What's a vehicle manufacturer to do?

The law requires more fuel-efficient vehicles -- but consumers appear to have an insatiable appetite for gasoline-thirsty SUVs, big cars and pickups, which also happen to be extremely profitable.

New car technology has come up with one answer: variable displacement engines that essentially enable a big six-cylinder motor to operate part-time more frugally on three cylinders, saving fuel consumption without hurting performance.

You only need about 50 horsepower to move a full-size SUV down a straight, level road at 100 km/h. More power is necessary to start the vehicle moving, to climb grades, accelerate or tow loads. Modern technology has made it possible to reduce engine size while continuing to have more power available on demand. The solution -- reducing the displacement of the engine in low- or no-load conditions by cutting the number of cylinders in operation. A 6.0-litre eight cylinder becomes a 3.0-litre four.

Variable displacement engines are not an original concept -- it was tried and abandoned more than 20 years ago. But there have been major advances in electronics and solenoids.

The small controllers used in today's adaptive suspension systems must react to road and other conditions within centimetres of wheel travel.

They are 50-times more powerful and 25-times quicker. Similar advances have been made in computers. The small engine control module in today's vehicle is quicker and more powerful than a desktop PC from a decade ago and able to handle a variety of inter-related chores simultaneously. The combination of fast, powerful solenoids and modern computers has allowed engineers to have a fresh look at cylinder deactivation.

Variable displacement is currently available on some V12 Mercedes models. GM and DaimlerChrysler will introduce it in a variety of high-volume 2005 models. The Multi-Displacement System (MDS), as Chrysler calls it, will be standard on the 5.7-litre Chrysler Hemi V8 in the new rear-drive 300C sedan and Dodge Magnum wagon. GM's Displacement on Demand (DOD) will be available on a variety of trucks and SUVs equipped with a version of the 6.0-litre Vortec V8. Annual production will run in the hundreds of thousands the first year and millions within a couple of years.

These engines start and run on all cylinders until a series of sensors report little or no power is required. Under a set of pre-determined conditions -- little or no throttle opening, level road or long decline etc. -- special hydraulic valve lifters collapse, making them unable to open the valves on four of the eight cylinders. With no air or fuel supply, there is no combustion and these cylinders are just along for the ride.

The special lifters have separate inner and outer sections or sleeves. A pin, actuated by those high-speed, high-pressure solenoids, couples or uncouples the two sections. When uncoupled, the inner portion slides within the outer like a telescoping pointer or the legs on a camera tripod. When engaged, the pin connects the two parts and the lifter opens the valve at the proper point during the rotation of the camshaft allowing air to enter or leave the cylinder.

The process is virtually undetectable, lightning quick (40 milliseconds) and has proven reliable over millions of test miles. Nobody is making mileage claims yet but the engineers are talking off the record about fuel economy gains of 10 to 20 per cent -- a gain that could not possibly be duplicated in any other way.

The fuel economy of a four and the power of an eight. And as a bonus -- reductions in exhaust emissions.