The higher compression ratio leads to greater efficiency.

• A. The higher the compression ratio the lower the efficiency
• B. Efficiency depends on displacement
• C. The higher the compression ratio the greater the efficiency
• D. Compression ratio has no effect on efficiency

Answer: (C)

Raising the compression ratio improves the engine’s thermal efficiency in ideal cyclical processes. When the air–fuel mixture is compressed to a smaller volume, the temperature and pressure before ignition are higher, so the subsequent combustion does more useful work during the expansion stroke. In the ideal Otto cycle, efficiency increases with compression ratio according to η = 1 − 1/r^(γ−1) (with γ around 1.4 for air–fuel). As r grows, the subtracted term gets smaller, pushing efficiency higher. Of course, in real engines there are limits—higher compression can cause knocking and requires higher-octane fuel—but the fundamental relation is that greater compression ratio tends to yield greater efficiency.