Small-engine and compressor performance
Engines for outdoor power equipment, ATV's, 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 OEM's 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 (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/cylinder fit and alignment. The precision piston/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.
Honing developed with early auto industry
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 1930's, 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 Sunnen hone so successful has since been developed into a uniquely capable 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 controlled 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 controlled hone, can easily achieve sizing accuracies of ±0.000010" (0.25 µm). The high resolution on the tool feed systems of today's machines 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.
Process development key to selecting machines, abrasives and automation
To meet the needs of OEM's 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/manual labor, cost objectives, part weights, operator skills, geometry and tolerance requirements, quality of upstream processes, quality of (die)castings, use of cylinder platings/liners, etc. Depending on the region of the world, an OEM might prefer to use manually loaded/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 consistent to achieve a consistent outcome with honing. Abrasives must be matched to balance the desired finish and quality with cycle time and consumable cost.
Where honing is going to be added to a production line, process development helps establish the right types of machines and abrasives to meet the OEM's overall partmaking 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/higher mix.
Honing systems that have proven particularly agile in meeting the requirements for the small-engine OEM are the Sunnen ML-5000, KGM-5000, SV-1000 and SV-300. They can all be used in manual or automated production environments. Using metal-bond abrasives with a tool feed system capable of adjustments in increments of 0.00001" (0.25 µm), the KGM and SV series machines are ideal for "setting" the bore diameter, straightness and roundness with a precision that cannot be matched with production boring machines.
A two-cycle engine application
Two cycle engine cylinders have all the characteristics that challenge honing. A two-cycle cylinder is often a blind bore which 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 non-uniformity in the surface which has to be "straightened out" first. The hardness of chrome can also lead to eggshelling 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 4,000,000 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 (two cycle) tool is equipped with a large number of stones aligned in a an array designed for a ported, blind, tapered bore. The large number of stones produces superior roundness and consistency of 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 a size and arrangement 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 metallurgically stable 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 0.25µm (0.00001") without operator intervention.
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-controlled, and even manual, honing systems are proving an important part of the technology behind the improved efficiency, life and performance of today's compressors and small engines.