How to produce more performance with less
The emissions targets for the future are ambitious. And so are modern diesel engines. In all likelihood, the power density of engines will therefore increase further.
For the 24 Hours of Le Mans, engines with around 109 kW/L were already in use in 2012. Today, sporty diesel engines in series production tend toward 100 kW/L. The diesel racing engines were equipped with steel pistons from MAHLE, just like today’s engines. And with good reason: steel pistons are not only stronger but also consume less fuel than aluminum pistons.
This advantage makes steel pistons viable even for engines with lower stresses. For this reason, MAHLE is now starting large-scale production of steel pistons for mid-range power density diesel engines for the first time. But what happens next? Are steel pistons really THE solution for new engines? MAHLE has compared the two materials.
The main challenges for passenger car diesel pistons
Where the piston is, things really heat up: it is subjected to some of the highest thermal and mechanical stresses occurring in the engine! The service life of the diesel engine piston is generally determined by the edge of the combustion bowl. This is placed under particularly high stress by the peak cylinder pressure and the accompanying high temperature. Depending on the material, this area can reach temperatures of up to 400°C and 500°C! The critical mechanical load at the bowl rim results from the alternating bending of the piston around the piston pin and the highest thermomechanical stress from the transition from full load to partial load or idling.
What the aluminum piston offers
Aluminum is not completely heatproof. From 300°C, its resistance declines dramatically. For this reason, the thermal conductivity of aluminum alloys is around three to four times greater than that of the steel varieties used for pistons.
In aluminum pistons, the heat arising from combustion is distributed relatively quickly and dissipated mostly through the cooling oil and the cooling water. The shape and location of the cooling channel, its fill level, and the flow rate of the oil are the decisive parameters for optimal cooling.
Thanks to new cooling channels, the temperature at the bowl rim can be reduced by around 35 K. Other measures increase strength at the bowl rim. Reinforcing the fibers of the aluminum piston makes it possible to benefit from the good thermal conductivity of the material without incurring the disadvantages relating to the mechanical load capacity.
The strengths of the steel piston
The stronger the material, the lower the overall height of the piston: in a steel piston, both the compression height and the ring spacings are smaller than in an aluminum piston.
Steel pistons therefore allow a significant reduction in the overall height of an engine; weight savings in the double-digit kilogram range are possible. This brings enormous advantages in terms of the position of center of gravity, pedestrian protection, aerodynamic resistance—and fuel consumption!
Steel pistons were initially developed to allow the peak cylinder pressure limit for highly stressed diesel engines to be raised to above 200 bar. They are extremely robust! Commercial vehicles in series production already have peak cylinder pressures of up to 240 bar. However, the power densities are usually lower than 35 kW/L. The speeds at rated power are also lower.
Tests on the friction power test bench have shown that a steel piston can provide fuel savings of three to five percent and CO2 emissions savings of three percent. If cast iron crankcases are used, an even greater frictional loss advantage can be expected than with aluminum crankcases.
If CO2 emissions are tested in accordance with the Worldwide Harmonized Light Vehicles Test Procedure (WLTP) cycle in the future, rather than using the New European Driving Cycle (NEDC), the advantages of the steel piston will have an even stronger impact.
Where the benefits of steel pistons prevail
It seems clear that steel is ideally suited as a piston material. Because of its robustness, higher power output is possible at increased peak cylinder pressures and emissions can be reduced. In addition, steel allows more compact dimensions and lower oscillating masses to be achieved. All these advantages can already be implemented for engines with low and medium power density!
The situation is a little different for higher power densities. In these cases, steel pistons cannot completely replace aluminum components and are certainly not a “plug-and-play solution.”
After all, steels also have a temperature limit, although it is considerably higher than that of aluminum. Steel forms a noticeable scale layer from 550°C, which weakens the material overall: cracks in the layer can lead to cracks in the substrate. It is also possible for the material to shatter. Bowl rim undercuts used to optimize combustion in passenger car diesel engines allow a much more significant rise in temperatures than with aluminum!
In addition, the lower thermal conductivity of the material may lead to problems with piston cooling. In contrast to aluminum, the heat is “trapped” in the piston crown. This can result in high surface temperatures in the cooling channel. If these temperatures exceed a threshold of around 350°C, the cooling oil can crack. Insulating oil carbon is produced and the cooling oil ages substantially!
Changes to the bowl shape, e.g., a smaller undercut or a stepped shape, may have a positive impact on cooling. Two cooling oil nozzles allow the heat to dissipate more evenly. An optimized material can result in greater strength at the bowl rim. Completely new solutions that solve the problem of oil cracking would need to be found for high power densities.
It is certain that we will see an increase in power density in diesel engines. The steel piston is a good solution for these high power densities and will therefore continue to establish itself.
The most important points at a glance
- The average power density of diesel engines will increase in connection with upcoming emissions targets.
- Future aluminum pistons, with measures to increase their strength, will be able to withstand peak cylinder pressures of over 200 bar and power densities of about 100 kW/L.
- Steel pistons have the potential for fuel savings of three to five percent in comparison with aluminum pistons.
- For steel pistons, the goal is therefore to find solutions for high power densities that solve the problem of oil cracking.