Variable Compression Ratio

Hannu Jääskeläinen

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• Introduction
• Two-Step Systems
• Continuously Variable Systems
• Diesel Engine Benefits
• Otto Cycle Engine Benefits
• Dual Fuel Engine Benefits

Introduction

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:

• Multilink mechanism between the crankshaft and piston, Nissan and MCE-5
• Movable head/cylinder, SAAB [3541] and Enerva [3542]
• Eccentric crankshaft journals, Caterpillar [1927][1921][1934]
• An eccentric link between the crankpin and connecting rod, Gomecsys [6041][6040]
• Movable piston crown [3543]

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 F_M and gas force F_G are used to lengthen and shorten the connecting rod. The translatoric joint shortens the connecting rod when the resultant shaft force F_R is toward the center of the crankshaft (F_G > F_M) and extends the connecting rod when it is in the opposite direction (F_G < F_M). 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].

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