Two Stroke Engine Work Explanation
(written by Joseph A. Schuster)
A two-stroke in its purest form is extremely simple in construction and operation, as it only has three primary moving parts (the piston, connecting rod, and crankshaft). However, the two-stroke cycle can be difficult for some to visualize at first because certain phases of the cycle occur simultaneously, causing it to be hard to tell when one part of the cycle ends and another begins.
Several different varieties of two-strokes have been
developed over the years, and each type has its own set of advantages and
disadvantages. This discussion has already prompted many of you to find
a more interesting web-site, so I won't go into the sordid details of each
type. The subject of the animated GIF (and this dissertation) is known
as a case-reed type because induction is controlled by a
reed valve mounted in the side of the crankcase.
The easiest way to visualize two-stroke operation is
to follow the flow of gases through the engine starting at the air inlet.
In this case, the cycle would begin at approximately mid-stroke when the
piston is rising, and has covered the transfer port openings:
As the piston moves upward, a vacuum is created beneath
the piston in the enclosed volume of the crankcase. Air flows through the
reed valve and carburetor to fill the vacuum created in the crankcase.
For the purposes of discussion, the intake phase is completed when the
piston reaches the top of the stroke (in reality, mixture continues to
flow into the crankcase even when the piston is on its way back down due
to the inertia of the fuel mixture, especially at high RPM):
During the down stroke, the falling piston creates
a positive pressure in the crankcase which causes the reed valve to close.
The mixture in the crankcase is compressed until the piston uncovers the
transfer port openings, at which point the mixture flows up into the cylinder.
The engine depicted here is known as a loop-scavenged two-stroke because
the incoming mixture describes a circular path as shown in the picture
below. What is not readily apparent in the picture is that the primary
portion of the mixture is directed toward the cylinder wall opposite the
exhaust port (this reduces the amount of mixture that escapes out the open
exhaust port, also known as short-circuiting):
Mixture transfer continues until the piston once again
rises high enough to shut off the transfer ports (which is where we started
this discussion). Let's fast-forward about 25 degrees of crank rotation
to the point where the exhaust port is covered by the piston. The trapped
mixture is now compressed by the upward moving piston (at the same time
that a new charge is being drawn into the crankcase down below):
Somewhat before the piston reaches the top of the stroke
(approximately 30 degrees of crank rotation before top-dead-center), the
sparkplug ignites the mixture. This event is timed such that the burning
mixture reaches peak pressure slightly after top dead center. The
expanding mixture drives the piston downward until it begins to uncover
the exhaust port. The majority of the pressure in the cylinder is released
within a few degrees of crank rotation after the port begins to open:
Residual exhaust gases are pushed out the exhaust port
by the new mixture entering the cylinder from the transfer ports.
That completes the chain of events for the basic two-stroke
cycle. The discussion is not complete, however, so if you've made it this
far and you are getting bored, then go ahead and scroll down the rest of
the way - the animation should be done loading.
The animated demonstration has an added device commonly
known as an expansion chamber attached to the exhaust port. The expansion
chamber (an improperly named device) utilizes sonic energy contained in
the initial sharp pulse of exhaust gas exiting the cylinder to supercharge
the cylinder with fresh mixture.
Picking up the discussion at the point shown by the
exhaust blowdown picture above, an extremely high energy pulse of exhaust
gas enters the header pipe when the piston begins to open the exhaust port:
The sonic compression wave resulting from this abrupt
release of cylinder pressure travels down the exhaust pipe until it reaches
the beginning of the divergent cone, or diffuser, of the expansion chamber.
From the perspective of the sound waves reaching this junction, the diffuser
appears almost like an open-ended tube in that part of the energy of the
pulse is reflected back up the pipe, except with an inverted sign (a
rarefaction, or vacuum pulse is returned). The angle of the walls of the
cone determine the magnitude of the returned negative pressure, and the
length of the cone defines the duration of the returning waves:
The negative pressure assists the mixture coming up
through the transfer ports, and actually draws some of the mixture out
into the exhaust header. Meanwhile, the original pressure pulse is still
making its way down the expansion chamber, although a considerable portion
of its energy was given up in creating the negative pressure waves. The
convergent section of the chamber appears like a closed-end tube to the
pressure pulse, and as such causes another series of waves to be reflected
back up the pipe, except these waves are the same sign as the original
(a compression, or pressure wave is returned). Notice that this cone has
a sharper angle than the diffuser, so that a larger proportion of energy
is extracted from the already weak pressure pulse:
This pulse is timed to reach the exhaust port after
the transfer ports close, but before the exhaust port closes. The returning
compression wave pushes the mixture drawn into the header by the negative
pressure wave back into the cylinder, thus supercharging (a bigger charge
than normal) the engine. The straight section of pipe between the two cones
exists to ensure that the positive waves reaches the exhaust port at the
correct time:
Since this device uses sonic energy to achieve supercharging,
it is regulated by the speed of sound in the hot exhaust gas, the dimensions
of the different sections of the exhaust system, and the port durations
of the engine. Because of this, it is only effective for a very narrow
RPM range. This explains why two-stroke motorcycles equipped with expansion
chambers have such vicious powerbands (especially in the old days before
variable exhaust port timing existed). With the design illustrated here
(i.e. a single divergent stage and a single convergent stage), the powerband
of the engine will be akin to a 'light switch' - once the expansion chamber
goes into resonance, there will be a HUGE, almost instantaneous increase
in power. The powerband can be softened somewhat by reducing the angles
on the cones, but this is simply due to a lower degree of supercharging.
In order to get the best of both worlds (a large power increase and a wide
powerband), the cones should consist of several sections, with a different
angle for each section. Proper design of even a simple expansion chamber
is somewhat of a black art, even though formulae exist that will get you
in the ballpark (there is quite a bit more to this than simply choosing
the appropriate angles and lengths based on sonic velocity - everything
about the pipe comes into play, including the headpipe diameter and length,
and the tailpipe ('stinger') diameter and length). Design of a multi-stage
expansion chamber becomes incredibly difficult - it basically comes down
to the old 'cut and try' approach in the end. This of course is not even
considering whether or not the exhaust and transfer port timings and outlet
areas have been optimized for expansion chamber use.
Anyway, here is the animation you came to see...
Copyright © 1997 Joseph
A. Schuster |