Chapter 19

TWO-STROKE ENGINES

Chapter 19 TWO-STROKE ENGINES

The following tutorial data files are used in this chapter:

TWO_STROKE/twoStrokeCylinder.dbs

TWO_STROKE/twoStrokePort.dbs

PANELS/training.pnl

This chapter describes a two-stroke engine set-up using the Partial Arbitrary Sliding Interface (PASI) feature. PASIs are attachment boundaries that help simulate the opening and closing of two-stroke-type ports on the cylinder wall. es-ice detects the interface between the ports and cylinder and also identifies which cell faces act as master or slave attachment boundaries when the model is imported into pro-STAR. During a two-stroke cycle, the piston movement exposes the port openings and

allows fluid to flow in or out of the cylinder. This process is simulated using attachment boundaries and sliding-interface events so that, when the piston exposes the port, the master and slave attachment boundaries are coupled, allowing fluid flow. To avoid a large pressure difference across a small opening, and thus enhance solver stability, you can apply an overlap tolerance. The latter forces the port to be effectively “closed” even when the piston exposes the port. As long as the opening distance is less than the overlap tolerance, the attachment boundary is treated as a wall, which in turn prevents fluid flow. When the opening distance exceeds the overlap tolerance, fluid is allowed to flow between port and cylinder.

The cylinder wall and port-opening meshes are likely to be different and therefore non-conformal. To address this issue, as Ice generates the mesh it also performs a CPMatch operation when the port is open. This creates one-to-one connectivity between the two meshes and therefore improves the solution stability and accuracy of the flow between ports and cylinder.

The piston movement keeps exposing new cell faces, some of which are likely to be partially exposed. In this instance, the unexposed part of the cell face is defined as a wall boundary to stop any flow through it. The rest of the cell face allows fluid flow between the port and cylinder.

The tutorial simulates the operation of a two-stroke, spark-ignition engine over one cycle (360 degrees). The engine has four transfer ports and three exhaust ports connected to the cylinder wall, with no poppet valves. Therefore, the piston movement determines the port opening and closing and this in turn requires the use of PASIs. The pressure of the intake charge supplied to the transfer ports is higher than atmospheric pressure, as it is assumed that a crankcase mechanism drives the flow to the transfer ports. This condition initiates the gas exchange between the transfer ports and cylinder and fills the cylinder with premixed fuel following the combustion phase. Table 19-1 summarises the engine characteristics and operating conditions.

Version 4.20

19-1

TWO-STROKE ENGINES			Chapter 19
Importing the Geometry
Table 19-1: Engine characteristics and operating conditions
	Bore	100 mm
	Stroke	130 mm
	Connecting Rod Length	160 mm
	Speed	2000 rpm

The steps needed for this tutorial are summarised below:

1.Import the engine geometry

2.Create the mesh using the trimming method

3.Set up the necessary Star Controls in es-ice

Importing the Geometry

The tutorial starts by importing a .dbs file containing the discretised surface that defines the engine geometry.

To import this surface:

•Check that file twoStrokeCylinder.dbs is in your current working directory and launch es-ice in the usual manner

•In the Select panel, click Read data

•In the Read Tool, click the ellipsis (...) button next to the DBase file box and select twoStrokeCylinder.dbs via the file browser

•Click the ellipsis (...) button next to the Get button and select 1 Cylinder from the database browser

•Click CPlot in the Plot Tool to view the imported geometry, shown in Figure 19-1

19-2	Version 4.20