Cylinders take a snooze: advanced electronic technology can turn a gas guzzling V-8 into a fuel-sipping 4-cylinder, saving two- to five-mpg in one easy step

Automotive Industries, May, 2004 by Don Sherman

During World War II, enterprising car owners disconnected a spark plug wire or two in hopes of stretching their precious gasoline ration. Unfortunately, it didn't improve gas mileage. Nevertheless, Cadillac resurrected the concept out or desperation during the second energy crisis. The "modulated displacement 6.0L V 8-6-4" introduced in 1981 disabled two, then four cylinders during part throttle operation to improve the gas mileage of every model in Cadillac's lineup. A digital dash display reported not only range, average mpg, and instantaneous mpg, but also how many cylinders were operating.

Customers enjoyed the mileage boost hut not the side effects. Many of them ordered dealers to cure their Cadillacs of the shakes and stumbles even if that meant disconnecting the modulated-displacement system.

Like wide ties, short skirts and $2-per-gallon gas, snoozing cylinders are back. General Motors, the first to show renewed interest in the idea, calls it Displacement on Demand (DoD). DaimlerChrysler, the first manufacturer to hit the U.S. market with a modern cylinder shut down system calls its approach Multi Displacement System (MDS). And Honda, who beat everyone to the punch by equipping Japanese market Inspire models with cylinder deactivation last year, calls the approach Variable Cylinder Management (VCM).

The motivation is the same as before--improved gas mileage. Disabling cylinders finally makes sense because of the strides achieved in electronic powertrain controls. According to GM, computing power has been increased 50 fold in the past two decades and the memory available for control algorithms is 100 times greater. This time around, manufacturers expect to disable unnecessary cylinders so seamlessly that the driver never knows what's happening under the hood.

Cylinder deactivation works because only a small fraction of an engine's peak horsepower is needed to maintain cruising speed. Passenger cars require 25 or so horsepower to cruise at 60 mph while 35 horsepower is enough to drive a large SUV at that speed. Using a reduced number of hard working cylinders to produce the necessary power is more efficient than employing the full complement of lightly loaded cylinders.

The energy savings come from reduced pumping losses. In all gasoline engines, energy is consumed drawing the air needed for combustion past a partially closed throttle plate. Shutting down half the cylinders demands a wider throttle setting to ingest the same amount of air drawn in when all the cylinders are working. The wider throttle position yields lower pumping losses and better mileage.

Operating half the valves also diminishes the energy spent turning the camshaft. Minimizing the load carried by half the pistons and connecting rods trims friction and reduces energy losses to the cooling system.

The mechanical components needed to disable cylinders are surprisingly simple. All throe systems scheduled for 2004 implementation use a computer-controlled squirt of oil pressure to slide a pin inside either selected valve lifters (DaimlerChrysler and GM systems) or half of the rocker arms (Honda). The normal transfer of motion from a cam lobe to a valve stem is there by interrupted. In DCX and GM pushrod applications, the outer portion of each disabled lifter telescopes over the inner portion to maintain contact with the cam lobe without opening the valve. The Honda approach is a variation of the VTEC (variable valve timing electronic control) cam lobe switching scheme used for more than a decade. Instead of skipping between high- and low-lift cam lobes, VCM selects a rocker-arm alignment that delivers no valve lift.

In every case, the engine control computer triggers cylinder deactivation after studying coolant temperature, vehicle speed and engine load parameters. Each cylinder is disabled by interrupting not only the operation of the intake and exhaust valves but also spark and fuel delivery. Deactivation requires only 40 or so milliseconds and is timed to occur immediately after the power stroke so the disabled cylinder remains filled with exhaust gas. That creates what amounts to a gas spring. As the piston rises and falls, nearly all of the energy required to compress the gas spring on the up stroke is returned to the crankshaft on the down stroke.

From this common ground, the three makers implementing cylinder deactivation go their separate ways.

GM will introduce DoD on 2005 model year Chevy TrailBlazer and GMC Envoy SUVs equipped with a 5.3L Vortec V-8. Extensive oil system changes were necessary to support cylinder deactivation. Concurrently, the shift was made from iron to aluminum for the cylinder block casting. According to Chris Meagher, GM's assistant chief engineer for small block truck engines, "In order to alert our manufacturing partners that the scope of these changes is truly substantial, we designated the new version of the Vortec 5300 a Gen IV [fourth generation] design."

In the valley between cylinder banks, a Lifter Oil Manifold Assembly (LOMA) routes oil pressure to four solenoid control valves. When commanded by the powertrain control computer, the solenoids direct oil to valve lifters equipped with the switching mechanism. (One solenoid operates both of the lifters for each disabled cylinder.) To maintain even firing intervals, the end cylinders on the left bank and the center cylinders on the right bank are disabled.