A new sensor that combines the signals from two probes may enable mechanical engineers to better characterize the distribution of motor oil in a running engine. Understanding the behavior of oil films could lead to engine designs with lower oil consumption. Motor oil reduces friction between the piston rings and the cylinder liners in an engine, but it can find its way into the exhaust system, contributing to particulate pollution. Evaluating the oil distribution in a running engine is a challenge, however, because the pressure can reach 150 atmos and the temperature can soar to twice that of boiling water. Moreover, the clearance between a piston ring and the cylinder liner is approximately 5 µm. Jaal B. Ghandhi, an associate professor in the Engine Research Center at the University of Wisconsin, has developed a fiber-based sensor to measure oil films, which he described in the Oct. 20, 2000, issue of Applied Optics. The approach incorporates both single- and dual-fiber probes and takes advantage of the natural fluorescent response of motor oil. Get your motor running In the single-fiber probe, the excitation light and the fluorescent response travel along the same fiber; the dual-fiber probe uses separate delivery and collection fibers. Because the technique utilizes oil's own fluorescence, it does not require the addition of dopants that could affect the behavior of the films. Ghandhi sent 442-nm light from a 30-mW HeCd laser down the fibers into one cylinder of a six-cylinder diesel engine running at 1000 rpm and then monitored the fluorescence. The single-fiber probe excited the oil on the wall of the cylinder and measured its response, while the dual-fiber probe tracked the motor oil on the more-distant piston. Comparing the two signals allowed him to compute the oil distribution.