Usually, what is the relationship between compression ratio and engine efficiency?

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

Answer: (B)

When you increase the compression ratio, you boost the thermal efficiency of an internal combustion engine. In the ideal Otto cycle that models spark-ignition engines, the efficiency rises as the compression ratio r increases, roughly following η ≈ 1 − 1/r^(k−1). This happens because higher compression means the fuel-air mixture starts from a higher pressure and temperature, so more of the added heat is converted into useful work rather than wasted as heat. In real engines, you can’t push compression ratios arbitrarily high due to knocking, heat losses, and mechanical limits, which cap the practical gains. Still, the fundamental trend is clear: higher compression ratio generally leads to greater efficiency.