Forty-nine concept iterations, multiple patents and pending patents, as well as 1200-plus hours of dynamometer testing underscore the progress being made by Achates Power engineers to develop a two-stroke, opposed-piston, compression-ignition engine.
"We have developed a scaleable technology that achieves clean combustion at very high thermal efficiencies," said David Johnson, CEO of Achates Power, a San Diego, CA-based firm formed in 2004 with more than $50 million from venture capitalists, including Madrone Capital Partners (an investment branch of the family who founded Walmart).
The opposed-piston engine technology developed by Achates Power's 50 engineers and scientists is the patented "power cylinder," essentially a pair of opposed pistons in a single cylinder.
Positioned at opposite ends of the cylinder, the intake and exhaust ports are opened and closed by the piston's motion before and after passing through reversal points at bottom dead center. Because the ports are at opposite ends of the cylinder, the gas flows in one direction with fresh air entering via one cylinder end and exhaust gases exiting at the other cylinder end.
"Our uni-flow scavenging process is highly effective, and our patented port designs enable precise control of the air-charge motion as well as control of the gas composition for clean, efficient combustion," explained Johnson.
Fuel injectors are located at the power cylinder's geometric middle—essentially the point where the two pistons come together at top dead center. "That means that the fuel is directly injected into the highly compressed air charge and combustion occurs," said Johnson, adding, "Our engineers have spent countless hours crafting how to define, design, and develop the fuel delivery, the air-charge motion, and the ports."
Achates Power's power cylinder can be made larger or smaller in its bore size to accommodate larger or smaller engine displacements, or the addition of multiple cylinders also can change the engine displacement.
"By using just a single cylinder, we're able to get a stroke-to-bore ratio of 2.5:1. But our technology is also appropriate for a multiple-cylinder engine, a configuration in which each cylinder would have a pair of opposed pistons—meaning one cylinder with two pistons, or two cylinders with four pistons, or three cylinders with six pistons," said Johnson.
Although Achates Power's opposed piston engine is being designed to operate on various fuels, No. 2 diesel has been the focal point.
"A diesel engine that doesn't meet emissions doesn't have a future," said Johnson adding, "We've demonstrated very clean emissions for NOx, diesel particulate matter, hydrocarbons, and CO."
Emissions reduction for the engine will be handled by multiple techniques.
"There's a power density advantage inherent with a two-stroke engine. But by not pushing the limits with respect to power density, we're able to lower the temperature of the combustion and that lowers the NOx formation right from the beginning," explained Johnson, whose work experience includes stints with Navistar, General Motors (serving as Chief Engineer of clean diesel engine programs for passenger vehicles), and Ford.
When Achates Power is production-ready with its engine—expected within the next three years-existing emission after—treatment technologies will have a role.
"The engine will use a diesel particulate filter and a diesel oxidation catalyst, although we think those components can be downsized by close-coupling them to the engine," said Johnson. Use of selective catalytic reduction (SCR) technology can be employed as an additional option.
Because excessive oil consumption is a common, troublesome issue with two-stroke engines, engineers are using hardware and software tools to map oil usage as the engine is running in real time.
"We've demonstrated the ability to design a two-stroke engine with low oil consumption and that's important because high oil consumption creates a huge problem for emissions after-treatment. The customized and off-the-shelf analytical tools we're using to simulate and model a range of engine functions have solved many engineering challenges," said Johnson.
Engineers are also gaining insights from the dynamometer testing of Achates Power's A40, a single-cylinder research engine.
Industry interest in advanced opposed-piston engines is on the rise, particularly in the military and off-highway sectors where the engines’ multifuel capabilities, high power densities, and relatively low mass make them potentially attractive alternatives to conventional diesels. Other possible applications include marine propulsion and aircraft. Some light-duty automakers have expressed interest in the technology for use in extended-range electric vehicles.
The Achates Power program is on a similar pace with that of EcoMotors. Founded by former Volkswagen and FEV powertrain engineer Peter Hofbauer, the company’s opposed-piston, two-stroke, compression-ignition engine development currently is in its sixth generation. Combustion-optimization work began last summer. EcoMotors CEO Don Runkle expects the “power module” to be production-ready in two to three years.
"The more than 1200 hours of dynamometer testing, which has been ongoing since 2005, is serving a dual purpose. We're able to focus on combustion development, especially as it relates to the mixing of fuel and air, and we're able to focus on mechanical development in terms of durability and oil control," Johnson said.
While concept development work has focused on steady-state operation of the engine, the next steps include focusing a spotlight on the engine's transient operational characteristics.
"We've solved the big problems. Now it's about doing refinements, including making the engine more durable and making the combustion process fully repeatable at all speed/load points," said Johnson.
The SAE book, Opposed Piston Engines: Evolution, Use, and Future Applications, by Martin L.S. Flint and Jean-Pierrre Pirault, provides a detailed look at OPOC technology. A link to the book’s web page can be found at http://books.sae.org/book-r-378.