Design and Simulation of Two Stroke Engines

Chapter 8
Reduction of Noise Emission
from Two-Stroke Engines
8.0 Introduction
The subject of noise emission from an internal-combustion engine, and its reduction, is a
specialized topic. Many textbooks and technical papers have been written on the subject, so it
is not possible to completely cover the matter in a single chapter in this book. Instead, it is
intended to orient you to noise emission pertaining to two-stroke engines and to the problems
of silencer design for this particular type of power unit. Note that the American word for a
"silencer" is a "muffler." Many texts discuss silencer design as if the only engine in existence
is the spark-ignited, multi-cylinder, four-stroke automobile engine and merely describe the
techniques applicable to that area. As that can create considerable misconceptions for the
designer of silencers for two-stroke engines, this provides sufficient justification for the inclusion
of this chapter within this book.
The opening section of this chapter gives some general background to the subject of
noise, indeed it repeats in brief what may be found more completely in other books and papers
which are suitably referenced for your wider education. This opening section is included so
that the remainder of the chapter will be more immediately meaningful to the novice.
The next section deals with the fundamental nature of pressure-wave-created noise emission
and to the theoretical methods available for its prediction. The latter part of this section
debates the future for the technology of silencer design.
The succeeding sections will cover the more empirical approaches to silencer design and
incorporate pragmatic advice on the design of such devices for two-stroke engines.
8.1 Noise
In many texts, you will find "noise" described is "unwanted sound." This is a rather loose
description, as that which is wanted by some may be unwanted by others. A 250 cm 3 Yamaha
V4 racing two-stroke, producing 72 unsilenced horsepower at 14,000 rpm, and wailing its
way up the Mountain in the Isle of Man TT race of 1967, produced a noise which was music
to the ears of a thousand racing motorcycle fans. A nearby farmer, the owner of a thousand
chickens vainly trying to lay eggs, viewed the same sound from an alternate standpoint. This
simple example illustrates the quite subjective nature of noise assessment. Nevertheless, between
the limits of the threshold of human hearing and the threshold of damage to the human
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