Introduction to Engine Exhaust Systems
Aero-modellers are demanding greater performance and power from their two stroke engines. Without doubt,
the future use of the 2 stroke engine in aero-modelling will be more closely coupled with tuned exhaust systems,
especially the ‘quiet pipe’ versions which reduce noise levels and add power.
The difference between mufflers and tuned exhaust systems is that mufflers do not improve engine power, infact
in most cases suppress a lot of power, whereas the tuned exhaust system improves a combination of power,
handling or noise. Some different exhaust system types are .....
Mini-Pipes
Basically a straight tube attached to the exhaust of the correct length and diameter to extract burnt gasses from
the cylinder. Power gains are small at the expense of a lot of noise. Such pipes are often called 1/4 wave pipes,
as it is about 1/4 the length of a ‘normal’ full pressure wave pipe.
Megaphones
Similar to the mini pipe, but tapered to an expanding diameter (as the name would suggest). They are more
powerful than mini pipes and very noisy.
Magic mufflers
Magic mufflers are half way between full tuned pipes and mini pipes. Basically a mini pipe within a larger
mini-pipe, which allows for the exhaust wave to travel the same distance as a full power pipe. A lot of new
‘conventional’ power enhancing mufflers are employing the principles of mini pipes / magic mufflers in their
design.
The Power PIPE
Power pipes are of the conventional double cone, tuned exhaust design. These pipes are designed for a specific
purpose, are of a set length and attach directly to the motors exhaust stack / adapter via a silicon sleeve / o’ring.
This technique makes for quick efficient sealing and installation.
Generally these pipes (often called ‘Nitro’ pipes) are used for racing only. They are very powerful, but noisy and
generally only work best at one rpm range.
The Quiet PIPE
Quiet pipes such as those used for R/C Aerobatics, are of the expansion type chamber (similar to the first
section of a power pipe), combined with a rear section a multiple baffle / muffler. Almost all of these have a flat
‘washer’ style first baffle as a reflective disk. To some degree these pipes are infact a combination of a muffled
megaphone and a muffled power pipe. Used in conjunction with an alloy header, these pipes develop good
power at low rpm (high torque) and are very quiet.
Not only is the engine more powerful, the handling characteristics are vastly superior to a non-piped motor. The
broad operating range of the pipe produces power anywhere above one third to half throttle, giving good vertical
climb power. When in a dive, the pipe will 'hold back' the engine (assuming the pipe is long enough). The pipe
becomes a constant speed governor. The 'hold back' symptom comes from the rpm exceeding the speed at
which the pipe can pump or reflect the gases back into the cylinder, causing a power loss exactly when it is
wanted - in a dive. The vertical climb power is obtained by setting the pipe at a longer length that used for
optimum power on a given prop load. Under high load condition (i.e. vertical climb), the rpm drop, creating a
situation where the length is at an optimum for those rpm.
The Quiet Power PIPE
The quiet power pipe is a combination of both the Power and Quiet pipes. It is basically a muffled power pipe,
or quiet pipe designed for high rpm use. These pipes are usually attached directly to the engine similar to a
power pipe. Often used in ducted fan models. Power, noise and handling are also between the power and quiet
pipes in use. Bolly also use the DF designation for this style of pipe. Shown abovw is the Bolly QP-90.
The 4 Stroke pipe
With the increase in popularity of 4 strokes for pattern use, has come the development of suitable exhaust
systems. They are of the Quiet pipe type of design. These pipes work on the exhaust extraction system similar to
car exhaust systems. They are nowhere as effective as 2 stroke exhaust systems (for power increase), and are
sometimes combined with a large volume muffler. The ideal muffler should have an outlet diameter similar to the
exhaust valve diameter. This type of muffler offers a very slight increase in power for a large noise reduction.
Their effectiveness is very noticeable when compared to a conventional 4 stroke muffler which work by inhibiting
the exhaust outlet.... (as do 2 stroke mufflers).
4 stroke headers
There is a major design fault with almost all 4 stroke engines. The exhaust valve size is usually much larger than
the outlet of the head, and the commonly used exhaust ‘headers’ are even smaller. Combine the above problem
with the fact that 4 strokes run very much hotter than 2 stokes, and you have a major problem.
For many years, throughout the world, manufacturers and modellers have been searching for the perfect 4 stroke
exhaust system. The heat makes for many problems, if the header pipes from the engine to the pipe were
aluminium, the alloy would break down or even melt. For this reason almost all headers are of stainless steel or
high temperature Teflon variants.
A problem of the stainless steel is finding the correct types of semi flexible tubing (rigid tubing will break with
vibration), the correct diameters and to then be able to weld or braze fittings without damaging the metal.
In the picture above, shown is the Bolly FS-122 pipe and After Muffler. This pipe was initially designed to suit
our ‘Convoluted exhaust flex’. This header worked well, but was prone to fatigue breakage, and we have
withdrawn it from the market.
We now provide the CD Engineering system of header pipe / Teflon exhaust flex. This system has become very
popular in Australia for 1 good reason, it works. The Teflon used is a very high quality aerospace grade not
normally available to the public.
2 stroke headers
As the 2 stroke engine using tuned quiet pipes have been around for some time, an industry has been established
to provide suitable exhaust headers. Usually it is the form of an alloy header plate that bolts to the exhaust flange
of the model, with an aluminium tube of the correct diameter and shape welded to the plate.
With such a wide variety of models, pipes and engines, it is often a problem to supply the correct header.
As with 4 stroke headers, metal fatigue is still a problem (but much less so) with 2 stoke headers. Several golden
rules apply ..... never use a long header, this only increases the vibration strain on the weld, and never thread a
pressure nipple into the metal on a curve (where the metal has been stressed).
Source : n/a
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