If you will notice the formula of thermal efficiency, you will definitely get an idea.Thermal efficiency=1-1/(r^(¥-1))


, where, r=compression ratio,
¥=specific heat ratio.
r increases then Thermal Efficiency increases.

Introduction:
In spark ignition (SI) engines, the thermal efficiency refers to the efficiency with which the engine converts the energy from the fuel into useful work. Higher thermal efficiency is desirable as it indicates that a greater proportion of the energy from the fuel is being converted into useful work rather than being wasted as heat.

Explanation:
Among the given options, the correct answer is option 'A' - the compression ratio should be high. This is because a higher compression ratio leads to higher thermal efficiency in SI engines due to the following reasons:

1. Increased Combustion Efficiency:
A higher compression ratio results in a higher air-fuel mixture temperature and pressure before ignition. This leads to a more complete combustion of the fuel, resulting in a higher combustion efficiency. A higher combustion efficiency means that a greater proportion of the fuel is being converted into useful work, thereby increasing the thermal efficiency.

2. Increased Expansion Ratio:
The compression ratio determines the maximum pressure and temperature reached during the compression stroke. These parameters directly affect the expansion stroke, which is responsible for producing work. A higher compression ratio leads to a higher expansion ratio, resulting in more work being produced per cycle, thus increasing the thermal efficiency.

3. Reduced Heat Losses:
A higher compression ratio also leads to a smaller combustion chamber volume at the end of the compression stroke. This reduces the surface area-to-volume ratio, which in turn reduces heat losses to the cylinder walls. Minimizing heat losses is crucial for achieving higher thermal efficiency as it ensures that a larger portion of the released heat is used to produce work rather than being lost to the surroundings.

Conclusion:
In conclusion, a high compression ratio is essential for achieving higher thermal efficiency in SI engines. It leads to increased combustion efficiency, higher expansion ratio, and reduced heat losses, all of which contribute to a more efficient conversion of fuel energy into useful work.