During the last decade, typical performance outputs for automotive diesel engines rose from 50 kW/liter (67 bhp/liter) to around 70 kW/liter (94 bhp/liter). With increasingly pressing legislative targets for CO₂ reduction in most major global markets, the trend is likely to accelerate, according to Rainer Jueckstock, Federal-Mogul senior vice president of Powertrain Energy.

Rising specific outputs place higher mechanical and thermal loads on many of the components where Federal-Mogul has considerable expertise. The DuraBowl piston process is an example of how we are succeeding in delivering specialized process technologies that help our customers successfully address these challenges across a range of growing market sectors.

—Rainer Jueckstock

Typical failure due to thermal stresses, showing (a) the temperature distribution in a hot piston, (b) typical location of crack origin at point I on the edge of the bowl, in line with the thrust and non-thrust faces of the piston. Click to enlarge.

The combustion in a diesel engine takes place in a hollow bowl in the top of the piston, where temperatures can reach above 400 °C (750 °F) and pressures above 200 bar. Under these difficult combustion conditions, the rim of the piston bowl has an increased failure factor. Federal-Mogul’s engineers have identified that both thermal and mechanical failures of the piston bowl can be traced to the presence of free primary silicon particles distributed throughout the aluminium matrix.

Aluminum expands eight times as much as silicon, therefore stresses are set up within the piston every time the temperature fluctuates. Furthermore, repeated mechanical loads, each time the cylinder fires, could result in fatigue failure from the corners of the silicon particles. Silicon is a necessary constituent of the aluminum alloy, offering favorable properties such as low expansion and good castability, so it cannot be eliminated. The only potential solutions to this problem, until now, have been fiber-reinforced pistons.

Fiber-reinforced pistons increase manufacturing complexity as the molten alloy has to infiltrate the fibers during casting. Furthermore, there is not yet a reliable, non-destructive way to test the integrity of the finished part whereas, with our DuraBowl process, we can do an Eddy Current test to ensure the quality.

—Frank Doernenburg, Federal-Mogul director of technology, pistons and pins

DuraBowl pistons have lasted up to seven times the life of standard pistons during validation tests carried out on actual engines. Click to enlarge.

Federal-Mogul’s solution is to pre-machine the cast piston and then re-melt the alloy around the rim of the bowl. Federal-Mogul’s unique re-melting process for the aluminum-silicon alloy, combined with a rapid cooling process (1000x faster than when originally cast, significantly changes the alloy’s microstructure by reducing the size of hardening phases such as silicon particles and intermetallics. The grain size of the re-melted part is approximately one tenth of the as-cast size.

The result is a piston bowl rim whose first few millimeters provide significantly improved aluminum strength, further enabling engine manufacturer’s efforts to downsize or turbo-boost engines for greater specific output.

The strength and efficiency of our solution is that the process is physically simple. The sophistication is in the control of key parameters, which ensure consistent quality. The result is a technologically advanced, high-performing and very cost-competitive product when compared to both fiber-reinforced and steel pistons.

—Frank Doernenburg

The technological and cost benefits have been validated during extensive engine testing, both by Federal-Mogul and its customers. The re-melting process increases piston life and performance substantially, while at the same time, serving as a contributor to improve fuel efficiency and reduce CO₂.

The first application of the DuraBowl process is on a high-performance diesel engine recently launched for a leading global vehicle manufacturer.

Resources

• Frank TH Doernenburg, Simon Reichstein, Rainer Weiss, Scott David Peter Kenningley, K. Lades (2007) High-Performance Cast Aluminum Pistons for Highly Efficient Diesel Engines (SAE 2007-01-1438)