**5. Uniflow scavenging**

**Figure 3.** Schematic of intake and exhaust ports in the Loop configuration#2

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Upsweep angle

**Figure 4.** Sketch of intake and exhaust ports in the Loop configuration

flow must turn to exit.

venient to have a stroke longer than bore.

As far as the exhaust ports are concerned, figure 4 shows that it is convenient to assign a downward angle to the bottom wall, in order to increase the maximum port effective area. In fact, around BDC, the streamlines within the cylinder tend to be almost tangential to the exhaust port, so that an inclination of the port bottom wall reduces the angle at which the

It is important to notice that the permeability of a loop scavenging system is related to the choice of the bore-to-stroke ratio. In fact, since engine speed is limited by combustion con‐ straints, the critical factor generally remains the average effective area of the ports, referred to the piston area (see equation 3). It can be easily demonstrated that a low bore-to-stroke ratio helps to have larger opening areas for both types of ports, so that it is generally con‐

As far as the Uniflow scavenging is concerned (piston controlled inlet ports and exhaust poppet valves on the cylinder head) some design guidelines are provided below.

EXHAUST VALVES In the Uniflow scavenging, the critical issue for permeability is the ef‐ fective area of the exhaust valves. Even if the engine speed is low, strong constraints are generally placed upon the maximum lift, since the optimum opening duration is at least 30% less than a corresponding 4-Stroke engine. From this point of view, the higher is the number of valves, the better. As an example, passing from 2-valve to 4-valve, the maximum geometric flow area increases by about 30%; furthermore, the valves are smaller and lighter, so that it is possible to define in a more free manner the valve actuation law (maximum lift and duration), and provide a more effective cooling; last, but not least, the injector can be placed on the cylinder axis, without penalization on the valve dimensions. The central posi‐ tion of the injector is particularly important when the combustion chamber is in the piston bowl, in order to guarantee a uniform distribution of the fuel within the cylinder. Obvious‐ ly, with more valves, the valvetrain is more expensive and heavy, while the flow losses may significantly increase, without a proper design of the valve ports and ducts.

INLET PORTS A comprehensive CFD study on the influence of the inlet ports geometry has been carried out by Hori [18]. A simple but effective configuration studied by this author is pre‐ sented in figure 5, where a set of 12 ports uniformly distributed along the cylinder bore is shown. The ports do not need an upsweep angle, since the piston skirt is already driving the flow toward the cylinder head. At BDC, the upward direction of the flow can be imposed by leaving a small step (1-2 mm) between the piston crown and the bottom wall of the ports. It may be noticed that the axis of each port forms an angle with the radial direction. As this angle in‐ creases, the swirl ratio grows up, along with the pressure drop across the cylinder. A large an‐ gle is desirable in order to sweep the exhaust gas along the circumference of the liner, but a pocket of exhaust gas may remain in the cylinder core. Conversely, near-radial ports are less ef‐ fective in the outer region, but they better sweep the cylinder bulk. In terms of scavenging effi‐ ciency (concentration of fresh charge at inlet port closing), the former solution is better, according to Hori. This outcome can be explained considering that a ring of exhaust gas trap‐ ped in the outer region of the cylinder contains more mass than a ring of similar thickness close to the cylinder axis. In order to achieve a good scavenging efficiency in combination with low swirl, Hori proposed an "alternate port" configuration, i.e. a sequence of one radial port and one swirling port, the former with an upward angle of elevation, the latter with a downward angle. Another important parameter investigated by Hori is the opening area ratio, that is the fraction of cylinder bore occupied by the ports. As this ratio increases, the flow losses goes down, along with the swirl ratio. A typical range for the opening area ratio is between 50 and 80%: the upper limit concerns problems such as durability of the piston rings and of the liner. Finally, Hori showed the importance of the chamfering radius of the ports: as this parameter in‐ creases, flow losses are diminished and the swirl ratio goes down (the portion of straight chan‐ nel is lower, so that it becomes increasingly difficult to impart the direction to the flow). For a liner 10 mm thick, an optimum chamfering radius of 3 mm was suggested.

attention was paid to the loop version. Here, a ports design as the one visible in figure 3 was

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**Table 2.** Main features of the WAM 100 engine, assumed as a starting base for the CFD study presented in [13] and [14].

A comparison between the scavenging parameters calculated under real engine operating conditions (2000, 2500 and 3000 rpm, full load) is presented in figure 6. Figure 7 presents a pictorial view of the fresh charge concentration on a plane passing through the cylinder axis,

The scavenging parameters are defined as follows. The Trapping Efficiency (TE) is the ratio of the mass of fresh air retained within the cylinder to the mass of fresh air delivered; the Scavenging Efficiency (SE) is the ratio of the mass of fresh charge retained to the total cylin‐ der mass (fresh+exhaust); the Exhaust Gas Purity is the mass fraction of fresh charge in the exhaust flow leaving the cylinder; finally, the reference mass is calculated considering the

Analyzing figures 6 and 7, it is observed that operating conditions affect Uniflow scaveng‐ ing very slightly, while the influence is more evident on Loop. It should be considered that these conditions are defined not only by speed, but also by the pressure traces forced at both the inlet and the outlet boundaries, which are obviously different from case to case for rep‐ resenting real engine operations. The lower data scattering of the Uniflow design may be

adopted, and optimized via CFD-3D simulations.

Scavenging type Uniflow

Displaced volume 1832 cc Stroke 95.0 mm Bore 90.5 mm Connecting Rod 167.0 mm Compression ratio 17:1

Exhaust Valves Open 83° before BDC Exhaust Valves Close 80° after BDC Inlet Port Open 53° before BDC Inlet Port Close 53° after BDC Maximum Brake Power 102 HP @ 2750 rpm

at different crank angle. Engine speed is 2500 rpm, full load.

average delivery density and the total displaced volume.

mainly explained by the more regular pressure traces.

Engine type 2-Stroke, 3-cylinder in-line Combustion Diesel, Indirect Injection

Number of Valves/Ports 2 Exh. valves/20 inlet ports Air Metering Turbocharger + Roots blower Fuel Metering In-line mechanical pump Injector nozzle type Single-hole (Pintle)

**Figure 5.** Computational mesh of a Uniflow design analyzed by Hori [18], showing the layout of the inlet ports
