Question

The air standard efficiency of an Otto cycle compared to diesel cycle for the given compression is
A) Same
B) Less
C) More
D) More or less depending on the power rating

Answer 1

Hint: An idealized thermodynamic cycle that gives the functioning of a typical spark ignition piston engine is known as an Otto cycle. This thermodynamic cycle is most commonly found in automobile engines. And the diesel cycle is a compression ignition engine rather than spark ignition engine.

Complete answer:
The Otto cycle is a description of what happens to a mass of gas when it is subjected to changes of temperature, pressure, addition of heat, volume and removal of heat. The mass of gas which is subjected to these changes is called a system. The system, in this case, is defined to be the gas or fluid within the cylinder.
 A diesel cycle is a compression ignition engine rather than spark ignition engine. At high pressure, fuel is sprayed into the cylinder. When the compression is complete, there is an ignition without a spark. A diesel cycle can operate with a higher compression ratio than an Otto cycle as there is no risk of auto-ignition of the fuel and only air is compressed.
But, for a given compression ratio, the Otto cycle has higher efficiency than the diesel engine. The area of Pressure-Volume diagram of the Otto cycle is greater than the diesel cycle for a given compression. Which means Otto cycle gives more work output than diesel cycle for same compression. In addition to this the subsidiary parts of diesel engines increase their weight and hence reduce the efficiency.

Therefore, the answer is option C.

Note:
Air standard cycles are thermodynamic cycles with air assumed as the working substance. In these cycles the combustion process and exhaust process are replaced by heat addition and heat rejection process. The efficiency of these cycles is known as air standard efficiency.