Modern diesel engines need to deliver more power for their mass and do so with reduced fuel consumption. Which pistons best meet this challenge? MAHLE has closely examined both aluminum and steel materials, arriving at a nuanced conclusion.
The future's emission goals are demanding, and so are modern diesel engines. Consequently, engine power density is likely to continue increasing.
At the 24 Hours of Le Mans race in 2012, engines already reached approximately 109 kW/L. Today, high-performance diesel engines in regular production are nearing 100 kW/L. Diesel race engines, just like today's standard engines, were equipped with MAHLE's steel pistons.
There's a good reason for this: steel pistons are not only stronger but also use less fuel compared to aluminum pistons. This advantage makes steel pistons suitable even for engines under lower stress. Therefore, MAHLE is now launching mass production of steel pistons for mid-range power density diesel engines. But what's next?
Are steel pistons fundamentally THE solution for new engines? MAHLE has compared the two materials.
Peak cylinder pressure and specific output of diesel engines will continue to increase. This will lead to greater demands on the pistons.
Where the piston resides, conditions are extreme: it's one of the most thermally and mechanically stressed components in the engine! The piston's lifespan, especially the edge of the combustion bowl, is heavily influenced by peak cylinder pressure and the associated high temperatures. Depending on the material, temperatures can reach up to 400°C to 500°C! The significant mechanical strain at the bowl rim is caused by the piston's alternating bending around the piston pin and the highest thermomechanical stress during the transition from full to partial load or idling.
Aluminum has limited heat tolerance. Its strength drastically reduces beyond 300°C. However, the thermal conductivity of aluminum alloys is about three to four times higher than the steel types used for pistons.
In aluminum pistons, heat generated from combustion is quickly dispersed and primarily dissipated through cooling oil and water.
The shape and location of the cooling channel, its fill level, and the oil flow rate are crucial for optimal cooling.
| Material Properties at Room Temperature | Aluminium | Steel | Impact on Pistons |
|---|---|---|---|
| Density [g/cm3] | 2,7 | 7,8 | Weight of the piston |
| Bending Fatigue Strength [MPa] | 110 | > 370 | Compression height, peak cylinder pressures much greater than 20 MPa |
| Thermal conductivity [W/mK] | 130 | < 45 | Component temperature |
| Thermal Expansion [10^-6/K] | 21 | 12 | Friction performance |
Material Properties of Aluminum and Steel
With new cooling channels, the temperature at the edge of the combustion bowl can be reduced by about 35 K. Other measures can increase the strength at this edge.
By reinforcing the fibers of the aluminum piston, it's possible to benefit from the material's good thermal conductivity while avoiding the disadvantages in mechanical load capacity.
The stronger the material, the smaller the overall height of the piston: in steel pistons, both the compression height and the ring spacings are less than those in aluminum pistons.
Steel pistons allow for significant reductions in engine height; weight savings of double-digit kilograms are possible. This results in considerable advantages regarding the center of gravity, pedestrian protection, aerodynamic resistance, and fuel consumption!
Steel pistons were initially developed to increase the peak cylinder pressure limit in highly stressed diesel engines to over 200 bar. They are extremely robust!
Commercial vehicles in production today already have peak cylinder pressures of up to 240 bar, though their power densities are usually below 35 kW/L, with lower speeds at rated power.
Friction power test bench trials have shown that a steel piston can provide fuel savings of three to five percent and a three percent reduction in CO2 emissions. With cast iron crankcases, an even greater reduction in frictional losses is expected than with aluminum crankcases.
If CO2 emissions are tested using the Worldwide Harmonized Light Vehicles Test Procedure (WLTP) in the future, rather than the current New European Driving Cycle (NEDC), the benefits of the steel piston will be even more pronounced.
It's evident that steel is ideally suited as piston material. Its robustness allows for higher performance at increased peak cylinder pressures while reducing emissions. Additionally, steel enables more compact dimensions and lower oscillating masses.
These benefits are already achievable in engines with low to medium power density! The situation is somewhat different at higher power densities, where steel pistons can't completely replace aluminum components and are certainly not a "plug-and-play solution."
Steel, too, has a temperature limit. Even though this limit is considerably higher than that of aluminum, at temperatures above 550 °C, steel forms a noticeable scale layer that weakens the overall material: cracks in this layer can lead to cracks in the substrate. Moreover, the material can become brittle.
In passenger car diesel engines, bowl rim undercuts – necessary for optimal combustion – cause temperatures to rise significantly more than in aluminum! Additionally, the lower thermal conductivity of steel can lead to cooling challenges for the piston. Unlike aluminum, heat accumulates at the piston head, resulting in high surface temperatures in the cooling channel.
If these temperatures exceed a threshold of around 350 °C, the cooling oil can crack, creating insulating oil coke and significantly aging the oil! Altering the shape of the bowl, such as reducing the undercut or using a stepped design, can positively impact cooling. Using two cooling oil nozzles allows for more even heat dissipation.
Increased strength at the bowl rim can be achieved with an optimized material. For high power densities, entirely new solutions that address the problem of oil cracking need to be developed.
The power density in diesel engines will definitely continue to increase. The steel piston is the ideal solution for these high power densities and will therefore continue to establish itself.