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Reciprocating Engines Never Had it so Good

Before a single piston part is produced, Zollner Pistons has run it through a complete design and analysis process to ensure optimized part design and high part quality—in record time.
#ANSYS #Ricardo


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Hypereutectic alloy pistons feature increased fatigue strength, decreased thermal expansion, lower thermal conductivity, increased scuff resistance, lower friction, and better wear properties than pistons made with other alloys.

These properties are the tail that wag the dog; they affect both piston and engine design. For instance, the alloy's increased fatigue strength allows for pistons with thinner wall sections and lighter weight. Lower thermal expansion helps minimize the clearance between piston and cylinder, which increases stability within the cylinder and decreases piston noise. Lower thermal conductivity causes more of the heat energy produced in combustion to be put into useful work, rather than wasted in the engine's cooling system.

All of these benefits are against the backdrop of ever-changing engine designs. New internal combustion engine designs are constantly being produced to meet requirements in emissions, durability, fuel economy, and customer satisfaction. In this case, the customers are both the vehicle manufacturers and vehicle drivers.

Because of this, explains Ray Istenes, Product Analysis manager for Zollner Pistons (Fort Wayne, IN), Zollner "has to pay close attention to the changes that engine designers are implementing. And we have to produce prototypes with those changes quickly so that the new designs can be put into an engine and tested."

Key to that quick turnaround at Zollner are a variety of computer-aided design (CAD) and computer-aided engineering (CAE) software products. Primary among them is the Pro/Engineer mechanical CAD system from Parametric Technology Corp. (Waltham, MA). 3D models are created of Zollner's pistons and the mold tools to manufacture them. Along the way, third-party finite element analysis (FEA) applications check the integrity of the pistons and tooling. The approved mold geometry is then passed to the tooling manufacturers, who generate the toolpaths to cut the piston molds on computer numerical control (CNC) machines.

Thanks to this up-front work, when the molds come back to Zollner, they are exact. "We do less `cut and try' and we have a much higher confidence level in the first parts we put into an engine," says Istenes.

The Very Model of a Model Piston

Founded in 1912, Zollner Pistons today is a member of a rather exclusive club: It's the only major American-owned original equipment piston manufacturer in the United States. Zollner designs, develops, and manufactures pistons for passengers cars, trucks, agricultural and industrial machinery, and marine engines. The company is both ISO 9001 and QS 9000 certified.

Here is a 3D electronic solid model of a finished piston Zollner designed in PTC's Pro/Engineer for a diesel engine.

Zollner engineers use CAD/CAE to create a "virtual piston" that matches its customer's requirements for weight, noise, performance, and cost. This virtualness starts with a product model of a cylinder that is the length and diameter of the overall piston casting. This model is then hollowed out to form the undercrown and skirt thickness of a piston. Protrusions are added for the ring belt, skirt open-end belts, and pin boss with its supports. Cuts are made for the skirt tails, pin-boss outer-reliefs, and cast-pin bores. Cast-piston top features are then added, along with radii on all the sharp corners of the piston casting. The electronic piston solid model is complete after machining cuts are added, such as ring grooves, skirt, and lands.

However, these new piston designs don't typically start from scratch. Instead, Zollner engineers start with primitive CAD geometry of an existing piston that closely matches customer requirements. They then alter this into a finished piston design.

Pre-existing geometries are contained in a library of existing pistons and generic mold shapes within Pro/Engineer. The engineer taps into this library, scans its inventory for specific parts, pulls out the appropriate part assembly files, and then starts modifying those design files. Most of the new piston design already exists, so modifications are relatively minor, such as redimensioning, changing piston shapes, and adding new features. "We basically start a mile-long race about a quarter of mile into it," comments Darren Bailey, Mold Design coordinator. The finished new designs can then be saved for future use.

Zollner is using Pro/Engineer to design both the piston and the tooling needed to manufacture the piston. But rather than "design from scratch," Zollner first checks an on-line library of piston and mold parts. It can retrieve a previously stored design model, such as this one of the core assembly that forms the inside of a piston, and revise it for a current customer's requirements.

Because Pro/Engineer is parametrically based, the relationships between design features are already defined so that a change to one feature is automatically reflected in others that are affected by that change. Moreover, because the software features full associativity, changes in the geometry and dimensions of the piston 3D model are automatically and immediately reflected in the associated 2D drawings for that piston and its molds—and vice versa. These 2D drawings are critical as a reference to inspect piston prototypes. The end result is a molded part of which probably 70% of the surface area gets machined. Design tolerances are critical because of the areas of the piston never get machined, such as the undercrown regions. Some pistons are cast-to-size; certain areas are at their desired weight and there's no excess material to remove. "Small variations in dimensions can add up to weights large enough to throw a piston out of tolerance," explains Istenes. "That's why it's critical that the core geometry of the piston-as-cast is identical to what we want molded for that part."

