“It would be very easy to turn straight away to the practical workshop aspects of improving the performance of two stroke engines. Eventually, of course, we must do so and we will. But, where two stroke engines are concerned, many people are fumbling in the dark, or, what is worse, weighed down with jargon and fatuous `theories` that are not even understood by the people who propagate them”.
The above is the opening paragraph of the first in a series of articles, by a very well- known and respected author, published in Classic Racer magazine back in 1985 entitled, `Two-stroke Tuning`, indeed I have one of his books and is an excellent and constant reference source. This first article established the author`s general overview of the specific subject and then concentrated on exhaust and inlet matters, with rather more emphasis on the former.
At the heart of the reasoning is the phenomenon of the so called `Kadenacy effect`, and you can be forgiven for raising an eyebrow and asking just what is that?
Michel Kadenacy, a Hungarian national, filed a patent during 1936 safeguarding his` invention`.
We now move on to 1945 and Paul Schweitzer who published a book entitled `Scavenging of two-stroke diesel engines`, in this book the author describes that the essence of the Kadenacy system is “ the depression created in the cylinder by a well-timed rapid opening of the exhaust”. The book goes on further to describe how the fresh charge from the transfer ports is not required to push out any of the exhaust gasses, on the contrary, the fresh charge is drawn in by the outrushing exhaust gas with no mixing of the two.
So; no mention at all of any effects from the remainder of the exhaust pipe, indeed throughout the whole magazine article there is only vague references made to any suitable pipe profile proportions with no reference at all of diameters?
The final paragraph of the magazine`s exhaust section of the article`s, informed instruction, is the most startling of all, and again I quote.
“At this point, let us demolish a myth----that of the `positive wave` arriving (from heaven knows where) just in the nick of time, to stuff escaping mixture back through the exhaust port. If such a wave existed----in defiance of the laws of acoustics-----it would have to travel through the exhaust gas, thus causing the very reverse flow that Kadenacy wished to avoid, and stuffing exhaust gas as well as escaping mixture back into the cylinder. Fortunately it does not happen”
In his 1936 patent, Kadenacy made extravagant claims stating that the exhaust gasses left the cylinder with ballistic velocities, leaving behind a vacuum that could be used for inducing fresh charge into the cylinder
These claims inspired many research workers to challenge the legitimacy of Kadenacy`s theory and, as a result the wider subject of fluid dynamic flow theory was examined and subsequently established and expanded on.
Work at Birmingham University around 1948 investigated fluid flow blowdown using a test rig together with the expanding theories of unsteady gas dynamics. This work showed clearly that there was no substance at all in Kadency`s claims!
There are a great many authoritative documents from throughout mainland Europe published by august research centres and universities, extending into the late 1950s, concurring with the pioneering, post-war work at Birmingham University and their collective opinion
as to the lack of validity of Kadenacy`s claims.
I`m sure we are all of us well aware of the SAE publication, in 1970, of Blair`s work at QUB and his and his teams research into a mathematical solution of exhaust pipe design. This work together with subsequent fine tuning has formed the basis for numerous commercial software packages readily available to would be two-stroke tuners.
If all of this information, with little research was readily available in 1985 why then should anyone attempting to assist budding two-stroke tuners embrace, then espouse a design concept that had been comprehensively discredited more than thirty years earlier?
If nothing else it serves to reinforce the advice that one should treat with some caution a good deal of the information that is offered from so called informed sources from last century!
The final sentence from the opening paragraph of this post perhaps says all we need to know?
Enjoy your day.