Disclosed are cathepsin S, K, F, L and B reversible inhibitory compounds of the formulas and where R2, R3, R4, R6, R8 and Y are defined herein. The system comprises a radiation source that emits radiation which is passed through the exhaust plume of a motor vehicle to one or more detectors arranged to receive the radiation. This diaphragm has a radially inner circumferential portion that seals with the bulkhead, a radially outer circumferential portion that seals with the exhaust hood, and a radially median resilient portion.