Low Exhaust Back Pressure Challenge – Rasp/Weak Shock Formation
While the trend is to reduce engine displacement for fuel economy, the need for power remains.
Subsequently the power density of newer advanced engines is on the rise. Additionally, aggressive fuel economy improvements are resulting in significant reduction of exhaust back pressure. One of the interesting challenges that this presents is the increased risk of exhaust rasp. Rasp is a sharp, impulsive & potentially metallic sound emanating from the tail pipe or exhaust structure or both. In the absence of mechanical excitations, the sharp & impulsive orifice noise, which has been described as similar to the 'blatty' sound of a trombone played double forte, is due to the presence of higher order harmonics. These harmonics are caused by the onset of weak shock waves in the exhaust pipe(s) while the exhaust pulses travel from the cylinder head to the tailpipe. The shock wave forms due to wave steepening phenomena. Wave steepening occurs due to nonlinear terms in the governing equations that cause the wave propagation speed to vary with pressure amplitude. The propagation velocity differences mean that a pressure peak will travel faster than the preceding pressure trough up until the wave steepens into a shock. If the wave steepens into a shock before encountering a large expansion (muffler or tailpipe exit), rasp is likely.
