Engine design — why is the compression ratio of a spark-ignition (petrol) engine generally kept lower than that of a compression-ignition (Diesel) engine?

Introduction / Context:
Compression ratio strongly influences thermal efficiency, power density, and tendency to abnormal combustion. Diesel engines rely on very high compression to auto-ignite fuel, whereas spark-ignition engines ignite a premixed charge with a spark. The limit for petrol engines is set by knock and pre-ignition behaviour of the fuel-air mixture under compression and temperature rise.

Given Data / Assumptions:

• SI engine uses spark ignition and a premixed, throttled or direct-injected charge.
• CI engine uses compression ignition with very high compression ratios.
• Fuel octane rating constrains knock resistance in SI engines.

Concept / Approach:

As compression ratio increases in SI engines, end-gas temperature and pressure rise. If these exceed the auto-ignition threshold for the fuel-air mixture before the flame front arrives, knock or pre-ignition occurs, risking severe engine damage. Therefore, SI engines are limited to lower compression ratios compatible with available octane ratings and cooling strategies. Diesel engines require high compression to achieve the temperatures needed for spontaneous ignition of injected fuel, so their compression ratios are much higher by design.

Step-by-Step Solution:

Verification / Alternative check:

Engine design texts present typical ranges: SI engines often 9:1 to 12:1 (higher with direct injection and EGR), Diesel engines 14:1 to 22:1 or more, aligning with the knock-limited nature of petrol engines.

Why Other Options Are Wrong:

Makes petrol engines lighter — Weight depends on many design factors, not simply compression ratio.
Less compression gives better performance — Higher compression increases thermal efficiency until knock limits are reached.
Just customary — The limit is physical and chemical, not arbitrary custom.

Common Pitfalls:

Confusing knock in SI with the intended auto-ignition in Diesel; assuming fuel octane solves all knock without design trade-offs such as cooling and combustion chamber shape.

Final Answer:

higher or equivalent compression ratio in petrol engines is not possible due to pre-ignition