Page 743 - Automotive Engineering Powertrain Chassis System and Vehicle Body

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

Fig. 22.1-8 Intake port flow loss coefficient (AVL, 2000).

Fig. 22.1-9 Exhaust port flow loss coefficient (AVL, 2000).

for sonic flow through the orifice (valve) which is also length of the primary runners (Winterbone and Pearson,
known as choked flow as this represents the maximum 1999). Generally, longer runner lengths are used to
rate of mass transfer possible through a single orifice maximise low-speed torque, but often with a penalty of
of negligible length. Note that this is no longer a function reducing high-speed power.
of pressure ratio across the valve, but is solely a function of There is a connection between ignition timing and
the conditions in the manifold and of the effective open runner length. Fifteen degree changes in advance may be
area of the valve. The sonic condition is reached when required at a given speed when changing runner length
g=g 1 (Harrison, 2003).
p 01 2 In addition to optimising spark advance, one really
(22.1.53)
p 2 g þ 1 needs to optimise the valve timing as well. IVC is impor-
tant but good intake manifold design is more important.
which is a pressure ratio of around 0.53. A good cam design cannot make up for a poor manifold.
The balance in flows between runners is important.
22.1.3.10.4 A note on intake manifold design With one restrictive intake runner, that cylinder will tend
to run rich at high engine speeds. As a result, the other
The volumetric efficiency of a four-stroke engine can be
cylinders will run lean. Intake port velocity is typically
improved through dynamic tuning whereby it is con- 1
70–90 m s .
trived to maximise the pressure in the intake port at
around IVC (Ohata and Ishida, 1982). This of course
maximises the pressure ratio in equation (22.1.49) and
hence the mass flow rate, and hence the volumetric 22.1.3.10.5 The motion of the piston during
efficiency (see equation (22.1.8)). the intake stroke
Achieving the dynamic tuning effect is strongly de- Earlier discussions show that volumetric efficiency is
pendent on intake manifold design, and in particular the strongly influenced by the pressure ratio across the inlet

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