NEW METHOD FOR DETECTING ENGINE KNOCK MAKES AUTOS MORE EFFICIENT
COLUMBUS, Ohio -- A new method for detecting engine knock holds promise for more efficient automobiles, according to Ohio State University researchers.
As part of an on-board knock-control system, this method would 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, or when the fuel octane rating is too low, or when 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.
"You hear a sound like someone is shaking marbles in a tin can, or tapping a hammer inside your engine," said Giorgio Rizzoni, associate professor of mechanical engineering and director of OhioState's Powertrain Control and Diagnostics Laboratory.
Millions of cars on the road today contain vibration sensors (usually located on the engine block) that are supposed to detect engine knock. But the software that processes the signals from these sensors often confuses knock with normally occurring engine 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," said Rizzoni. "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 combustion progresses the temperature in the combustion 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.
The researchers verified the method on a production-grade 3.5 liter V6 engine from project sponsor Chrysler Corporation, and published the results in a recent issue of the Proceedings of the IEEE. During these tests, TF analysis picked up on knock vibrations even when the engine was running at high speeds. In most cases, the signal-to-noise ratio obtained using TF was more than six times greater than that of traditional knock frequency analysis.
Knock has always posed a major problem to auto makers, because the most fuel-efficient engines burn hot and fast, just on the brink of knock. The only engine incapable of knock is one that doesn't run hot, and wastes fuel. Meanwhile, knocking combustions create temperatures high enough to melt holes in a piston.
"It's a tradeoff -- you want to get as much power from the engine as you can, but your limit is knock," said Rizzoni. "You need to operate an engine as close to the knock limit as possible."
An automobile computer system that uses the TF method would detect low levels of knock quickly and accurately. It could then instruct the engine control system make the appropriate adjustments (typically retarding the spark), and virtually eliminate knock altogether.
Drivers wouldn't notice this exchange taking place, but they would notice better gas mileage, longer engine life, and lower maintenance costs.
Right now, Rizzoni and his colleagues are working to shorten the computation time of the software, and find new applications. The university would like to collaborate with commercial companies to apply this technology to engine knock or other mechanical malfunctions.
Rizzoni said TF analysis can diagnose vibrations in all kinds of machinery. That includes any moving equipment on an automobile, aircraft, or boat, as well as manufacturing machinery.
Contact: Giorgio Rizzoni, (614) 292-3331; Rizzoni.email@example.com
Written by Pam Frost, (614) 292-9475; Frost.firstname.lastname@example.org
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