Detonation in a Gasoline Automotive Engine

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 specification.

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 sides.

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.

(P88-89)

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