Noise Control of Internal Combustion Engines

A: Adding packing shifts attenuation upward in frequency and risks fiber blowout at elevated temperature without touching the discrete order. Increasing backpressure raises pumping losses and can amplify structure-borne excitation into the body. Retuning the effective length moves the notch back onto the firing order that drifted with gas temperature and addresses the root cause. Increasing averaging time hides nothing in a regulated metric and burns test time.

A: Using ambient speed underestimates wavelength at hot conditions and mistunes the notch upward. A half-wave assumption doubles length and lands the attenuation between orders. Double-counting end correction bloats length and shifts attenuation too low. Temperature-corrected speed with one end correction puts the quarter-wave notch on the intended order.

A: Assuming near-zero penalty ignores perforate and turning losses that scale with mass flow. Treating dB as a direct proxy for pressure drop confuses acoustic power with steady flow resistance. Estimating added volume and perforate losses lands in the mid-single-digit kPa range at rated power. Perfect tuning doesn't cancel viscous losses or mean flow effects.

A: Zinc coating volatilizes and glass fiber erodes under high temperature and oil fouling. Polyurethane degrades thermally and becomes an odor source long before durability targets. Ferritic stainless with basalt wool tolerates temperature and resists oil fouling while maintaining mid-band absorption. Glued mineral wool delaminates and shifts attenuation upward after thermal cycling.