State the airstandard assumptions and the coldairstandard as

State the air-standard assumptions and the cold-air-standard assumptions. An ideal Otto cycle operates using the cold-air-standard assumptions with a given compression ratio. What gas would provide the highest thermal efficiency, air, argon, or ethane?

Solution

a)

The following assumptions are commonly known as the air standard assumptions:

1- The working fluid is air, which continuously circulates in a closed loop (cycle). Air is considered as ideal gas.

2- All the processes in (ideal) power cycles are internally reversible.

3- Combustion process is modeled by a heat-addition process from an external source.

4- The exhaust process is modeled by a heat-rejection process that restores the working fluid (air) at its initial state.

Assuming constant specific heats, (@25°C) for air, is called cold-air-standard assumption.

b)

Thermal efficiency of Otto cycle is given by 1 - (1/r)^k-1

Here r is compression ratio and k is the ratio of specific heats.

From this expression, we can deduce that for a given r, the higher the k, the higher the thermal efficiency.

In the given problem, r is same for all the 3 gases.

We have

k_air = 1.4

k_Argon = 1.66

k_Ethane = 1.18

Since Argon has highest k among all, it\'ll have highest thermal efficiency.