This position is at the end of
the exhaust stroke
TDC
Inlet valve
opens
Exhaust
valve closes
The inlet valve
is now open
4. Exhaust
1. Induction
3. Power
2. Compression
Ignition
And is 45 degrees
into the induction
stroke
Inlet valve
closes
BDC
Exhaust
valve opens
Operating Cycle
The exhaust valve
is now closed
Lag
TDC
Inlet Lead
verlap
valve
Valve o
opens
Lag
Exhaust
valve
closes
Simple isn’t it!
Charge ignited
Suck
Exhaust
4
1
Induction
Power
3
2
compression
30
Squeeze
Bang
Inlet
valve
closes
PROPULSION 33-11
Lag
BDC Lead
Exhaust
valve
opens
Operating Cycle
Blow
But, this is happening in every cylinder
in all 4 stroke engines
no matter how many cylinders there are.
Lets look at the most typical vehicle engine,
the
‘Inline 4’
Operating Cycle
cylinders
from
And The
the firing
orderare
is:-numbered
1–3–
4-2
to the
rear
the
front
Operating Cycle
Operating Cycle
Operating Cycle
Operating Cycle
Operating Cycle