The trend toward downsized, higher-specific-output engines places huge demands on suppliers to produce components that can endure greater mechanical and thermal loads. Take, for example, BMW's new 3.0-L triple-turbocharged diesel. Its two small variable geometry (VG) high-pressure turbos, plus one larger low-pressure unit enable a specific power output of 93 kW/L—making it, as the automaker claims, the world’s most powerful in-line six-cylinder diesel car engine.
Combustion pressure in this new diesel reaches a maximum 200 bar (2900 psi) and its piezo injectors operate at 2200 bar (31,908 psi). For each of the engine’s power strokes, three pre-injections, one main injection, and four post-injections of fuel occur. Such high output and extreme operating demands required a new type of series-production piston and new piston-testing process.
The BMW development team selected Federal-Mogul’s aluminum raised gallery piston design developed specifically for such applications. The raised gallery places cooling oil as near to the piston bowl as feasible to facilitate optimum cooling and required durability while reducing friction.
Modern diesel pistons have a cooling gallery through which oil flows continuously. The position and design of the gallery have a significant impact on the component’s operating temperature and durability. The closer the gallery is to the piston bowl, the more heat can be removed, allowing engine manufacturers to increase combustion temperatures and pressures to improve fuel economy and CO2 emissions.
The BMW piston's reinforced combustion bowl rim was created using Federal-Mogul's Durabowl process which re-melts the alloy around the piston’s combustion bowl, refining the aluminum’s microstructure and raising the material’s fatigue strength.
New 2D ultrasonic inspection solves the problem
The raised gallery configuration was supported by Federal-Mogul’s new 2D ultrasonic inspection system developed at its Nuremberg, Germany, Technical Center. It called for a piston testing technique that would help the company achieve cooler running, higher performance diesel pistons capable of contributing to smaller capacity power units that could meet all required criteria.
According to Dr. Frank T.H. Doernenburg, Director of Technology, Pistons and Pins, historically it was a major challenge to cast gallery pistons with precise control of the size and location of the cooling gallery. He explained that the new 2D ultrasonic analysis process has solved the problem.
“Our process is nondestructive, completely controls casting process quality and aids advanced casting process development, giving engine designers substantially more freedom to increase engine efficiency,” Dr. Doernenburg told AEI.
He said regular 1D ultrasonic testing can identify defects but cannot quantify their size and position. But the 2D process generates some 125,000 data points in 30 seconds, enabling accurate understanding of the size and position of any defects, providing data for casting process development.
“The detailed information provided also ensures consistent quality in the finished high-precision components,” he explained.
Validation of the new 2D ultrasonic technology was vital, so the company dissected and sampled several hundred pistons, correlating the ultrasonic images against destructive testing methods as it developed required software tools. It also developed physical parameters including wave length, probe geometry, and beam geometry and focus.
Creating superior aluminum pistons
The first application of the technique has resulted in what Federal-Mogul engineers claim to be "greatly improved” cooling capability of the raised-gallery piston. They said this facilitates introduction of a series of measures to significantly reduce CO2 when compared to a piston without a raised gallery.
It also provides the ability to generate more engine power output. As the first production application of the raised gallery piston, the BMW tri-turbo diesel achieves a 25% increase in specific power. Piston temperatures are markedly lower than what Dr. Doernenburg described as the “acceptable limit” of 400°C. “In the same conditions, a standard piston’s bowl rim stresses are 43% higher and its temperature reaches 440°C, close to the metal’s melting point,” he noted.
Gian Maria Olivetti, Federal-Mogul’s Vice President, Technology and Innovation, Powertrain Energy, added: “With the development of new, highly loaded engines, the risk of piston failure has increased substantially as past improvements in materials, design, and cooling concepts have reached their physical limits.
“Our aluminum piston can operate efficiently under higher thermal and mechanical loads than was previously possible without the risk of engine oil cracking and carbon deposit formation associated with a steel piston," he continued. "These advances mean that aluminum pistons can retain their leading position in diesel engines for light vehicles for some time to come.”