Field Testing to Validate Models Used in Explaining a Piston Problem in a Large Diesel Engine

Two crankcase explosions occurred within one month in diesel engines that drive large emergency generator sets at a nuclear power plant in Eastern Pennsylvania. As a result, the electric utility conducted an extensive investigation to determine the root cause(s) of the problem. Initial inspections confirmed that the crankcase explosions were the result of pistons and liners becoming overheated. The technical challenge was to establish why the pistons and liners were overheating when other engines of the same type did not appear to have the problem in the same duty. Analytical models of piston motion, engine start, and run thermodynamics, and a finite element analysis of piston distortion during engine start and load transients were developed. Preliminary work with these models predicted a feature of the piston design that could adversely affect lubrication conditions during a rapid start and load transient. Final input data to refine the models were needed and these were obtained from tests carried out on a similar diesel generator operated by a municipality in Iowa. This paper describes the successful accomplishment of the field tests using state-of-the-art instrumentation and recording equipment. It also shows how the modeling and test work identified wear at certain locations on the piston skirt as the origin of distress leading to the crankcase explosions. Unfavorable engine starting and loading conditions as well as less than desirable piston skirt-to-liner lubrication conditions in the engines at the nuclear power plant have been identified as the root causes and corrective action has been initiated.