Based on NVH tests conducted on a heavy-duty turbocharged DI diesel engine, noise relevant differences between steady-state and transient operating condition were investigated. A vehicle drive-by test simulating the effects of vehicle mass and inertia was performed, followed by transient NVH measurements in a semi-anechoic test cell.
Steady-state noise was exceeded by 5 dBA during transient operation due to broadband increase of noise excitation combined with structure resonance amplification. Transient noise results mainly from “harsher” combustion as a consequence of enlarged ignition delay indicated by significant increase in maximum cylinder pressure gradient. Variation of geartrain excitation and combustion excitation revealed that geartrain noise is of minor importance in this context.
Transient noise improvement must focus on reducing ignition delay, for instance by increasing combustion chamber temperature, optimization of injection rate (controlling the amount of premixed fuel at start of combustion) and advancing begin of injection in compliance with adequate NVH and excellent drivability.
Transient tests should be implemented in the diesel engine NVH development process, since steady-state parameter variations do not reflect the individual transient NVH behavior of the engine.