Page 695 - Automotive Engineering Powertrain Chassis System and Vehicle Body

CHAP TER 2 1. 1 Interior noise: Assessment and control

use. However, it is far less reliable at low frequencies and Cylinder block – engine blocks with separate crank-
commonly results in the under estimation of the contri- shaft bearing endcaps at the bulkheads between cyl-
bution made by engine components at these frequencies inders exhibit the lowest frequency for the first
(Crocker et al., 1980). bending mode of the engine. For larger engines this
Therefore, at low frequencies, sound intensity may be as low as 200 Hz. The use of a ladderframe or
(Crocker et al., 1980) or noise from vibration techniques bedplate bottom end to join the bearing caps to-
might be preferred (Dixon and Phillips, 1998). The noise gether with a locally stiff structure can push the first
source rankings for contemporary powertrains are as bending mode above 300 Hz (Querengasser et al.,
follows, with the noisiest item at the top of the list 1995), reduce the axial excursion of the endcaps at
(March and Croker, 1998). resonance (commonly around 1000 Hz) and gener-
ally reduce the low-frequency modal density (March
and Croker, 1998). Careful design of the crankcase
and the block to reduce the effect of panel modes is
Diesel engines Gasoline engines
also beneficial for frequencies around 800þ Hz
Oil pan Transmission (Russell, 1972).
Other sources Other sources Intake system – the avoidance of large planar surfaces
Fuel system Intake system on intake components can reduce noise emissions
along with general stiffening of the structures.
Similar Cylinder block Ancillary drive Noise shields – well-damped, isolated engine covers
effect Ancillary drive Oil pan can reduce noise radiated by the engine structure
Transmission Exhaust manifold (Russell, 1972).
Intake system Cam cover
Exhaust manifold Front cover Engine bay enclosures – engines may be effectively
Front cover Fuel system enclosed within their engine bay in the vehicle, thus
Cam cover Cylinder block encapsulating the noise sources. Problems with ven-
tilation and cooling are common (Thien et al., 1984).

21.1.5 Road noise
See also Section 20.1.6.2.1 for an alternative noise source
ranking.
21.1.5.1 Introduction to road noise
21.1.4.7 Engine noise control The term road noise might be replaced by the more
The options for controlling engine noise are the usual complete description ‘road and tyre noise’ as it is taken
ones available to the noise control engineer, namely: here to include:
stiffen structures to push resonant frequencies above Interior noise resulting from the contact between the
the highest forcing frequency; tyres and the road, being transmitted to the interior
isolate components from sources of excitation; by both airborne and structure-borne paths. This is
encapsulate noise sources with massive panels; often labelled as road noise and is the subject here.
add damping where resonances occur. Exterior noise resulting from the contact between the
tyres and the road. This is often labelled as tyre noise
Engine-specific noise control measures include the andwasthesubjectofanearliersection(Section22.1.4).
following:
Oil pan – the use of an isolating gasket between the 21.1.5.2 Interior road noise
oil pan and the crankcase. The adoption of struc-
tural aluminium oil pans to replace the traditional Vehicle interior road noise is mainly a low-frequency
pressed steel components has made oil pan noise noise problem (<1000 Hz). Contributions are made by
more significant in spite of improvements to the
crankcase to reduce its noise radiation. structure-borne noise paths through the vehicle sus-
Rocker cover – the use of rubber isolating gaskets pension (<500 Hz);
(Querengasser et al., 1995). direct airborne noise paths from the tyre through the
Fuel injection equipment – the adoption of common vehicle structure (>500 Hz); often confused with
rail systems and unit injector systems which are more wind noise.
compact and quieter have brought about significant The structure-borne components tend to dominate
improvements (March and Croker, 1998). overall noise levels except on the smoothest of road

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