Design and Simulation of Two Stroke Engines

Chapter 8 - Reduction of Noise Emission from Two-Stroke Engines
the intake system contains a reed valve, it produces its own unique high-frequency noise
components, much like the vibrating reed in an oboe or a clarinet which has a special quality
described occasionally as "honking." This adds to the design complexity of the intake silencer
for such engines.
Remember that any vibrating metal surface can act as a noise source, in the same manner
as does the vibrating diaphragm of a loudspeaker, hence the pressure signal emanating from
the combustion pressure rise is transmitted through the cylinder walls and can be propagated
away from the outer surface of the cylinder and the cylinder head. If the engine is air-cooled,
the cylinder and head finning are ideally suited to become metal diaphragms for this very
purpose. The need to control this form of noise transmission is obvious; observe that aircooled
motorcycle engines have rubber damping inserted between the cooling fins for this
very purpose. The problem is greatly eased by the use of liquid (water) cooling of a cylinder
and cylinder head, as the intervening water layer acts as a damper on the noise transmission.
That does not totally solve this problem, for the noise can be transmitted through the piston to
the connecting rod, via the bearings to the crankcase walls, and ultimately to the atmosphere.
The use of pressure-fed plain bearings on an engine crankshaft is superior to ball, roller, or
needle roller bearings as a means of suppression of this type of noise transmission because the
hydrodynamic oil film in the plain bearing does not as readily transmit the combustion vibrations.
However, the employment of pressure-fed plain bearings does not lend itself to the use
of the crankcase as the air pump of the simple two-stroke engine.
The rotating bearings of an engine produce noise characteristics of their own, emanating
from the vibrations of the mechanical components. Another mechanical noise source is that
from piston slap, as the piston rocks on the gudgeon pin within the cylinder walls around the
tdc and bdc positions.
However, the discussion regarding noise suppression in this chapter will concentrate on
the gas pressure-wave-generated noise from the exhaust and the inlet systems.
8.3 Silencing the exhaust and inlet system
In the matter of silencing, the two-stroke engine has some advantages and disadvantages
by comparison with an equivalent four-stroke cycle engine. These are catalogued below in
the first instance and discussed at greater length in succeeding sections as the need arises.
The disadvantages of the two-stroke engine
(i) The engine operates at double the frequency of creation of gas pressure waves and
humans dislike exposure to higher-frequency noise.
(ii) The ports in a two-stroke engine open faster than the poppet valves of the fourstroke
engine and so the pressure wave fronts are steeper, thereby creating more
high-frequency noise components within the sound spectrum.
(iii) Many two-stroke engines are used in applications calling for an engine with low
bulk and weight, thereby further reducing the space available for muffling and
potentially giving the engine type an undeserved reputation for being naturally
noisy.
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