An Enduring Test Legacy

| Transport

Mallard helped shape the way testing and measurement is performed in the railway industry (Courtesy NRM)

A chain of testing technology leading to today’s “train zero” concept lies within the story behind the record breaking run of Mallard as far back as 1938

Like any world record holder, Mallard didn’t merely slip into its laurels effortlessly to bask quietly in the world’s admiration for the next 83 years, it gained its position through the tireless efforts of its designer, engineers, technicians, driving crew and dozens of other specialists and workers toiling under the gaze of the locomotive’s creator, Sir Nigel Gresley.

To understand more about the testing and measurement technology used by Gresley and how that evolved, Jonathan Newell spoke to Bob Gwynne, Associate Curator at the National Railway Museum in York.

The 1930s was a decade rich in engineering innovation and industrial design. Transport technology was making giant strides in air, sea, road and rail. There was a strong element of rivalry between Gresley, who was Chief Mechanical Engineer of the London North Eastern Railway, and his west coast counterpart, Sir William Stanier. Both engineers were prolific locomotive designers and both had their flagship large express locomotives with which they headed for the Scottish cities of Edinburgh and Glasgow respectively in record time.

Iterative designs

Such competition spawns rapid innovation and Gresley was in constant search for greater fuel efficiency, higher drawbar forces, improved steam pressure cycles etc. These were just some of the parameters that determined the performance of a steam locomotive and the complexity was staggering, particularly given the tools available to engineers at the time.

Today, we have computational fluid dynamics, simulation, mathematical modelling tools and a sophisticated array of sensors, accelerometers, data acquisition systems and of course dynamometers. In Gresley’s day, instrumentation was available but in a much more rudimentary form and without the off-the-shelf availability that we enjoy now.

As such, the design process was highly iterative and that was problematic because you couldn’t just take a design from the drawing board, build it as specified and expect it to work. The requirement for skill, craftsmanship and experience was pervasive. The huge whirling and reciprocating masses had to be balanced perfectly, every pressure joint sealed, all alignments precisely judged; even the driving crew had a huge influence on performance.

Gresley understood very well that testing and measurement were key elements in perfecting designs. His first venture into the express “Pacific” locomotive was the A1, the first iteration of the famous “Flying Scotsman”.

According to Gwynne, although the A1 was good, and certainly better than what had come before it, there was still work to do to ensure the steam produced by the boiler was better used. “There are lots of factors and design variables that could be incorrectly specified and so had to be refined. The result was the A3, the version of the Flying Scotsman that had been modified and went on to break the 100mph barrier”.

This iterative approach led to the A4, the class to which Mallard belongs, some 8 years later. “The A4 was a progression beyond the Flying Scotsman and by this time the design had become very well defined,” says Gwynne.

Dynamometer

Refining the design to this extent required detailed knowledge of how each variable was performing. At the time, there was no national network and each of the main four railway companies in the UK had different resources available to them. The LNER had a mobile dynamometer car, a coach towed as part of a train and endowed with the capabilities of measuring an range of variables under motion.

Constructed a full three decades before Mallard was built, the dynamometer car was a heavyweight teak and brass legacy from the turn of the century but was nonetheless a sophisticated data acquisition and analysis tool that gave Gresley all the information he needed to perfect his designs.

With graphs, a chronometer and an large number of instruments, the dynamometer could measure speed, draw-bar force, energy and an array of very detailed parameters such as compression, back pressure and exhaust characteristics. It could even be used to measure throttle position and coal / water consumption.

For this, it needed a substantial crew both in the dynamometer car itself and also on the locomotive tending to the instruments that supply the dynamometer with the data it needs. To this end, a “shed” was often constructed on the very front of the locomotive so that technicians could sit on the walkway sheltered from the buffeting winds while they tapped into the cylinders to measure steam pressures that could be as high as 220psi.

“This was an extraordinarily difficult and dangerous task but it was the only way of obtaining accurate pressure measurements, which were very important to understand the entire pressure cycle of the cylinder, which has a significant bearing on efficiency and performance,” Gwynne tells me.

Gresley’s Dream

Although the dynamometer car provided large quantities of important data and analytics for locomotive development at the time, it had its shortcomings. According to Gwynne, it was a frustration to Gresley that he didn’t have access to a static dynamometer.

The visionary engineer knew that the future lay in having access to test facilities that the locomotive could be taken to rather than dragging the facility with it to take the measurements.

Such a facility existed in France at the time, commissioned by Gresley’s French contemporary André Chapelon.

“When Gresley was working on an earlier design of his, the P2, in Darlington, he took the prototype with him to Chapelon’s facility in France along with local coal, to try it out for himself,” Gwynne says.

Chapelon was a proponent of the scientific approach to design rather than the artisan approach that prevailed at the time. He wanted to understand exactly what effect parametric changes would have on performance and the only way to achieve that was through experimentation, simulation and measurement.

In this respect, Gresley and Chapelon shared a similar vision, one which would ultimately lead to the approach used for today’s designs.

“He was right. Gresley knew that fixed testing facilities were the correct approach but he never saw it come to fruition. The first fixed facility opened in Rugby in 1948, seven years after Gresley’s death,” says Gwynne.

Towards Train Zero

Since those pioneering days of railway test engineering, technology has advanced dramatically on every available front but some of the principles are very similar.

The record breaking run by Mallard was recorded by a group of engineers surrounded by teak and brass at a recording table with pens tracing graphs on paper. Speed measurements can now be made instantaneously at levels of accuracy that are meaningless in such macro-transport applications.

Network Rail have mobile dynamometers and measuring equipment that monitor the track, provide data for maintenance operations and a host of other tasks.

The industry also now has the benefit of a number of academic and research establishments, such as the Institute of Railway Research at Huddersfield University, all of which provide advanced facilities and an incredible body of knowledge that’s available to the industry to keep the innovation flowing in the modern railway age.

With design and development residing in the private sector, companies have commercial incentives to invest and innovate. One such innovation is “Train Zero”, the state of the art in prototyping and testing.

This life-size pseudo product has a virtual existence for simulating its life cycle. It uses Hardware-in-the-Loop enabling prototype components to be “plugged” into the simulation to try them out under the range of variable values they’re likely to encounter in the real world on the finished product.

Gresley’s dream was to have a fixed test bed and he could not have foreseen at the time what the available technology would be like for something like Train Zero. However, he did always have his eye on the future.

“Gresley was certainly visionary. He was involved in data processing, analysis and even electrification projects, he was a futurist,” concludes Gwynne.

Jonathan Newell
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