Limitations of rigid body descriptions for heavy-duty diesel engine vibration

Internal combustion engine vibration modeling commonly relies on assuming t he engine is a linearly reacting rigid body, thereby ignoring rotating, rec iprocating, and nonsolid engine components. Limitations of this approach ar e identified from a series of experiments on a heavy-duty in-line six-cylin der Diesel engine typical of Class VIII trucks. Measurement of all three or thogonal vibration force components were mane at each of three engine mount s during standard impact-excitation modal identification tests on the quies cent engine and during engine operation. The running-engine vibration force s, measured throughout the test engine load and speed operating envelope, w ere projected onto the quiescent-engine rigid body modes to determine the m odal content and residual vibration as a function of frequency. Modal decom position results for the running engine show that the quiescent-engine rigi d body modes, with modal frequencies between 5.6 and 26.3 Hz, account for 8 0 percent or more of the measured engine vibration forces for all engine sp eeds and loads in a bandwidth from zero to 200 Hz. The likely origins of th e residual vibration within this bandwidth are discussed.