Diagnosing Engine Problems with Magnetostrictive Sensors

Spectral analysis of the signals detected using a single MsS placed on an automobile engine allows easy identification of detonation in any of the engine's cylinders. Data shown here were taken from a four-cylinder engine: (1) under normal conditions, (2) with inaudible detonation occurring in one of the cylinders, and (3) with audible detonation occurring.

The U.S. Environmental Protection Agency has passed regulations that require all 1994 and later model light-duty vehicles and trucks to carry an onboard misfire detector to ensure that the exhaust emissions system is functioning properly.

High emissions levels caused by misfire in gasoline engines generally occur during start-up or during transient events associated with low engine speeds. At higher engine frequencies, misfire can be a useful indication of problems with spark plugs, ignition wires and coils, fuel quality, or fuel system components.

Although magnetostrictive sensor (MsS^®) technology was previously used only in the nondestructive evaluation of structural components, SwRI engineers believed it had promise for evaluating engine firing characteristics by measuring the resultant stress waves.

An internal research project was initiated in 1994 by Kwun and Roy Meyer, formerly with SwRI's Engine, Emissions and Vehicle Research Division, to investigate the usefulness of MsS technology for engine evaluation using prototype sensors and analysis techniques.

"The results have been very interesting," says Meyer. "The number of faults detectable with magnetostrictive sensors is almost limitless. It provides the potential for engine diagnostics at a level we did not believe possible."

Several prototype sensors were designed, fabricated, and tested on automobile engines. The degree of signal analysis required to discern the characteristics of individual events, such as misfire and detonation, from the rest of the available signal was determined. Detonation was easily identifiable with frequency analysis. Early results showed that engine irregularities caused by mechanical faults were also readily detectable.

"Early analysis," says Meyer, "indicates that while the sensors have the ability to detect a wide range of phenomena, misfire proves one of the hardest signals to recognize. The prototype sensors were more attuned to the high frequency stresses associated with mechanical problems than combustion problems such as misfire that are associated with low frequency responses. For misfire detection, the sensor and signal conditioning electronics must be modified to increase their sensitivity to low frequencies.

"We learned," he adds, "that the sensor acts rather like a doctor's stethoscope. If a doctor puts the instrument on your chest he can hear a lot of things going on in there. It is training that allows one to separate the significant from the insignificant noises. Right now, we lack the signal analysis sophistication to identify everything we are looking for, but we can acquire it. No currently available sensor is capable of detecting such a range of engine characteristics."

Kwun and Meyer's successor Dr. Jongmin Lee, also of the Engine, Emissions and Vehicle Research Division, are continuing to develop the misfire detection capabilities of MsS.