1. PISTON ENGINE PROPULSION
Chapter 2
The Four Stroke Cycle
2014
Hyundai
Engine
1933 Alvis Engine

2. BUT HOW DOES IT WORK EXACTLY? LET’S LOOK IN MORE DETAIL
The Four Stroke Cycle
BUT HOW DOES IT WORK EXACTLY?
LET’S LOOK IN MORE DETAIL

3. 1. INDUCTION 2. COMPRESSION 3. COMBUSTION 4. EXHAUST
The Four Stroke Cycle
Operating Cycle
1. INDUCTION
Air and fuel mixture is sucked into the cylinder.
Known as SUCK
2. COMPRESSION
Air and fuel mixture is compressed by the piston.
Known as SQUEEZE
3. COMBUSTION
Air and fuel mixture is ignited and the piston is
pushed down.
Known as BANG
4. EXHAUST
Burnt gas is pushed out by piston.
Known as BLOW
SUCK, SQUEEZE, BANG, BLOW
Which gives:

4. The Four Stroke Cycle
Operating Cycle

5. The Four Stroke Cycle Operating Cycle Down Up Up Down
Therefore a ‘Four’ Stroke engine
is 2 revs of the crankshaft
1 ‘Stroke’ = the piston sliding either up or down the cylinder

6. The Four Stroke Cycle Operating Cycle STROKE 1
AIR/FUEL DRAWN (INDUCED - SUCKED) INTO CYLINDER
TDC
PISTON MOVES DOWN CYLINDER
CRANK ROTATES
STROKE 1
BDC

7. CRANK CONTINUES TO ROTATE
The Four Stroke Cycle
Operating Cycle
TDC
BDC
STROKE 2
AIR/FUEL TRAPPED (COMPRESSED - SQUEEZED) IN CYLINDER
PISTON MOVES BACK UP CYLINDER
CRANK CONTINUES TO ROTATE

8. The Four Stroke Cycle Operating Cycle STROKE 3
TDC
BDC
STROKE 3
AIR/FUEL IGNITED AND BURNS (COMBUSTION - BLOW) IN CYLINDER
PISTON FORCED DOWN CYLINDER
CRANK STARTS 2ND ROTATION

9. CRANK CONTINUES 2ND ROTATION
The Four Stroke Cycle
Operating Cycle
TDC
BDC
STROKE 4
BURNT AIR/FUEL PUSHED OUT OF CYLINDER (EXHAUST – BLOW)
PISTON MOVES BACK UP CYLINDER AGAIN
CRANK CONTINUES 2ND ROTATION

10. And the whole process starts again
The Four Stroke Cycle
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

11. The Four Stroke Cycle
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 are’nt 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.

12. The Four Stroke Cycle Operating Cycle
The start of Induction – Stroke 1
We’ll start the process at 30 degrees before TDC
TDC
BDC
At which point, the Inlet valve begins to open

13. The Four Stroke Cycle Operating Cycle
The start of Compression – Stroke 2
The induction stroke ends at 15 degrees after BDC.
TDC
BDC
1. Induction
Inlet valve opens
When the Inlet valve closes
Trapping the air/fuel mixture in the cylinder
Inlet valve closes
Called Valve Lag because it is after BDC

14. The Four Stroke Cycle Operating Cycle The start of Power – Stroke 3
TDC
BDC
Inlet valve closes
1. Induction
2. Compression
Inlet valve opens
Ignition occurs at 30 degrees before TDC.
At this point, both valves are closed
Ignition
And is the start of the Power stroke

15. The Four Stroke Cycle Operating Cycle The start of Exhaust – Stroke 4
TDC
BDC
Inlet valve closes
1. Induction
2. Compression
Inlet valve opens
Ignition
3. Power
Exhaust valve opens
Burning air/fuel mixture reaches maximum expansion
15 degrees before BDC, the exhaust valve opens
And is the end of the Power stroke and start of the Exhaust stroke
Called Valve Lead because it is before BDC

16. The end of Exhaust – Stroke 4
The Four Stroke Cycle
Operating Cycle
The end of Exhaust – Stroke 4
So as the exhaust valve starts to close,
TDC
BDC
Inlet valve closes
1. Induction
2. Compression
Ignition
3. Power
Exhaust valve opens
4. Exhaust
This position is towards the end of the Exhaust stroke
the inlet valve starts to open
Inlet valve opens
And is ALSO the beginning of the Induction stroke

17. Valve Overlap - Stroke 4 to 1 This is called ‘Valve Overlap’
The Four Stroke Cycle
Operating Cycle
Valve Overlap - Stroke 4 to 1
TDC
BDC
Inlet valve closes
1. Induction
2. Compression
Inlet valve opens
Ignition
3. Power
4. Exhaust
Exhaust valve closes
Exhaust valve opens
Both the inlet and exhaust valves are partially open
This is called ‘Valve Overlap’
Therefore the Induction stroke starts 45 degrees before the end of the Exhaust stroke

18. The start of Induction – Stroke 1
The Four Stroke Cycle
Operating Cycle
The start of Induction – Stroke 1
The exhaust valve is now closed
TDC
BDC
Inlet valve closes
1. Induction
2. Compression
Inlet valve opens
Ignition
3. Power
4. Exhaust
Exhaust valve closes
Exhaust valve opens
This position is at the end of the exhaust stroke
The inlet valve is now open
And is 45 degrees into the induction stroke

19. The Four Stroke Cycle Operating Cycle Simple isn’t it! Suck Squeeze
Bang
Blow

20. The Four Stroke Cycle Operating Cycle
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’

21. The cylinders are numbered from
The Four Stroke Cycle
Operating Cycle
The cylinders are numbered from
the front
to the rear

22. The Four Stroke Cycle Operating Cycle And the firing order is:-
1 – 3 – 4 - 2

23. The Four Stroke Cycle
Operating Cycle

24. The Four Stroke Cycle
Operating Cycle

25. The Four Stroke Cycle
Operating Cycle

26. The Four Stroke Cycle
Operating Cycle