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Creating a Green Engine

By Achates Power Contributed Article

The backstory of how one company helped develop a greener engine

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Sergio Ramirez programs the toolpaths for a piston.

If San Diego-based Achates Power is ahead of its time now, it was completely revolutionary when it was founded in 2004 by James U. Lemke, former physics professor and founder of Spin Physics Inc. The company designs green internal combustion engines for passenger and commercial vehicles, and for military, marine, and stationary use. Achates Power’s success in designing high efficiency, low cost engines has resulted in working relationships with nearly every passenger and commercial vehicle original equipment manufacturer in the industry.

The engines meet emissions standards such as EPA 2010, Euro 6, and Tier3/LEV 3. In addition to various industry awards, Achates Power was awarded $9 million from the U.S. Department of Energy’s ARPA-E program to create their new gasoline compression ignition opposed-piston engine.

This innovative compression ignition engine design is a result of a partnership with Argonne National Laboratory and Delphi Technologies. The 2.7-liter gasoline engine will be used in full-sized pickup trucks and boasts 37 miles per gallon, 270 horsepower, and 480 pound-feet of torque. The opposed-piston design means that no cylinder head is necessary, reducing heat loss and increasing efficiency. Most impressive, though, is that the engine will cost $1,000 less than a comparable engine. The next phase of production is getting the engine out on the road. “By the end of 2019, we’ll have thousands of test hours and miles on that engine,” said Sergio Ramirez, shop manager at Achates Power.

Achates Power focuses its resources on design and prototype creation rather than manufacturing finished parts. The licenses to manufacture are sold to engine manufacturers after the prototypes are perfected and tested. For the most part, the only prototype components that are machined in-house are pistons, the pistons’ internal pieces (i.e. any components that go within the piston for telemetry issues or diagnostic wiring harness brackets), and the cylinder liners along which the pistons ride. “Anything to do with the combustion of the engine is what we try to keep in-house because those are really the intellectual property components,” Ramirez said.

Prototyping Process

Much of the prototyping process involves producing a few pieces to evaluate before making minor design changes. According to Ramirez, production pistons will be produced as two-piece welded forgings or castings. Forging or casting prototypes are cost-prohibitive due to the material used; the material used in rapid prototyping won’t stand up to the heating and welding processes, so he machines them using 4140 and 4340 steel billet.

Additionally, machining undercuts and cavities in pistons comes with many more difficulties than are associated with casting them. One of these is the different thermal properties associated with casting materials and billet. According to Ramirez, once the two pieces are welded, even with the use of 5-axis machines and lollipop endmill tools, it was difficult to reach the underside of the crown of a piston to cut the cavities. Ramirez turned to San Diego CAD/CAM technical support specialist Trevor Bailey, who showed him how Mastercam CAD/CAM software (CNC Software, Inc., Tolland, Conn.) could help solve his problem.

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Manny Rodriguez works on a prototype of the 2.7 liter gas engine that boasts 37 mpg, 270 horsepower and 480 pound-feet of torque.

The solution was to machine all the different undercuts that were needed in the pre-welded crown portion of the piston. Using the software’s 5-axis toolpaths–including Multiaxis Flowline, Swarf Milling and Rotary toolpaths—the crown could be machined in a shorter reach setup.

Once the crown was welded to the bottom portion of the piston, machinists needed to machine all the internal cavities and undercuts for this part. Ramirez relied heavily on the software’s Port Expert add-on which uses collision checks along the tools and the cutting surfaces. The add-on, initially made to CNC program engine cylinder head porting toolpaths, has other uses.

“Port Expert allows us to tilt more in a tangent cutting toolpath along the surface. We just have more vectoring control of the tool and are able to utilize the optimum cutting flute surfaces on the lollipop endmills,” Ramirez said. When he or another programmer has questions, they can easily turn to Bailey for support. “He gives me insight on what else this Port Expert add-on can do for us. Not just making cylinder ports, but using it for pockets and for all kinds of different shapes and surfaces.”

“Machining this assembly has been the biggest challenge because of the deep cavities and radii in the corners; small cutting tools just don’t like it. And, any time you weld two pieces together, there is the possibility of a failure mode. You don’t want the piston to break apart during machining, let alone in an engine. But we’ve surpassed it and we haven’t had any failures. That was probably the biggest climb we had to take – to try to mimic the performance of a production-ready steel piston using billet,” Ramirez said.

The software provides Ramirez with countless options for setting parameters, programming different stepovers, and specifying different depths of cut as needed. “There are lead in, lead out cuts and different roughing parameters that the software allows you to use,” Ramirez said. “We’ve used trochoidal toolpaths where it’s more of a peeling toolpath versus just a straight cut, and we can see the amount of tool and material burial on the screen. The parameters give us tons of control and helps us achieve the surface finishes we are looking for.”

After the pistons are machined in-house, they are then sent out for Wire EDM cutting of wrist pin bore. The pistons are further machined with desired combustion bowl geometry, piston profiles, and ring grooves. They get the final coatings and are ready to install in an engine and head over to one of Achates Power’s three engine test cells.

The Future

The cylinder liner is also a two-piece design that starts with a cast iron cylinder machined with all the cooling features; a 4140-steel sleeve is then pressed over it. The cooling features are proprietary, but they contain a combination of hundreds of pegs and grooves, each with different angles and geometric planes. Ramirez enlisted Bailey to help in machining these.

Bailey suggested a few combination toolpaths for roughing the pegs and grooves. He started with a flowline to rough in between the pegs while rotating the 4th axis rotary. He continued with a multiaxis/roughing toolpath. For finishing, they used a multiaxis morph/curved toolpath that proved to work best. Ultimately there were many toolpaths and additional geometry that had to be created as well as transformed, translated, rotated, and mirrored toolpaths, but in the end, they achieved exactly want was designed.

“Mastercam provides a really good way of seeing your parent stock or the stuff that you’re cutting, so we are able to follow the process of a piston or liner or other parts because we have updated models for each step of the process, and that has been really helpful for us when programming,” said Ramirez.

The software’s Verify feature allows Achates Power to program offline when a machine is cutting another part, or the machinists are tending to another job because all toolpaths and cutting strategies have been simulated and proven out within the software to ensure that the tool will not hit something it shouldn’t while running at high speeds. Programmed jobs can be immediately run on the machines after they finish previous jobs, minimizing downtime.

After road testing, Achates Power will turn over manufacturing to a facility with bigger capabilities. “We prove it out by producing a handful of engines, doing all the test hours that the customer would like,” Ramirez said. “The customer will then be responsible for manufacturing engines for their products.”

Edited by Yearbook Editor Bill Koenig from information supplied by Achates Power.

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