Evening star. [volume] (Washington, D.C.) 1854-1972, 1922-02-26
~ ALBERT \. ATKINSON

!
There is nothing more" annoying,
than the insidiouss knock. You chase
1t from one end of the engine to the
other. but still it eludes you. II may
be harmless and run on for months
without giving trouble. like piston
! slap. or 1t may wreck the engine the
, mex; instant like a loose connecting
rod. In any ease. it must be located
and remedied. if possible. before your
! mind is at rest
Engine knocks may he classified
roughly under four heads-spark.
overheating, carbon and loose or worn
parts. We shall first explain these in
detail and then give directions for
eliminating .oae afte; another until
t,le trouble is found
\ spark knock is produced by ad-
vancing the spark too far for the
speed Of the engine We retard the
spark when starting an engine 50 as
TO have it occur at the beginning of
| the power stroke.
Adjustment Of spark.
After the engine starts we advance
J, II to give more power. This brings
the spark OVER dead center into the
end of the compression stroke which
means that the piston is travelling
! upward when the spark occurs. But
| the flywheel turns SO fast that the
piston is brought TO upper dead cen
! ter and started down ON the pow or
stroke before the fun force OF the
| explosion is developed. Now, if the
| spark is too far advanced or the en-
CINE slows down under load. such as
when negotiating steep hill. the ex-
:. suit is a knock. The force OF the
/ explosion pressing downward meets
! the piston coming up. and the result
/ is what is known as q spark knock.
+. This can be readily proved when go
| ing from a level road to a hill The
! engine will begin to knock unless the
! spark is retarded. OF course. this
/ will occur only il the spark has been
kept advanced as 1t should be up to
! that time. The rule for spark ad
vance is to advance it as far as it
/ will go without a knock. Then when
/ heavy roads or grade are met the
| knock will start. but must be stopped
| Immediately, as it reduces the pow er
! and tends to fatten out the bearings.
!; SO retard the spark. but only enough
!, to stop the noise.
The next type OF knock is one due
| to overheating. Because of some fail
| ure of the cooling system the cylinder
| walls and pistons get SO hot that they
set fire to the gas in the cylinders
| without the electric spark. At first
| this self-ighition comes at about the
| same time as the electric ignition, and
| so produces no bad effect. Eut as the
/, temperature rises the gas is explod ed
| sooner, and it then occurs so much
before dead center as to make a
! knock. This is always accompanied
! by other symptoms of overheating.
such as steaming radiator. etc, and
! if the switch is shut off the engine
! will continue to run by self-ignition.
| This symptom is usually called pre-
| ignition" because the charge is ig-
| nited before dead center. This trou-
| BILE can be readily recognized by the
! symptoms of overheating already
| mentioned.
| Carbon Knock.
! Carbon knock is another case of
preisnition but due to an accumula-
| ton of carbon in the combustion spa ce
| and on top of the piston. When the
deposit becomes fairly thick it catches
! fire from the repeated explosions GO
l ing on around it and sets fire to the
mixture before 1t should. If it comes
| with the electric spark 1t makes no
difference. but as the engine heats up
and the carbon glows more intensely
! the gas is exploded before dead cen
| ter. as the piston rises, and the ex-
plosion meeting the piston makes a