Variable Compression: Small Engine, Big Power

GENEVA - An astonishing 30% reduction in fuel consumption and in consequent emissions is claimed for a variable-compression engine concept developed by Saab Automobile AB and revealed at the recent Geneva motor show here.

Saab, long recognized for its out-of-the-box engine engineering, has a history of bringing cutting-edge engine designs to production. The company is largely responsible, for example, for putting gasoline-engine turbocharging on the map.

With the new variable-compression engine, a unique hydro-mechanical cylinder-head "tilting" system - combined with high-pressure super-charging - permits calculated downsizing of the basic engine. A smaller-displacement engine generally must work at close to full capacity - and thus maximum thermal efficiency. Also, the design is enhanced by low pumping losses.

Thus only 1.6L displacement delivers a remarkable 225 hp at 6,000 rpm and a peak torque of 225 lb.-ft (305 Nm). That equates to 150 hp and 148 lb.-ft. (200 Nm) per liter of displacement.

Compression ratio is altered between 14:1 and 8:1 by separating the cylinder block (with fixed head) from the crankcase assembly, and tilting the block relative to the crank centerline. This effectively increases the combustion chamber volume, providing a numerically lower compression ratio. One side of the block is directly hinged to a fixed pivot inside the enlarged crankcase, while the other side is linked to eccentrics on a control shaft.

Rotation of the shaft is handled by a sleeve-like hydraulic actuator with internal helical engagement. Axial movement of the sleeve tilts the cylinder block by as much as four degrees, when it is hydraulically locked in position to counter compression and combustion forces.

Inducted air is compressed to 40 psi (28 bar) by a belt-driven, twin-screw super-charger. This pressure is much higher than the boost applied by most turbochargers, avoiding the throttle lag typical with turbo-chargers.

The blower is combined with a water-cooled intercooler, which in turn feeds directly into the inlet manifold. This integrated construction helps suppress the compressor whine generated by speeds up to 30,000 rpm, although there is a magnetic-particle clutch to decouple the turbo drive at light loads, when pressurized air is not needed.

Variable compression avoids the need for the "compromise" fixed compression ratio of conventional engines, and can be continually adjusted to the optimum value for the prevailing load and power demand. The highest compression ratio of 14:1 - abnormally high by existing practice - gives the most efficient fuel utilization at light loads. When engine load increases, the risk of pre-ignition or knocking is avoided by reducing the compression ratio toward the 8:1 limit at the bottom of the spread. The resultant increase in combustion space permits greater cylinder filling, with a larger fuel-air charge, for maximum power delivery.

Stepless compression ratio changes are controlled by an advanced variant of the Saab Trionic engine management system that also regulates all other engine operating parameters.

The cylinder block is joined to the vertically-extended crankcase by a rubber bellows-type seal that accommodates the four-degree tilt and contains splashing engine oil. Although the block's angle change has some effect on the camshaft - and hence valve timing - the resultant shift of up to 2 degrees is thought by Saab engineers to be insignificant.

Called a "monohead" by Saab, the 4-valve-per-cylinder block is a one-piece aluminum casting with steel cylinder liners that required some special tooling to machine the valve seat recesses at the top of the blind bores.

However, tool access is eased by the short and fully exposed cylinders.

Also, despite the unique separation of the cylinder block and the crankcase, Saab says one objective was to retain as many standard production components as possible. The crankshaft, connecting rods, valves and pistons all are conventional.

Several experimental prototypes are now undergoing further development at Saab's Sodertalje, Sweden, engine plant. But despite the use of a number of conventional internal components, the relative complexity of the design, compared to conventional fixed-compression ratio engines, is acknowledged by the company as affecting manufacturing cost.

"While that is certainly a factor, the economic balance could well be tilted toward the 30% saving in fuel consumption," says Lars Bergsten, manager of Saab's Advanced Engineering and Technical Development.

"There also is the added bulk, although this engine is actually lighter and more powerful than our 3L V-6," he offers. Volume production still is some years ahead.

Meanwhile, General Motors Corp., which now fully owns Saab, reportedly is taking a positive interest in the project.