Inside Look

Fev’s Variable Compression Connecting Rod

- Thom Cannell

It has been years—over a decade, in fact— since reducing emissions or improving fuel economy was either easy or cheap. In the early days of engine efficiency upgrades, you could get a 1-percent improvement at low cost. An early example: Simply changing piston ring design for less blow-by dramatically improved hydrocarbon emissions.

Today, we’re at a point in engine technology and advancements where every single percentage is hard won and costly. Yet, there are still solutions that result in over 5-percent improvement in CO2 on the New European Driving Cycle, like this connecting rod that delivers variable compression ratio.

FEV is an engineering consultant company based in Aachen, Germany, with an office in Auburn Hills, Michigan. We’ve been following development of this connecting rod for five years, but the idea of variable compression ratio goes back more than a decade.

There was an attempt to continuously vary compression ratio by moving the entire crankshaft up and down. Moving the crank vertically meant the transmission had to move along with it, or involve some energy transfer mechanism. The last straw in that development was that there was no way to easily adapt a fixed center line change with an existing transmission (think about it). A later solution moved the head itself, but both had significant challenges. Why continuously variable compression? At the time, engineers thought it necessary to linearly move the compression ratio across the entire engine map.

The next evolution was to look at the connecting rod. Earlier designs were based on an eccentric bearing at the crankshaft. Now, FEV’s con-rod design uses an eccentric bearing at the wrist pin to move the piston up and down from its peak in the cylinder bore. If you are using an eccentric bearing you deliver what becomes a two-step scenario.