PROPWASH
November 2012
15
data. Getting enough blow down area is tough. The blow down period is the time
between exhaust port opening and transfer port opening. This period partially exhausts
the cylinder and, more importantly, creates a powerful exhaust pulse. Since the
maximum width of an exhaust port is around 70% of the cylinder bore, divided and
triple exhaust ports were developed. The remaining wall area is used for the transfer
ports.
The second requirement is good
scavenging of the cylinder. In a four stroke
the piston forces the combustion products
out the exhaust during a full upstroke of the
piston. In a two stroke this process has less
than half the time at the same rpm as a four
stroke and is accomplished by the incoming
air. A two stroke’s power is a sensitive
measure of its scavenging efficiency.
Experimenting with transfer port layouts
and angles has still not completely solved
scavenging issues in the Schnürle style
scavenging system. However, most current
high power engines use four transfer ports with a small single rear boost port. The
transfer passages need to be gently curved for good directional control of the incoming
mixture. The up angles on the transfers are partly determined by the bore/stroke ratio
and piston speed. The goal is to have the flows collide just above the piston to create a
rising column that doesn’t mix with the combustion products. This column needs to
stay along the back side of the cylinder away from the exhaust to avoid too much loss
out the exhaust port. A very interesting series of visualizations of this process in a
piston port non piped engine can be found at: http://aam.mathematik.uni-freiburg.de/
IAM/homepages/trescher/2StrokeEngine/movie/2StrokeEngine_MPEG1.mpg
The third requirement is a tuned exhaust system that creates a low pressure in the
cylinder during the time the transfers are open and a high pressure pulse after they
close. The low pressure pulse helps evacuate the cylinder. It also encourages flow out
of the crankcase through the transfers
and into the crankcase through the
intake. The high pressure pulse
returns fresh mixture that has flowed
out the exhaust port and supercharges
the cylinder. The tuned pipe is
responsible for more of the simple
two stroke’s power than any other
part of the engine. Notice in the graph
below that this supercharge amounts
to 2.4 atmospheres absolute or a boost
pressure of 20 psi above atmospheric
pressure.
Today the Aprilia 125 represents
the peak of modern small two stroke
development. This engine develops
54 horsepower at the transmission output shaft. That’s over 400 horsepower per liter,
twice what Kaden’s engines produced. From the lessons learned in their 125 cc race
engine program, engineer Jan Thiel predicted that a two cylinder 50 cc engine could
develop 39 horsepower at 23,000 rpm. That would be over 19 horsepower from a 25cc
racing engine, about three times the power developed by the industrial type engines
now used in model boat racing.
Is this the best that can be done? What old and new ideas promise to give two
strokes even more power? Part 2 will explore the future.
Bill Wisniewski is generally
attributed with bringing this technology
to model engines. In 1964 he brought
his Theobald-Wisnewski
Association “wart” engine (from the
bump on the casting for the boost port)
to the FAI control line speed
championships and won. In 1966, he
added the tuned pipe, and the Americans
were 1, 2, 3 with this technology. Bill
gave a talk at the event explaining the
details of his engine and pipe. He
published a pipe design article in the
March 1967 issue of Model Airplane
News. Bill’s Pink Ladies dominated
control line speed events for years. He
was responsible for the design of many
of K&B’s high performance engines and
worked there from 1961 almost until his
death in 2007. The Russians petitioned
the FAI to make tuned exhaust systems
illegal, but they, along with Schnürle
porting, are still a design feature of most
high performance model engines.
What distinguishes the modern, high
power small two stroke? The first
requirement of a high rpm engine is
sufficient flow area in the intake,
transfer, and exhaust ports. Empirical
methods have identified the time area
required to generate this flow for a
particular power at a given rpm. Time
area is the “average” open time and area
of a port that determines how much
fluid can flow through it in an engine
cycle. The program I use, Engine
Mod2T, calculates this from the input
Bill Wisniewski in 1966
The Aprilia designed ports
The Aprilia transfer passages