The following blog hopefully
gives an insight into the research and development work carried out in the
early 1970’s into the efficient silencing of motorcycle internal combustion
engines.
Dr Geoffrey Roe MA PhD and his
colleague Dr Terry Thorpe Bsc PhD were both Academics in the Department of
Mechanical Engineering, University of Manchester. They worked as a research and
development team within the Simon Engineering Laboratories and made their
expertise available to the worldwide motorcycle industry.
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| Geoffrey Roe and Terry Thorpe |
The team were involved in many diverse aspects of motorcycle design and development, from suspension and road-holding issues to engine design and tyre contact area optimisation. However it is the concept of the annular discharge silencer that concerns us here today.
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| Geoffrey testing in the anechoic chamber |
Geoffrey Roe had long been
interested in the understanding and performance of engines and in 1974
published as co-author with Walter Annand, a definitive book on the subject:
Gas Flow in the Internal Combustion Engine. In his research he had found that in
passing exhaust gas through an annulus and over a series of various capacity
chambers, it was possible to effectively silence a single cylinder engine
without a serious loss of performance. Furthermore it was possible to ‘tune’
these internal chambers by altering the volume and surface hole configuration,
in order to dampen the specific frequencies required.
Geoffrey was a keen motorcyclist
and rode a Norton Commando at this time. Terry had made the better choice and
rode a Velocette Venom.
In June 1973 Geoffrey
published a paper entitled: The Silencing of a High Performance Motorcycle. In
it he describes how the noise level of a 750cc Norton motorcycle was reduced
from 98 decibels to 86 decibels by silencing of both the induction and exhaust.
This was quite some achievement considering the mechanical noise emanating from
the air-cooled engine. Later under MIRA testing it was found the antiquated engine
produced more noise than the exhaust!
Geoffrey’s initial idea behind his annular discharge type silencers, was that noise reduction by viscous damping could be achieved by passing the exhaust gas down a channel with a high width/height ratio, with height being very small compared to the wavelength of the sound to be suppressed. In order to turn this into practice, a small peripheral (annular) gap between two co-axial cylinders was used. The peripheral volume would also act as a quarter-wavelength resonator (closed end/open end), which was hoped would balance the half-wavelength resonance of the feedpipe. With sufficient width, the flow area of this type of silencer could still be maintained to at least equal to that of the feedpipe from the engine. For perfect mutual cancellation of resonance, the peripheral volume would have needed effectively the same length as the feedpipe (approximately 39 inches), however a silencer of this length is not practical for installation on a motorcycle. For practical installation purposes a length of 26 inches was chosen, with an outside diameter of 3 ¼ inches. The feedpipe and peripheral volume were connected by a short expansion chamber of low expansion ratio and adjustable length. With this initial design a noise reduction of 10 dB(A) was observed compared to the standard Norton silencer.
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| Initial experimental 'annular discharge' silencer produced by Roe |
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| Prototype Commando silencer produced by Roe |
Back-to-back power tests were completed with Roe’s
new silencer. It was found that the ‘annular discharge’ silencer gave a large
boost in low speed torque. The net effect was an increase in power from
2000-4000rpm, a slight loss from 4000-5000rpm, and a gain above 5000rpm. The
increase in low speed torque allowed the Norton to be fitted with a higher gear
ratio, which also meant a reduction in noise due to the lower engine speed
utilised during noise certification testing.
In October 1973 Norton Villiers filed a US Patent on
Exhaust Silencers. Geoffrey is named as joint inventor along with John Favill
of Norton Villiers in US Patent Number - US3888332A, but the designs and
technical argument submitted are all based on his work at Manchester.
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| US Patent Number - US3888332A |
This patent was subsequently
granted and published in June 1975. Initially these annular discharge silencers
were fitted to the Norton Commando 850 Mk3 and were very effective in use. Following
Norton Villiers takeover of BSA/Triumph in 1973, a similar design of annular
discharge silencer was fitted to the Triumph Trident T160.
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| Norton Commando Mk3 |
Geoffrey was also interested in
all aspects of motorcycle sport and in the mid-seventies his silencer design
was approved by the British Speedway Promoters Association (BSPA). Speedway as
a sport was coming under severe environmental pressure to reduce noise levels
and the Auto Cycle Union (ACU) had finally now imposed an upper limit. As a
consequence scrutineers were issued with a sound meter and testing took place
on a regular basis, riders had to comply or they were ejected from the meeting.
It is no easy task to silence a
high performance 500cc single cylinder air-cooled engine and it was though this
interest that our paths crossed.
As had happened in Speedway, the ACU were
about to impose noise limits in all branches of the sport and being involved in
Road Racing, did not wish to be caught unawares. At International level the FIM
had already introduced upper noise limits in an attempt to quell the onslaught
of the 2 stroke. In practice it had completely the opposite effect to that
intended, the expansion boxes of the 2 strokes were easy to muffle with an absorption
tail-pipe. However the 4 strokes struggled to meet the new limits without a
serious loss of power and the dominant MV’s were rendered non-competitive and retired
overnight.
