Page 693 - Automotive Engineering Powertrain Chassis System and Vehicle Body

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

210
1000 Rev/min
1500 Rev/min
2200 Rev/min
200

190

SPL (dB) 180

170

160

150
10 2 10 3 10 4
Frequency (Hz)
Fig. 21.1-6 Effect of engine speed on cylinder pressure spectra. NA-DI diesel engine at full load: data obtained from Nelson (1987) and
Russell (1979).

It should be appreciated that the slope of the cylinder very high peak cylinder pressures) and so variants have
pressure spectrum provides an indication of the speed been developed that require smaller swings in timing
dependence of combustion noise. Typical slopes per (Atkins and Challen (1979), reported in Nelson (1987)).
decade (tenfold change in frequency) are given as (Lilly, Of course the opposite applies and a retarding of in-
1984): jection produces a reduction in combustion noise. This
NA-DI diesel 25–30 dB/decade noise control technique may be used only to a small
NA-IDl diesel 40–50 dB/decade extent due to the impact on engine performance and
Turbo DI diesel 40–50 dB/decade emissions (retarding the injection increases bsfc and the
formation of smoke/soot/particulates). A compromise
Gasoline 50–60 dB/decade
between smoke and noise emissions may be found by
The higher the slope, the greater the speed dependence. retarding the injection timing but increasing the injection
It can be seen that the noisiest engines for combustion rate (Glikin, 1985).
noise have the lowest speed dependency. This explains The modelling of the wave dynamics in the fuel in-
why heavy trucks remain noisy even when used at low jection system that strongly influences the rate of fuel
speeds, and why this effect is not noticed in gasoline- injection is reported in Russell and Lee (1994) and the
powered vehicles. effect that the rate of delivery has on combustion is
The portion of each spectrum in Fig. 21.1-6 in the reported in Russell (1997). The recent advent of
1–4-kHz range is responsible for the diesel knock com- common rail diesel fuel injection equipment with sole-
monly associated with diesel engines. The rate of pres- noid controlled injectors allows more than one pulse of
sure rise is greatest in diesel engines with the greatest diesel fuel per cylinder per cycle and therefore fuel flow
ignition delay. This ignition delay is extended when the rate modulation as a means of noise control (as well as
injection timing is advanced. With advanced injection, emissions control) can be explored readily.
the fuel has more time to pre-mix with air before com-
bustion occurs, yielding a larger pre-mixed charge which
will burn quickly producing a rapid pressure rise.
So, advancing injection timing increases combustion 21.1.4.3 Mechanical noise
noise. This effect is commonly used to separate combus-
tion noise from mechanical noise. The injection timing can The crank mechanism (pistons, conrods, crankshaft,
be slowly advanced until the change in exterior noise bearings) experiences externally applied forces due to gas
spectrum matches the change in cylinder pressure spec- forces and internally generated forces due to its own in-
trum. At that point the exterior noise is dominated by ertia. The reaction of the engine structure to the sum of
combustion noise. This technique may not be practical these forces produces mechanical noise by an indirect
with certain fuel injection equipment (it also may yield noise-generating mechanism.

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