Saturday, November 15, 2014

Minimalist Four Stroke Engine (codename: Batwing)

A while back I built a four-stroke engine kit for fun.  I had a few thoughts ...
  1. Hmm, so that's how they work  :-)
  2. That's how it works ?!
  3. All the circular to linear conversion of motion seems inefficient to me.
  4. The whole design seems to have too many parts for what it needs to do.
So, I started thinking about how I'd like to see a four stroke engine designed (in theory).  A couple design goals were:
  1. minimize the number of parts
  2. avoid the circular to linear conversion of motion
  3. increase the symmetry of design
So, to start, I wanted to use both ends of a piston for combustion.   This immediately cuts the components in half.  Also, this way the engine could reuse momentum for compression in the next cycle.

Though it can do better.  The whole mechanism can be reduced to use only one irregularly shaped piston.

Here's a rough sketch of the idea.  Suppose the piston and case are both circular (like a torus).  The piston nests inside the case.    Since both are circular, the piston can move freely inside the case, but create a seal.  The piston could oscillate in a clockwise and counter-clockwise motion inside the case.

While I sketched the piston as having flat sides, the original thought was more of a torus shape.  I thought a flat design would be simpler to machine though.   

Compression occurs between the end of the piston and the points A B C D.

Ignition occurs in the cycle:

    A  B  C  D

Vent and fuel lines are between B and C.  Also, between A and D.   If cams are used, they can be driven directly from the rotations off the piston axle (no need for timing belt).

Example of operation, focusing on A:
  1. Piston rotates clockwise compressing the fuel at A, igniting A
  2. Piston rotates counter-clockwise (away from A).  Fuel at B compresses and ignites
  3. Piston rotates clockwise (away from B) compressing A.   The spent fuel at A vents out.  C ignites.
  4. Piston rotates counter-clockwise (away from C and A, and towards B and D).  Fuel is injected into A as the space opens up.  It's ready for re-compression.  Goto 1
The other cases follow the same pattern (just re-label A B C D).

Since all the motion is circular, this simplifies the cams needed to control the vent valves.  Everything is moving around the same point of symmetry.

Power is tapped from the piston's axle as it rotates.

Possible issues I see would be:
  1. The compression force would want to bend the piston slightly, causing possible wear on the inside of the case.
  2. The piston would fire like a bullet into the next compression cycle  (diesel fuel might make more sense in this case).  The power would have to be tapped in a very controlled fashion, otherwise you don't want excess force slamming the piston into the other side.  
  3. It may be difficult to get a tight 3D seal.

Let me know if you see any issues with the concept/design.  I don't plan on building it... but the fun part for me is in the design part.

I call this thing a "Batwing Engine," named after that weirdly shaped piston.  :-)

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