This little wonder is quite the ingenious design.  I'm still somewhat baffled as to how Kevin thought it up and he even admitted that he is surprised it worked.  Like the previous 3 designs, this engine will run in any position - upsidedown, rightsideup, sideways... it doesn't matter.  But the one thing that this engine does that the other 3 can't - it can run forwards AND backwards.

      You'll notice right away that there are no external valve assemblies or cams.  In comparison to the previous 3 designs, you can see the similarities though, such as the flywheel, the intake and exhaust ports, and the crankshaft.  But how does this engine run apparantly without any valves?  Well the secret is that the valves are INSIDE the cylinder and are triggered by the piston - a very unconventional design to say the least.  The valve assembly actually forms one wall (a sealing surface) of the cylinder and as the piston hits the extremes of its travel, it hits stoppers that are located on the ends of the slider valve.  The slider is dual acting  - meaning that when the piston pulls the slider valve (either up or down), the ports are automatically switched into the correct position - either in the "intake open exhaust closed" position or the "intake closed exhaust open" position.
 
 

    Here I removed the head of the cylinder to reveal the "secret" of the internal slider valve design.  The valves are actually pulled out father than they would normally travel.  The picture doesn't show it that well but the two red 4x2x1 pieces are not "level" with each other. Likewise, the intake and exhaust ports are not level or straight across from each other as is the case with the previous 3 engine designs.  The red piece to the left sits lower than the one on the right and that is how, when the valve is either all the way up or all the way down, it will have either port in the correct position to keep the engine running.
 
 
 
 
 
 
 
 
 
 
 
 

    The intent of these photos was to show the bottom part of the valve and how the piston operated it.  Unfortunately, the pictures were deeply shadowed in the area I wanted you to see.  However, it still illustrates the full travel and motion of the whole crankshaft/connecting rod/piston assembly.  This engine also has a longer stroke than the previous designs because the valve operation is dependant upon the piston travel.  The long stroke makes for a slower running engine but with the benifit of a lot of torque.  If all these engines were set up to power something, I seriously believe this engine, although winning no races, would win a longevity award - it's a very simple and reliable design which surprisingly, uses a bare minimum of LEGO Technic parts.

    When building an engine like this, everything (and I mean EVERYTHING) has to work with the other components in harmony..  At first, this sounds like a statement that would qualify me for the Masters of the Obvious award but think about it.  The travel of the piston has to coincide with the travel of the valve.  If the valve got pushed up too far, it would bust out the top of the cylinder.  If it got pulled down too far, it would rip off the bottom stopper of the valve or break the piston.  Also, your piston and valve travel may work okay but then the valves may not line up with the ports correctly, causing the engine to not run at all.  This requires either modifying the cylinder, the piston travel, the valve travel, or all of the above.  Changing one part of the engine more often than not requires modifying the whole engine so that everything works together.  This is the case in real life as well - often modifying one part of an engine can require modifying many other parts of the engine so everything works together in harmony - once again demonstrating that LEGO can teach real life lessons.