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.
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For more information, please contact:-
Ford Motor Company, Essex UK
November 2000
