A Wear Model for Diesel Engine Exhaust Valves

Abstract

Executive SummaryThe work summarized here comprises the concluding effort of a multi-year project, funded by the U.S. Department of Energy, Office of Vehicle Technologies. It supports the development of a better understanding of advanced diesel engine designs in which enhanced power density, energy efficiency, and emissions control place increasing demands upon the durability of engine materials. Many kinds of metallic alloys are used in engines depending on the operating stresses, temperatures, and chemical environments. Exhaust valves, for example, are subjected to high temperatures and repetitive surface contacts that place demands on durability and frictional characteristics of the materials. Valves must continue to seal the combustion chamber properly for thousands of hours of cyclic engine operation and under varying operating conditions. It was the focus of this effort to understand the wear processes in the valve-seat area and to develop a model for the surface deformation and wear of that important interface. An annotated bibliography is provided to illustrate efforts to understand valve wear and to investigate the factors of engine operation that affect its severity and physical manifestation.The project for which this modeling effort was the final task, involved construction of a hightemperature repetitive impact test system as well as basic tribology studies of the combined processes of mechanical wear plus oxidation at elevated temperatures. Several publications resulted from this work, and are cited in this report. The materials selected for the experimental work were highperformance alloys based on nickel and cobalt. In some cases, engine-tested exhaust valves were made available for wear analysis and to ensure that the modes of surface damage produced in experiments were simulative of service. New, production-grade exhaust valves were also used to prepare test specimens for experimental work along with the other alloy samples.Wear analysis of valves and seats run for hundreds of hours in heavy-duty diesels provided insights into the kinds of complexity that the contact conditions in engines can produce, and suggested the physical basis for the current approach to modeling. The model presented here involves four terms, two representing the valve response and two for its mating seat material. The model's structure assumes that wear that takes place under a complex combination of plastic deformation, tangential shear, and oxidation. Tribolayers form, are removed, and may reform. Layer formation affects the friction forces in the interface, and in turn, the energy available to do work on the materials to cause wear. To provide friction data for the model at various temperatures, sliding contact experiments were conducted from 22 to 850 o C in a pin-on-disk apparatus at ORNL. In order to account for the behavior of different materials and engine designs, parameters in all four terms of the model can be adjusted to account for wear-in and incubation periods before the dominant wear processes evolve to ...

Design of a life prediction tool for high speed diesel engines

“…and Figures 35 to38 it is observed that valve head radius, valve seat width, engine cycles, valve closing velocity, impact wear constant and specific heat constants (Cp, Cv), influence wear outcome more than 20%, for a given change in input of 25%. Important result is that friction coefficient changes the valve recession, only by 1% for a given change of 25%.…”

“…Figure 19:formation of tribo layers (a) [40], uneven wear profile in the valves (b,c) [38] Recession is one of the most prominent wear phenomena that occur in engine valves [52]. If other operational conditions are met, recession is the single greatest reason for wear, as [41], ridges due to impact(c) [39] Loading of the valve seat interface (VSI) at the instant of closing, as shown in Figure 21a, generates time-variable stresses at high frequencies and relative micro-sliding between valve and valve seat insert, due to the elastic deformation of the valve head caused by combustion pressure.…”

“…As discussed, physical mechanisms that govern wear are extremely complex and it is difficult to predict wear in mechanical components using analytical methods. Rather than attempt to incorporate all of the foregoing surface damage mechanisms based on the wear mechanisms observed, the general valve wear model in literature [39] consists of two contributions, one for displacement and one for abrasion [38]. Both of these can contribute to the wear profile of the valve and seat combination.…”

“…Blau et.al. [38] also considered abrasion as a contribution to valve sealing surface wear. They considered slip per cycle not as constant, because as the seating surface wears the clearance increases and therefore slip amplitude will also increase.…”

“…The differences & similarities between the Lewis and Blau models can be summarised as follows [38]:  Both models contain empirical constants as well as physically-measurable quantities, so neither can fairly be called a 'first-principles' model.…”