Page 748 - Automotive Engineering Powertrain Chassis System and Vehicle Body

Exterior noise: Assessment and control C HAPTER 22.1

bmep is a useful measure of torque output. Typical gasoline engines). This is because the turbocharged
maximum bmep are: engine produces more torque at a given engine speed
(Watson and Janota, 1982). As friction losses in the
engine are speed-dependent, these form a smaller pro-
NA gasoline 8–11 bar
portion of the turbocharged IMEP than that of the NA
Turbo gasoline 12–17 bar
IMEP. Thus the overall efficiency of the turbocharged
NA diesel 7–9 bar
engine is greater.
Turbo diesel 10–12 bar
Watson and Janota have an interesting perspective on
turbocharging (Watson and Janota, 1982).
Based on these data, it appears that the turbocharger is
being used to raise inlet air densities by a factor of z1.5 Turbocharging is a specific method of supercharging.
in the gasoline engine and by z1.3–1.4 in the diesel An attempt is made to use the energy of the hot
engine. exhaust gas of the engine to drive the supercharging
compressor. The user is not getting something for
nothing, but is merely using energy that would
normally go to waste; however it is clear that it is no
22.1.3.10.8 The rationale for turbocharging
longer necessary to debit the power requirement of
Turbochargers are now routinely fitted to diesel engines. the compressor from the indicated power of the
The reasons for this are: engine.
Specific output (power/swept volume) of the NA
diesel is poor, leading to large, heavy expensive 22.1.3.10.9 On turbocharger noise
engines (due to the need to run lean to avoid exces- The addition of a turbocharger to an engine is known to
sive smoke (Heywood, 1988)). reduce the levels of intake noise. The silencing mecha-
Power output of the diesel is smoke-limited. This nism is believed to be simple with the compressor
restriction is relaxed if more air mass is added to
housing behaving as a small, reactive silencer element.
a given cylinder size by turbocharging.
The presence of the spinning rotor will have little effect
There are no knock problems to overcome even when except to add some flow-induced noise. Reductions in
turbocharging diesel engines (unlike gasoline narrow-band intake noise of the order of 3–8 dB should
engines). be expected depending on the frequency content of the
Diesel engines are more costly, allowing the addi- intake noise.
tional costs of a turbocharger to be absorbed
(Watson and Janota, 1982).
22.1.3.11 Sources of intake
The best-known referencework on turbocharging (Watson (and exhaust) noise
and Janota, 1982) is mostly devoted to diesel engines. The
rationale for turbocharging the gasoline engine is, to be The noise due to the operation of the intake and exhaust
honest, rather less compelling than for the diesel engine. systems can be classified as follows:
If the designer needs/wants to boost the torque Primary noise sources being the unsteady mass flow
output from a restricted swept volume then they might through the valves, which causes pressure fluctuations in
choose to turbocharge. However, there are two limits to the manifold and these propagate to the intake orifice
this. The first is that the increased air density at inlet, (or exhaust tailpipe) and are radiated as noise. It should
when combined with the correct ratio of fuel will pro- be noted that the mechanism that causes primary (or
duce higher cylinder pressures and temperatures thus engine breathing) noise is the same mechanism that is
increasing the tendency to knock (Heywood, 1988) – this usefully harnessed to improve the volumetric efficiency
can be controlled by reducing the compression ratio and of the NA engine by wave action tuning (see Section
accepting the loss of thermal efficiency caused. Even 22.1.3.10.6).
with the loss of efficiency, the work achieved per cycle is Harrison and Stanev (2004) propose the following
greater in the turbocharged engine than achieved by the hypothesis to explain the fluctuating pressure time his-
NA engine because more fuel can be burnt whilst pre- tory found in the intake manifold:
serving the desired air–fuel ratio. The second limitation
to turbocharging the gasoline engine is speed. The tur- The early stages of the intake process are governed
bocharger is ideally matched at one engine operating by the instantaneous values of the piston velocity and
point (speed/load combination) and achieving adequate the open area under the valve. Thereafter, resonant
performance at other operating points is difficult. wave action dominates the process. The depth of the
The turbocharged diesel engine may be more fuel ef- early depression caused by the moving piston governs
ficient than its NA counterpart (unlikely to be the case in the intensityofthewave actionthat follows.A pressure

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