Why use steel pistons in diesel engines?


aluminum Piston (left) against steel piston (right) diesel engine Mercedes-Benz

In the sector of commercial vehicles the use of steel pistons is not new, but it is relatively in diesel passenger cars. Traditionally -from the years 20- have been using aluminum pistons, a material of lower density than steel, and that is sometimes used to lighten weight as compared with the steel.

But the techniques in the metal does not stop to move forward, and the steel still has a lot to say, especially in the heart of the engines. MAHLE presented at the iaa in Frankfurt 2011-new steel pistons for diesel engines for passenger cars, with specification 42CrMo4, more rigid and resistant than aluminum.

Renault introduced diesel engines 1.5 and 1.6 as of 2014

The properties of the aluminum are very interesting. In the diesel engine the pistons suffer high mechanical stress and temperature, and assumes a high useful life. By using the pistons of steel, the reliability increases, since it is a material that is better able to resist the long-term fatigue.

in Addition, the steel expands less, so that may bring more shirts to the piston.


steel Pistons MAHLE

In principle this is good, but has a side effect. As the steel resists heat better, the oil becomes the weak point of the system. At temperatures to which the steel endures, some oils do not, so it is necessary to choose very well the type of oil to not lose its properties and lubricants.

With high-strength steel, you can make a piston more chato, so that you can get two benefits, one or the other:

  • It lowers the height of the pavilion: those millimetres that are earned to the piston can result in engines of a lower height, so that they can lower the hoods, or place the engines down, improving the center of gravity of the car.
  • increases the length of the connecting rod: in this way the lateral forces are reduced, so that there is less need to balance the engine so that it is comfortable, ergo saves weight and friction mechanical internal components.

on the other hand, the union piston-connecting rod may be smaller.


By lowering the height of the piston connecting rod can be longer, reducing the lateral forces and hence the vibrations

they are Also lighter that aluminum in spite of its greater density per unit of volume. Less weight means less loading for the crankshaft and the rest of the engine. On the other hand, to reduce the height of the piston also reduces the friction between the piston and the liners, which saves energy. Little by little, we will achieve a savings on several fronts.

The combustion is a little faster with steel because of its lower thermal conductivity

according to the suppliers MAHLE and Kolbenschmidt can be achieved fuel savings between 3 and 5% with respect to the aluminum pistons, which in a diesel engine power, with an approval of close to 4.5 l/100 km, assumes a reduction of up to 0.25 l/100 km May seem like very little, but over the life of the engine is much fuel.

For example, Mercedes-Benz used steel pistons in the E 350 BlueTEC in 2014, making 258 HP. The engine of the previous generation, which gave the same power, it consumed 3% more with aluminum pistons. Also reduce a little the emissions, since the expenditure of oil is smaller and burns less in the camera combustión.


performance Comparison between aluminum pistons and steel, approximately 3% less consumption with the past

Several manufacturers are behind this technology is to incorporate it into their diesel engines. For example, the supplier Kolbenschmidt occur from 2019 aluminum pistons for a manufacturer Premium (has not wanted to mencional) that have diesel engines six cylinders, in a plant within the Czech Republic. Could be Audi, Jaguar, BMW…

however, consider some of the disadvantages. It is more expensive to produce these pistons due to the necessary supplies, and you also have to invest more in development to deal with the high concentration of temperature that supports the oil between the cylinder sleeves and piston, a secondary effect of the lower thermal conductivity of the steel.

The combustion engine does not have long to live, two or three decades of leadership, but the manufacturers have to make savings of any side. Despite the fact that the basic design of internal combustion engines has more than a century, small improvements can have a positive effect on performance, since the component most basic to the most sophisticated.