1. Four Stroke Engine Operation
Automotive Technology 1
Mr. Wasacz

2. Objectives
The student will be able to describe the four stroke engine operation
The student will be able to explain the purpose of each stroke
The student will be able to explain the concept of valve timing
The student will be able to identify TDC and BDC

3. What is Stroke?
Stroke is the movement of a piston from one end of the cylinder to the other
Per stroke, the piston either moves towards the crankshaft, or away from it
Basically it’s the distance between TDC and BDC
Each stroke is identified by the job it performs

4. Stroke

5. Four Stroke Engine (Otto Cycle Engine)
In a four Stroke engine, 4 strokes are needed in order to complete one operating cycle
The crankshaft requires 2 revolutions to complete one operation cycle
The strokes are as follows
Intake
Compression
Power (ignition)
Exhaust

6. Intake Stroke The piston travels from TDC to BDC
This movement causes the creation of a partial vacuum.
This vacuum draws in the air fuel mixture into the cylinder
The larger the bore and stroke, the more air that can be drawn in

7. Intake Stroke Valve Timing
During intake, the intake valve must open at the correct moment to permit the entry of air and fuel
Must close at the proper time for correct compression
The shape must be streamlined to increase the flow of gases into the combustion chamber

8. Intake Stroke

9. Compression Stroke The piston moves from BDC to TDC
Both valves must be closed
As the piston moves upwards, the air fuel mixture is compressed into a smaller space

10. Why do we need Compression?
When you compress atoms of air and fuel, heat is created
Fuel almost reaches a flash point, making combustion practically instantaneous the entire mixture
In addition to this, upon combustion, the atoms want to spread out from each other, creating more power pushing down on the piston

11. Compression Stroke

12. Power Stroke Piston moves from TDC to BDC
Triggered by an electrical spark that jumps the gap of the electrodes of the spark plug
Spark is timed to ignite on the compression stroke right before the piston reaches TDC (Done to ensure even burning and pressure on the piston)
Air / Fuel is ignited, pushing down on the piston

13. Power Stroke
The amount of power produced is directly related to the compression ratio
Compression ratio is the proportionate difference between the volume of the cylinder from BDC to TDC
If the compression ratio is to high, fuel may reach flash point and spontaneously combust

14. Power Stroke

15. Exhaust Stroke The piston Moves from BDC to TDC
Exhaust Valve Opens, allowing the escaping of air
As the piston moves up, it pushes out the exhaust gasses
The Removal of gasses is called scavenging

16. Exhaust Stroke Valve
The valve must also be streamlined to provide for easy air flow
Exhaust gasses are extremely hot! The valve needs to withstand this heat
Heat is transferred from the stem to the valve guides
When the valve is closed, heat from the head is transferred to the valve seat

17. Exhaust Stroke