1. Engine Operating Principles
Small Engines

2. Internal Combustion Engine
Is an engine where air and fuel are burned with the release of this energy taking place within the engine.
Their operation can be divided in compression, ignition, and carburetion.

3. Four Stroke Cycle Four Stroke
The four events in the operation of the four-stroke engine are:
Intake, compression, power, and exhaust.
Each event occurs with a linear movement of the piston which is called a stroke.
Each cycle of events is completed after two revolutions of the crankshaft.

4. Four Stroke Cycle
The first event in the operation of the four stroke is the intake stroke.
At the beginning of the intake stroke, as the piston moves downward, the intake valve will start to open.
This allows the air-fuel mixture to flow into the combustion chamber above the piston.
The intake valve cam lobe high point on the camshaft has turned to open the intake valve and the exhaust valve is held shut by its valve spring.

5. Four Stroke Cycle
The downward movement of the piston creates a partial vacuum in the combustion chamber.
The air-fuel mixture from the carburetor rushes into the combustion chamber as long as the intake valve is open and the pressure in the cylinder is lower than the pressure at the carburetor outlet.
At the bottom of the piston stroke, bottom dead center (BDC) the camshaft has turned until the cam lobe no longer holds the intake valve open and the valve is closed by the valve spring.

6. Four Stroke Cycle The second event is the compression stroke.
During the compression stroke both the intake and exhaust valves are closed and the piston moves upward compressing the air-fuel mixture.
The top of the stroke is called top dead center (TDC).
The piston rings form a seal between the piston and the cylinder wall which prevents the escape of the air-fuel mixture.

7. Four Stroke Cycle The third event is the power stroke.
The compressing of the air-fuel mixture in the combustion chamber increases the temperature of the mixture making it easier to ignite.
The third event is the power stroke.
The intake and exhaust valves are both closed.
When the piston approaches or is at the top of the stroke a spark jumps the spark plug gap igniting the heated and compressed air-fuel mixture.

8. Four Stroke Cycle The forth event is the exhaust stroke.
The fuel mixture burns rapidly and the expanding gases create pressure in all directions.
The piston is propelled downward which in turn turn the crankshaft producing rotary motion.
The forth event is the exhaust stroke.
When the piston reaches the bottom of the power stroke the momentum of the flywheel and the crankshaft moves the piston up the cylinder.

9. Four Stroke Cycle
The exhaust valve is opened by the high point of the exhaust valve cam lobe on the camshaft.
The intake valve is held shut by the valve spring.
Burned or exhaust gases are forced out of the combustion chamber past the exhaust valve into the exhaust passage.
When the piston is near the top of its stroke in the cylinder the exhaust valve closes.
One cycle of events has been completed.

10. Two Stroke Cycle Two Stroke The First event is:
The four events in the operation of the two stroke engine are:
Intake, compression, power, and exhaust.
All these events must occur but more than one event takes place during each piston stroke.
The First event is:
The air-fuel mixture is compressed in the combustion chamber.

11. Two Stroke Cycle
The spark jumps the plug gap igniting the air-fuel mixture.
This again forces the piston downward in the cylinder.
The downward travel of the piston uncovers the exhaust port first and than the intake port.
The incoming air-fuel mixture forces the exhaust gases out the exhaust port.
This is called scavenging.

12. Two Stroke Cycle
During the downward movement of the piston all the power event has been completed and part of the intake and exhaust event has been completed.
The piston starts its upward movement in the cylinder covering the intake and exhaust ports thus trapping the air-fuel mixture in the combustion chamber.
The four events are completed in tow strokes and the crankshaft has made one revolution (360).