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Eliminating engine knock increases operating efficiency.

A new method for detecting engine knock holds promise for more efficient automobiles, according to researchers at Ohio State University, Columbus. As part of an on-board knock-control system, the method could help engines consume gasoline more efficiently and last longer.

Knock begins in the combustion chamber, where spark plugs ignite gasoline to power the engine. When the spark plugs fire too early, the fuel octane rating is too low, or carbon deposits inside the chamber grow hot enough to ignite the gasoline, the fuel doesn't burn gradually -- it detonates all at once. This explosion sends out violent shock waves that vibrate the entire engine. In addition, the phenomenon, also referred to as "pinging," causes combustion chamber temperatures to soar, with the risk of permanently damaging valves and pistons.

Millions of cars on the road today contain vibration sensors (usually located on the engine block) that are supposed to detect engine knock. However, the software that processes the signals from these sensors often confuses knock with normally occurring vibrations, especially at higher engine speeds. Many production knock-control systems are incapable of detecting engine knock until the levels are very high.

"Everything vibrates on an engine. You've got things rotating, things shaking, everything is moving," explains Giorgio Rizzoni, associate professor of mechanical engineering and director of Ohio State's Powertrain Control and Diagnostics Laboratory. "The difference between a normal and knocking condition is just not that obvious." This difficulty is made even greater by the fact that the vibration induced by knocking combustion occurs over the span of a few milliseconds.

Rizzoni and his colleagues wrote software that uses a different kind of mathematical technique -- time-frequency (TF) analysis -- to extract knock vibration from background noise more effectively. The researchers took advantage of the fact that the pitch of the sound (acoustic resonance) induced by knock changes with the combustion temperatures. As fuel combustion progresses, the temperature in the chamber rises and then falls.

Knock typically starts when combustion chamber temperature is near its peak. Thus, the pitch of the knocking sound becomes lower with time, and this gives rise to a characteristic vibration signature. The software tells the computer to look for vibrations with this characteristic drop in pitch and ignore all the others.

An automobile computer system that uses the TF method would detect low levels of knock quickly and accurately. It then could instruct the engine control system to make the appropriate adjustments (typically retarding the spark) and virtually eliminate knock altogether. Drivers would get better gas mileage, longer engine life, and lower maintenance costs.
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Title Annotation:Ohio State Univ researchers have developed software that better differentiates between automobile engine knock and background noise
Publication:USA Today (Magazine)
Article Type:Brief Article
Date:Jun 1, 1997
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