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Using shock physics to improve engine efficiency

05 October 2014

The National Physical Laboratory (NPL) has developed a system to provide accurate characterisation of high-speed pressure sensors.

NPL’s plastic shock tube installation
NPL’s plastic shock tube installation

Pressure sensors are used in thousands of everyday processes in the medical, aerospace and transportation sectors. For a lot of these applications, only slow pressure changes occur.

However, for uses such as monitoring turbines and in research into improving the engines powering cars and aircraft, rapidly changing pressures need to be measured, and accurate, high-speed pressure sensors are required. In combustion engines for example, the in-cylinder pressure varies periodically from 0.1MPa to 10MPa, with frequency content up to 30kHz. 

Even if they are taking dynamic measurements, pressure sensors are usually only calibrated using static pressures – the pressures exerted by a still liquid or gas. The dynamic behaviour of a sensor deviates from its static characteristics as the frequency increases. So, the use of a sensor in a different frequency range can affect the reliability and uncertainty of the measurement result.

Last year NPL opened a new facility to allow the accurate characterisation of high-speed pressure sensors using shock tube techniques. 

A simple shock tube consists of two straight tubes, separated by a diaphragm, normally a thin metal plate. One tube contains a low pressure ‘driven’ gas and the other contains a ‘driver’ gas. Gas is added to the driver side until the diaphragm ruptures. This generates a series of compression waves that join together to form a shock wave that travels through the low pressure tube. 

Shock physics and shock tubes are largely used for aerospace and materials research. Often scientists create a shock wave to investigate what it does to materials or structures. But, crucially, for industrial purposes, a shock wave provides a very rapid (of the order of a nanosecond) pressure step with calculable amplitude. This pressure step can provide the basis for the calibration of pressure sensors used in highly dynamic applications. 

A plastic shock tube
Scientists at NPL’s Dynamic Pressure Sensor Facility recently announced that they had developed a plastic shock tube, capable of working at 14 times the pressure of the atmosphere (1.4MPa). The shock tube is made from PVC-U tubing, providing a low-cost, light and easily modifiable system, in contrast to the standard approach of using steel shock tubes which can be extremely heavy and unwieldy.

Because the PVC-U tubing is so light, the longest (6m) section can be easily manoeuvred by one person, and it can be constructed and modified using standard laboratory tools

NPL’s plastic shock tube works in similar way to conventional metal shock tubes. First, the driver section is pressurised using bottled gas; initially, nitrogen but helium and argon can also be used.

The entire pressurisation and measurement system is computer controlled. Two pressure sensors in the side wall of the driven section are used to derive the velocity of the shock wave by measuring the time delay between shock detections.

The velocity is calculated from the known 400mm separation between the sensors and this measured time interval. The pressure step reflected from the end wall of the tube, where the pressure sensor being calibrated is located, is then determined from a gas theory equation relating it to the initial temperature, pressure and type of gas in the driven section, and the velocity of the incoming shock wave.

Scientists at NPL investigated the effect of diaphragm material and configuration, and the driven section length, on the operation of the plastic shock tube and none were found to affect the generated pressure step significantly. As such, the novel plastic shock tube has been proven capable of acting as a primary dynamic pressure standard and one that is significantly more flexible and low cost than existing methods.

Improving efficiency
There is a need to ensure that dynamic measurements are giving true real-time pressure values, as they are essential for optimising the industrial process being controlled- saving money and reducing environmental impact.
One major area of need for dynamic pressure measurement is in gas turbine engines. The need to meet emissions targets and improve reliability and performance sees engines undergo many improvement programmes and run with ever-leaner fuel/air mixtures. This can lead to instabilities and excessive pressure pulsations that can result in mechanical failure.

Improved dynamic pressure measurement could help lower the costs of mechanical repair, downtime, and environmental fines. The improvement that accurately calibrating dynamic pressure sensors can have isn’t huge for an individual engine, but for a fleet of trucks, for instance, making engines just a few percent more efficient could potentially save millions of pounds, and make a significant difference to their environmental impact.

The NPL team recently published a paper on the technology - Towards a shock tube method for the dynamic calibration of pressure sensors - in a special issue of Philosophical Transactions of the Royal Society A, celebrating the centenary of Bertram Hopkinson’s seminal paper, A method of measuring the pressure produced in the detonation of high explosives or by the impact of bullets.

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