We were currently involved in a
Final Year Project on exhaust silencing with a student from Stannington
Technical College in Sheffield. Through this a visit to meet Geoffrey in the
Simon Engineering Laboratory was hastily arranged. We spent a very interesting
afternoon in Manchester and it was instantly apparent that Geoffrey was both a
very able Academic and an enthusiast. He was extremely helpful, offering advice
and the benefit of his experience to help us on our way. In return we agreed to
make available our findings to him, not to produce the silencers for commercial
gain and before we departed a letter was signed to this effect.
We hurried
home to start work on our first designs. Clearly we were influenced by what we
had seen and heard that day and it was not long before the first silencer was
fabricated.
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| 2valve annular discharge silencer |
As always time
was of the essence and in an equally short period we were ready to test on a
cold and bright day at Cadwell Park early in the season. A series of designs
were produced, varying both the annulus width and the diffuser angle. The primary
pipe length could also be altered easily on the day, so that the effects of any
back-pressure change could hopefully be mitigated. All the initial testing was
done on a 2valve engine, as the characteristics and setup were well known.
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| 2valve testing at Cadwell Park |
Initial
results were good, too good if I am honest and as a rider you could hardly hear
the exhaust note through your helmet. This made starting and gear changes more
difficult but there was a useful increase in bottom end power, which was helpful
in pulling out of Cadwell’s hairpin bend. This was no doubt caused by an
increase in back-pressure in the exhaust due to the fixed plate at the rear of
the diffuser chamber. This acts as an end-stop to the incoming flow, before it
passes out through the primary holes and into the annulus. However there was
also a slight loss of power at the top of the range which was considered
unacceptable. We played with different primary pipe lengths and the alternative
silencers we had available throughout the afternoon and wrote up our results
accordingly. It was interesting to note that the silencers when fitted made
very little difference to the carburation of the engine. This is always a good
sign in that the engine is continuing to breathe well and shows that things
were not a million miles away. As far as we were concerned it had been a most
satisfying afternoon and even after this brief session, from the changes that
were made a pattern was forming. Further silencers were fabricated in the light
of our findings, and this whole development sequence was repeated many times. Eventually
we began to understand what made things tick and it was time to report our
findings to Geoffrey.
Another
visit to Manchester was arranged and once again Geoffrey made us most welcome.
We had gone for the full day this time and for him to make the time available
to see us was quite telling. He was exceedingly interested in our results and
in what we had found out during testing. As agreed, we passed on this
information freely and showed him all the modifications that had been made as
the silencer was developed. When three enthusiasts are in conversation about
something that interests them, food and drink become irrelevant.
Eventually a
break was taken and when we returned to the laboratory it was via his office.
This time we were sworn to secrecy before he took us over to his covered drawing
boards. It was here we saw the initial designs of a front suspension system for
the BMW motorcycle. As you might expect the associated ‘brief’ discussion lasted
well over an hour, after which he took us to a second drawing board, which if I
remember correctly, was secured by a lock. Geoffrey raised the cover and asked
what we saw. It was clearly an air-cooled, OHC 4valve single cylinder head,
other than that not much was obvious. He informed us that it was a new design
for Yamaha and a development of the single cylinder, 2valve SR500 engine
currently in production. Again he asked us what we thought.
Before
anyone could speak, Eric my companion informed him that it would not work.
Now Eric was
always forthright and often out-spoken, but even I dreaded this last comment,
straight away imagining this is the end of a flourishing relationship. Geoffrey
asked why?
Eric asked
what size the spark plug would be used and where it would be situated relative
to the single OHC. Geoffrey produced another drawing from the bundle, at which
point Eric shakes his head and says “the plug breaks through into the inlet
port”. A close investigation takes place, before Geoffrey backs away wiping his
eyes and says “you are absolutely right….how did I miss that?”
So there you
go, even the mighty can have a day off.
Before we
left, Geoffrey was asked what further developments he would make to his
discharge silencer given the chance. Armed with his reply we bid farewell and
headed home. Using the
information gleaned from our testing and advice from Geoffrey, a second series
of silencers were designed and fabricated.
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| 4valve annular discharge silencer |
Thought had
been given as to how the incoming charge is diffused into the annulus and the surface
effect on flow along the silencer length. The second was a subtle change, the
first more dramatic in concept, but the pair combined to greatly reduce the previous
loss of performance at the top of the range. Once again significant testing was
carried out, but this time using a 4valve engine.
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| 4valve silencer (LH) |
As might be
expected, the exhaust noise reduction was nothing like as great as when using
the original designs inspired by Geoffrey. However this did not concern us
greatly as the ACU had set an upper limit of 105 decibels at a mean piston
speed of 11m/s. On an 86mm stroke engine this equates to an engine speed of
3837rpm and at this figure we were well within the imposed limit. In practice
it is difficult to accurately measure the noise emitted from an exhaust other
than in a laboratory. Many factors like air density, moisture content and
ambient sound levels come in to play, meaning that any values obtained are
highly compromised. However it is possible to make a comparison under similar
conditions, as long as one accepts the readings as not being absolute.
It was an
interesting project from start to finish and a wonderful opportunity to work
with such an able man as Geoffrey Roe.
