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 InductionDownSuck 2 CompressionUpSqueeze 3 CombustionDownBang 4 ExhaustUpBlow That means that one cycle is completed for every two revolutions of the crankshaft. Introduction The Operating Cycle BUT EXACTLY HOW DOES IT WORK? Down Down Up Up 1Induction 2Compression 3Combustion 4Exhaust

1Induction TDC BDC Crank Rotates Piston moves DOWNCylinder Air/Fuel drawn (Induced/Sucked) Into Cylinder The First Stroke - Induction The Operating Cycle

The Second Stroke – Compression or Power TDC BDC Crank continues to Rotates Piston moves UPCylinder Air/Fuel trapped (Compressed/Squeezed) in Cylinder

The Operating Cycle The Third Stroke - Combustion TDC BDC Crank starts Second Rotation Piston FORCED DOWNCylinder Air/Fuel ignited & burns (Combustion/Bang) in Cylinder

The Operating Cycle The Fourth Stroke - Exhaust TDC BDC Crank continues Second Rotation Piston moved back UP Cylinder again Burnt Air/Fuel pushed out of Cylinder (Exhaust/Blow)

The Operating Cycle All 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 Cycle TDC 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 Shaft Start of Induction - Stroke One TDCBDC We’ll start the process at 30 o before TDC At which point, the Inlet valve begins to open And the Exhaust valve is almost closed

Angular Positions of the Crank Shaft Start of Induction - Stroke One TDCBDC We’ll start the process at 30 o before TDC At this point, Induction begins Inlet valve opens

Angular Positions of the Crank Shaft Start of Compression - Stroke Two TDCBDC When the Inlet valve closes Inlet valve opens Induction stroke ends at 15 o after BDC. Trapping the air/fuel mixture in the cylinder

Trapping the air/fuel mixture in the cylinder Angular Positions of the Crank Shaft Start of Combustion - Stroke Three TDCBDC At this point both valves are closed Inlet valve opens Inlet valve closes Ignition occurs 30 o before TDC. Ignition Which is the start of the combustion stroke

Angular Positions of the Crank Shaft Start of Exhaust - Stroke Four TDCBDC 15 o before BDC the Exhaust valve opens Inlet valve opens Inlet valve closes Ignition The burning air and fuel mixturereachesmaximumexpansion Exhaust valve opens

Angular Positions of the Crank Shaft End of Exhaust - Stroke Four TDCBDC Inlet valve closes Ignition Exhaust valve opens Inlet valve opens This is the end of the Exhaust stroke And the beginning of the Induction stroke At which point, the Inlet valve begins to open As the Exhaust valve starts to close

Angular Positions of the Crank Shaft The Overlap - Stroke Four to One TDCBDC Inlet valve closes Ignition Exhaust valve opens Inlet valve opens Both the inlet and exhaust valves are partially open This is called ‘Valve Overlap’ Exhaust valve closes Therefore the Induction stroke starts 45 o before the end of the Exhaust stroke

Angular Positions of the Crank Shaft Start of Induction – Stroke One TDCBDC Inlet valve closes Ignition Exhaust valve opens Inlet valve opens Exhaust valve closes This is at the end of the Exhaust stroke and is 45 o into the Induction stroke The Inlet valve is now open And the Exhaust valve is now closed

Angular Positions of the Crank Shaft Start 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 angle before 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 is very 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 SUCK BLOW SQUEEZE The cylinders are numbered from FRONT to REAR FRONT to REAR

FRONT REAR BANG BLOW The Operating Cycle SUCK BLOW SQUEEZE BANG SQUEEZE SUCKSUCK BLOW BANG SQUEEZE SUCK BLOW BANG SQUEEZE And the cycle runs..... And the cycle runs..... BANG SUCK BLOW 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 bigger to 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 SUCK BLOW SQUEEZE So the cycle now runs The Operating Cycle BANG SQUEEZE SUCK BLOW BANG SQUEEZE SUCK BLOW BANG SQUEEZE SUCK BLOW BANG 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

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 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 of the Crank Shaft Valve Overlap The Ineffective Crank Angle Check of Understanding

In the 4-stroke cycle, where does ignition take place? At TDC on the ignition stroke 15 o after TDC on the combustion/power stroke At TDC on the compression stroke 30 o before TDC on the combustion/power stroke Check of Understanding

In a piston engine, in which region does ineffective crank angle occur? 90 o 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? 15 o before TDC 15 o before BDC 15 o after TDC 15 o after BDC Check of Understanding

What is the valve operating angle just 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? Check of Understanding

PISTON ENGINES End of Presentation