Research on advanced heat engine concepts, such as the low-heat-rejection
engine, have shown the potential for increased thermal efficiency,
reduced emissions, lighter weight, simpler design, and longer
life in comparison to current diesel engine designs. A major obstacle
in the development of a functional advanced heat engine is overcoming
the problems caused by the high combustion temperatures at the
piston ring/cylinder liner interface, specifically at top ring
reversal (TRR). TRR is the most critical part of the engine cycle
because the ring and liner undergo a majority of their wear at
this location. In a conventional engine, where TRR temperatures
are near 200 °C, the cylinder kit materials consist of chrome-coated
piston rings and cast-iron liners. These materials usually provide
adequate service for about 500,000 miles before a major overhaul
is needed. The TRR temperature in an advanced heat engine; however,
has been predicted to be in excess of 300 °C, with some estimates
as high as 650 °C. These high temperatures preclude the use
of chrome-coated rings and cast-iron liners because the extreme
temperature severely degrades their wear life. Therefore, advanced
cylinder liner and piston ring materials are needed that can survive
under these extreme conditions.
To address this need, researchers at the NASA Lewis Research Center
have designed a tribological test method to help evaluate candidate
piston ring and cylinder liner materials for advanced diesel engines.
The selected test method uses a commercially available, pin-on-plate,
reciprocating wear test rig with specially modified specimens
machined from conventional top compression piston rings and cast-iron
liners. Loads, speeds, and temperatures are selected to approximate
engine wear conditions present at the ring-liner interface at
TRR. It is intended that this test setup be used as a screening
tool to eliminate poor coating combinations before any effort
is expended on costly engine tests.
As a way to validate the test method, repeated baseline tests were run with conventional chrome-coated ring and cast-iron cylinder liner specimens, and the results were compared with used engine hardware. On both the used and test specimens, the worn areas had a smooth glossy finish, which indicated the presence of a fine polishing wear mode. In addition, wear factors, which quantify the amount of wear produced over a given time, were calculated for the used hardware and test specimens. As shown in the figure, the baseline wear factors for both specimens were very repeatable from test to test. When individual components are compared, the ring specimen wear factors are very similar to that of the used ring. The baseline liner wear factors, on the other hand, are an order of magnitude greater than for the used liner, which suggests that the test conditions with respect to the liner are more severe than actual engine experience. Since studies have shown that ring wear is of greater concern, the corroboration exhibited between the wear factors of the used ring and the ring specimen suggests that the test rig and established procedures can be used to conveniently screen material candidates for advanced heat engine applications.
Previous articleLast updated April 30, 1997
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