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