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.