Greencarcongress – Achates Power, the developer a two-stroke, compression-ignition (CI) opposed-piston (OP) engine, is presenting performance and emissions results of the Achates engine used in a medium-duty application, as well as the results of a detailed thermodynamic analysis comparing the closed-cycle thermal efficiences of three engine configurations: a baseline 6-cylinder, 4-stroke engine; a hypothetical 3-cylinder opposed-piston 4-stroke engine; and a three-cylinder opposed-piston two-stroke engine (the Achates engine).
In a closed-cycle simulation, they found that the 4-stroke (4S) showed an indicated thermal efficiency (ηfuel) of 47.5%; the opposed-piston 4-stroke (OP4S), 50.1%; and the opposed-piston 2-stroke (OP2S, the Achates engine), 53%.
They selected five mode points to assess the OP2S efficiency advantage over a simulated operating map, and returned with a weighted average indicated thermal efficiency over the five points of 52.6% for the OP2S, compared to 47.7% for the 4S.
They found that the Achates engine demonstrated an 19% fuel consumption improvement over the conventional engine at similar engine-out emissions levels
Achates Power was a top Clean Energy Pick by Business Week
According to Business Week, the companies to look for are Achates Power (which is testing a clean-burning diesel engine), AltoRock Energy (which has bored wells as deep as four miles to tap geothermal energy), and BioFuelBox (which converts oil-laden sludge from sewage systems, industrial plants, and animal processing factories into clear, odorless diesel fuels).
A detailed thermodynamic analysis was performed to demonstrate the fundamental efficiency advantage of an opposed-piston two-stroke engine over a standard four-stroke engine. Three engine configurations were considered: a baseline six-cylinder four-stroke engine, a hypothetical three-cylinder opposed-piston four-stroke engine, and a three-cylinder opposed-piston two-stroke engine. The bore and stroke per piston were held constant for all engine configurations to minimize any potential differences in friction. The closed-cycle performance of the engine configurations were compared using a custom analysis tool that allowed the sources of thermal efficiency differences to be identified and quantified. The simulation results showed that combining the opposed-piston architecture with the two-stroke cycle increased the indicated thermal efficiency through a combination of three effects: reduced heat transfer because the opposed-piston architecture creates a more favorable combustion chamber area/volume ratio, increased ratio of specific heats because of leaner operating conditions made possible by the two-stroke cycle, and decreased combustion duration achievable at the fixed maximum pressure rise rate because of the lower energy release density of the two-stroke engine. When averaged over a representative engine cycle, the opposed-piston two-stroke engine had 10.4% lower indicated-specific fuel consumption than the four-stroke engine.
Historically, the opposed-piston two-stroke diesel engine set combined records for fuel efficiency and power density that have yet to be met by any other engine type. In the latter half of the twentieth century, the advent of modern emissions regulations stopped the wide-spread development of two-stroke engine for on-highway use. At Achates Power, modern analytical tools, materials, and engineering methods have been applied to the development process of an opposed-piston two-stroke engine, resulting in an engine design that has demonstrated a 15.5% fuel consumption improvement compared to a state-of-the-art 2010 medium-duty diesel engine at similar engine-out emissions levels. Furthermore, oil consumption has been measured to be less than 0.1% of fuel over the majority of the operating range. Additional benefits of the opposed-piston two-stroke diesel engine over a conventional four-stroke design are a reduced parts count and lower cost.