Kistler supports Continental Aerospace Technologies with aircraft engine development

KiBox, the universal combustion analysis system, was mostly used for terrestrial vehicles in the past. But now it is literally soaring to new heights: as part of the development process for a new generation of aircraft engines, engineers from Continental Aerospace Technologies ventured up into very thin and cold air. And with the help of KiBox, they succeeded in optimizing the engines' performance.

More power and less fuel consumption – aircraft engines are expected to meet more or less the same requirements as those in automobiles. But conditions prevailing above the clouds are far from usual – making it more difficult to develop engines for the aviation industry. Continental Aerospace Technologies GmbH, which belongs to globally operating Continental Aerospace Technologies Ltd. ranks as one of the world's leading manufacturers of engines for small aircraft. The company operates facilities in the US and in Germany. Starting in the first decade of this century, the company developed its kerosene piston aircraft engine based on a diesel engine for passenger cars, with a redundant electronic control. This engine uses about 40 percent less fuel, so the range of the aircraft is extended by as much as 30 percent. This success was a quantum leap in innovation for Continental Aerospace Technologies, setting a new benchmark in the industry.

'As we continue to develop our engines, we literally want to soar to new heights!' The speaker is Dr. David Dörner, Application Engineer for Test and Application at Continental Aerospace Technologies GmbH. Some background information: as density decreases with altitude, air resistance is also reduced – so fuel consumption is cut. However, there are limits on the possibilities for flying high: as the aircraft gains height, the engine must increase its altitude performance so the plane will always have sufficient lift. Unlike large jets, engines in small aircraft have no turbocharging or only single-stage charging, so they have a much lower service ceiling (the maximum flight altitude in continuous operation). This is why optimizing altitude performance is such a key aspect of development.

To ensure that the engine also runs smoothly while the plane is descending from a high altitude and when power output from the engine is low, there must be sufficient compression energy for combustion to take place. Otherwise, the result can be what is known as flameout, which is similar to loss of flame in jet engines. Another requirement is the ability to actively turn the engine off at altitudes above 10,000 feet and then to restart it reliably – functions that cannot be taken for granted at temperatures in the high two-digit minus range: the decisive factor here is the reliability of the combustion process.

Knowledge gained in real time

Development work on the new generation of engines already showed promising results at an early stage: for example, the cruising altitude where 100 percent of maximum continuous output is still available can be increased from 2,500 to more than 3,900 meters. 'Thanks to this improvement alone, fuel savings of several percent could be achieved – and the new engine is more efficient in other respects too,' Dörner explains. A comparison with competitors also proves the point, he adds.

To arrive at a reliable determination of the flameout limit, the engineers depend on data collected during test flights: 'With our previous solution, we had to return to land every time so we could read the data and use it to reset the engine. Then we had to take off again for another flight,' Dörner continues. The result was that test phases often became long, drawn-out affairs, especially when changeable weather conditions made it even more difficult to take off again.

"In order not to endanger our projects and the ones of our customers, we can't afford any delays. Since this is a familiar problem in the automotive industry, my colleagues recommended me to contact Kistler. That's how I became aware of the KiBox."

Dr. David Dörner, Application Engineer Test and Application at Continental Aerospace Technologies GmbH

KiBox breaks new ground

The inquiry from Continental Aerospace Technologies meant that the engineers at Kistler really had to break new ground: a KiBox had never before operated at such altitudes, with ambient pressure of only 375 mbar. The small aircraft were not equipped with pressurized cabins, so the pressure would act directly on the instrument. 'Until then, we'd only used the KiBox in the automotive sector. As the basis for our altitude specification, we took journeys in mountainous areas where the pressure more or less corresponds to 750 mbar,' according to Jörg Ruwe, a Sales Engineer at Kistler. So they could collect empirical values, the Swiss sensor experts initially loaned a KiBox to the customer. 'We're used to sounding out the limits from our experience with motor racing. And here too, we saw an opportunity to gain new knowledge,' Ruwe continues.

The first experiments were completed in a cooling chamber, and then came the acid test: the engineers took a winter trip to Sweden – where temperatures were still below zero – to carry out the test flights. 'We simulated the worst-case scenario here: temperatures at altitude of below –40 degrees Celsius, poor fuel with low cetane ratings, and altitudes of over 7,000 meters. 85,000 meters was the total altitude covered by the team on two days of testing,' Dörner continues. The KiBox completed the experiments with no complications and proved its merits across the board. The crew (one test pilot and an engineer) were able to read the data in real time, and they performed various settings on the engine software while they were still airborne. Thanks to a 28-volt connection, the KiBox could operate easily from the onboard power supply with no need for an additional unit. By eliminating take-offs and landings, the engineers saved many hours of flying time as well as the landing charges and parking fees that would otherwise be incurred.

With its compact dimensions and easy handling, the KiBox also made work much more efficient back on the ground. The developers only needed a few minutes to transport the instrument between its measuring points in the aircraft and on the stationary test stand, and they could start operating it immediately at both locations. This gave them added flexibility with planning their tests, because they could decide where to operate the KiBox according to the weather at the time.

Boost for new developments

As well as appreciating the practical advantages of the KiBox, the aircraft engineers were convinced by its technical equipment: with eight measuring channels, it has twice as many inputs as the solution they were using previously. This pays off for the aircraft engine manufacturers in two ways. First, they obtain extra data that could not be captured before, or could not be recorded at the same time as other measurands: 'As well as the pressures in the four cylinders, we're also able to measure current signals and injection pressures now. We can apply this knowledge to other models, and that saves us several test cycles,' Dörner points out. And second, the KiBox is a genuine all-round solution for Continental Aerospace Technologies: it is suitable for the company's entire portfolio of engines, including the six-cylinder models.

The engineers at Continental Aerospace Technologies were utterly convinced by Kistler's performance – and after just a few tests, the company purchased the KiBox that was initially on loan to them. 'Our working relationship with Kistler was smooth. Even though the conditions were unknown, they took on the risk involved in the tests and they provided excellent support for us throughout this phase,' Dörner sums up. This places Continental Aerospace Technologies in an ideal position not only to introduce its pioneering engines into new aircraft, but also to supply engine replacement kits to plane owners all over the world who will thus benefit from increased performance and better flight characteristics.