One of the most viable solutions to economically and environmentally-sustainable transportation has been around for more than a century. The opposed-piston, two-stroke engine was first manufactured successfully in 1890 and has since been used in a variety of ground, marine and aviation applications.
Architecturally, the design is unlike any standard engine available today. Instead of one piston per cylinder, the opposed-piston architecture has two pistons per cylinder, working in opposite, reciprocating motion. With this architecture, parts such as the cylinder head and valvetrain are eliminated. This makes the overall engine more efficient, since these components are primary contributors to heat and friction losses. The cylinder head and valvetrain systems are also among the most complex and costly elements of conventional engines.
Instead of one piston per cylinder, the opposed-piston architecture has two pistons per cylinder, working in opposite, reciprocating motion. With this architecture, parts such as the cylinder head and valvetrain are eliminated.
The two-stroke cycle compounds the efficiency benefits of the opposed-piston engine architecture. Because of the two-stroke cycle, each combustion event is shorter in duration and, therefore, closer to optimum timing, as compared to four-stroke cycle engines. Additionally, two-stroke engines will be smaller in displacement and size compared to four-stroke engines for similar performance. It is further possible to use some of the two-stroke power density to lower in-cylinder pressures and temperatures and, in so doing, reduce emissions. This, in combination with a direct injection fuel system, provides the ideal platform for a compression ignition engine – an architecture that, in itself, is more efficient than conventional spark-ignited engines due to higher expansion ratios and the ability to not only run on diesel (which has 100 times the gravimetric energy density of a lithium-ion battery) but also alternative fuels, such as biodiesel.
So, why aren’t opposed-piston, two-stroke engines in every passenger car and commercial vehicle on the road? Despite their architectural benefits, the opposed-piston engines of the mid-1900s were not able to meet modern emissions requirements including those first enacted by the US in 1970 with the passage of the Clean Air Act. In fact, opposed-piston engines at that time had high NOx and soot emissions as a result of inferior fuel injection, air charging and controls systems and unoptimised combustion chamber geometry. They also suffered from a lack of oil control and the resulting high hydrocarbon emissions.
With a steady increase in fuel prices and more stringent fuel economy and carbon pollution standards for commercial and passenger vehicles in the US and around the world, the need for a clean, more efficient and lower cost engine has never been greater.
Unlike battery-electric and hybrid alternatives, the opposed-piston, two-stroke engine provides the best fuel economy and emissions levels at the best price.
Using the industry’s leading-edge testing, simulation and analysis tools, it is possible to solve the well-known challenges of the opposed-piston engine architecture. Techniques such as computational fluid dynamics (CFD) and high-pressure fuel injection were not available 40 years ago. Today, however, they are essential to optimising the complex gas exchange process inherent in a two-stroke engine and to take advantage of the unique combustion geometries of the opposed-piston design. Furthermore, recent advancements in computing have reduced the analysis of complex multi-million cell models from weeks to days, significantly shortening the time for engine design and development.
Achates Power has applied these innovative tools in combination with rigorous science and engineering methods, resulting in an opposed-piston, two-stroke engine that, when compared to one of the best medium-duty engines available, demonstrates more than 20% lower cycle average brake-specific fuel consumption; similar engine-out emissions levels; low fuel-specific oil consumption and reduced cost, weight and complexity.
To make an impact on the global transportation industry, a new engine or technology needs to be sustainable – economically and environmentally. The opposed-piston, two-stroke engine meets this sustainability requirement and, unlike battery-electric and hybrid alternatives, provides the best fuel economy and emissions levels at the best price.
The opinions expressed here are those of the author and do not necessarily reflect the positions of Automotive World Ltd.
David M. Johnson is President and Chief Executive of Achates Power, a US developer of opposed-piston, two-stroke engine technology. Formerly, Johnson was vice president of product operations for military and export markets at Navistar as well as programme manager and chief engineer for multiple clean diesel engine programs at GM. He began his career at Ford Motor Company where he led the development and launch of the SuperDuty with an all new diesel engine, the highest volume diesel-powered pickup truck on the market. To learn more about Achates Power, visit www.achatespower.com.
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