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Sensors
enable real time measurement of engine piston displacement.
A UK-based automotive consultancy is using miniature, eddy current
displacement sensors built into pistons on gasoline engines,
to measure the displacement between the piston and the cylinder
bore - in real time. This means engineers can look closely at
how the piston moves with critical engine operating parameters,
such as cylinder pressure and engine speed.
Mahle Powertrain
Ltd, formerly Cosworth Technology, is an automotive consultancy
based in Northampton. The company is part of Mahle Group, a leading
supplier of piston systems, cylinder components, valve train
systems and air and liquid management systems to the automotive
industry.
 One of Mahle Powertrain's
recent internal research projects involved the use of Micro-Epsilon
sensors to measure the displacement between the piston and the
cylinder bore on a gasoline engine. The sensors were mounted
within the piston itself, so that movement could be viewed throughout
the complete combustion cycle. Mahle has worked successfully
with Micro-Epsilon for a number of years and so when this complex
project emerged, Mahle consulted with Micro-Epsilon for its expert
advice.
Two objectives
of automotive engine development are to minimise harmful emissions
and to reduce oil consumption. To obtain reliable measurements,
which can then be used to help develop the engine and components
for high volume production, tests need to be conducted on real
engines and under realistic operating conditions.
At Mahle Powertrain,
the test environment was harsh and so selection of the sensor
was critical. Two Micro-Epsilon 'UO5' eddy current displacement
sensors were used for the project, due to their high precision
in extreme environments and sub-miniature design (2mm diameter
by 4mm in length with a 0.5mm diameter sensor cable).
 Because of the piston
design, the 'UO5' sensors needed to be positioned as high and
as low as possible on the piston. In this case, the uppermost
sensor was mounted between the first and second piston ring on
the thrust face. The other sensor was vertically aligned, as
low as possible on the piston skirt. As Carl Godden, Senior Development
Engineer at Mahle Powertrain explains: "The position of
the sensors enabled the measurement of any 'rocking' movements
on the gudgeon pin as it travelled up and down the bore. Effects
of varying the cylinder pressure, engine speed, piston squirt
jet flow and other potentially influential parameters could then
be viewed in real time."
To carry out these measurements, Mahle Powertrain utilised its
own mechanical linkage mechanism. This enabled cables to be routed
from the piston area, down the connecting rod, and across two,
pivoted beams and out via the sump. This system enabled Mahle
engineers to acquire a diverse range of measurements from a wide
range of miniature transducers.
Mahle's linkage
system can be utilised in both gasoline and diesel engine applications.
Its ability to give real time data via hard wiring, means data
acquisition resolution is only limited by the maximum speed of
the amplifiers or the acquisition system. In this particular
application, the output of the Micro-Epsilon 'U05' sensors was
plotted against crank angle by using a high resolution crank
angle encoder.
According to
Godden, the UO5 sensor's miniature, 0.5mm coaxial cable "proved
more than durable for the duration of the tests, even under such
an extreme environment". Mahle Powertrain's test schedule
meant that the U05 sensors were subjected to sustained speeds
of up to 6,500 rpm with a total test duration of 35 hours, with
oil temperatures reaching well over 100°C.
The sensor
has a high temporal resolution (0.1° per crankshaft rotation)
and a very high spatial resolution (~ 1µm) and is not affected
by fuel, oil, exhaust fumes or other contaminants.
Micro-Epsilon (www.micro-epsilon.co.uk)
is a major global manufacturer of sensors, headquartered in Germany.
The company's range of sensors measure everything from displacement
to distance, position, vibration, dimensions and thickness, using
both contact and non-contact measurement techniques. These techniques
include inductive, differential transformer, eddy-current, capacitive,
laser-optical, potentiometric and draw-wire principles.
With more than
30 years' experience in the industry, Micro-Epsilon isn't just
a sensor manufacturer. The company is highly innovative and understands
the importance of providing complete solutions and support services
for its customers. The firm is renowned for its expertise in
consulting, development and application of industrial sensors
to complex, customer-specific solutions for measurement, inspection
and automation. The focus is on selling technical advantage to
its clients.
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