Piston motion path optimization of crankless two-stroke engines based on a virtual crank-connecting rod mechanism

The piston motion path (PMP) strongly influences heat-work conversion by governing the rate of cylinder volume change in internal combustion engines. In crankless two-stroke engines, the PMP can be actively designed, but obtaining its optimal configuration is challenging due to complex kinematic constraints. To overcome this challenge, this study proposes an optimization strategy based on a virtual crank-connecting rod mechanism. By mapping the piston linear motion to a virtual crank rotational motion, the kinematic constraints are significantly simplified, enabling successful optimization of the PMP using a Sequential Quadratic Programming (SQP) algorithm. Results demonstrate that under an acceleration constraint of 2 × 104 m/s2, the optimized PMP increases the indicated thermal efficiency (ieff_c) by 2.72% compared to a conventional engine. Physically, this improvement originates from a tailored motion pattern. During the compression stage, the PMP closely follows an adiabatic path, minimizing heat exchange. During combustion, the piston experiences a dwell at top dead center (TDC), which increases the degree of constant-volume combustion and enlarges the area of the pressure-volume (P-V) diagram. During the expansion stage, rapid piston motion sharply reduces the in-cylinder high temperature, leading to decreased heat transfer losses. Furthermore, the study reveals that matching the piston dwell duration at TDC and the expansion speed is critical, where a higher degree of constant-volume combustion demands a greater expansion speed and vice versa. These findings offer design guidelines for active motion control systems in crankless engines to enhance thermal efficiency.