(a) 0.9＜P_me/P_me0≦1.0

(b) 0.8＜P_me/P_me0≦0.9

Fig. 2 Load characteristics of NOx

The calculated E3 mode NOx values by using the data in Fig. 2 are shown in Fig. 3 compared with the values at 100%L. In relation to the load characteristics of NOx, the E3 mode NOx in case of higher rating engines with p_me/p_me0 of 0.9〜1.0 is little bit higher than the NOx at 100%L, on the other hand, the E3 mode NOx in lower rating engines with p_me/p_me0 of 0.8〜0.9 is little bit lower than the NOx at 100%L, but these E3 mode NOx in either case coincides well with the NOx at 100%L within the deviation of ±5%. Therefore the values of NOx at 100%L instead of E3 mode NOx are adopted in the following discussion.

Fig. 3 Relation between E3 mode NOx and NOx at 100%L

　

3. Characteristics of engine performance on the layout diagram and the max. average temperature in cylinder T_max

　

3.1 layout diagram and matching method

Fig. 4 shows the lay out diagram. The line I in this figure represents the condition with max. n (n=n₀), the line II represents that with max. p_me (p_me=p_me0), the line III represents that with p_me/p_me0 of 0.64, the line IV represents that with n/n₀ of 0.75. The p_max within this area is generally kept constant to get the higher thermal efficiency.

Fig. 4 Layout diagram for large bore 2 stroke diesel engines

　

The engine manufactures have their own planning diagram of the scavenging pressure ratio p_s/p_me which increases gradually along with the decrease of p_me/p_me0, and they accomplish such values of p_s/p_me in the shop trials by applying the adequate turbocharger matching. Also, they arrange the shim thickness at the bottom part of piston rod to keep the pressure difference between p_max and the compression pressure p_c at NOR within 30〜35 bar in order to avoid the hard combustion condition. Furthermore, they set the exhaust valve open timing as late as possible to get maximum effective work. at the same time, they set this timing to make the cylinder pressure at the scavenging port open being lower than p_s in order to avoid blow back phenomenon.

According to these matching method. when the values of p_max/p_me and p_s/p_me were set, then the shim thickness and the exhaust valve timing are determined. consequently the volume ratio at compression beginning V₁/V_H and that at the end of compression V₂/V_H are also determined.

　

3.2 Setting of p_max/p_me, p_s/p_me and the value of T_max

It is said that the thermal NOx is generated according to the expanded Zeldovich mechanism, and the value of NOx is depend closely on the local max. flame temperature.

BACK　　　CONTENTS　　　NEXT