The Multiple Displacement System (MOS) provides cylinder deactivation during steady-speed, low-acceleration and
shallow grade climbing conditions to increase fuel economy.
MDS can provide a 5·20 percent fuel economy benefit when operating in four-cylinder mode, depending on driving
habits and vehicle usage. For EPA rating purposes, fuel economy is 8-15 percent higher than if the engine was
operating on eight cylinders at all times.
MDS is integrated into the basic engine architecture, requiring a minimum of additional parts – four additional solenoids, an oil temperature sensor and a wire harness. Eight unique valve lifters and a modified camshaft are also
required.
OPERATION
The Multiple Displacement System (MDS) provides cylinder deactivation during steady·speed, low-acceleration and shallow grade climbing conditions to increase fuel economy. Both four- and eight-cylinder configurations have even firing intervals, providing smooth operation. Two cylinders on each bank are ‘active when the engine is in four-cylinder mode every other cylinder in the firing order. All of the cylinders that are deactivated have unique hydraulic valve lifters that collapse when deactivated to prevent the valves from opening. Engine oil pressure is used to
activate and deactivate the valves. It is delivered through special oil passages drilled into the cylinder block. Solenoid valves control the flow. When activated, pressurized oil pushes a latching pin on each valve lifter, which then becomes a “lost motion” link.
Its base follows the camshaft, but its top remains stationary, held in place against the pushrod by light spring pressure but unable to move because of the much higher force of the valve spring.
NOTE: It is critical to use the recommended oil viscosity in S.7L engines that use MOS.
Deactivation occurs during the compression stroke of each cylinder, after air and fuel enter the cylinder. Ignition then occurs, but the combustion products remain trapped in the cylinder under high pressure, because the valves no
longer open. No air enters or leaves. During subsequent piston strokes, this high-pressure gas is repeatedly compressed and expanded like an air spring, but fuel is not injected.
