How does variable compression ratio (VCR) technology work and what challenges does it present?

VCR enables real-time adjustment of compression ratio to optimize efficiency at each operating point. Technologies include: Multi-link crankshaft mechanisms (Infiniti VC-Turbo), eccentric connecting rods, variable piston height, and two-stage systems. Benefits include high compression (14:1) for efficiency at low loads, reduced compression (8:1) for knock avoidance under boost. Challenges include mechanical complexity, friction and weight penalty, packaging constraints, durability of moving parts, and control system response speed. Calibration must coordinate VCR with spark timing, boost, and VVT for optimal combustion phasing across all conditions.