Also critical is that the piston works! Zollner uses PTC's Pro/Mesh to move the 3D solid model from Pro/Engineer to Ansys Mechanical from Ansys Inc. (Canonsburg, PA). Ansys Mechanical helps Zollner determine how the piston will perform in an engine. If the FEA system shows an area of the piston as either under- or over-designed, the analyst can recommend adding or subtracting weight from the solid model. "If we know we're going to make a design change, we can make it right there, early in the game, before actually making prototypes," says Bailey.

Complete piston mold assemblies can be designed in PTC's Pro/Moldesign module, which lets Zollner also check the mold for design interferences and to ensure the proper fit among mold components.

Once the piston is shown to perform at acceptable stress levels, Zollner ports the design over to another software package: Pisdyn from Ricardo Consulting Engineers Ltd. (West Sussex, England). Pisdyn is a dynamic analysis software package for piston, pin, and connecting rod simulations. It displays the piston moving inside a cylinder bore. It also performs vibration, motions, and contact analysis, giving Zollner an estimate about how much noise the piston might make. The results of these analyses are also incorporated in the piston design, well before the associated mold design is completed.

Tooling Up For Production

Once a 3D model of the piston design is approved, work begins on the mold that will produce that piston. Zollner's piston molds contain five major components. Again using the Pro/Engineer library of piston and mold parts, Zollner engineers select the proper pre-designed mold components they want to use. Because the size and shape of these mold parts never change, 3D models are never generated from scratch, which helps reduce overall design time.

Pro/Moldesign uses the piston geometry to make a reference part that incorporates a defined shrink factor. The final mold geometry is then ported to ProCAST from UES, Inc. (Dayton, OH). ProCAST helps Zollner visualize how molten metal will flow into the molds—all before machining any metal. In fact, ProCAST simulates the entire casting process—from filling to solidification to ejection. ProCAST shows the locations of incomplete filling caused by trapped gas, displaying hot spots and porosity. It also lets engineers optimize their mold designs by simulating process modifications on-line, such as in vent location, mold spray, cooling and heating lines, and chill locations. Optimizing the mold designs reduces the scrap rate in production.

The finished mold design files are sent to outside suppliers as an IGES file, along with 2D blueprints of the mold components. Because of customer requirements, Zollner may require as few as two or three molds of a piston, or as many as 55 molds (40 of which might be in production at any one time while the rest are kept as spares for maintenance). Consequently, Zollner might need several of the same mold geometry from several different suppliers.

Each mold supplier generates toolpaths to machine the finished molds. They can do that in any way they want—as long as they use Zollner's geometry.

The Measure of Good Quality

"People don't want to take four to five years to develop a new engine," says Istenes. "They want to do that faster and they want their first prototype parts faster. The best way to do that is to take time out of the design process."

By no small coincidence, the actual Zollner piston and the mold to produce the piston match what was designed in CAD. The weight of finished pistons differ from what was designed by 1% or less.(Source: Zollner Pistons)

CAD/CAE lets Zollner do exactly that. The time for mold design alone has been reduced by about 30%. In one year, the time from concept to manufactured piston was halved; what took 21 weeks now takes 10 weeks.

The tooling suppliers are realizing some benefit, too. In the past, they used to keep on their shelves blanks of core mold components generically machined to a certain dimension—and ready for final machining when an order came in from Zollner. Now they keep rough steel stock on hand, which they then machine into molds while still making the lead times Zollner requires.

Because Zollner gives the same IGES files of mold geometry to its suppliers, the finished molds are virtually identical. This is critical, explains Istenes, "because all of our customers, automotive and diesel, have relatively stringent requirements. We need to ensure that all the molds in production are making pistons within, for example, the weight limitation set by the customer, which might be plus/minus a few grams." Nor can Zollner afford making "overweight" pistons, which would generate scrap and increase material costs.

So how identical are these molds? For one piston contract, Zollner ordered 30 molds from three tool manufacturers. What came back had a variance of 0.002 inches. "These are the kind of results we are coming to expect working off a solid model," says Bailey. Apparently these stringent requirements manifest themselves throughout Zollner's design process and its tooling suppliers' machining operations. Tests on the piston weight of finished pistons differ from design predictions by 1% or less.