1. BUT HOW DOES IT WORK EXACTLY?
THE FOUR STROKE CYCLE
BUT HOW DOES IT WORK EXACTLY?
LET’S LOOK IN MORE DETAIL
WE KNOW ABOUT:-
WHICH WE KNOW AS:-
1. INDUCTION
SUCK
2. COMPRESSION
SQUEEZE
3. COMBUSTION
BANG
4. EXHAUST
BLOW

2. AND RELATE IT TO THIS DIAGRAM
WE’LL SEE A STEP BY STEP EXPLAINATION OF THIS DIAGRAM
Operating Cycle

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

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

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

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

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

8. And the whole process starts again
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

9. Because valves don’t open and close instantly, and
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

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

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

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

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

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

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

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

17. Simple isn’t it!
Suck
Squeeze
Bang
Blow
Operating Cycle

18. 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’
Operating Cycle

19. The cylinders are numbered from
And the firing order is:-
The cylinders are numbered from
1 – 3 – 4 - 2
the front
to the rear
Operating Cycle

20. Operating Cycle

21. Operating Cycle

22. Operating Cycle

23. Operating Cycle