Variable Compression Ratio

Hannu Jääskeläinen

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Abstract: Variable compression ratio can offer a number of benefits, such as limiting the required peak firing pressure in diesels and counteracting the efficiency loss due to knock in spark ignited engines. The mechanisms for varying engine compression ratio include a two-step system where a low or a high compression ratio can be selected or a continuously variable compression system. Most two-step systems utilize a variable length connecting rod, while a variety of mechanisms have been proposed for continuously variable systems.


Variable compression ratio offers a number of benefits for diesel and gasoline engines. While the concept has been studied for many years [3537][3538][3539][3540][1942], the added cost and complexity have been difficult to justify for many applications. Developments that have provided a simpler mechanism through the use of variable length connecting rods appear to make this a viable option for volume production.

The mechanisms for varying compression ratio include either a two-step system where a low or a high compression ratio can be selected or a continuously variable system where any compression ratio between a low and high value can be selected.

Two-step systems include AVL’s and FEV’s variable length connecting rod. Continuously variable systems can be implemented through a variety of mechanisms including:

Two-Step Systems

AVL System

AVL’s two step variable compression ratio system uses a telescopic connecting rod, Figure 1. Actuation is by gas or mass forces. Inertia force FM and gas force FG are used to lengthen and shorten the connecting rod. The translatoric joint shortens the connecting rod when the resultant shaft force FR is toward the center of the crankshaft (FG > FM) and extends the connecting rod when it is in the opposite direction (FG < FM). Limit stops define the minimum and maximum connecting rod length. To “hold” one of the two positions, oil is transferred into the volumes below or above the translatoric joint. A control system signals when a change in connecting rod length is required [3518][3544].

Figure 1. AVL’s telescopic connecting rod