MoTeC Knock OKM - Manual (Page 33)

MoTeC Appendices 33
Detonation causes a very large, very rapid, pressure spike of very short
duration in the combustion chamber. The pressure trace of the combustion
chamber process would show the normal burn as a steady pressure rise, and
then all of a sudden a very sharp spike when the detonation occurred. The
pressure spike creates a force on the combustion chamber causing the
structure of the engine to ring or resonate (much as if it were hit by a
hammer). Resonance, which is characteristic of combustion detonation,
occurs between 4 to 12 kHz resulting in the audible pinging. This noise or
vibration is what the knock sensor detects.
Detonation Indicators
The best indication of detonation is the pinging sound that cars, particularly
old models (pre 1980) make at low speeds and under load. It is very difficult to
hear the sound in the well insulated luxury interiors of today's cars. An
unmuffled engine running straight pipes or a Rally Car racing on a gravel road
can easily mask the sound.
In some cases, the engine may smoke but more often the driver is not aware
detonation is occurring.
Typical results of detonation are broken piston ring lands, broken spark plug
porcelains or broken ground electrodes. However these signs are usually not
spotted externally.
It is also difficult to detect detonation while an engine is running in a remote
and insulated dyno test cell.
To help hear detonation a very elementary technique often proves successful:
run a copper pipe bolted flat to the side of the engine block into the control
room, place a funnel at the end to amplify the sound through the pipe and
listen. This allows the operator to hear all mechanical noises within the engine
and helps to identify detonation should it occur.
Also commonly used for knock detection are electronic amplifiers.
These devices either connect directly to a knock sensor or use an alligator clip
placed on the engine block. The engine sounds are amplified and filtered and
then routed to the operator via headphones.
Detonation Failures
Detonation causes three main types of failure:
1. Mechanical damage (broken ring lands, hammered big end bearings)
The high impact nature of the pressure spike can also cause
fractures; it can break the spark plug electrodes, the porcelain around
the plug, cause a clean fracture of the ring land and in severe cases
can actually cause fracture of valves-intake or exhaust.
2. Abrasion (pitting of the piston crown).
The sandblasted appearance to the top of the piston near the
perimeter the piston is typical if detonation occurs. Examined with a