Engine Fairbanks Morse Opposed Piston
My Dad served on the SSR-312 Burrfish during Korea. I forget if he qualified on the Congor or the Burrfish… he kept the forward engines doing their job (Fairbanks-Morse). The Burrfish was one of the WWII Fleet Type subs converted to radar picket duty.
He tells some interesting stories from those days. The Burrfish had a sticky poppet valve on one of the forward ballast tanks (which were filled with seawater to submerge by opening valves at the bottom and tops of the tanks, and were then blown full of air to surface).
I’ll probably muddle the details here, but what I remember is that they had a visiting dive officer (possibly trying to qualify), who ordered the boat down. The valve stuck closed, keeping a bubble of air in the forward tank, which made it hard to get the nose underwater. So, instead of stopping to see what the problem was, he ordered more speed and full down on the dive planes (little winglets at the bow and stern), which gave the desired result, until the valve decided to unstick itself.
At this point, things took a literal nosedive, and Dad said he saw the depth gauge pass 400′ (dangerously deep for that class of boat), and about 35° down angle (about a 75% grade in road terminology). They finally got things leveled out, surfaced, and the officer was packed off to some surface ship assignment.
The other story I remember is when Dad’s boat had been in the Mediterranean Sea, and hit a rapid seawater temperature change while running surfaced through the Gibraltar Straits when they hit the cold Atlantic waters. He said it cracked most of the cylinder liners on the engines. They had to tear apart all four engines to get enough usable parts to put one engine back on line…
In 2001, I took my Cub Scout Pack to Patriot’s Point, where we camped on the Yorktown. Dad and my two then-young sons came along, and one Saturday afternoon, he walked us through the Clamagore over the course of about two hours, telling these and other stories.
Dad could still point out all the different systems in the boat (which is required to qualify). That’s a cherished memory for both me and my sons…
Study finds Achates two-stroke opposed-piston engine shows indicated thermal efficiency of 53%
6 October 2011
Achates Power, the developer a two-stroke, compression-ignition (CI) opposed-piston (OP) engine (earlier post), 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).
Achates began presenting the results at the SAE 2011 Commercial Vehicle Engineering Congress, along with two SAE papers in Chicago; continued on to Der Arbeitsprozess des Verbrennungsmotors (The Working Process of the Internal Combustion Engine) in Graz; and then moved to the 2011 Directions in Engine-Efficiency and Emissions Research Conference in Detroit.
Thermodynamic analysis. Working with David Foster from the University of Wisconsin, the Achates Power team found in the thermodynamic review that combining the opposed-piston architecture with the two-stroke cycle increased the closed-cycle 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 (γ) 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.
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.
Averaged over a simple engine operating map, the opposed-piston two-stroke had 10.4% lower indicated-specific fuel consumption than the four-stroke engine and was accomplished with significantly lower peak cylinder pressures and temperatures.
Medium-duty application. The Achates team compared the performance of an inline 6-cylinder 4-stroke conventional diesel (Ford 6.7L V8) with an inline 3-cylinder 2-stroke opposed piston engine.
They found that the Achates engine demonstrated an 19% fuel consumption improvement over the conventional engine at similar engine-out emissions levels (EP 2010). Furthermore, oil consumption was measured to be less than 0.1% of fuel over the majority of the operating range. (Historic OP engines, and two-strokes in general, suffer from high oil consumption, on the order of ~1% or more.)
The Achates team notes that the two-stroke cycle and its double firing frequency provides the opportunity of decreasing brake mean effective pressure (BMEP) levels and increasing power density compared to four-stroke engines of equivalent power output. The lower BMEP levels can be accomplished with lower peak cylinder temperatures, which lead to lower mechanical stress on engine components, which can therefore be designed to be of lighter weight.
Lower cylinder temperature result in decreased NOx formation during combustion, lowering the requirements for aftertreatment. The increased power density leads directly to smaller engine package size and weight, the Achates team suggested, both beneficial to decreasing fuel consumption and manufacturing costs.
Resources
Randy E. Herold, Michael H. Wahl, Gerhard Regner, James U. Lemke, David E. Foster (2011) Thermodynamic Benefits of Opposed-Piston Two-Stroke Engines (SAE 2011-01-2216)
Gerhard Regner, Randy E. Herold, Michael H. Wahl, Eric Dion, Fabien Redon, David Johnson, Brian J. Callahan, Shauna McIntyre (2011) The Achates Power Opposed-Piston Two-Stroke Engine: Performance and Emissions Results in a Medium-Duty Application (SAE 2011-01-2221)
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