Page 692 - Automotive Engineering Powertrain Chassis System and Vehicle Body

Interior noise: Assessment and control C HAPTER 21.1

BS 4196-7:1988 (ISO 3747:1987) combustion noise is a more significant contributor to
Sound power levels of noise sources. Survey method the engine noise produced by compression ignition
for determination of sound power levels of noise sources (diesel) engines.
using a reference sound source. Engine noise is dependent on engine speed and may also
BS 4196-8:1991 (ISO 6926:1990) depend on engine load for some types of engine (the
Sound power levels of noise sources. Specification for the normally aspirated direct injection diesel and the gasoline
performance and calibration of reference sound sources. engine in particular).
BS EN ISO 3743-1:1995
Acoustics.Determinationofsoundpowerlevelsofnoise 21.1.4.2 Combustion noise
sources. Engineeringmethodsforsmall,movablesources in
reverberant fields. Comparison for hard-walled test rooms. Combustion noise results from gas forces in the cylinders
BS EN ISO 3743-2:1997 applied to the structure of the engine, causing vibration
Acoustics. Determination of sound power levels of to occur which is then radiated as noise. It is produced
noise sources. Engineering methods for small, movable therefore by an indirect noise-generating mechanism.
sources in reverberant fields. Methods for special re- The gas forces in each cylinder vary during the working
verberation test rooms. cycle of the engine (two or four stroke). They are highest
during the combustion period where the cylinder
BS EN ISO 3744:1995 pressure is rising quickly.
Acoustics. Determination of sound power levels of The vibration response of the engine is greatest when
noise sources using sound pressure. Engineering method the forcing caused by the rate of pressure rise is greatest.
in an essentially free field over a reflecting plane.
This is intuitively obvious: if the rate of pressure rise is
BS EN ISO 3746:1996 zero, then the forces due to cylinder pressure will be in
Acoustics. Determination of sound power levels of equilibrium with the restraining forces in the engine
noise sources using sound pressure. Survey method using structure, and hence with no net force there will be no
an enveloping measurement surface over a reflecting plane. net acceleration of the structure. However, with a rapidly
BS EN ISO 9614-1:1995 changing cylinder pressure, the response of the structure
Acoustics. Determination of sound power levels of lags behind the causal force, equilibrium is never
noise sources using sound intensity. Measurement at reached, and a net force results producing vibration. The
discrete points. more rapid the rate of change of pressure, the greater
the net force and hence the greater the vibration and the
BS EN ISO 9614-2:1997 noise. A more rapid rise in pressure also increases the
Acoustics. Determination of sound power levels of high-frequency content of the force, and hence of both
noise sources using sound intensity. Measurement by the vibration and the noise.
scanning.
The tendency to produce combustion noise of different
engine types can be reasonably ranked according to their
typical rates of cylinder pressure rise during combustion.
21.1.4 Engine noise Starting with the noisiest for combustion noise:

21.1.4.1 Introduction to engine noise NA, DI diesel (4þ bar/degree crank)
NA, indirect injection
In this section, the term engine noise will be taken as the (IDl) diesel (3–4 bar/degree crank)
noise produced by a combination of the gas loads in the Turbocharged DI diesel (2–3 bar/degree crank)
cylinders and the mechanical motions in the base engine. Gasoline engine (<2–3 bar/degree crank)
Intake and exhaust noise shall be considered as separate The spectrum of the cylinder pressure is a more useful/
problems as will be the noise caused by engine ancillaries reliable indicator of combustion noise. Typical spectra for
(alternators, fans, pumps, motors, etc.). the NA-DI diesel engine at full load are shown in
Engine noise is the sum of two elements: Fig. 21.1-6 (data taken from Nelson (1987), originally
1. combustion noise published in Russell (1979)).
The effect of increasing speed can be seen in
2. mechanical noise. Fig. 21.1-6 as:
The relative mix of the two will vary between engines but a shift in the spectrum towards the higher fre-
as a general rule: quencies;
mechanical noise dominates the engine noise pro- an increase in spectral levels in each third octave
duced by spark ignition (gasoline) engines; band as a result of the shift to the higher frequencies.

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