"All corporate politics aside, the engineers at GM Powertrain have designed yet another mechanical marvel, and it's
all due to some remarkably modest changes to the very robust Gen III architecture on which the Gen IV is based.
The new DOD-specific hardware includes two-stage switching lifters, a lifter oil manifold assembly (located in the
valley of the engine), a redesigned lube circuit and oil pump, electronic throttle-by-wire operation, a
pressure-activated muffler valve, and an improved E40 engine controller running DOD-specific software.
"In order to eliminate the pumping losses," says Meagher, "you need to disable both the intake and exhaust
valve." This results in a completely sealed, deactivated cylinder, which is essentially an air spring being acted
upon by a piston. Virtually all the work put into it during compression is returned to the crank during
decompression, finally giving credence to the old joke about piston-return springs. (That's nothing. Wait 'til
you hear about the muffler valve...)
"Currently, we could disable just the fuel delivery," says Meagher, "but the valves would still be opening and
closing and each cylinder would still be doing work pumping air in and out. So there would be no net gain in
efficiency--you wouldn't have eliminated the pumping losses at all.
In support of cylinder deactivation is some very interesting choreography from things ranging from throttle
valve modulation to active exhaust tuning, but it all starts with the additional job tasked to the lifters.
"We disable the valves through a device called a switching lifter," explains Meagher. "This differs from a normal
lifter in that there is an inner body and an outer body connected by a spring-loaded pin. For V-8 operation,
the pin is fully expanded by the spring so the two pieces act as one and the lifter acts like a regular lifter.
When we want to disable the valve operation, we deliver high-pressure oil to a groove in the lifter that leads
to the outside end of the pin, forcing the pin to collapse the spring. Now the two parts of the lifter are free
to move relative to one another and as the cam lobe pushes on the follower the inner portion of the lifter
pushes against another spring at the top of the lifter and does not transfer force to the pushrod."
A look at the lifter cross-section reveals an elegant, yet simple design that has the potential to change the way
we think about traditional pushrod engines. (Ironically, when DOD is working, it hinges on lifters that do not lift!
Something we never thought we'd ever want.) In order for the switching lifter to work effectively, the engine
needed a redesigned oiling system. Both iron and aluminum versions of the engine block have redesigned oil galleries
o support DOD oiling requirements. Those oil galleries are supplied by a lifter oil manifold assembly (LOMA) located
in the lifter valley of the engine. Under cylinder deactivation, the LOMA routes oil to the applicable lifters by means
of four lifter oil solenoids, which are controlled by a new E40 engine management controller. To supply the additional
needs of the cylinder deactivation circuit, a higher capacity oil pump is fitted to the LH6 engine."