Evaluating and Enhancing Engine Durability

Meeting emissions standards, improving fuel efficiency and lowering manufacturing costs is critical to the success of any new engine design. So too is durability. In the commercial vehicle market, for example, truck owners expect their vehicles to log one million miles or more. As a result, engine developers are investing significant resources in durability analyses—before ever putting their engines into a vehicle.
 
At Achates Power, we use advanced modeling and simulation tools to evaluate and enhance the durability of our engine. The opposed-piston, two-stroke engine is unique in that it has inherent thermodynamic advantages—no heat rejection into the cylinder heads and faster combustion, just to name a few. It also has cost and weight advantages that result from having fewer parts and a less complex architecture compared to four-stroke engines.
 
In the Achates Power engine, two facing pistons in a single cylinder come together at top dead center and move apart under combustion. The pistons are connected to two crankshafts and the crankshafts are then connected by a gear train. The opposed motion of the pistons results in partial cancellation of the imbalance forces and, thus, in low engine vibrations.
 
To evaluate the durability of the design, we employ a hybrid approach of multi-body simulation (MBS) and finite element (FE) analysis. Our first step is to perform a modal reduction of the crankshaft, conrods and engine block FE models using the Craig-Bampton method, which allows for the selection of a defined subset of degrees of freedom to be preserved. These can then be used as interface nodes in the multi-body simulation model. The advantage of modal reduction is a reduced number of degrees of freedom while maintaining near complete modal information along with the modal stress.
 
Next, component modal-neutral files are imported into ADAMS VEngine. This model helps us achieve the best representation of the mechanism’s structural stiffness. From there, we apply boundary conditions like gas pressure traces, temperatures and oil viscosity. Finally, we perform the MBS analysis, carefully analyzing different aspects of engine dynamics including:
 

• Torsional and bending vibrations
• The effect of torsional vibration damper and flywheel layouts
• Hydrodynamic bearing analyses
• Load histories of the conrods and engine block
• Crankshaft modal participation factors ≡ Contribution of the modal stresses

 
Finally, there is the actual durability analysis. In the case of linear components like the crankshaft, modal participation factors are combined with the modal stress and superimposed to the 360° stress state using a fatigue code to determine life cycles and fatigue safety factors. In the case of nonlinear components—like the conrod or engine block—the load history is applied in a set of FE analyses to determine worst case stress states that, in turn, will be fed to the fatigue code to again calculate life cycles and fatigue safety factors.
 
By performing this type of simulation, we’re able to validate the soundness of our engine design as well as identify areas where we can make structural improvements that will enhance durability while reducing the engine’s size and dimensions. And, since we’re continually refining the process and applying it to all stages of development—including sensitivity studies and root cause analyses—we can ensure the structural integrity of the Achates Power opposed-piston, two-stroke engine. In addition to meeting the world’s most stringent emissions standards, demonstrating a significant fuel efficiency improvement and reducing manufacturing costs, the Achates Power engine is very well positioned to meet today’s commercial vehicle demands for one-million-mile durability.