APPLICATION STORY

 New Spark-Plug has Pressure Sensor built in.


Engineers from Ford in Essex have developed a spark-plug
mounted sensor capable of directly measuring combustion
pressure inside an engine.


 The thick-film piezo-resistive material
 is mounted on analumina substrate
 and protected from the harsh combustion
 conditions by a glaze.
 Two electrodes connect the resistor
 to the outside world.
Ford's transducer is simplicity itself - a layer of piezoresistive material between 10 and 40µm thick and two electrodes are printed and fired onto an alumina substrate. These are protected by an encapsulating layer of glaze. The piezo material is Heraeus 8241, which comprises particles of ruthenium dioxide and glass powder suspended in a solvent and organic binder. One of the problems encountered in an engine is obviously temperature, which could affect the sensor's output. This problem is heightened by the fact that a 10 Bar change in pressure only results in a 0.03 per cent change in resistance. The pressure transducer is mounted inside an otherwise normal spark-plug. Wires connect the transducer to the engine management system via holes milled in the threaded portion of the plug.
To increase the resistance change with pressure, the designers increased the porosity of the piezo-resistive material from six to around 50 per cent, so its modulus of elasticity was reduced. This was achieved by adding ito 10pm diameter gas filled silica spheres to the mixture before firing. This process doubles the sensitivity of the sensor to around 0.06 per cent per 10 Bar change. Though still small, this avoids the use of expensive temperature compensation circuits in the design. The sensor was then designed into a spark-plug with only minor modifications to the plug. Ford says the sensor could have other applications, for example, in tyre pressure sensing, crash detection and as a microphone for use in harsh environments. Why add a pressure sensor? Modern engine management systems use a number of inputs from the engine to determine the fuel injection and ignition timing settings. These are critical to meet the latest emissions regulations. A sensor measuring the pressure inside the combustion chamber which changes with engine load and speed will allow for better engine control. Today this pressure can only be inferred from transducers mounted on the outside of the engine. However, these are severely affected by noise. Mounting conventional diaphragm-type pressure sensors inside the combustion chamber has proved too expensive, moreover such sensors are unreliable for production use. ____________________________________________ For more information, please contact:- Ford Motor Company, Essex UK November 2000
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