1. BRIGGS & STRATTON
4-Cycle Theory

2. 4-Cycle Theory - Introduction
A crystal clear understanding of 4-Cycle theory is key to accurate and successful troubleshooting.
During this presentation we will be breaking down the four cycles into easy to understand terms. Following will be more advance information about these four cycles.
There are four individual cycles required for the engine to operate:
Intake Stroke - Compression Stroke - Power Stroke - Exhaust Stroke
One of the most over looked things in troubleshooting is the basics… You need to know the fundamentals of this to be successful and time effective. There are certain things to know here that will help you figure out hwy and engine may not be running right. It can be traced to fuel, compression or ignition in most cases.

3. 4-Cycle Theory - Intake Stroke

4. The first step needed for the engine to operate is the Intake Stroke.
4-Cycle Theory - Intake Stroke
The first step needed for the engine to operate is the Intake Stroke.
As the engine rotates towards this stroke, the camshaft opens the intake valve to open a path for air to get in.
The camshaft is the brains of the operation here and is made to open these valves at just the right time with piston movement.

5. The first step needed for the engine to operate is the Intake Stroke.
4-Cycle Theory - Intake Stroke
The first step needed for the engine to operate is the Intake Stroke.
Now that the intake valve is open, the piston begins traveling DOWN very rapidly and creates low pressure in the cylinder (vacuum).
Since higher pressure outside the engine will try to fill the low pressure in the cylinder, air will flow through the carburetor picking up a fine mist of fuel along the way and will fill the cylinder with this volatile mixture of air and fuel. Much like using a straw to drink!
When the piston goes down its just like taking a breath of air… Your lungs expand and create a low pressure area that air outside you rushes in through your nose and mouth to fill them. The engine is essentially an air pump.

6. 4-Cycle Theory - Compression Stroke

7. 4-Cycle Theory - Compression Stroke
The second step needed for the engine to operate is the Compression Stroke.
Once the piston reaches the bottom of the cylinder, both valves are now closed.
The piston starts traveling up the cylinder and begins to squeeze the air/fuel about 8.5 times SMALLER in size which makes that mixture extremely volatile, ready for the next step!

8. 4-Cycle Theory - Power Stroke

9. The third step needed for the engine to operate is the Power Stroke.
4-Cycle Theory - Power Stroke
The third step needed for the engine to operate is the Power Stroke.
Now the real business of power generation finally occurs!
The mixture is now compressed and at its highest point of ignitability, the ignition coil releases the generated 20,000 volts to the spark plug which jumps the gap.
The mixture now starts to burn very rapidly and expands out and down, the expansion of the flame front pressure wave is what pushes the piston down. There is NO “explosion” happening during this process.

10. 4-Cycle Theory - Exhaust Stroke

11. The third step needed for the engine to operate is the Exhaust Stroke.
4-Cycle Theory - Exhaust Stroke
The third step needed for the engine to operate is the Exhaust Stroke.
The engine has now turned most of that mixture into useful work and heat with some spent gasses left behind.
The camshaft now will open the exhaust valve as the piston travels upward allowing these spent gases out of the cylinder.
Now the whole process starts over again, and again and again….

12. 4-CYCLE THEORY - ADVANCED INFORMATION

13. 4-Cycle Advanced Info - Intake Stroke
During the intake stroke the incoming air/fuel charge is entering at about a rate of 25 MPH.
Depending on the engine and cam configuration, the intake valve will begin to open roughly 8 to 40 degrees before TDC.
Multi-angle valve jobs can increase airflow into the combustion chamber, yielding better cylinder filling and ultimately more power.

14. 4-Cycle Advanced Info – Compression Stroke
The compression stroke is essential for efficient combustion to occur. Any breach in pressure will drop the engines performance and ease of starting substantially. When the mixture of fuel and air are compressed, it begins to increase in potential energy and will pack more punch when ignited. The tighter it can be squeezed, the energy potential also increases.
Typically the valve train is the culprit in most cases whether it be poorly seating valves or improper valve lash. Its not common for head gasket failure or worn rings to be the first suspect when troubleshooting a loss of compression. The leak down test is the easiest, most effective troubleshooting method