The exhaust gases in a supercharged two-st
题型：单项选择题
问题：
The exhaust gases in a supercharged two-stroke/cycle diesel engine are expelled from the cylinder by （）
A.pumping action of the piston
B.pressure of the fuel charge
C.vacuum developed in the manifold
D.pressure of the fresh air charge
则z=2x+y的最大值为( )