transition and closing both of the other ports 50 and 54 as in the transition from fig3 to fig4 thus the piston 63 has one extreme position fig5 7 in which it closes at least one port such as 50 and a second extreme position fig2 in which it closes at least one other port 54 fuel is withdrawn sequentially from the manifolds 33 31 a and 31 b there are three manifolds 31 a 31 b and 33 and three disjoint time intervals one for each manifold during which fuel is withdrawn from or supplied to exactly one manifold both withdrawing from and supplying fuel to any one manifold is substantially completed before the withdrawal from or supplying to another manifold commences 0020 fig2 shows the pressurizing valve 17 closed blocking the p 2 p 3 flow path with its switching function connecting pxe pressure on line 45 fig1 to pin pressure on line 47 by way of the groove 59 in piston 57 as illustrated in fig2 this low pressure pin at the ecology valve control port 46 exerts a force on the piston 63 which is less than the force exerted by spring 48 to urge the piston 63 toward its uppermost position as illustrated a condition indicative of a quiescent engine condition the ecology valve 43 is thereby shown filled with fuel and the engine manifolds are purged set amounts of fuel have been retracted from the manifolds into the spring cavity 55 of the valve the flow divider valves 39 and 41 and head effect valve 53 are also in their closed.