Analysis of Aluminum and Steel Pistons: Comparison of Friction, Piston Temperature, and Combustion

Abstract

While steel pistons have been in use for a long time in commercial vehicle diesel engines, the first series production applications for passenger car diesel engines are currently imminent. The main reason for the use of steel pistons in high speed diesel engines is not, as maybe initially hypothesized, the increasing requirements on the component strength due to increasing mechanical loads, but rather challenges based on the actual CO2-legislation. The increasing requirements to reduce the fuel consumption necessitate new innovative technologies. The imminent penalties for exceeding the prescribed CO2 emissions seem to make the steel piston a viable alternative today, despite its higher manufacturing costs. So far, the CO2-benefits using steel pistons were mainly ascribed to the reduced friction between piston and cylinder liner due to no thermal interference. Fuel consumption measurements at vehicle manufacturer and research institutes hypothesize also an influence of the steel piston on the thermodynamic efficiency. MAHLE uses engine tests to investigate one piston variant made of aluminum (series production piston with cooled ring carrier) and one of steel (MAHLE TopWeld) in a detailed system comparison. Using a fully indicated engine, a combustion process analysis is performed and used as the basis for a loss analysis. The engine set-up parameters can be adjusted fully variable using a flexible ECU. The effect that the piston variant has on the combustion process is captured and balanced, e.g., by adjusting the parameters to obtain identical emissions. The analysis records the potential of the variants for each engine operating map area. The thermal conditions for the piston and the piston wall temperature on the combustion chamber side are varied over a wide range using a conditioning device for piston cooling. The influence of this intervention on the thermal load of the piston and the combustion and also the influence of different combustion mappings is measured directly by telemetric piston temperature measurement. MAHLE recently completed a system comparison [3] between aluminum and steel pistons with detailed measurements on a fully indicated engine, covering friction and temperature behavior as well as influences on combustion.

Friction Reduction Tested for a Downsized Diesel Engine with Low-Viscosity Lubricants Including a Novel Polyalkylene Glycol

“…The engine is used in a wide range of passenger cars and light commercial vehicles and is mechanically efficient. Amongst others, it employs steel pistons that reduce the friction power losses of the piston assembly due to their reduced thermal expansion [27,28].…”