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 threecylinder opposed-piston four-stroke engine, and a threecylinder 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 twostroke 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. In a second analysis, the closed-cycle simulation was extended to a engine system model to estimate the pumping work required to achieve the operating conditions needed to reach a specified NOx emissions rate. Because the opposed-piston two-stroke engine has inherently lower peak incylinder temperatures than the four-stroke engine, lower intake pressure was required to meet the NOx emissions constraint and as a result lower pumping work was needed. At the simulated condition considered, the opposed-piston two-stroke engine had approximately 9.0% lower brakespecific fuel consumption than the four-stroke engine.
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Opposed-piston two-stroke diesel engines have an inherent thermodynamic efficiency advantage and, by virtue of having double the firing frequency, allow for increased power density and lower NOX emissions while improving fuel efficiency, when compared to a four-stroke engine of equivalent power. However, opposed-piston two-stroke engines are piston-ported and, as such, are often erroneously dismissed for use in emissions compliant, on-highway vehicle applications over oil control concerns. The results presented in this paper show that oil control at levels acceptable for combustion and emissions control purposes is attainable with crankcase-lubricated, piston-ported opposed-piston diesel engines. Lubricant oil consumption was measured for the 13 test modes of the European Stationary Cycle using a real-time Da Vinci lubricant oil consumption measurement system. Repeatability of the measurement process was demonstrated. Oil consumption was measured during engine warm-up and shown to be reduced 30% compared to the fully warm condition. Furthermore, an increase of the oil control ring tension resulted in 16% lower oil consumption compared to the baseline. An optimization involving measurements with different cylinder kits resulted in a weighted average fuel-specific lubricant oil consumption of 0.18%. These data represent the first measured lubricant oil consumption maps for any contemporary two-stroke diesel engine ever reported.
The cooling system design for a two-stroke, opposed-piston (OP) engine is substantially different from that of a conventional four-stroke engine as the opposed-piston engine requires efficient cooling at the center of the cylinder where the heat load is highly concentrated. A thermally efficient design ensures engine durability by preserving the oil film at the top ring reversal zone. This is achieved by limiting the surface temperature of the liner to below 270° C at this location. Various water jacket designs have been analyzed with computational fluid dynamics (CFD) using a "discretized" Nusselt number approach for the gas side heat flux prediction. With this method, heat transfer coefficients are computed locally given the flow field of the combustion gases near the liner surface and then multiplied by the local gas/liner temperature difference to generate the heat flux distribution into the cylinder liner. The heat flux is then averaged over the cycle before being applied as a boundary condition to the CFD simulation. The baseline design consists of a simple water jacket with coolant flowing axially from the inlet near the intake port to the outlet near the exhaust port. This approach yields uneven cooling both longitudinally and circumferentially about the cylinder liner. A greatly improved thermal response has been achieved by introducing the coolant at the hot center section of the liner with roughly half of the coolant flowing toward either end of the cylinder. A detailed analysis shows that liner surface temperatures well below 270° C can be achieved for an engine with a power density of 50kW/liter by carefully optimizing the coolant velocities in the center section of the liner.
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