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
 or combustion roughness .
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
. Pilot injection allows you to slow that down and you don't have all the combustion noise.
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
For the SKYACTIV-D 1.5, Mazda has developed the Natural Sound Smoother(4) which reduces combustion noise known as "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.