The following figures show CFD simulation results for gas admission and mixing the gas with air using the valve lift range from 5 mm to 15 mm. Diameters of poppet valves are 40 mm and three valves have been used. For precise lifts, flow velocities and air excess ratios (Lambda) are presented. As the results show, velocity of mixture and air excess ratio depend on valve lift. If valve lift is very small, gas will not flow into the cylinder. However, this depends on the shape of the gas edges. When the air excess ratio is about 2.1, the ratio is optimal. This is also seen in the figure 2.1.
7.2.1 Valve lift 5 mm
Figure 7.2 presents flow velocity at the poppet valve lift 5 mm. The highest flow veloci-ty is achieved just before air flows into the cylinder.
Figure 7.2. Valve lift 5 mm, side view. [47].
Figure 7.3 shows that only air flows through the poppet valves at the valve lift 5 mm.
The valves are not opened enough that gas would flow into the cylinder. Thus, air ex-cess ratio is 3.
Figure 7.3. Air excess ratio at poppet valve lift 5 mm. [47].
Because of the shape of the gas edges and valve lift, cylinder scavenging is possible.
The valve should be opened enough much that enough air flow to the cylinder.
Howev-er, too large valve lift should be avoided that gas is not possible to flow into the cylin-der.
7.2.2 Valve lift 6 mm
Figure 7.4 presents flow velocity of air-gas mixture when the poppet valve lift is 6 mm.
As the figure shows, maximum velocity of mixture is higher than using poppet valve lift 5 mm.
Figure 7.4. Valve lift 6 mm, side view. [47].
Figure 7.5 shows the air excess ratio at the poppet valve lift 6 mm. Compared to the valve lift using 5 mm, also methane gas flows into the cylinder.
Figure 7.5. Air excess ratio at poppet valve lift 6 mm. [47].
As the figure 7.5 shows, methane flows mainly from the left side of the poppet valve.
Thus, on the left side of the poppet valve gas mixture is much richer than on the right side. It is also noticeable that gas mixture is not homogenous in the cylinder. For the valve lift 6 mm in steady-state simulation, uniformity index is 0.168 [47].
7.2.3 Valve lift 7.5 mm
Figure 7.6 presents flow velocity of the air-gas mixture when the poppet valve lift is 7.5 mm. In comparison to the previous flow velocity presented in figure 6.4, the maximum velocity value of the gas mixture is higher at valve lift 7.5 mm.
Figure 7.6. Valve lift 7.5 mm, side view. [47].
Figure 7.7 shows air excess ratio at the poppet valve lift 7.5 mm.
Figure 7.7. Air excess ratio at poppet valve lift 7.5 mm. [47].
At the poppet valve lift 7.5 mm in steady-state simulation, uniformity index is 0.168 [47].
7.2.4 Valve lift 8.5 mm
Figure 7.8 presents flow velocity of the air-gas mixture at the poppet valve lift 8.5 mm.
Velocity of air-gas mixture is close to the results using poppet valve lift 7.5 mm but the maximum value of velocity is a little bit lower than using valve lift 7.5 mm.
Figure 7.8. Valve lift 8.5 mm, side view. [47].
Figure 7.9 shows the air excess ratio at the poppet valve lift 8.5 mm. The figure shows that the area where air excess ratio is 3.0 is larger than using valve lift 7.5 mm. This area is on the right side of the cylinder.
Figure 7.9. Air excess ratio at poppet valve lift 8.5 mm. [47].
At the poppet valve lift 8.5 mm in steady-state simulation, uniformity index is 0.075 [47]. Thus, the uniformity index value is lower than at the poppet valve lift 7.5 mm.
7.2.5 Valve lift 10 mm
Figure 7.10 presents flow velocity of air-gas mixture at the poppet valve lift 10 mm.
Maximum flow velocity value is higher than using lower poppet valve lifts.
Figure 7.10. Valve lift 10 mm, side view. [47].
Figure 7.11 presents the air excess ratio at the poppet valve lift 10 mm. Compared to valve lift 8.5 mm, on the right side of the cylinder where air excess ratio is 3, the area is larger. Also on the top of the cylinder there is obviously area where air excess ratio is 3.
Figure 7.11. Air excess ratio at poppet valve lift 10 mm. [47].
At the poppet valve lift 10 mm in steady-state simulation, uniformity index is 0.104 [47]. The value is a little bit higher than using valve lift 8.5 mm but still lower than us-ing poppet valve lift values 6 mm and 7.5 mm.
7.2.6 Valve lift 15 mm
Figure 7.12 presents flow velocity of air-gas mixture at poppet valve lift 15 mm. As the figure shows, maximum velocity value is higher than using lower poppet valve lifts.
Figure 7.12. Valve lift 15 mm, side view. [47].
Figure 7.13 shows the air excess ratio at the poppet valve lift 15 mm. Compared to pre-vious air excess ratio figure, air excess ratio is much lower on the right side of the cyl-inder.
Figure 7.13. Air excess ratio at poppet valve lift 15 mm. [47].
As figure 7.13 shows, on the left side of the poppet valve the value of air excess ratio is near zero and on the right side ratio is three. Thus, gas flows still from the left side of the poppet valve and air flows from the right side. For the valve lift 15 mm in steady-state simulation, uniformity index is 0.84 [47]. The value is significantly higher than using lower valve lifts.