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Figure 1. MonoLite laser-welded steel piston with low compression height.

Steel piston and coating system reduce cost of ownership for CV engines

Requirements for heavy-duty diesel engines are driving a system approach for Mahle’s piston and coating development. The MonoLite piston combined with a new carbon-based piston pin coating contributes significantly to fuel consumption reduction by minimizing the friction losses and reducing the oscillating masses. This new heavy-duty power cell unit (PCU) system helps to achieve the future emissions requirements and to produce a competitive total cost of ownership (TCO).

The MonoLite piston, shown in Figure 1, is made with a laser welding process and offers high durability for peak cylinder pressures (PCPs) beyond 23 MPa while providing a compression height down to approximately 45%. Motivations for the piston development were a high degree of freedom for the design of the general compression height regarding load requirements as well as the need for cooling gallery shapes and cross sections that provide the best benefit in terms of bowl rim temperatures. A reduced cross section has positive effects on the cooling performance by increasing catching efficiency and reducing the dwell time of oil in the gallery. In addition, shapes such as a kidney-like design can be manufactured with the laser welding process to improve the overall cooling situation.

The laser weld seams experience minimal displaced material during the joining process and therefore offer smooth gallery surfaces over the entire circumference. The aspect of a potential blockage of either the cooling gallery cross section or the oil inlet can be completely avoided with this technology. Comparison of a laser-welded piston to a friction-welded design for the same application and PCP requirement shows a weight reduction of the former by approximately 1.1 kg for a typical 130-mm diameter class piston. A study of higher PCPs and the expected increase in compression height for durability purposes showed that even for up to 30 MPa, the laser-welded piston would still offer a slight weight benefit of 80 g compared to the 24-MPa friction-welded solution.

Another key technology advancement of the piston is the ta-C diamond-like carbon (DLC) coating. The thermal resistance of ta-C was compared to that of a standard a-C:H layer. After five hours in a furnace at 450°C, the a-C:H layer completely disappeared, and the ta-C coating was preserved. A second key feature of the new carbon coating was a low level of friction coefficient, at least as good as the common a-C:H. Test results showed that the friction coefficient for the ta-C coating measured 60% lower than that for the hydrogenated a-C:H carbon coating (the initial friction coefficient at very low speeds was 35% better for the ta-C).

In a system approach, the advantages of the two components can be combined to enable two different paths of the TCO. The weight reduction potential can be transformed into a significant improvement of the drivetrain. A study with a typical 16 HD engine block showed a weight savings of approximately 15 kg just by a reduced block and liner height of 20 mm (see Figure 2). Additionally, by optimizing the entire PCU including the pin, the oscillating masses can be reduced significantly. The weight benefit of an optimized PCU, which includes the piston pin diameter reduction potential enabled by the coating, is another 2.3 kg. This has a direct impact on further components of the drivetrain: up to approximately 25% of the crank mechanism weight can be reduced.

The overall reduced engine size and weight also enables a better arrangement of the peripherals and improves the package situation in the engine compartment. This path creates either an improvement in the curb weight or makes the application of additional systems such as waste heat recovery possible without negatively impacting overall vehicle weight.

Conversely, a reduced compression height of the piston can be used to increase the length of the connecting rod and therefore reduce the piston side forces in the crank mechanism. Investigations on a passenger car engine have shown up to 3-5% of fuel consumption reduction by applying a reduced-compression-height steel piston and a longer connecting rod. This benefit can be extended even further by applying the newly developed piston pin coating. The combination of these two component technologies represents the latest advancements for friction reduction and consequently fuel consumption and TCO reduction.

This article is based on SAE technical paper 2016-01-8066 authored by Marco Maurizi and Daniel Hrdina of Mahle GmbH. The paper will be presented at the SAE 2016 Commercial Vehicle Engineering Congress.

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