Two-sided honing process for opposed-piston engine cylinder liners

Title of Invention:
Two-sided honing process for opposed-piston engine cylinder liners

Name of Inventor or Inventors:
Brian Callahan, William McHargue and Iain Read

Date of Disclosure:
April 8, 2011

Background:
When designing a surface for a piston to run within, it is necessary to control the surface roughness of the interface so that friction is managed and oil retention is achieved to prevent bore scuffing. There are a number of processes used to accomplish this, including:

  • Pre-machining to a diameter close to the final diameter
  • A rough hone process, which creates rough “peaks and valleys” within the material
  • A fine honing process, which removes the extreme “peaks” of the rough hone and achieves a plateau within the surface roughness

Novelty:
A limiting factor of honing is the stroke of the honing machine. Certain engine layouts have longer cylinders that require a long hone stroke. It is not uncommon during long honing strokes to lose some control over the cylindricity of the cylinder. In the case of an opposed-piston engine, it can be possible to allow the cylindricity of the intake bore to be “out of position” of the cylindricity of the exhaust side of the bore. Each piston can move independent of the other within the same bore and total control along the full length can be relaxed.

Invention:
This invention changes the honing process of the cylinder such that each side is honed in a difference process from the other pair. The first process is to machine and hone the intake side, then flip the part around and hone the exhaust side. This reduces the stroke of the honing machine and allows better control of cylindricity of each side.

The machining and honing can either stop at the top of the stroke (at the injector plane, for example) or it could stop short or pass through. Since honing stones are run through a cyclic movement, the extreme of the stone on the stroke does not achieve as much contact time as surfaces along the mid stroke. As long as the area level with the top ring on the piston is honed, the process will achieve the desired surface texture in the critical areas. Stopping before top dead center (TDC) can allow for extra material or a rougher surface finish in the combustion chamber and, therefore, can reduce blow-by. Passing through TDC can remove the local taper associated with stone contact frequency. There is a possibility to rough hone past TDC. This establishes large “peaks and valleys” in the surface roughness, which is normally advantageous for scuffing control. If the second hone stops short of TDC, the surface finish in the ring area can be smoother than the combustion area, which can reduce blow-by. A third honing pass at another penetration depth can provide a very smooth surface in the main length of the cylinder where friction control is critical. Using this method, the cylinder (per side) can have three discreet surface roughness specifications to provide the best blow-by control, scuffing control at top ring reversal and a smooth surface for friction.

By changing the diameter of the part locally, below the ports, it is possible to control a rougher surface finish at the bottom ring reversal zone. Increasing the diameter in this area reduces ring pressure, an advantage to friction, scuffing and oil control.

Furthermore, the process can be achieved with a single hone and flipping the cylinder. It can also be achieved with two opposed honing machines, with the honing tools running in or out of phase. An additional benefit of this method is that doubling the number of machines reduces the process cycle time.