Aspen 4WD V8-5.7L Hybrid (2009)
Lifter / Lash Adjuster: Description and Operation
Hydraulic Lifters - Operation
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 provide 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 5.7L engines that use MDS.
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.
