Historically, opposed-piston, two-stroke (OP2S) engines have set combined records for fuel efficiency and power density. But, because they are piston ported, the engines were mistakenly dismissed for use in emissions-compliant, on-highway vehicle applications due, in part, to oil control concerns.
 
In 1998, however, Achates Power founder Dr. James Lemke began investigating the opposed-piston engine—seeking to prove that the architecture was not only fuel efficient, but could also overcome the inherent challenges with oil consumption and emissions.
 
Using state-of-the art equipment and cutting-edge scientific and engineering methods, Achates Power has since developed a more fuel efficient OP2S diesel engine that has demonstrated oil control at levels acceptable for combustion and emissions purposes. And we’ve done it across a range of engine speeds and loads.
 
How?
 
We start by measuring oil consumption with the Da Vinci Lubricant Oil Consumption (DALOC) measurement system, which uses a sulfur tracing technique. The core of this technique is to measure the concentration of atomic sulfur in the exhaust gas stream while running sulfur-free fuel (actually 1.0 to 1.2 ppm) and sulfur-containing oil (around 4000 ppm), formulate a sulfur mass balance over the engine, and then calculate oil consumption. Among the engine parameters calculated are:
 

  • Air flow
  • Fuel flow
  • Exhaust gas sulfur concentration
  • Fuel sulfur concentration
  • Oil sulfur concentration

 
The first three quantities are measured simultaneously. This enables real-time oil consumption calculations for steady-state and transient engine testing.
 
When testing this way, the lower the fuel sulfur concentration compared to oil sulfur concentration, the better signal-to-noise ratio for the measurement. For example, fuel sulfur levels less than 3.0 ppm work well on engines with fuel-specific oil consumption (FSOC) levels around 0.10% and oil sulfur between 3000 and 4000 ppm. Note that we have the fuel sulfur level measured (per ASTM D5453) rather than estimating it.
 
To determine the sulfur concentration in the lubricating oil, we draw an oil sample from the oil sump for sulfur analysis by the ASTM D5185 method, which detects all sulfur-containing species in the oil. This is done at the beginning and end of each day’s testing.
 
After formulating sulfur balance over the engine control volume and assuming the engine was not generating or storing any sulfur, we algebraically calculate engine-out oil consumption. We also directly measure air flow and fuel mass flow rates—all while paying careful attention to conditioning the exhaust gas sample.
 
The sulfur tracing technique allows us to obtain three or more steady-state oil consumption maps per day with results that suggest sufficient repeatability of both the engine and instrument:
 

  • Weighted average variation of ± 2.6% intra-day
  • Weighted average variation of ± 4.2% inter-day

 
The sulfur tracer technique also helps us quickly understand how engine design considerations—such as oil ring tension, ring end gaps, land chamfers and more—impact engine oil consumption.
 

Fuel-Specific Oil Consumption for the Achates Power Engine
Fuel-specific oil consumption (FSOC) results are plotted on a contour map, similar to BSFC and BSNOx. Achates Power has successfully demonstrated FSOC lower than any opposed-piston or two-stroke engine reported in open literature.

To date, we’ve successfully demonstrated a weighted average FSOC of 0.18% across the entire operating map—significantly lower than the 0.25–0.60% FSOC of historic OP2S engines and the lowest FSOC ever reported in open literature for any two-stroke diesel engine. We’ve also shown oil consumption to be 30% lower during warm-up compared to the fully warm condition. With further refinement, we’re working to reach a weighted average FSOC of 0.05–0.10%—a level low enough for use in any application.
 
Content from this post was published in the technical paper Oil Consumption Measurements for a Modern Opposed-Piston Two-Stroke Diesel Engine and presented at the ASME 2011 ICE Division Fall Technical Conference.

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