Using MTII Capacitive Sensors to Precisely Measure Engine Oil Film Thickness

Marine engines are often exposed to heat, cold, salt spray and other aggressive elements. Even when these engines are operating at low speeds, they continue to remain under strain. This can be attributed to the variable operating conditions and the constantly changing loads. As a result, manufacturers of marine engines often find it difficult to design and develop engines that are reliable, long lasting, and operate under the harshest conditions. Figure 1 shows a typical marine engine.

Marine engine

Figure 1. Marine engine

Non-contact Sensor

A leading marine engine manufacturer challenged MTI Instruments to develop a non-contact sensor to determine the thin layer of oil film present between the piston ring and the wall of the engine cylinder. During the development of engines, the thickness of oil film is a vital parameter and directly corresponds to oil consumption, engine performance, engine life, and subsequent emission reduction.

Engine testing is usually carried out at high RPMs with piston linear speeds of 500”/s. In order to reduce engine machining, a measuring system with a small sensor size and high frequency response was needed. This would also ensure that the operating characteristics of the engine are not affected and yet dynamic measurement information is obtained as the piston ring passes across the face of the sensor.

Miniature Sensor Design

Engineers at MTI Instruments developed an exclusive miniature sensor design that had the following features:

  • Measurement range of 50µm
  • Diameter of 5.5mm
  • Resolution better than 0.25um
  • Response time of 20kHz

The sensor was accurately machined so that the sensor face is flush or recessed into the cylinder bore. For easy mounting inside the cylinder bore, the sensor was also threaded. MTI’s Accumeasure capacitive sensors can measure even the small capacitance change caused by the difference in oil thickness and transforms this capacitance change into a thickness measurement.

The sensor provided a rectangular sensing area that measured 0.12mm x 1.25mm. The ensuing footprint was designed to be significantly smaller than the thickness of the piston ring so that only the oil film thickness of the piston ring is determined. During the development of probe, high temperature materials and epoxy were utilized so that the probe can tolerate high pressure and operate at high temperature of 2850C.

High Frequency Dual Channel Amplifier

MTI’s Accumeasure 5000 precision high frequency dual channel amplifier with a response time of 20kHz (Figure 2) was found to be suitable for the specified application. However, since the application demanded a high frequency response, engineers created a customized amplifier filter, which prevented overshoot of signal and also reduced phase delay in the electronics.

Accumeasure 5000 precision high frequency dual channel amplifier

Figure 2. Accumeasure 5000 precision high frequency dual channel amplifier

Applications of Accumeasure Sensors

In addition to measuring engine oil film thickness, Accumeasure sensors can also be used for determining the amount of air and oil found in an engine passage. Depending on whether air, oil or a combination of both is present, these sensors provide a clear voltage output. The sensors can also be used in transmission research to measure the ratio of cavitation in the fluid.

When foaming occurs, it means that the mixture contains air and would affect the output signal significantly. Such kind of information is useful to ascertain the effect of combining different additives into transmission fluids and allows researchers to develop superior and durable products.


For complex noncontact sensing requirements, MTI Instruments has a team of application specialists who precisely and systematically assess specific requirements and provide a viable and cost-effective solution.

About MTI Instruments

MTI Instruments is a worldwide leader in the design, manufacture and engineering of non-contact measurement systems and sensors.

MTII’s main products consist of computerized general gauging instruments for position, displacement, thickness and vibration applications based on laser triangulation, fiber-optic and capacitance measurement technologies.

The Semiconductor Products sensor group manufactures manual, semi-automated and fully automated wafer characterization tools designed to measure wafer thickness, total thickness variation (TTV), bow, warp and flatness of semi-insulating and semiconducting materials.

MTII’s Aviation Balancing Instruments group provides state-of-the-art portable balancing and vibration analysis systems for turboprop and jet aircraft engines.

This information has been sourced, reviewed and adapted from materials provided by MTI Instruments.

For more information on this source, please visit MTI Instruments.


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