Thermodynamic Analysis of Novel 4-2 Stroke Opposed Piston Engine 2021-24-0096

In this work, a novel opposed piston architecture is proposed where one crankshaft rotates at twice the speed of the other. This results in one piston creating a 2-stroke profile and another with a 4-stroke profile. In this configuration, the slower piston operates in the 2-stroke CAD domain, while the faster piston completes 2 reciprocating cycles in the same amount of time (4-stroke). The key benefit of this cycle is that the 4-stroke piston increases the rate of compression and expansion (dV/dθ), which lowers the combustion-induced pressure rise rate after top dead center (crank angle location of minimum volume). Additionally, it lowers in-cylinder temperatures and pressures more rapidly, resulting in a lower residence time at high temperatures, which reduces residence time for thermal NO_x formation and reduces the temperature differential between the gas and the wall, thereby reducing heat transfer.

In this work, a custom 0D thermodynamic model was used to study the sensitivity of this architecture to engine speed, load, and combustion phasing and duration. The results quantify the benefits and tradeoffs of this cycle compared to a standard opposed piston 2-stroke engine. Details of the mechanical design are not the objective in this work; rather, this work aims to investigate the closed portion of the cycle and understand the thermodynamic impact of the unique kinematics on instantaneous work, heat transfer, etc. Three initial operating points were analyzed to understand the effect of the differing crankshaft speeds on critical engine parameters. It was found that the novel architecture can reduce heat transfer losses by up to 5.2% for these three points of interest, leading to a nearly 3% absolute gain in thermal efficiency. Secondarily, a sweep of engine speed and load at optimal combustion timings was conducted to compare the efficiency difference between the two architectures. The novel architecture has a significant advantage at low speeds and loads, achieving up to 4.1% absolute indicated gross thermal efficiency improvement compared to a traditional opposed piston 2-stroke engine.