Correct Answer - Option 2 : ideal efficiency
Explanation:
Air standard efficiency:
To compare the effects of the different cycles, it is of paramount importance that the effects of the calorific value of the fuel are altogether eliminated and this can be achieved by considering air (which is assumed to behave as a perfect gas) as the working substance in the engine cylinder.
The efficiency of the engine using air as the working medium is known as an "Air standard efficiency". This efficiency is often called ideal efficiency.
Relative efficiency:
The actual efficiency of a cycle is always less than the air-standard efficiency of the cycle under ideal conditions. This is taken by introducing a new term "Relative efficiency" which is defined as:
\({\eta _{relative}} = \frac{{{\bf{Actual}}\;{\bf{thermal}}\;{\bf{efficiency}}\;}}{{{\bf{Air}}\;{\bf{standard}}\;{\bf{efficiency}}}}\)
Assumptions in Air-standard cycles:
The analysis of all air standard cycles is based upon the following assumptions:
- The gas in the engine cylinder is a perfect gas i.e., it obeys the gas laws and has constant specific heat.
- The physical constants of the gas in the cylinder are the same as those of air at moderate temperatures i.e., the molecular weight of the cylinder gas is 29, cp = 1.005 kJ/kg-K, cv = 0.718 kJ/kg-K
- The compression and expansion processes are adiabatic and they take place without internal friction, i.e., these processes are isentropic.
- No chemical reaction takes place in the cylinder. Heat is supplied or rejected by bringing a hot body or a cold body in contact with the cylinder at appropriate points during the process.
- The cycle is considered closed with the same 'air' always remaining in the cylinder to repeat the cycle.
