The engine also adopts Natural Sound Smoother and Natural Sound Frequency Control, proprietary technologies that reduce
diesel knock sound for a quieter and more pleasing sound.
Designed to reduce
diesel knock noise during starting and low-speed acceleration, NSS suppresses resonance by using a dynamic damper inside the piston pin to suppress the three critical frequency bands in which engine components typically vibrate most loudly.
This noise is often referred to as
diesel knock [1] or combustion roughness [2].
If you have one major event, there is a lag time and then the big combustion event, that's how you get the
diesel knock.
Good sound insulation has obviously been a consideration and although there is a bit of
diesel knock under severe acceleration it is otherwise a quiet runner with a minimum of road and wind noise coming into play.
Natural Sound Frequency Control, another new technology featured on the updated Axela, reduces
diesel knock in the SKYACTIV-D clean diesel engine for a more pleasant engine sound.
Power flows seamlessly to the wheels and even on tick-over there is no
diesel knock.
I honestly couldn't detect the slightest
diesel knock.
Despite being a diesel there is hardly any
diesel knock, and wind and road noise are well suppressed.
Noise levels are impressively low and even at start-up there's barely a sign of the once-familiar, low-speed
diesel knock.
You can't entirely get rid of the
diesel knock at tick-over, but once under way the Alfa sounds and performs as well - if not better - than its petrol brothers.
I'm not convinced the efforts to reduce
diesel knock are totally successful because the engine in my test car could be quite noisy initially.
