evaporation from the liner wall is not changed since it is not affected by the left oil film thickness, indicating that the left film  is not evaporated completely。 The oil blow through the top ring end gap is independent of the elastic force。 However, 

the  increase  of  the  elastic  force  significantly  increases  the friction  between  the  piston  ring  and  the  cylinder  liner。 Therefore, a balanced scheme needs to be considered。 Considering  the  oil  ring  as  the  greatest  influence  factor on the oil consumption, the oil consumption is investigated 

by just varying the elastic force of the oil ring to 50 N, 75 N, 100  N,  125  N,  which  is  also  compared  with  the  average  oil consumption  when  the  elastic  force  of  three  rings  are  all changed。 As shown in Fig。 12, two curves have basically the same  trend  and  there  is  only  a  small  numerical  difference, which  indicates  that  the  variation  of  the  elastic  force  of  the oil ring plays a significant role in the oil consumption。 

4   Optimal Design by BP Neural Network 

4。1   Experimental design and calculations 

It can be seen that in the four oil consumption modes, the oil evaporation on the cylinder liner wall is mainly controlled by the  engine  working  condition  and  is  independent  of  the piston  rings,  which  dominates  in  the  four  oil  consumption modes。 Therefore, it will be more comparable and practical to  select  the  cylinder  oil  consumption  after  removing  the cylinder  liner  wall  evaporation  as  the  optimization objective。  With  the  fixed  groove  size  of  the  piston  ring, three  factors,  they  are,  the  ratio  (e/B)  between  the  barrel surface  of  the  top  ring  (e)  and  the  effective contact  height of the ring (B), the closed gap of the second piston ring and the  stretch  force  of  oil  ring  are  selected  as  the  optimizing variables。 Four values of different levels are taken for each parameter。 As shown in Table 2, the second line is the value of each piston ring in the original engine。 Four levels of the three  factors  are  crossed  and  combined,  and  64  different schemes  are  formed。  The  oil  consumption  values  after removing  the  cylinder  liner  wall  evaporation  are  obtained by  substituting  various  schemes  into  the  calculation  model of the oil consumption。 

(a）Evaporation from liner

(b) oil blow through ring end gap

(c) Throw-off at top ring

(d) oil sctaping of piston top land

(e) Total ring friction

(f) left oil film thickness

Figure 11 Four modes of oil consumption with differernt ring tangential forces

Table 2。   Factors and levels of piston ring

Order          e/B of top ring       ♂S /mm        Fs/N

          1                0。004                 0。3                50