Abstract:
The intake
system is one of the components on the internal combustion
engine most linked with the achievement of the high volumetric efficiency
required of naturally aspirated engines. High performance racing engine
intake
systems have unusual geometry with separate intake pipes (often
known as intake
trumpets) housed in a common airbox. These intake
trumpets are short pipes that are sometimes cylindrical but often conical.
The flow within the intake
system is very unsteady with high frequency wave
action
taking place. A carefully developed intake trumpet can use the wave
action to tune the
engine and therefore increase the performance distinctly.
The intake
tuning is strongly dependent on the acoustic properties of the
intake
system. Two important parameters are the pressure wave reflection
coefficient at the
open inflow end of the intake pipe (to tune the engine
effectively) and the acoustic length of the intake pipe (to tune the engine at
the
appropriate engine speed).
Acoustic measurements show that the
open inflow end reflection coefficient
decreases with inflow but increases with external
(coaxial) flow. CFD
calculations show that the vena contract a which
gets created at the open
inflow to the
pipe disappears with coaxial flow. A conical inflow shape
decreases the reflection coefficient at certain frequencies and influences the
phase angle of the acoustic waves. One dimensional prediction models for
the wave transfer inside these conical intake
pipes are validated against
acoustic measurement results.
4
The airbox decreases the reflection coefficient
distinctly due to three
dimensional resonances inside the airbox and limits the use of one
dimensional
prediction models, but the acoustic length can still be predicted
accurately. Fuelling inside the intake pipe decreases the speed of sound by
10% and thus increases the acoustic
pipe length. Therefore it influences the
engine tuning and needs be considered when developing intake systems.
A new acoustic measurement method is introduced which allows the
measurement of the acoustic
pipe length within the conical part of the intake
pipe.
Finally, the time varying nature of the intake flow and intake acoustics is
explored on a running racing engine by means of hot-wire and pressure
measurements and shows the influence of intake acoustics on the
fluctuating intake airflow velocity especially during the period when the
intake valves are closed.