Air intakes perform the following functions: they filter the air; they guide the air prior to it reaching the impeller of the compressor of the turbocharger; and they attenuate the noise produced by the turbocharger as emitted by the air inlet to the air intake.
The noise attenuation is achieved through the use of baffle structures and acoustically absorbent lining in the air intake.
A problem arises that the geometrical form of the baffle structures is not ideal across the full operating range of the turbocharger/engine. The directing of the air by the baffle structures is such that the air approaches the impeller in a certain direction. This certain direction is not ideal at the extremes of the operating range of the turbocharger/engine. This results in a reduction in turbocharger/engine efficiency as well as increased noise at these extremes.
It is known to add a variable guide vane arrangement to the air intake to direct the air dependent on operation of the turbocharger/engine so that the air approaches the impeller in a direction best suited to turbocharger/engine operation. However, the addition to the air intake of a further component, together with its associated aerodynamic loss, has the undesirable effect of increasing overall aerodynamic loss in the air intake. Further, existing variable guide vane arrangements are complex and expensive.