In a vehicle there are hundreds of moving parts that get you from the point A
to point B. The modern automobile is an intricate piece of machinery and a wonder
of the technological processes. One of the main processes that produces a vehicle's
power is called compression. Compression is the pressure of the air/fuel mixture
that has been compressed in the cylinder of the engine by the piston. As the piston
travels upward this mixture is compacted down into a smaller volume so it can be
ignited by a spark plug. In a four stroke engine compression occurs on the upward
stroke or the upward push of the piston through the cylinder bore before the spark
plug ignites the mixture.
Engine Configuration with Camshaft Operation
Octane Rating
Why should you read the octane rating at the gas pump? The compression ratio
of your engine determines the octane required for optimum performance. The octane
rating of gasoline states the level fuel that can be compressed before it ignites.
Under extreme conditions such as heavy acceleration fuel can ignite without the
use of the spark plug resulting in a "ping" or "pinging" and under ultra heavy condition
a knocking sound. When this condition is left un-treaded it could result in sever
engine damage.
The compression ratio is the number that can be used to help predict the engine
performance. It is calculated by taking the volume of the cylinder, when the piston
is at the bottom of its stroke, and then dividing it by the volume when the piston
is at the top of the stroke, the higher the compression ratio the more power your
engine can produce, The extra power produced is coupled with added efficiency in
turn help improve gas mileage.
Checking Compression
To check compression you must first disable the ignition system to keep the engine
from starting. Locate the ignition coil and disconnect it, or unplug the ignition
module connector. To test if the ignition is disabled crank the engine over, the
engine should not start. With the engine warm remove one of the spark plugs and
insert the compression tester into the spark plug hole. Now turn the key of the
engine for at least 5 to10 seconds to get an accurate reading. Repeat this for each
cylinder of the engine and record your results relating the reading to the individual
cylinder. If the individual readings vary by more than 10% between cylinders you
may have a problem.
Optional compression testing methods include an electric starter tester. This
test measures the battery amperage draw to each cylinder while the engine is cranking.
It will show you which cylinders have good compression and which have the least
compression. In a gasoline engine a moderate compression ratio is needed, around
140 to 160 pounds per square inch. Each manufacturer produces specific compression
ratios for their engines to closely control the power output and emissions. If high
compression exists it can cause a problem known as pre ignition or detonation. A
compression test can detect internal combustion malfunctions such as, bad valves,
bad piston rings, or excessive carbon build up. These problems if left un-repaired
can cause more extensive engine damage and even catastrophic failure.
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