Detonation in a Gasoline Automotive
Detonation is a form of abnormal or
improper combustion. It is commonly referred to as "spark
knock" or "pinging". When detonation is present
cylinder firing pressures and temperatures are elevated with
resulting engine damage. This engine damage varies depending
on the degree of detonation.
With this in mind Hastings Manufacturing
conducted testing using a 350 C.I.D. Chevrolet engine and
developed a cycle where the engine drifts into and out of
a detonation condition. The objective was not to destroy the
engine, but rather to operate in what would be borderline
abnormal combustion and stop before serious engine damage
resulted. Oil economy was observed during the test.
The engine was set up on a dynamometer
using premium unleaded fuel and S.A. E. 20W engine oil. The
engine was cycled from 1200 - 2400 R.P.M. every 25 seconds.
It was run with the ignition timing advanced beyond the factory
During the acceleration part of the
cycle light to medium spark knock was audible.
After 80 hours the engine was disassembled,
one piston was removed, and the rings examined. There was
distressing on the bottom side of the compression rings and
the sides of the oil ring rails. The sketch below illustrates
the shape the ring assumes from the pounding.
The engine was re-assembled and the
test was continued for another 80 hours at which time the
test was concluded and the engine was torn down and examined.
Oil economy declined approximately
30% when the engine was operating under detonation conditions.
The graph shows oil control deterioration, and the photographs
illustrate the damage to the rings.
All compression rings exhibited the
pounded condition shown in the foregoing sketch. The oil ring
expander had indented Itself into the lower oil ring rail.
The upper rail also has indentations although to a lesser
degree. The top compression rings had light to medium scuff
on their O.D. face. No piston damage was apparent although
some cylinder scoring was present on major and minor thrust
Detonation can be caused by:
1. Lean fuel mixture.
2. Fuel octane too low.
3. Improper ignition timing.
4. Lugging the engine.5. Excessive milling of heads or block
which will increase compression ratio.
Detonation is a form of abnormal combustion
in the combustion chamber. During normal operation of the
engine, the burning of the fuel-air charge produces a steady,
smooth push on the pistons of each cylinder. At the instant
of ignition by the spark plug, the flame of combustion moves
rapidly outward from the plug very much like the waves when
a stone is dropped into a pool of water.
Abnormal operation may allow combustion
pressures to develop so fast that the heat and pressure will
"explode" the remaining unburned fuel. This produces
the knock, often called ping, carbon knock, etc. Actually
this is detonation. The knock results from the violent explosion
when the normal flame front runs into the secondary flame
front. Detonation will cause piston and ring damage, top ring
groove wear, scoring, sticking rings, loose head gaskets and
possible complete engine failure.
The goal was accomplished in this experiment
as testing did not destroy the engine but did piston ring
damage and verify that oil control is adversely affected when
detonation is present.
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