Otto Cycle vs Brayton Cycle: 5 Facts You Should Know

Brayton and Otto cycles generate mechanical energy out of thermal energy. This article discusses in detail on the topic Otto cycle vs Brayton cycle.

Brayton cycle is used in jet engines whereas Otto cycle is used in SI engine vehicles. Lets find out what other differences and similarities exist between these cycles.

Major working parts used in Brayton cycle

A set of machines work together to make Brayton cycle possible.

The different working parts used in Brayton cycle are compressor, mixing chamber and turbine. Compressor compresses the air, fuel is added in mixing chamber where the compressed air and fuel interact. Finally, thermal energy is converted to mechanical energy by turbine.

Working of Brayton cycle

Air is used as working fluid in Brayton cycle. Minimum three processes are required to complete this cycle (Three processes for open cycle and four processes for closed cycle).

Following processes combine to make up Brayton cycle-

• Isentropic compression- Process 1-2 represents isentropic compression in which air is compressed without changing its entropy.
• Isobaric heat addition- Process 2-3 represents isobaric heat addition in which heat is added to the mixing chamber; heat combined with compressed air produces high thermal energy.
• Isentropic expansion- Process 3-4 represents isentropic expansion in which the thermal energy is converted to mechanical energy. Rotation of turbine shaft represents mechanical energy.
• Isobaric heat rejection- Process 4-1 represents isobaric heat rejection where the heat is removed from the working fluid and is sent further to get compressed for next cycle.

Major parts used in Otto cycle

The parts used in Otto cycle are much smaller than those used in Brayton cycle.

The parts used in Otto cycle are-

• Piston- Piston performs up-and-down reciprocating motion that compresses the working fluid inside the cylinder.
• Cylinder- Cylinder is the foundation of Otto cycle. Cylinder is the place where all the energy conversion takes place.  
• Valves- The suction and delivery valves are used for intake of working fluid and exit of exhaust gases respectively.

Working of Otto Cycle

Otto cycle uses steam as its working fluid.

Following processes take place in Otto cycle-

• Isentropic compression- Process 1-2 shows isentropic compression of working fluid. The piston moves from BDC to TDC. The entropy of system is constant during this process hence it is called as isentropic compression.
• Isochoric heat addition- Process 2-3 represents heat addition in the system. The piston remains at TDC and shows ignition of the working fluid.
• Constant entropy expansion- Process 3-4 represents isentropic expansion (constant entropy expansion) where the piston moves from TDC to BDC. Since, the entropy remains constant throughout this process it is called as isentropic expansion.
• Isochoric heat addition- Process 4-1 represents heat addition to constant volume. The piston remains stationary at BDC while heat gets rejected to atmosphere.

This cycle keeps repeating as piston moves to TDC.

Brayton cycle vs Otto cycle efficiency

Both cycles different processes and different working fluids. This affects the efficiency of the cycles.

The comparison of thermal efficiencies of Brayton cycle and Otto cycle is shown in the table below-

Thermal efficiency of Brayton cycle	Thermal efficiency of Otto cycle

Where,

rp is the compression ratio and Y is specific heat ratio.

Hence, for constant values of compression ratio, both the efficiencies have same values.

But in practice, Brayton cycles are used for larger values of compression ratios and Otto cycle is used for small values of compression ratio. Hence, the formula of efficiency may be same but their applications are different.

Why is Brayton cycle more suitable than Otto cycle?

Brayton cycle uses a gas turbine and compressor whereas Otto cycle uses piston cylinder arrangement for its working. Otto cycle is preferred for SI engines where one cannot fit a gas turbine and compressor in the vehicle.

Following points explain in detail about advantages of Brayton over Otto cycle-

• For same values of compression and work output, Brayton cycle can handle a larger volume at small range of temperature and pressure.
• A piston cylinder arrangement can’t handle large volume of low pressure gas. Hence, Otto cycle is preferred in vehicles.
• In Otto cycle, the working parts are exposed to maximum temperature for a very short period of time and also it takes time to cool down. Whereas in gas turbine cycle, the working parts are exposed to high temperature all the time. In steady state process, the heat transfer from the machinery is difficult in constant volume process (ie Otto cycle) than at Constant pressure (ie Brayton cycle).