Gas turbines are widely used in power generation and industrial applications due to their high power-to-weight ratio and compact size. The overall efficiency of a gas turbine refers to the ratio of the useful work output to the energy input. In this context, the correct answer is option 'D', which states that the overall efficiency of a gas turbine is less than the efficiency of a diesel cycle.

Explanation:

Gas Turbine Basics:
A gas turbine operates on the Brayton cycle, which is an idealized thermodynamic cycle for gas turbines. It consists of four main processes: compression, combustion, expansion, and exhaust. Air is compressed in the compressor section, mixed with fuel and burned in the combustion chamber, and then expanded in the turbine section to produce work.

Factors Affecting Efficiency:
Several factors contribute to the overall efficiency of a gas turbine, including its design, operating conditions, and the properties of the working fluid. The key factors affecting efficiency are discussed below:

1. Compression Ratio:
The compression ratio is the ratio of the absolute pressures at the end of the compression and the beginning of the compression processes. A higher compression ratio leads to higher efficiency by increasing the pressure and temperature of the working fluid before combustion.

2. Combustion Efficiency:
The combustion efficiency refers to the ability of the combustion process to convert the chemical energy of the fuel into heat. Higher combustion efficiency results in more energy being available for conversion into useful work.

3. Turbine Efficiency:
The turbine efficiency is a measure of how effectively the turbine converts the thermal energy of the working fluid into mechanical work. Higher turbine efficiency results in more work output for a given amount of energy input.

Comparison with Other Cycles:
Gas turbines have lower overall efficiencies compared to diesel cycles due to several reasons:

1. Higher Exhaust Temperature:
Gas turbines have higher exhaust temperatures compared to diesel engines, resulting in more energy being lost in the exhaust gases. This reduces the overall efficiency of the gas turbine.

2. Lower Compression Ratio:
Gas turbines typically have lower compression ratios compared to diesel engines. This limits the amount of energy that can be extracted from the working fluid during expansion, leading to lower overall efficiency.

3. Different Combustion Process:
Gas turbines rely on an open combustion process, where the fuel is mixed with a large amount of air. This results in lower combustion efficiencies compared to the closed combustion process used in diesel engines.

Conclusion:
In summary, the overall efficiency of a gas turbine is less than the efficiency of a diesel cycle due to factors such as higher exhaust temperatures, lower compression ratios, and lower combustion efficiencies. However, gas turbines still offer advantages in terms of their power-to-weight ratio and compact size, making them suitable for various applications in power generation and industry.