Thermal Efficiency

For a heat engine, thermal efficiency is the ratio of the net work output to the heat input. 

For a heat pump, thermal efficiency (also known as the coefficient of performance) is the ratio of the net heat output (for heating) or the net heat removed (for cooling) to the energy input.

For a heat engine, thermal efficiency is the ratio of the net work output to the heat input (external work). 

The efficiency of a heat engine is always going to be negative because the output will be less than the input, but the coefficient of performance (COP) of a heat pump will always be more than 1.    

The theorem of Carnot places further constraints on these value ranges.

Air standard cycle

The actual processes that take place within an internal combustion engine are quite complicated.

As a result, the air-standard cycle is a frequent instrument for the examination of internal combustion engines.

When modelling a real engine, it is possible to examine the influence of only the most important operating variables on engine performance thanks to the simplification of the modelling technique. 

Even if the numerical values that are produced from such models only provide a qualitative picture of the actual process, 

The information that they provide is extremely valuable and analytical.

The air-standard cycle is an idealised version of a cycle that is based on the following approximations: 

(1) The working fluid throughout the entire cycle is only air

(2) The air behaves as if it were an ideal gas. 

(3) The combustion processes are replaced by well-defined heat addition processes. 

(4)The exhaust process is replaced by a heat rejection process that returns the air of the cycle to its intake conditions.

 It is common practise to assume constant values of specific heat at constant volume and pressure when discussing air

 because it is generally considered to be an ideal or perfect gas.

Effectiveness of the air standard cycle

The concept of air standard efficiency is predicated on a Carnot cycle, 

also known as an air standard cycle, which can be seen in the following link, which discusses the thermodynamics of internal combustion engines.

When comparing various kinds of engines to one another, the air standard efficiency serves as a baseline for comparison since it offers a point of reference that is consistent.

It is of the utmost importance that the effects of the calorific value of the fuel are entirely eliminated in order to make a fair comparison of the effects of the various cycles. 

This can be accomplished by considering air (which is assumed to behave as a perfect gas) as the working substance in the engine cylinder. 

Air is assumed to have the same properties as a perfect gas.

“Air standard efficiency” refers to the level of productivity achieved by a machine that makes use of air as the working medium.

It is common practise to refer to this efficiency as ideal efficiency.

Presumptions in Air-standard cycles 

The following presumptions serve as the foundation for the study of any and all air standard cycles:

• The gas that is contained within the engine cylinder is a perfect gas, which means that it abides by the gas laws and maintains the same level of specific heat

• The molecular weight of the gas in the cylinder is 29, cp = 1.005 kJ/kg-K, and cv = 0.718 kJ/kg-K; these values are the same as those for air at mild temperatures

The physical constants of the gas in the cylinder are the same as those for air.

• Because there is no internal friction involved in either the compression or expansion processes,

we may say that these processes are isentropic, which means that they are adiabatic.

• Within the cylinder, there is not going to be any chemical reaction

At the proper points in the process, heat is either introduced into the cylinder or removed from it by bringing a body that is either hot or cold into touch with it.

• The cycle is deemed to be closed when the same amount of ‘air’ remains in the cylinder at all times in order to repeat the cycle

Conclusion

Even the most effective heat engines have a poor efficiency rating, which is often below 50 percent and frequently much lower. 

Therefore, the energy that is released into the atmosphere by heat engines is a significant example of wasted energy resources.

Even though modern cogeneration, combined cycle, and energy recycling schemes are beginning to use this heat for other purposes,

a significant portion of the useful energy produced worldwide is lost due to the inefficiency of heat engines. 

This loss could account for as much as half of the useful energy produced worldwide. 

There are three factors that contributed to this level of inefficiency. 

The Carnot efficiency describes an overall theoretical limit that may be placed on the amount of work done by any heat engine as a function of temperature.

Second, the intrinsic irreversibility of the engine cycle utilised by certain types of engines places a lower limit on the amount of efficiency that can be achieved by those engines. 

Thirdly, extra efficiency losses are caused by the nonideal behaviour of real engines, such as mechanical friction and losses in the combustion process.

These factors contribute to the non ideal behaviour of real engines.