Quality Exclusives

Applying Eddy Current

June 2, 2008
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Monitoring interactions in a running engine presents a genuine challenge; enormous pressures and temperatures are present, along with multiple moving parts-making the integration of sensors almost impossible.

Applications for this sensor include monitoring the performance of internal combustion engines. Source: Micro-Epsilon Messtechnik


Modern internal combustion engines now are being tuned to provide high performance, while still being kind to the environment. Therefore, the interaction of the various engine components plays an increasingly important role. Monitoring these interactions in a running engine presents a genuine challenge; enormous pressures and temperatures are present, along with multiple moving parts-making the integration of sensors almost impossible.

Few sensors are able to meet these challenges. In interesting development projects, new ways are continuously being discovered to improve the efficiency of internal combustion engines. The sensors that are considered for these applications are based on the eddy current measuring principle.

Eddy current sensors measure displacement, distance, position and spacing to sub-micrometer accuracy. These sensors have been used for many years in numerous measurement applications on internal combustion engines.

Special miniature eddy current sensors have been produced, the smallest having an external diameter of 2 millimeters with an installation length of 4 millimeters. The cable between sensor and controller also is miniaturized with a cross section of half a millimeter. In contrast to larger standard sensors, these sensors have a ceramic case that enables them to withstand high temperatures and pressures. Distances of up to 6 meters between the controller and sensor can be bridged with a connection cable. In this way, the controller can be securely installed in the passenger compartment or at a safe distance from the engine during measurements. In addition, the operating controls for the controller are located within the case. This means that the controller is fully sealed and resistant to oil, water and dirt.

Harsh Environment

There are space constraints with the applications involving an engine. The casing of an engine usually has to incorporate multiple coolant channels. Therefore, sensor integration and cable routing are difficult because no channel can be modified or damaged. In the heart of the engine-at the piston, connecting rod or the crankshaft-the cable routing is even more difficult, as all parts are moving constantly. However, the desired measurements and values only can be obtained inside an engine. As well as difficult installation, the sensor also has to withstand an unfavorable environment for sustained periods. Temperatures of up to 200 C, pressures of up to 2,000 bar and contact with fuels, oils or air-fuel mixtures are typical.

Eddy current sensors have been used for many years to measure engine performance in many locations. Source: Micro-Epsilon Messtechnik

Different Applications

The number of applications is increasing. For example, the secondary movement of the pistons during the different strokes is now measured. To do this, several sensors are directly integrated in the pistons, forming a flat surface on the piston wall. The cables are routed along the connecting rod and drive shaft to the outside via a swing arm. During operation, particularly when the engine is producing torque under load, it can be established, for example, whether the piston in the cylinder has too much “play” and whether this would adversely affect the service life. The sensors are integrated in the engine casing for measuring the movement of the piston rings.

If, for example, the sensors are moved to another place in the engine housing, the elongation of the cylinder head gasket during the stroke can be tested. Pressures of up to 50 bar are produced for every ignition of a cylinder, which slightly raise a tightly bolted cylinder head. The cylinder head gasket compensates for this movement, which is why it is referred to as “cylinder head gasket breathing.” Eddy current sensors measure how far the cylinder head moves. This data provides information about the durability of the gasket and prevents fatal engine damage.

The measurement of the lubricating gap in the crankshaft bearing is another interesting application. For this, miniature sensors are integrated in the bearing, which measure the distance through the plain bearing on the crankshaft. Using this distance, it can be established whether sufficient oil is lubricating the shaft in the bearing. If the film of oil breaks down, the distance to the shaft must be almost zero. This is because a certain minimum clearance from the bearing to the shaft is required due to the viscosity of the oil.

In the engine itself, sensors also can be used to measure, for example, the axial movement of the crankshaft or the oil film thickness on the connecting rod bearing. The top dead center (TDC) of the piston stroke and the movement of the valves also can be measured.

Eddy current sensors have applications in turbochargers to measure speed. Source: Micro-Epsilon Messtechnik

Speed Monitoring

In addition to the engine, other important engine components can be monitored. For example, a special sensor is available for measuring the speed of the turbocharger.

Turbochargers are particularly interesting at present because they can be found in practically every vehicle with a diesel engine. Apart from the angle of the guide vanes, the speed of the vanes on the turbine wheel is important for the performance of the turbocharger. For this, the rotation of the vanes has already been measured on the face side for many years. The sensor is directly located in the turbocharger housing and has a clearance of approximately 1.5 millimeters from the vanes. This apparently small clearance is considered very large in this application. Due to increasing material stresses and speeds, titanium vanes are now being used, which presents a challenge in terms of measurement technology.

Because titanium is a very poor electrical conductor, eddy current sensors cannot easily be used on this material. With speeds varying from 200 to 400,000 rpm, it is extremely difficult to measure precisely. However, using special linearization and an advanced electronic system, operators will be able to accurately measure the turbocharger speed over the whole range.

Needle Movement

Modern engines use injectors for moving the fuel into the combustion chamber. The fuel is atomized directly into the cylinder. Basically, the injector acts as a valve, comprising a sleeve, a needle (located in the sleeve) and a control unit for the needle. If the needle is moved back, fuel is injected into the engine. In order to save fuel and reduce exhaust emissions, it is critical that precisely the correct amount of fuel is injected at the exact, specified time. This means that the nozzle needle must only move for a specified time and specified distance. The movements are, therefore, becoming faster because up to five injections per ignition process is typical for most engines today.

An eddy current sensor also is used for this application and can be integrated in the central area of the injector, measuring the nozzle needle directly. In this position, the sensor withstands pressures of up to 700 bar. The controller for the nozzle needle movement is optimized using the data obtained, so that the fuel consumption of the engine can be reduced.

Eddy current sensors are used in fuel injectors to measure nozzle needle stroke. Source: Micro-Epsilon Messtechnik

Eddy Current Measuring Principle

The eddy current principle is based on the extraction of energy from an oscillating circuit, which is required for induction of eddy currents in an electrically conductive metal measuring object. If a metal plate approaches a coil charged with high-frequency alternating current, eddy currents will be induced in this plate using the electromagnetic field from the coil.

According to the Lenz Rule, the field of these eddy currents is opposed to the excitation field. Therefore, the energy extraction induced causes a change of the alternating current resistance of the sensor coil as a function of the distance to the measuring object (metal plate). Distance-dependent signals can be derived from this measurement using demodulation.

Effects

By using miniature sensors directly in the engine, data can now be obtained that enables further optimization of the performance yield and service life of the engine. The control of critical components is essential, particularly for engines where the performance has reached its peak.

These results contribute to the development of modern engines with smaller displacements that produce greater performance and have a longer service life than their predecessors.

Tech Tips

  • Eddy current sensors measure displacement, distance, position and spacing to sub-micrometer accuracy.

  • For measuring the performance of internal combustion engines, miniature eddy current sensors have been produced, the smallest having an external diameter of 2 millimeters with an installation length of 4 millimeters.

  • In contrast to larger standard sensors, the miniature sensors have a ceramic case that enables them to withstand high temperatures and pressures.
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