Reciprocating Engine Exhaust Dynamics

The design of reactive silencers for reciprocating internal combustion engines is one of the most daunting tasks in the noise control field; especially, for low rpm units typically used for driving constant speed equipment including generators. Manufacturers typically provide basic engine operating data but what is missing is information about engine orders that may be controlling the exhaust noise. This missing information is especially important in order to assure adequate transmission or insertion loss at those frequencies. Most novice designers base silencer performance on the engine's firing frequency and harmonics but an engine order, unrelated to the firing frequency, can produce significant exhaust tones that are generally not discovered until after the installation and the engine is operating. To further the challenge, the engine exhaust sound power level is affected by the exhaust system design (manifold, turbocharger, cross connections) and especially the measurement location. Another handicap is most engine data are reported in full octave bands that obscure the controlling exhaust tone or tones; an octave band can have two or more engine orders. Engine dynamics, reactive silencer design and field measurements are reviewed to provide insight to reactive silencer design to hopefully avoid disappointing performances.