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issue: March 2008 APPLIANCE Magazine

Small Engine and Compressor Performance
Honing Can Improve Small Engine and Compressor Performance


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by Rich Moellenberg, Sunnen Products Co.

Precision size, geometry, and surface of cylinder bores improve sealing and component life, while IC engines increase power density with lower exhaust emissions.

Two-cycle honing tool is designed for work inside interrupted, blind bore.

Engines for outdoor power equipment, all-terrain vehicles, snowmobiles, motorcycles, and outboard motors, along with many piston-type air and refrigeration compressors, share a common operating principle—a piston reciprocates in a cylinder bore. And like their big brothers on the NASCAR circuit, these engines are also beginning to share a “performance” technology used for race engines and most other auto engines—honing. Honing optimizes the cylinder bore’s geometry, surface finish, and dimensional accuracy like no other machining process can, resulting in higher power density, longer life, and lower emissions.

A changing regulatory climate that targets emissions and noise has led to a convergence of design goals for small engines and automotive engines in the last decade. Small-engine OEMs are striving for higher power densities and lower fuel consumption, lower emissions, all while controlling cost. As a result, they are coming to appreciate what the automotive industry has known for decades: The fit between the piston and bore, along with the quality of the surface contact between the two, influences power output, combustion efficiency, emissions, component life, and vibration, to name just a few.

The precision of bore geometry (i.e., roundness, straightness, cylindricity, diameter) and surface finish play important roles in engine life and performance. Tests have shown that precision bore geometry and finish reduce vibration, friction, and blowby because of improved piston and cylinder fit and alignment. The precision piston and cylinder interface contributes to better sealing and lubrication, greater combustion efficiency, more usable power, and lower emissions. Honing is the only process capable of producing this precise geometry and surface finish with the consistency needed for mass production.

Die-cast two-cycle chainsaw engine with interrupted bore is chrome plated and then honed to tolerance of 0.0002 in./ 0.005 mm for diameter, straightness, and roundness.

Early Honing Developments

Honing is an abrasive machining process whereby a tool with expanding abrasive stone assemblies rotates in the cylinder bore, while the tool or the part reciprocates rapidly during the process. Because the cutting points of the honing abrasive are so small and numerous, heat and stress in the workpiece are minimal. As a result, the surface integrity of the bore is excellent and can be finished to a specified level of roughness.

Cylinder honing first became popular in engine rebuilding during the 1930s, shortly after Joseph Sunnen invented and patented the offset hone head for deglazing cylinder walls during engine rebuilding. He sold this product to shops to “touch up” cylinders when they installed new piston rings. Where cylinders were resized by boring, Sunnen’s hone was used as a finishing operation. The principle that made the first hone so successful has since been developed into a uniquely capable computer numerical control (CNC) machining process that adds many favorable properties to new cylinders.

One of the keys to the manufacture of engine and compressor cylinders is the crosshatch pattern produced on the bore surface. The honed finish combines a specific quality of bearing surface with a defined crosshatch pattern that enhances axial lubrication transfer. These traits are particularly beneficial for engines and compressors. Today’s CNC honing systems can also correct a multitude of errors in bore geometry, such as barrel, taper, centerline bow, roundness, and straightness.

Honing, especially on a CNC hone, can easily achieve sizing accuracies of ±0.000010 in. (0.25 µm). The high resolution on today’s tool feed systems minimizes process variability, so honing is ideal for high Cpk process control. In a production environment, honing is a more capable process than reaming or boring. The latter two processes have sometimes been considered adequate finishing steps for small engine bores, compressors, and hydraulic components. However, the drive for performance improvements in power and efficiency—with lower emissions and fewer maintenance problems—is leading small-engine and compressor makers to adopt honing to set the final bore geometry, size, and finish.

One of the keys to the performance of engine and compressor cylinders is the crosshatch pattern that honing produces on the bore surface. The honed finish combines a specific quality of bearing surface with a defined crosshatch pattern that enhances axial lubrication transfer.

