A Two Stroke Engine Cold Flow CFD simulation is a powerful computer model that helps engineers see inside a working engine. Unlike four-stroke engines, a two-stroke engine completes its power cycle very quickly, making the internal gas flow complex. A Piston-cylinder CFD analysis using ANSYS Fluent is essential to understand this process. This Engine Simulation uses an advanced technique called In-cylinder Dynamic Mesh to model the piston’s rapid up-and-down movement. This report details a Two Stroke Engine fluent analysis that focuses on the gas exchange process. By using an In-cylinder Dynamic Mesh fluent model, we can see exactly how fresh air pushes old exhaust gas out—a process called scavenging. This Engine CFD study helps designers improve port timing and shape, which leads to better fuel efficiency and lower emissions, all without building expensive physical prototypes. For more advanced dynamic mesh simulations and moving boundary problems, explore our Dynamic Mesh CFD tutorials.

Figure 1: The 3D geometry model of the two-stroke engine’s cylinder, piston, and ports used for the In-cylinder CFD simulation.

Simulation Process: Fluent-CFD Setup, The In-Cylinder Dynamic Mesh Module with a Piston UDF

The simulation process for this Two-Stroke Engine CFD analysis began with the 3D geometry of the engine. To save computer time, only half of the model was used, applying a symmetry condition. A very specific mesh structure was created. The area where the piston moves, known as the combustion chamber, was meshed with high-quality hexahedral (brick-shaped) cells. This is essential because the layering technique within Fluent’s Dynamic Mesh module, which adds and removes cell layers as the piston moves, works best with this type of structured grid.

Inside ANSYS Fluent, the simulation was set to be transient to capture the changes over time. The In-Cylinder module was activated to define the basic engine parameters, like the crankshaft speed and connecting rod length. To model the gas exchange process, the Species Transport model was enabled to track the movement of fresh air and the exhaust gas (represented as CO2). The most critical part of the setup was a custom User-Defined Function (UDF). This UDF, written in C code, calculates the exact position of the piston at every single time step based on the engine’s crankshaft angle. This Piston UDF is necessary because it provides the precise, complex motion that the Dynamic Mesh module needs to update the moving grid correctly, ensuring the simulation perfectly mimics the mechanics of a real engine.

Figure 2: The generated mesh for the Fluent CFD simulation, showing the hexahedral cells in the combustion chamber required for the layering dynamic mesh technique.

Post-processing: CFD Analysis of In-Cylinder Dynamics and Scavenging

The simulation results completely show a direct cause-and-effect relationship between the piston’s motion and the resulting changes in pressure, temperature, and gas flow inside the cylinder. From an engineering viewpoint, the analysis begins with the compression stroke. The pressure contour at time 0.00178 s in Figure 4 shows a low pressure of around 331 Pa as the piston starts moving up. By the time the piston reaches near the top of its stroke at 0.025 s, this pressure has dramatically increased to a peak of 272,464 Pa (approximately 2.7 bar). This pressure rise is a direct result of the piston squeezing the gas into a smaller volume, a fundamental part of the engine cycle. The temperature contour in Figure 6 shows the direct consequence of this compression: the gas temperature rises from a uniform 293 K to 423 K (150°C) due to adiabatic heating. This ability to accurately predict peak cylinder pressure and temperature without combustion is a crucial first step in engine design, as it determines the mechanical and thermal loads on the components.

Figure 3: Pressure contours at four different time steps from the Two-Stroke Engine fluent simulation, illustrating the pressure change during the compression and expansion strokes.

The second half of the analysis focuses on the expansion and scavenging process. As the piston moves down at 0.05 s, the pressure drops to around 126 Pa. The velocity contour in Figure 5 at this exact moment shows what is really happening: high-velocity jets of fresh air, moving at up to 12.74 m/s, are entering through the intake ports. This is the scavenging process in action. These powerful jets are designed to push the hot, residual exhaust gases out through the exhaust port. The temperature contour at 0.05 s provides visual proof of this success. We can see the cooler, 291 K fresh air entering and mixing with the warmer gas inside, effectively cooling the cylinder. By the end of the cycle at 0.0696 s, the temperature has become almost uniform at 293 K, showing that the fresh charge has successfully replaced the hot gas.

The most important achievement of this simulation is the successful visualization and analysis of the entire gas exchange process. For an engine designer or manufacturer, this data is invaluable. It provides a “virtual engine” to:

1. Optimize Port Design: They can see the exact flow patterns from the velocity contours. This allows them to change the angle and shape of the intake ports to create a better swirl or tumble, which improves scavenging efficiency and ensures no fresh charge is lost out the exhaust.
2. Improve Fuel Efficiency: By ensuring all exhaust gas is removed, the next combustion cycle will be cleaner and more complete, which directly leads to better fuel economy.
3. Reduce Emissions: Poor scavenging leaves behind unburnt fuel and exhaust, which are major sources of pollution. This simulation allows designers to minimize trapped exhaust gas, leading to a cleaner-burning engine before a single physical part is made.

Figure 4: Temperature contours from the Fluent simulation, showing the temperature rise due to adiabatic compression and the cooling effect of the incoming fresh air.

Figure 5: Velocity contours from the dynamic mesh CFD analysis, highlighting the high-speed flow during the scavenging process.