Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
level. The identification of the frequency band of that major noise component will be shown
later as the first step toward its eradication as a noise source.
The measurement of noise is a tedious experimental technique in that a set procedure is
not just desirable, but essential. Seemingly innocent parameters, such as the height of the
microphone from the ground during a test, or the reflectivity of the surface of the ground in
the vicinity of the testing, e.g., grass or tarmac, can have a major influence on the numerical
value of the dB recorded from the identical engine or machine. This has given rise to a plethora
of apparently unrelated test procedures, such as those in the SAE Standards [8.16]. In actual
fact, the logic behind their formulation is quite impeccable and anyone embarking on a silencer
design and development exercise will be wise to study them thoroughly and implement
them during experimentation.
8.2 Noise sources in a simple two-stroke engine
The sources of noise emanating from a two-stroke engine are illustrated in Fig. 8.1. The
obvious ones are the intake and the exhaust system, where the presence of gas pressure waves
has been discussed at length in Chapter 2. As these propagate into the atmosphere they produce
noise. The series of photographs in Chapter 2, Plates 2.1-2.4, illustrate the rapid nature
of the pressure rise propagating into the atmosphere and toward the ear of the listener. The
common belief is that the exhaust is the noisier of the two, and in general this is true. However,
the most rudimentary of exhaust silencers will almost inevitably leave the intake system
as the noisier of these two sources, so it also requires silencing to the same level and extent. If
NOISE CREATED BY
COMBUSTION PRESSURE ^
RISE TRANSMITTED
THROUGH THE WALLS
MECHANICAL NOISE
FROM PISTON SLAP
AND BEARINGS SENT
THROUGH THE VALLS
MECHANICAL NOISE
FROM BEARINGS AND
COMBUSTION FORCES V
TRANSMITTED THROUGH
VALLS TO ATMOSPHERE
NOISE CREATED BY
EXHAUST PRESSURE PULSES
INLET
Fig. 8.1 Various sources of noise from a two-stroke engine.
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NOISE CREATED BY
INDUCTION PRESSURE PULSES
AND VIBRATING REEDS