Question

a) Why does an automobile Diesel engine typically run with a higher efficiency than an engine based on the Otto cycle?

b) describe the Carnot cycle and its significance.

c) What properties must a working fluid ( such as HFC-134a) have in the refrigeration cycle?

Answer 1

Diesel engine is more efficient than an engine based on the Otto cycle. it is because of higher compression. Some amount of heat energt will be gnerated during combustion stroke in an engine. But not all of the generated energt will ve transformed to mechanical form. :arge part of he heat energy will be expelld through the exhaust pipe and therefore lost.Because it rses higher copression ratio, the diesel engine will be able to extract more mechanical from heat that is diesel engine will attain higher efficiency.

if you look at he engine in largely simplified way you can see that combustion stroke is a simple adiabatim process. During the combustion stroke. you start with high presure and high teperature of the combustion gas in the cylinder then the poston travels downward for some length to its bottom position and now yu have lower porssure and lower tempreature of the gas. obviously, the energy contained in the gas is now lower . the difference is converted to mechanical energy of the piston.

B) the cycle followed by the working substance.As indicated by the arrows,the cycle is traversed in the clockwise direction. Imagine the working substance to be a gas, confined to an insulating cylinder with a weighted, movable piston.The cylinder may be placed at will on either of the two thermal reservoirs, as in or on an insulating slab. shows that, if we place the cylinder in contact with the high temperature reservoir at temperature TH,heat |QH| is transferred to the workingsubstance from this reservoir as the gas undergoes an isothermal expansion from volume Va to volume Vb. Similarly, with the working substance in contact with the low-temperature reservoir at temperature TL, heat |QL| is transferred from the working substance to the low-temperature reservoir as the gas undergoes an isothermal compressionfrom volume Vc to volume VD. In the engine ,we assume that heat transfers to or from the working substance can take place only during the isothermal processes ab and cd . Therefore, processes bc and da in that figure, which connect the two isotherms at temperatures TH and TL, must be (reversible) adiabatic processes; that is,they must be processes in which no energy is transferred as heat.To ensure this,during processes bc and da the cylinder is placed on an insulating slab as the volume of the working substance is changed. During the processes ab and bc, the working substance is expanding and thus doing positive work as it raises the weighted piston.This work is represented in by the area under curve abc. During the processes cd and da ,the working substance is being compressed,which means that it is doing negative work on its environment or,equivalently,that its environment is doing work on it as the loaded piston descends.This work is represented by the area under curve cda.The net work per cycle, which is represented by W in both, is the difference between these two areas and is a positive quantity equal to the area enclosed by cycle abcda .This work W is performed on some outside object,such as a load to be lifted.

C)

A refrigerant is a substance or mixture, usually a fluid, used in a heat pump and refrigeration cycle. In most cycles it undergoes phase transitions from a liquid to a gas and back again. Many working fluids have been used for such purposes. Fluorocarbons, especially chlorofluorocarbons, became commonplace in the 20th century, but they are being phased out because of their ozone depletion effects. Other common refrigerants used in various applications are ammonia, sulfur dioxide, and non-halogenated hydrocarbonssuch as propane.^[1]

The ideal refrigerant would have favorable thermodynamic properties, be noncorrosive to mechanical components, and be safe, including free from toxicity and flammability. It would not cause ozone depletion or climate change. Since different fluids have the desired traits in different degree, choice is a matter of trade-off.

The desired thermodynamic properties are a boiling point somewhat below the target temperature, a high heat of vaporization, a moderate density in liquid form, a relatively high density in gaseous form, and a high critical temperature. Since boiling point and gas density are affected by pressure, refrigerants may be made more suitable for a particular application by choice of operating pressures.