SpeedyV6

Goldilocks V-8's, Always Just Right
By DON SHERMAN
The Hemi V-8 in the Dodge Magnum becomes a V-4 when cruising on the highway.


DETROIT

THIS winter, homeowners in the Northeast may be tempted to cut energy bills by shutting off rooms they use only occasionally. Come spring, they will be able to buy cars that apply the same principle to engines, cutting off cylinders when full power is not needed. Driving on half an engine can bring fuel savings of up to 20 percent, automakers say.

Of course, there are times when nothing will substitute for the brawn of a large V-8 engine - towing an Airstream over the Rockies, for example. But, in most driving situations, that power reserve is unused potential and a waste of fuel. Buying a vehicle that is powerful enough to handle the most severe loads imposes the full-time penalty of a big, thirsty engine.

That inefficiency may be relieved somewhat by the emergence of technology that enables engines to shut down some cylinders when the power demand is minimal, reforming a gluttonous V-8 into a thriftier, part-time 4-cylinder. This development, effectively adapting engine size to suit the task, continues a trend toward variable engine controls already under way.

In today's vehicles, microprocessors reduce the need to settle for rigid choices in the way an engine performs: power vs. economy; high performance vs. low emissions; low-speed muscle vs. high-r.p.m. zing.

Increasingly, drivers can have the best of all possible worlds thanks to onboard computers that vary the amount of fuel injected into each cylinder and the timing of the spark that ignites the fuel mixture, along with many settings that were once established by the manufacturer or adjusted by mechanics.

In many cars, the size and shape of the air intake system is automatically adjusted to deliver pulling power over a wider range of speeds; the actions of the engine's valves are constantly fine-tuned to improve performance without hurting fuel economy. Rather than engineers' fixing a compromise setting that strikes a balance between power and economy, the car's computer now makes many of the choices depending on the current conditions - and like the bowl of porridge that finally satisfied Goldilocks, the engine is always operating "just right."

One specification that has not been variable is the engine's size, usually expressed in liters or cubic inches of displacement. Buyers can choose large or small based on their needs, but they can have only one at a time.

Systems to adjust the engine size - or more precisely, the number of cylinders in use - will appear this year on models from three automakers, all incorporating mechanisms to idle half the engine's cylinders by shutting off the fuel injection, the sparkplugs and the valves for intake and exhaust.

General Motors calls its system Displacement on Demand, DaimlerChrysler's name is Multi-Displacement System and Honda's label is Variable Cylinder Management.

The concept of deactivating cylinders to save fuel has been studied for some time, at least back to the gasoline rationing era of World War II. In 1981, Cadillac introduced what it called a "V8-6-4" engine that dropped two cylinders at a time to improve mileage. But the attendant stumbling and shaking did not sit well with owners of luxury cars. A more refined engine was rushed to the rescue and V8-6-4 went the way of the Edsel.

Mercedes-Benz resurrected the idea years later for the V-12 engines in its largest cars, but it does not now use such a system.

The current revival of variable engine displacement systems is made possible, in part, by advances in electronic controls. In the last two decades, engine-control computers have become 50 times more powerful and the memory available for engine-control software is 100 times greater.

Cylinder deactivation is an effective way to reduce fuel consumption because only a small fraction of a vehicle's maximum horsepower - typically less than 30 for today's aerodynamic cars - is needed to maintain highway cruising speeds. When the demand is so low, fewer hard-working cylinders are inherently more efficient than the full complement of lightly loaded cylinders.

This is true because of an underlying phenomenon that engineers call pumping loss - the energy expended in drawing air, needed for combustion, past a partly closed throttle plate.

The effort required to move the air consumes fuel, but it is possible to minimize the pumping loss by opening the throttle wider - exactly what is needed to maintain speed with fewer cylinders in operation. Also, less power is consumed because fewer valves are being used.

While the computer controlling the part-time cylinders is quite sophisticated, the mechanical components that do its bidding are surprisingly simple. G.M.'s Displacement on Demand uses two-part valve lifters for the cylinders that switch between active and idle modes. When all eight cylinders are in use, the two sections of the switching lifter are locked together by a sliding pin, and the motion of the camshaft is transferred by the pushrod to open the valve.

When the computer calls for the cylinder to be deactivated, the sliding pin is retracted by engine oil pressure and the lifter sections are unlocked; the internal section of the lifter simply telescopes within the outer sleeve and no motion is transferred to the valve. DaimlerChrysler's system operates similarly.

With the valves closed, pistons in the deactivated cylinders must compress the air trapped inside as they stroke upward. That requires some effort, but most of the energy is given back on the piston's downstroke, except for a small loss due to friction.

In spite of the mechanical simplicity, implementing Displacement on Demand required G.M. to thoroughly revamp its 5.3-liter Vortec V-8 and convert its existing 3.5-liter V-6 into a new 3.9-liter engine. Chrysler engineers planned ahead when they designed the Hemi V-8, incorporating the necessary oil passages into the engine's cylinder block.

In addition to modifications of the valve components, other vehicle systems must be altered to support cylinder deactivation. The engine-control computer is reprogrammed to interrupt and restore fuel delivery and ignition spark to each cylinder. An electronically controlled throttle, rather than one that uses mechanical linkage, provides the precision needed for smooth operation.

DaimlerChrysler disclosed its plans to introduce variable-displacement technology at the Detroit auto show this month. It will be fitted to the optional 5.7-liter Hemi V-8's that will power the Chrysler 300C and Dodge Magnum RT models arriving in showrooms this spring.

Honda will follow, later this year, with a hybrid gasoline-electric Accord that switches its V-6 to an in-line 3 by shutting down the rear bank of cylinders.

G.M., the first to promote cylinder-shutdown technology, will deliver Displacement on Demand in 2005 Chevy TrailBlazers and GMC Envoys equipped with the 5.3-liter V-8 and in Pontiac G6 sedans with the 3.9-liter V-6.

Manufacturers say the only place that owners will notice the on-off cylinder routine will be at the gas pump. Sure that variable-displacement will work as intended this time, G.M. is predicting that more than two million cars and trucks will be equipped with these systems by 2008.

Slimey

My uncle owned one of these POS in a 1980s El Dorado. It didn't really work too well as stated above. The car was serviced numerous times for electrical and other problems. It seemed to rarely use the deactivated cylinder mode and spent >90% of its life in the 8 cylinder mode -- seemed like a poor application of the technology.