Advances in Determination of Piston Group Friction Losses at High Speeds and Loads using the AVL FRISC Single-Cylinder Engine (Zuverlässige Messung der Kolbengruppenreibung bei hohen Drehzahlen und Lasten mit dem AVL-FRISC-Einzylindermotor)

Zusammenfassung

Future engine development places crucial demand on understanding the details of components in order to provide maximum power output and maximum efficiency in terms of fuel consumption and CO2 emissions.

Following these requirements, AVL provides detailed piston group simulation capabilities in terms of piston and ring dynamics, strength, thermal loading, blow-by, lube oil consumption and friction losses. The highly complex system of the piston and rings is very sensitive to the equilibrium between the inertia-, gas- and friction forces incorporating the thermal expansion of each component. In addition, nearly the complete Stribeck-curve is reflected in the lubricating conditions during one engine cycle. Therefore continuous validation work is mandatory to ensure the model quality and accuracy. Using the AVL FRISC single-cylinder engine enables comprehensive validation through direct comparison of measured and simulated values and characteristics. This paper provides an overview of the successive measurement to simulation comparison and method development for the piston group simulation.

To identify the friction contribution of the piston group AVL uses a single-cylinder engine based on a floating liner concept. This engine has a newly developed sealing on top of the inner liner cylinder, which avoids the influence of pressure from the chamber. The measurement of the pure friction forces coming from the contact of the piston ring pack and the piston skirt with the liner is done under motored as well as fired conditions. Using this technology indicated friction force measurements are obtained with high absolute quality and good reproducibility. As a result AVL is capable to optimize a part matrix consisting of liner, piston and rings with different materials, surfaces, honing’s, coatings and part geometries with respect to the reduction of friction losses.

In this paper, various indicated friction force signals are presented which are resolved by stroke thus enabling further investigation and optimization of the piston – ring – liner system in more detail. Integration of the area beneath the friction – force curve yields the friction power due to the piston group. Doing this for different load and speed points enables to calculate the friction power for a specific driving cycle and show the differences caused by different piston group assemblies. Based on the maximum difference the impact on fuel consumption and CO2 emissions will be presented with an example.

In the past, our signal processing was capped at speeds of 2500 rpm for Diesel engines and 3000 rpm for Otto engines. Loads were restricted to around 100 bar. This limitation was due to an assumed resonance issue with amplitudes that falsify the signals in such a manner that make them unusable.

Nowadays operating our AVL FRISC up to 4500 rpm and at loads above 160 bar is possible. Results from accompanying vibration measurements are given here as Campbell diagrams to provide further insight. The data presented here show amplitudes over the frequency spectrum and vibration order lines (integer) as usual. But main reason for analyzing these diagrams is to detect resonance bands and evaluate their influence on the measurement signals.