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Measuring pressure in tough conditions

FPT Motorenforschung, a Swiss manufacturer of engines for commercial vehicles, construction equipment and agricultural machinery, uses Keller’s M5 pressure transmitters in its test benches to achieve reliable, precise measurement results

 

“The engines that we develop and test in our facility need to meet the most stringent quality requirements in terms of materials, emissions and efficiency,” noted Rainer Linsenmeyer, head of test engineering and electronic measurement at FPT Motorenforschung.

“This means that when we examine the engines on our test benches, we have to consider all the variables that influence how well they work, so we need precise measuring equipment that delivers reliable readings even in the conditions of an engine, which can sometimes vary considerably and be extremely tough.”

Keller developed its sensors to meet the company’s requirements in engine development. This work focuses primarily on reducing exhaust emissions and achieving a high power density, low fuel consumption, a long service life and maximum reliability. An engine’s efficiency depends largely on the extent to which the flow of air into the combustion chamber and that of the exhaust gases back out again are unimpeded. The pressure here will be under 10 bar. Extremely accurate and highly dynamic measurement is essential.

Several of Keller’s M5 devices are used at various places on the engine during tests. To obtain a reliable analysis of pressure conditions and losses, it is important to take measurements as close as possible to the individual cylinders on the inlet side. The varying geometry of the intake manifold poses an additional challenge as it means that each cylinder is supplied with a different amount of air, which should be avoided as much as possible. On the outlet side, too, the combustion gases in the individual cylinders can cause unwanted interactions between pressure surges or pulses. The pressure sensors measure these processes on the inlet and outlet sides locally and with a high level of phase fidelity.

The sensors must not be sensitive to structure-borne noise. The measuring slot should be sealable with a plug once measurements have been taken and should be as resistant as possible to air flows. This application can also lead to compensating currents of up to 40A being generated by the engine battery – these must not influence the sensor.

Keller’s sensors do not require bonding wires in the vicinity of the medium and there are no connectors between the sensor and the electronic system. This removes any additional contact resistances and sources of faults. The cable gland is hermetically sealed (IP67) to minimize external influences on the cable. Most of the electronic system is fitted externally, enabling it to be positioned in a noise-free area.

Potential uses

FPT Motorenforschung uses M5 sensors with electronic systems on all of its 30 engine test benches. The company takes measurements inside components such as oil, fuel and water pumps, injector lines, intercoolers, heat exchangers, and intake and exhaust ducts. Logging the pressure conditions enables any fluctuations, peaks or other generally unusual pressure phenomena to be detected.

The data obtained makes it easier to extrapolate the dynamic conditions inside the engines. For instance, a pressure loss of 1Pa at a volume flow of 1m³/s – typical values for the intake air volumes of large HGV engines – translates into 1W of lost performance. In practice, this means that a 50mbar fall in pressure results in a flow loss of as much as 5kW. Consequently, this value must be kept as low as possible to make the engine more efficient.

M5 sensors can also be used in the return pipes in injectors. Because of how they are designed, injectors generate a return flow through leaking and cooling. The pressure conditions in these pipes are also measured and optimized, with a pressure range from 1 to 5 bar. Oil pressure is a further critical factor in engine efficiency.

“When measuring oil pressure it is particularly important to find a sensor that is small enough. This is because smaller sensors have less of an impact on the actual flow conditions. Also, there is often very little space to install them with this kind of application, so a standard sensor was never an option. The M5 sensor ticks all the boxes,” said Linsenmeyer.

“One particular benefit is the fact that, after testing is complete, the openings for the sensors can be sealed using a simple M5 plug almost as if nothing had changed. This means that any modifications to the system won’t distort the results of other tests. We’re delighted that, in the M5 sensor, we’ve found a reliable measuring instrument that can be put to a wide range of uses on our test benches.”

The M5 series
Pressure sensors in the M5 HB series are designed to deliver extremely fast and precise measurements to an accuracy of 0.1% FS up to 180°C. The pressure connection to the M5 fine thread is designed for both static and dynamic measurements such as pressure pulses. Thanks to their miniature size, these sensors are ideal for local measurements.

The M5 sensor also has a low dead volume and good venting capability. It uses a rear-side technology without a transfer medium that also helps to minimize the attenuation of the measurement signal. No sealants, adhesives or capillaries are required.

To prevent any unnecessary impact on flow conditions, the process connection should be as short as possible and, in terms of handling, be able to be installed easily even in narrow tubes. The sensor is thus also very lightweight and able to withstand rapid acceleration.

The M5 series is available with or without an electronic system and offers signal fidelity up to the limit frequency of 50kHz. The M5 HB pressure transmitters (i.e. with an electronic system) were optimized in line with FPT Motorenforschung’s technical specifications and have been in use for over two years.

February 6, 2017

 

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