the four stroke cycle

THE FOUR STROKE CYCLETHE FOUR STROKE CYCLE

BUT HOW DOES IT WORK EXACTLY?BUT HOW DOES IT WORK EXACTLY?

1. INDUCTION1. INDUCTION

4. EXHAUST4. EXHAUST3. COMBUSTION3. COMBUSTION2. COMPRESSION2. COMPRESSION

SUCKSUCK

BLOWBLOWBANGBANGSQUEEZESQUEEZE

WE KNOW ABOUT:-WE KNOW ABOUT:- WHICH WE KNOW AS:-WHICH WE KNOW AS:-LET’S LOOK IN MORE DETAILLET’S LOOK IN MORE DETAIL

WE’LL SEE A WE’LL SEE A STEP BY STEP STEP BY STEP EXPLAINATION EXPLAINATION

OF THIS OF THIS DIAGRAMDIAGRAM

PROPULSION 33-11

In letva lve

closes

In le tvalveopens

Charge ign ited

BDC

TDC

Exhaustva lveopens

Exhaustva lve

closes

1Induction

Pow

er3

Exh

aust

42co

mpr

essi

on

Lead

LeadLag

Lag

Lag

30

Valve overlap

1 2 3 4

BANG SQUEEZE SUCKBLOW

AND RELATE IT TO THIS AND RELATE IT TO THIS DIAGRAMDIAGRAM

Operating CycleOperating Cycle

1

2

3

4

INDUCTION (first downstroke)

CO MBUSTION (second downstroke)

EXHAUST (second upstroke)- Exhaust valve open- P iston m oves back up the cylinder (from BDC to TDC)-The burnt (exhaust) gases, having now perform ed their useful work on the Power Stroke, escape into the atm osphere via the exhaust pipe.

- Inle t valve open- P iston m oves down the cylinder (from TDC to BDC)- Fuel air m ixture (the charge) is drawn into the cylinder

- - Spark occurs, igniting the compressed fuel/air m ixture- R apid expansion of the burning m ixture forces the p iston back down the cylinder (from TD C to BDC ).

Both valves are closed

CO MPRESSIO N(first upstroke)- Both valves are closed- Piston m oves back up the cylinder (from BDC to TDC)- Fuel air m ixture is com pressed in to the top of the cylinder (the com bustion cham ber).


AND THIS DIAGRAMAND THIS DIAGRAM

1 ‘Stroke’ = the piston sliding either up or 1 ‘Stroke’ = the piston sliding either up or down the cylinderdown the cylinder

Therefore a ‘Four’ Stroke engine is 2 revs Therefore a ‘Four’ Stroke engine is 2 revs of the crankshaftof the crankshaft

DownDown

DownDown

UpUpUpUp

TDC

BDCSTROKE 1

PISTON MOVES DOWN CYLINDER

CRANK ROTATES

AIR/FUEL DRAWN (INDUCED - SUCKED) INTO CYLINDER


TDC

BDCSTROKE 2

PISTON MOVES BACK UP CYLINDERAIR/FUEL TRAPPED (COMPRESSED - SQUEEZED) IN CYLINDER

CRANK CONTINUES TO

ROTATE


TDC

BDCSTROKE 3

PISTON FORCED DOWN CYLINDER

CRANK STARTS 2ND ROTATION

AIR/FUEL IGNITED AND BURNS (COMBUSTION - BLOW) IN CYLINDER


TDC

BDCSTROKE 4

PISTON MOVES BACK UP CYLINDER AGAIN

CRANK CONTINUES 2ND ROTATION

BURNT AIR/FUEL PUSHED OUT OF CYLINDER (EXHAUST – BLOW)


When the end of the exhaust stroke is reached

It is of course the start of the induction stroke And the whole

process starts again


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.


We’ll start the process at 30 degrees before TDC

The start of Induction – Stroke 1

TDC

BDC

At which point, the Inlet valve begins to open


The induction stroke ends at 15 degrees after BDC.

TDC

BDC

Trapping the air/fuel mixture in the cylinder

When the Inlet valve closes

1. Induction

Inlet valve closes

Inlet valve opens

Called Valve Lag because it is after BDC


The start of Compression – Stroke 2

Ignition occurs at 30 degrees before TDC.

And is the start of the Power stroke

At this point, both valves are closed

TDC

BDCInlet valve closes

1. Induction

2. C

ompr

essi

on

Inlet valve opens

Ignition


The start of Power – Stroke 3

Burning air/fuel mixture reaches maximum expansion

And is the end of the Power stroke and

start of the Exhaust stroke

TDC


1. Induction

2. C

ompr

essi

on

Inlet valve opens

Ignition

3. Power

15 degrees before BDC, the exhaust valve opens

Exhaust valve opens

Called Valve Lead because it is before BDC

The start of Exhaust – Stroke 4


This position is towards the end of the Exhaust stroke

And is ALSO the beginning of the Induction stroke

TDC


1. Induction

2. C

ompr

essi

on

Inlet valve opens

Ignition

3. Power

Exhaust valve opens

4. E

xhau

st

the inlet valve starts to open

So as the exhaust valve starts to

close,


The end of Exhaust – Stroke 4

This is called ‘Valve Overlap’‘Valve Overlap’

TDC


1. Induction

2. C

ompr

essi

on

Inlet valve opens

Ignition

3. Power

4. E

xhau

st

Both the inlet and exhaust valves are partially open

Exhaust valve closes

Therefore the Induction stroke starts 45 degrees45 degrees

beforebefore the end of the Exhaust stroke


Valve Overlap - Stroke 4 to 1

Exhaust valve opens

This position is at the end of the exhaust stroke

And is 45 degrees into the induction

stroke

TDC


1. Induction

2. C

ompr

essi

on

Inlet valve opens

Ignition

3. Power

4. E

xhau

st

The inlet valve is now open

The exhaust valve is now closed

Exhaust valve closes

Operating CycleOperating CycleExhaust

valve opens

The start of Induction – Stroke 1

PROPULSION 33-11

In letva lve

closes

In le tvalveopens

Charge ign ited

BDC

TDC

Exhaustva lveopens

Exhaustva lve

closes

1Induction

Pow

er3

Exh

aust

42co

mpr

essi

on

Lead

LeadLag

Lag

Lag

30

Valve overlap Simple isn’t it!

Suck

Squeeze

Bang

Blow


in all 4 stroke engines

no matter how many cylinders there are.

Lets look at the most typical vehicle engine,

the ‘Inline 4’

But, this is happening in every cylinder


The cylinders are numbered from

the front

to the rear

And the firing order is:- 1 – 3 – 4 - 2


the four stroke cycle

Documents

stroke cyclebut

diagram1 stroke

compression stroke

powercalled valve lead

powerexhaust valve opens4

stroke enginesno matter

operating cycletdcbdcstroke

exhaust strokeand