Cylinders Take a Snooze

To make a DoD engine, new oil galleries are required (above) to feed the Lifter Oil Manifold Assembly (LOMA) that feeds oil to special lifters. The system requires a new gerotortype oil pump with 25-percent higher capacity to handle increased flow and pressure needs.

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 of 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 but 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.

DaimlerChrysler 5.7L OHV Hemi V-8 was designed from scratch to incorporate the MDS. The oil galleries normally used to lubricate valve lifters handle the control function — commanding half of the engine’s lifters (below) to disable their cylinders on cue. Lifter lubrication is delivered down the pushrods from the galleries that oil the rocker arms.

The mechanical components needed to disable cylinders are surprisingly simple. All three 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 thereby 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.

Associated engine changes include new galleries supplying the LOMA, moving the knock sensors from the valley area to the sides of the cylinder block to clear space for the LOMA, a new gerotor-type oil pump with 25 percent higher capacity to handle Gen IV flow and pressure needs, and a more powerful powertrain control computer. The E40 controller — what GM terms a key DoD enabler — was introduced on some 2003 models. Key features are a Motorola Excalibur 32-bit microprocessor operating at 32 MHz, one Mbybte of flash memory, and 32 Kbytes of random access memory.

To integrate DoD into the truck platform, GM engineers refined torque converter lockup and transmission shift characteristics, programmed the electronically-controlled throttle, retuned the engine mounts, and added a special valve in the muffler. The latter device helps optimize resonance characteristics for both operating modes.

Mileage gains attributable to DoD are substantial. GM touts an 8 percent improvement in EPA ratings (about 1.5 mpg) and up to 25 percent improvement (4 or more mpg) during highway cruising.

The two truck applications are merely the first stage of GM’s grand DoD plans. A similar system will follow under the hood of 2005 Pontiac G6 sedans equipped with a new 3.9L 60-degree V-6 capable delivering a 6-8-percent mileage improvement when only one bank of three cylinders is firing. By 2008, GM expects to have at least two million DoD vehicles on the road.

The Japanese-market Honda Inspire (above) uses a 3.0L V-6 with Variable Cylinder Management (VCM) (below) that makes 247 hp while still getting 27 mpg. A 3.5L V-6 with VCM will be an option on the Honda Odessey coming this fall to the U.S. market.

DaimlerChrysler selected a different path to achieve similar results. Instead of adding MDS to an existing engine, DC’s 5.7L OHV Hemi V- 8 was designed from scratch to incorporate the feature. The oil galleries normally used to lubricate valve lifters handle the control function — commanding half of the engine’s lifters to disable their cylinders on cue. Lifter lubrication is delivered down the pushrods from the galleries that oil the rocker arms.

One advantage the DCX V-8 enjoys over GM’s engine is two spark plugs per cylinder. According to Robert E. Lee, vice-president of powertrain product engineering at DCX, “In the four-cylinder mode, we use additional EGR to curtail the NOx produced by harderworking and potentially hotter cylinders. Dual plugs are a very helpful means of firing air-fuel mixtures containing extra EGR.” (As EGR is added, the throttle must be opened wider to admit air needed for combustion, incrementally improving mileage.)

An electronically controlled throttle — a luxury Cadillac didn’t enjoy 23 years ago — is critical to smoothing the transition from eight to four cylinders. Notes Lee, “We pre-position the throttle during the cylinder deactivation and reactivation sequences so that the torque is precisely the same in both four- and eight-cylinder modes. Without this measure, the driver would perceive a bump in acceleration or deceleration. “Tuning the engine to operate smoothly in the car required one of the closest working relationships powertrain has had with a vehicle team. It was an intense learning experience, partly because our modeling studies didn’t anticipate all the refinements that were necessary.”

Lee expects the mileage improvement to vary between a 10 percent gain in all-around driving and up to 20 percent during highway cruising. The extra 2 or so mpg is just enough cushion to keep the new Chrysler 300C sedans and Dodge Magnum wagons equipped with the MDS Hemi and AWD from wearing the gas guzzler stigma.

Honda has disclosed few details related to the two hybrid-electric VCM vehicles scheduled for fall introduction. This much is known: a V-6 Accord is one product to benefit from the advancement and the goal is performance superior to the conventional 240-horsepower Accord V-6 combined with the 30+ mpg fuel efficiency of a four-cylinder Civic.

Missing information can be inferred from the Inspire, an Accord variant launched last summer in Japan. That model has impressive credentials: 247 horsepower, more than 27 mpg in urban driving, and ultra-low emissions.

In the efficiency mode, one bank of the Honda V-6’s cylinders takes a break. An active engine mount strokes out of phase with engine vibrations to minimize powertrain disturbances transmitted to the passenger cabin. An active noise control signal is broadcast through entertainment speakers to cancel audible engine noise.

Cylinder deactivation may be the most painless 2-5-mpg mileage gain in automotive history. Thanks to the advent of intelligent engine controls, engineers have been able to tune their systems to work with the driver oblivious to the fact that his or her luxury car or truck is operating on a mere three or four cylinders. Assuming that customers embrace the three pioneer applications, cylinder deactivation is bound spread throughout light-duty gasoline world quicker than you can say “$40 per fill-up.”

Key Cylinder Deactivation Players

Mercedes-Benz: optional on certain European market V-8s since 1998; U.S. market V-12 mileage improved by a claimed 20 percent in 2001 and 2002 model years. Technology retired (in U.S.) with 2003 arrival of turbocharged V-12 engines.

INA: German bearing and valvetrain specialist supplies switching lifters for DC’s 5.7L Hemi V-8 and GM’s 3.9L V-6.

Eaton Automotive: supplies a portion of the LOMAs and switching lifters for GM’s 5.3L V-8. Also developing valve deactivation systems for light- and heavy-duty European diesel engines.

Siemens-VDO Automotive: exploring cylinder deactivation for possible future use. Delphi Automotive: supplies a portion of the LOMAs and switching lifters for GM’s 5.3L V-8.

Ford Motor Company: has investigated cylinder deactivation but no applications planned for near future.

Nissan and Toyota: no near-term plans to implement cylinder deactivation.