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BRIGGS & STRATTON

4-Cycle Theory

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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

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4-Cycle Theory - Intake Stroke

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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.

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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!

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4-Cycle Theory - Compression Stroke

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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!

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4-Cycle Theory - Power Stroke

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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.

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4-Cycle Theory - Exhaust Stroke

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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….

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4-CYCLE THEORY - ADVANCED INFORMATION

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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.

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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