CUSTOMER
EDUCATION
BRIGGS & STRATTON
4-Cycle Theory
CUSTOMER
EDUCATION
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
CUSTOMER
EDUCATION
4-Cycle Theory - Intake Stroke
CUSTOMER
EDUCATION
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.
CUSTOMER
EDUCATION
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!
CUSTOMER
EDUCATION
4-Cycle Theory - Compression
Stroke
CUSTOMER
EDUCATION
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!
CUSTOMER
EDUCATION
4-Cycle Theory - Power Stroke
CUSTOMER
EDUCATION
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.
CUSTOMER
EDUCATION
4-Cycle Theory - Exhaust Stroke
CUSTOMER
EDUCATION
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….
CUSTOMER
EDUCATION
4-CYCLE THEORY - ADVANCED
INFORMATION
CUSTOMER
EDUCATION
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.
CUSTOMER
EDUCATION
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