Porting and Cylinder Scavenging part.1

TODAY, when we take a look down the cylinder of a two-stroke engine, we find
its walls literally filled with ports to handle the induction, transfer and exhaust phases
of gas flow through the engine.
Those of us who have grown up in the Japanese twostroke
era take it for granted that every cylinder has a huge exhaust port flanked by
anything from four to six transfer ports' However, it hasn't always been this way. As
far back as 1904 Alfred Scott patented his original two-stroke vertical twin. Then in
1906 the French Garard motor appeared with a rotary disc inlet valve. Scott also
developed a rotary valve engine in 1912, winning the Senior TT in that year and the
following year. However in spite of some very innovative designs being incorporated in
two-stroke engines they continued to be embarrassingly unreliable and this single
factor stifled development right up until the time of World War II.
In the mid-1930s, the DKW company set out to make two-strokes respectable.
They were in the business of manufacturing economical two-stroke motorcycles and
stood to profit from changing the two-stroke's image. They engaged the services of an
engineer named Zoller to build a 250 racer, which ultimately won the Isle of Man TT in
1938. This led to the development of a 125 single employing a porting arrangement
originally invented for two-stroke diesels by German engineer Dr.E.Schneurle. It was
this concept which ultimately brought success to the two-stroke, both as an economical
power source for transport and as a powerful, light-weight power source for
competition. Schneurle's loop-scavenging method, patented in 1925, employed a single
exhaust port flanked by two small scavenge or transfer ports, whose air streams were
aimed to converge on the cylinder wall opposite the exhaust (FIGURE 3.1). Being
aimed away from the exhaust, the transfer streams had a natural resistance to shortcircuiting
straight out the exhaust. Earlier designs had used deflector-dome pistons to
keep the fuel/air charge away from the exhaust port. This increased the piston's heat
gathering area and meant that only low power outputs could be aimed for without
continually risking piston seizure.
After the war DKW moved to Ingolstadt in West Germany, while their old plant atZschopau in East Germany was rebuilt as Motorradwerke Zschopau, or MZ. In 1952
Walter Kaaden joined MZ to take over development. His early work concentrated on
exhaust development and alternate scavenge methods. After much experimentation he
proved that the Schneurle loop-scavenge system yielded the best power and reliability.
Then in 1957 he added a third transfer port, opposite the exhaust. Its air stream joined

with the two main transfer ports, directing flow up toward the head (FIGURE 3.2) Contemporary two-stroke technology was introduced initially to Suzuki, and later
to Yamaha in Japan when Ernst Degner defected from East Germany to join Suzuki.
By combining designs which Degner brought from MZ with Japanese technology in the
field of metallurgy two-stroke power outputs and reliability took a leap forward.