It was at the recent Society of Automotive Engineers World Congress when Franz Pischinger, CEO of engine developer FEV Motorentechnik GmbH, stressed the importance of downsizing automotive engines to improve fuel economy and reduce emissions.

Now, FEV is doing more than just talking about smaller engines. The engineering supplier is demonstrating a prototype engine with a continuously variable compression ratio (VCR). The company replaced the 3L V-6 base engine in an Audi A6 with a smaller 1.8L turbocharged I-4. Even with a considerably downsized powerplant, the switch makes for a highly entertaining driving experience.

The 1.8L turbo from Volkswagen AG is a fine engine and a multi-year Ward's 10 Best Engine award winner. But FEV added VCR with a few machining tweaks to the bottom end and added an electrically actuated eccentric bearing that allows the crankshaft to move vertically while the engine is running.

In normal driving, the crankshaft is in the “up” position, which raises the compression ratio of the engine by reducing the squish volume above the piston at top-dead-center. Pushing the piston higher into the cylinder compresses the air-fuel charge more tightly and produces more efficient combustion.

Most gasoline engines operate at a fixed compression ratio of less than 10.5:1. But FEV, and other companies dabbling in variable compression, contends that the compression ratio of an engine should change depending on the demands of the driver.

The VCR concept engine achieves compression ratios of between 8:1 and 16:1, says David Gian, department manager of vehicle integration at FEV North America.

During normal driving, idling at stop lights and even while cruising the freeway at 87 mph (140 km/h), the concept engine delivers ratios of between 14:1 and 16:1 (comparable to the high compression ratios of diesels). It's only during hard acceleration that the compression ratio drops below 9:1, when the engine needs a larger air-fuel charge.

FEV says its VCR concept works best on inline engines, and that the efficiency gain is diminished in a V-6 or V-8 because the compression ratio differs between each cylinder bank. An I-6 truck engine equipped with VCR could achieve compression ratios of between 9:1 and 12:1 while pulling a load uphill, says Martin Whitehead, manager of business development for FEV North America.

In the demanding European driving cycle, FEV's concept achieves 9% better fuel consumption than the same 1.8L 4-cyl. without VCR. Compared to the 3L V-6, the 1.8L concept achieves a 25% improvement in fuel economy. But Gian warns that jack-rabbit starts at every stop light will wipe out much of the potential fuel savings.

FEV is shopping the concept to auto makers everywhere. Recently, the concept car was at FEV's Auburn Hills headquarters for test drives by customers and journalists.

The engine performs so admirably that Audi fans will barely miss the not-too-shabby 3L normally aspirated V-6 usually found in the A6. The crankshaft on the concept engine can be moved vertically as much as 0.25 in. (6 mm) with the help of a 200-watt electric motor, and the movement is neither felt nor heard by the driver.

During hard acceleration, the system needs only 0.1 of a second to reduce the engine's compression ratio from 16:1 to 8:1. During deceleration, the system needs a bit more time — 0.5 of a second — to raise the compression ratio from 8:1 to 16:1.

The only minor annoyances are a hint of turbo lag and the torque steer that comes with this powerplant mated to the front-wheel-drive A6. But underpowered it's not.

Enhanced low-end torque is a key benefit of VCR, as the technology allows higher compression ratios to be maintained at lower speeds — without the annoying engine knock, Whitehead says.

Saab Automobile revealed in 2000 its variable-compression engine concept, with a “tilting” hydro-mechanical cylinder-head. But sources say Saab does not intend to produce the system because of durability issues.

FEV's VCR will be production-ready around 2005, Whitehead says.

Gian says VCR can be applied to an existing engine with minor manufacturing changes. The system adds about 15 lbs. (7 kg) to the weight of the concept engine, but Gian suggests that weight could be cut significantly on a production engine program.

Depending on volume, the technology can cost an OEM between $140 and $160 per engine, say FEV engineers.