AIR STANDARD CYCLE

ER. BANOJ KUMAR BEHERA

3^RD SEMESTER MECHANICAL ENGINEERING

THERMAL ENGINEERING-I

MAYURBHANJ SCHOOL OF ENGINEERING (MSE),

LAXMIPOSI, BARIPADA

Process 1🡪 2 Isentropic compression

Process 2 🡪 3 Constant volume heat addition

Process 3 🡪 4 Isentropic expansion

Process 4 🡪 1 Constant volume heat rejection

v₂

TC

​

TC

​

v₁

BC

​

​

BC

Q_out

Q_in

Air-Standard Otto cycle

Compression ratio:

First Law Analysis of Otto Cycle

1🡪2 Isentropic Compression

AIR

2🡪3 Constant Volume Heat Addition

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​

​

​

AIR

Q_in

TC

3 🡪 4 Isentropic Expansion

AIR

4 🡪 1 Constant Volume Heat Removal

AIR

Q_out

​

BC

Effect of Compression Ratio on Thermal Efficiency

• Spark ignition engine compression ratio limited by T₃ (autoignition)

and P₃ (material strength), both ~r^k

​

• For r = 8 the efficiency is 56% which is twice the actual indicated value

Typical SI engines

9 < r < 11

k = 1.4

Process 1🡪 2 Isentropic compression

Process 2 🡪 3 Constant pressure heat addition

Process 3 🡪 4 Isentropic expansion

Process 4 🡪 1 Constant volume heat rejection

Air-Standard Diesel cycle

Q_in

Q_out

​

​

Cut-off ratio:

v₂

TC

v₁

BC

​

​

TC

​

BC

​

For cold air-standard the above reduces to:

Thermal Efficiency

recall,

Note the term in the square bracket is always larger than one so for the

same compression ratio, r, the Diesel cycle has a lower thermal efficiency

than the Otto cycle

​

Note: CI needs higher r compared to SI to ignite fuel

Typical CI Engines

15 < r < 20

When r_c (= v₃/v₂)🡪1 the Diesel cycle efficiency approaches the

efficiency of the Otto cycle

Thermal Efficiency

Higher efficiency is obtained by adding less heat per cycle, Q_in,

🡪 run engine at higher speed to get the same power.

Thermodynamic Dual Cycle

Process 1 🡪 2 Isentropic compression

Process 2 🡪 2.5 Constant volume heat addition

Process 2.5 🡪 3 Constant pressure heat addition

Process 3 🡪 4 Isentropic expansion

Process 4 🡪 1 Constant volume heat rejection

Dual Cycle

Q_in

Q_in

Q_out

1

1

2

2

2.5

2.5

3

3

4

4

Thermal Efficiency

Note, the Otto cycle (r_c=1) and the Diesel cycle (α=1) are special cases:

The use of the Dual cycle requires information about either:

1. the fractions of constant volume and constant pressure heat addition

(common assumption is to equally split the heat addition), or

ii) maximum pressure P₃.

​

Transformation of r_c and α into more natural variables yields

For the same inlet conditions P₁, V₁ and the same compression ratio:

For the same inlet conditions P₁, V₁ and the same peak pressure P₃

(actual design limitation in engines):