UPDATE: Researchers Tout Variable-Compression Engine System

Carmakers eager to improve fuel efficiency increasingly are turning to smaller, highly boosted gasoline engines. But doing so means reducing the engine's compression ratio to avoid pre-ignition under high loads and that reduces fuel economy during low-load conditions.

Researchers at FEV GmbH are developing a way to enjoy both advantages: a system that automatically lowers a a gasoline engine's compression ratio during high-pressure turbocharging. They presented the technology earlier this week at the SAE 2013 World Congress in Detroit.

[Earlier editions incorrectly reported that the FEV system was developed for diesels. The technology is intended for gasoline engines.]

FEV says its system can improve overall fuel consumption by about 4%. The greatest gains are during highway cruising conditions, when demand for maximum power is low.

The system is intended for the industry's new generation of smaller and more fuel efficient direct-injection gasoline engines that use turbocharging to produce the power of larger-displacement engines when needed.

The FEV device fits two small pistons within each piston's connecting rod. Operating from engine oil picked up in the crankcase, they hydraulically actuate an eccentric cam in the wrist pin that connects the rod to the piston. Rocking the cam back and forth changes the length of the connecting rod, thus modifying the compression ratio.

The two-ratio system can detect and make its adjust in about 0.6 seconds, according to FEV. It says the device is almost as effective as continuously varying the compression ratio but at lower cost and a considerably simpler design. The company estimates the technology could be fitted in a four-cylinder engine for about $150.