Thermodynamic Benefits of Opposed-Piston Two-Stroke Engines

Abstract

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.

Effects of Scavenging System Configuration on In-Cylinder Air Flow Organization of an Opposed-Piston Two-Stroke Engine

“…However, modern emission regulations stopped widespread development of most two-stroke engines in the latter half of the 20th century [1]. In recent years, with the application of advanced design technology, modern analytical tools, materials and engineering methods, the emission problems no longer limit the successful design of a clean and efficient OP2S [2], so OP2S engines are once again attracting intensive attention to improve engine efficiency and emission performance [3][4][5][6][7]. Compared with conventional engines, OP2S engines have many fundamental advantages [8].…”

Scavenge flow analysis of opposed-piston two-stroke engine based on dynamic characteristics

“…Meanwhile, coupling simulation characteristic of 1D and 3D can be adopted for model testing. 9 The experiment carried out 1D and 3D scavenging process comparison validation on the above five cases at 6000 r/min. As shown in Table 2, given scavenging model index i, delivery ratio and scavenging efficiency derived from 1D performance prediction and 3D numerical simulation show a good agreement.…”

“…5 In recent years, with the application of advanced design technology, modern analytical tools, materials, and engineering methods, the emission problem is no longer limiting the successful design of a clean and efficient OP2S, 6 so OP2S engine has again attracted intensive attention to improve engine efficiency and emission performance. [7][8][9][10][11] Compared with conventional engines, OP2S engines have many fundamental advantages. 12 The opposed piston (OP) structure characterized by two pistons reciprocating opposite to each other in a common cylinder canceled the cylinder head and valve mechanism and leads to lower heat loss for higher wall temperature of the two piston crowns compared to a cylinder head.…”

Simulation Modeling Method and Experimental Investigation on the Uniflow Scavenging System of an Opposed-Piston Folded-Cranktrain Diesel Engine

“…In the first half of the 20th century, OP2S engines were developed in multiple countries for a wide variety of applications, however, modern emissions regulations stopped the widespread development of most two-stroke engines in the latter half of the 20th century [1]. In recent years, with the application of advanced design technology, modern analytical tools, materials and engineering methods, the emission problem no longer limits the successful design of a clean and efficient OP2S [2], so OP2S engines have again attracted intensive attention to improve engine efficiency and emission performance [3][4][5][6][7]. Compared with conventional engines, OP2S engines have many fundamental advantages [8].…”