Process Development Is the Key

To meet the needs of OEMs producing <50-hp engines and compressors in volumes of 300,000 to millions per year, honing system builders have developed solutions that differ from the hard automation that works well in the automotive industry. Makers of small engines and compressors differ widely in their part volumes, use of automation and manual labor, cost objectives, part weights, operator skills, geometry and tolerance requirements, quality of upstream processes, quality of (die) castings, and use of cylinder platings and liners, etc. Depending on the region of the world, an OEM might prefer to use manually loaded and unloaded machines, even in a plant producing millions of cylinders per year.

Cylinders might be lined with cast iron, Nikasil, or chrome plating, or the cylinder might be made of high-silica aluminum, all of which will affect the selection of abrasives and machines. The quality of the castings must be well established and reliable to achieve a consistent outcome with honing. Abrasives must be matched to balance the desired finish and quality with cycle time and consumable cost.

If an OEM is going to add honing to a production line, process development helps establish the right types of machines and abrasives to meet the OEM’s overall part-making objectives with an output that synchronizes with upstream and downstream processes (takt time). Changeover time and flexibility are important, too, because the work tends to be lower volume and higher mix.

Cast-iron refrigeration compressor has three bores that are honed with a precision-head (PH) hone head.

A Two-Cycle Engine Application

Two-cycle (TC) engine cylinders have all the characteristics that challenge honing. A TC cylinder is often a blind bore that is sometimes tapered. There are sizable ports in the cylinder wall, so it is critical to avoid washout around the ports, a characteristic honing defect caused by improper tooling. Most of these cylinders are chrome or Nikasil plated, which creates its own set of challenges because there is some initial nonuniformity in the surface that has to be resolved first. The hardness of chrome can also lead to “egg shelling” if the abrasive grit is too large.

All of these issues have been addressed in a very successful product improvement program for a leading maker of small engines. The company is currently using 11 cells of Sunnen machines at a plant that turns out 4 million engine cylinders a year in 20 different models.

The basic engine cylinder starts as an aluminum die casting. The castings are aged to normalize them, then bored and machined on CNC machining centers. The parts are then deburred before being chrome plated.

For this application, a custom TC tool is equipped with a large number of stones aligned in an array designed for a ported, blind, tapered bore. The large number of stones produces superior roundness and consistent finish. The TC tooling is specifically designed to avoid washout around the cylinder ports, which helps maintain the cylinder’s seal characteristics.

Washout occurs when the honing abrasive tries to bulge out through the port opening as it is forced against the cylinder wall. This could produce a radius on the edge of the port opening where a sharp 90 degrees is needed, resulting in some irregularity of roundness and a small degradation of ring seal.

To hone a bore with a keyway or open hole in its wall, the stones must be sized and arranged in a manner that bridges the opening. A very rigid tool also eliminates the tendency to washout.

As a final step, the bore is plateau honed. Plateau honing increases the life of both cylinders and piston rings by producing a surface finish on the cylinder wall that is “broken in” before the engine starts the first time. Plateauing creates a stable metallurgical surface on the wall of the cylinder, removing loose, torn, or folded material. A machined metal surface typically consists of microscopic peaks and valleys. Plateauing cuts down the peaks to produce a bearing area that meets an exact spec, while leaving the valleys. This produces a very smooth surface with high sealing contact between the cylinder and piston ring, while leaving the crosshatch valleys to retain lubrication.

After proving the value of honing on cylinder bores, this manufacturer is now evaluating improvements gained from honed connecting rods. The plant recently purchased a vertical CNC honing system, which can automatically control hole size to accuracies of 0.25 µm (0.00001 in.) without operator intervention.

Conclusion

The challenges in honing small engines and compressors are little different from those involved in any advanced machining process. While process development is always crucial, CNC—and even manual—honing systems are proving to be an important part of the technology behind the improved efficiency, life, and performance of today’s compressors and small engines.


www.sunnen.com

About the Author

Rich Moellenberg has a BSME degree from the University of Missouri-Rolla. He has 25 years of honing application development experience and is currently manager, global sales services, at Sunnen Products Co. If you would like to contact Moellenberg, please e-mail lisa.bonnema@cancom.com.

 

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