2 Stroke Engine Diagrams and Animation

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[h=2]2 Stroke Engine Diagrams and Animation[/h][h=3]2 Stroke Engine Animation[/h]     This article and pics courtesy of www.southernskies.net    
 As the 2 stroke engine animation below shows, a two-stroke engine 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-stroke engines have been  developed over the years, and each type has its own set of advantages and  disadvantages. The subject of the 2 stroke engine animation (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. As in the 2 stroke engine animation and diagram, 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, as shown in the 2 stroke engine animation, 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 in the 2 stroke engine animation and diagrams 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) as shown in the 2 stroke engine animation and the diagram here:

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 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. If you watch the 2 stroke engine  animation you will see 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. In the 2  stroke engine animation you can see the gases moving out of the exhaust  at the same time new mixture is entering the cylinder.
  That completes the chain of events for the basic two-stroke  cycle. The discussion is not complete. The 2 stroke engine animation 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. This device is also known as a tuned exhaust.

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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, you can see these pulses in the 2 stroke engine animation:
   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). Watch the 2 stroke engine animation closely  to see the waves reflected back up the pipe. 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.  Watch the fresh green mixture in the 2 stroke engine animation get drawn  into the expansion chamber before the returning waves "squeeze" it back  into the combustion chamber:  
   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. As illustrated in the 2 stroke engine animation, the pipes build specifications are extremely important on a 2 stroke engine:
   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. As the 2 stroke engine animation illustrates, the  timing of the pressure waves is perfect at a certain RPM resulting in  the 2 stroke engine "coming on pipe". 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.
    The 2 stroke engine animation is a tool to help you understand the importance of port timing and exhaust waves in a 2 stroke engine.