PISTON ENGINES
Part 2
The Four Stroke Cycle
The four stroke piston engine is so called because one ‘Stroke’ is the piston sliding up or down the cylinder;
and to complete one operating cycle, the piston must travel along the cylinder four times as follows :-
1 Induction Down Suck2 Compression Up Squeeze3 CombustionDown Bang4 Exhaust Up Blow
That means that one cycle is completed for every two revolutions
of the crankshaft.
IntroductionThe Operating Cycle
BUT EXACTLY HOWDOES IT WORK?
Down
Down
UpUp 1Induction
2Compression
3Combustion
4Exhaust
1Induction
TDC
BDC
Crank Rotates
Piston moves DOWN Cylinder
Air/Fuel drawn (Induced/Sucked)
Into Cylinder
The First Stroke - Induction
The Operating Cycle
The Operating CycleThe Second Stroke – Compression or Power
TDC
BDCCrank continues
to Rotates
Piston moves
UP Cylinder
Air/Fuel trapped (Compressed/Squeezed)
in Cylinder
The Operating CycleThe Third Stroke - Combustion
TDC
BDCCrank starts
Second Rotation
Piston FORCED DOWNCylinder
Air/Fuel ignited & burns (Combustion/Bang)
in Cylinder
The Operating CycleThe Fourth Stroke - Exhaust
TDC
BDCCrank continuesSecond Rotation
Piston movedback UP
Cylinder again
Burnt Air/Fuel pushedout of Cylinder (Exhaust/Blow)
The Operating CycleAll Four Strokes
TDC
BDC
When the end of the Exhaust Stroke
is reached . . .
It’s the start of the
Induction Stroke
and the whole process starts again
and the whole process starts again . . .
The Operating CycleTDC and BDC
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 occur at other positions called, Angular Positions of the Crank Shaft.
Angular Positions of the Crank ShaftStart of Induction - Stroke One
TDC
BDC
We’ll start theprocess at 30o
before TDCAt which point,the Inlet valvebegins to open
And the Exhaust valve
is almost closed
Angular Positions of the Crank ShaftStart of Induction - Stroke One
TDC
BDC
We’ll start theprocess at 30o
before TDCAt this point,
Induction beginsInlet valve
opens
Angular Positions of the Crank ShaftStart of Compression - Stroke Two
TDC
BDC
When theInlet valve
closesInlet valve opens
Induction strokeends at 15o after BDC.
Trapping the air/fuel mixture in the cylinder
Trapping the air/fuel mixture in the cylinder
Angular Positions of the Crank ShaftStart of Combustion - Stroke Three
TDC
BDC
At this pointboth valvesare closedInlet valve
opens
Inlet valve closes
Ignition occurs 30o before TDC.
Ignition
Which is the start of the
combustion stroke
Angular Positions of the Crank ShaftStart of Exhaust - Stroke Four
TDC
BDC
15o before BDCthe Exhaust valve
opensInlet valve opens
Inlet valve closes
Ignition The burningair and fuel
mixturereaches
maximumexpansion
Exhaust valve opens
Angular Positions of the Crank ShaftEnd of Exhaust - Stroke Four
TDC
BDCInlet valve closes
Ignition
Exhaust valve opens
Inlet valve opens
This is the end of
the Exhaust stroke
And the beginning ofthe Induction stroke
At which point,the Inlet valvebegins to open
As the Exhaust valve starts to close
Angular Positions of the Crank ShaftThe Overlap - Stroke Four to One
TDC
BDCInlet valve closes
Ignition
Exhaust valve opens
Inlet valve opens
Both the inlet and exhaustvalves are partially open
This is called ‘Valve Overlap’Exhaust
valve closes
Therefore the Induction stroke
starts 45o before the end of the Exhaust stroke
Angular Positions of the Crank ShaftStart of Induction – Stroke One
TDC
BDCInlet valve closes
Ignition
Exhaust valve opens
Inlet valve opens
Exhaust valve closes
This is at the end ofthe Exhaust stroke
and is 45o into the
Induction stroke
The Inlet valveis now open
And the Exhaust valve is now closed
Angular Positions of the Crank ShaftStart of Induction – Stroke One
‘Valve Overlap’ is the brief period when the
inlet valve opens before TDC,and the exhaust valve does
not close until after TDC.
‘Valve Lead’is the valve operating anglebefore either TDC or BDC.
‘Valve Lag’is the valve operating angle
after either TDC or BDC.
‘Ineffective Crank Angle’is an arc of travel at both
TDC and BDC where there isvery little piston movement.
The Operating Cycle
The cycle 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’
BANG
The Operating Cycle
1 3 42
SUCKBLOW SQUEEZE
The cylinders are numbered from
FRONT to REAR
FRONT REAR
BANGBLOW
The Operating Cycle
1 3 42
SUCKBLOW SQUEEZEBANG SQUEEZESUCKSUCK BLOW BANGSQUEEZE SUCK BLOWBANGSQUEEZE
And the cycle runs . . . . .
BANG SUCKBLOW SQUEEZE
The Firing Order
The Operating Cycle
So far we have seen the operating cycle as it takes place in one cylinder,
but it is necessary to ensure that all cylinders in an engine fire at different times.
What do you think would happen
if all of the cylinders fired at the same time ?
The moving parts would be subjected to 4 times the load.
Fuel and inlet manifolds would need to be biggerto cope with 4 times the fuel flow.
The exhaust would need to be bigger to cope with 4 times the flow of the exhaust strokes.
BANG
1 3 4 2
SUCK BLOWSQUEEZE
So the cycle now runs - 1 - 3 - 4 - 2
The Operating Cycle
BANGSQUEEZE SUCK BLOWBANG SQUEEZESUCK BLOWBANG SQUEEZE SUCKBLOWBANG SQUEEZE SUCK BLOW
The Firing Order
The Operating Cycle
This illustration includes the inlets and exhaust,and shows the same
firing order and cyclic sequence as before, but looking from above.
and so it goes on . . . .
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
12341234
The Operating Cycle
This illustration includes the inlets and exhaust,and shows the same
firing order and cyclic sequence as before, but looking from above.
1
2
3
4
The Firing Order
Check of Understanding
In a 4-stroke cycle,which is the second stroke?
Induction
Combustion / Power
Compression
Exhaust
Valve operation and air/fuel ignition isn’t set at the TDC and BDC positions.
These events occur at other positions called what?
Valve Lead and Lag
Angular Positions ofthe Crank Shaft
Valve Overlap
The Ineffective Crank Angle
Check of Understanding
In the 4-stroke cycle,where does ignition take place?
At TDC on theignition stroke
15o after TDC on the combustion/power stroke
At TDC on the compression stroke
30o before TDC on the combustion/power stroke
Check of Understanding
In a piston engine,in which region does
ineffective crank angle occur?
90o after both TDC and BDC
Only at TDC
At both TDC and BDC
Only at BDC
Check of Understanding
In a 4-stroke cycle,where does the induction stroke end?
15o before TDC
15o before BDC
15o after TDC
15o after BDC
Check of Understanding
What is the valve operating anglejust before both TDC and BDC called?
Valve Overlap
Valve Lead
Valve Lag
The Ineffective Valve Angle
Check of Understanding
What is the firing order in the 4-stroke cycle?
1 - 3 - 4 - 2
1 - 2 - 4 - 3
1 - 3 - 2 - 4
1 - 2 - 3 - 4
Check of Understanding
PISTON ENGINES
End of Presentation