🧊Thermodynamics II Unit 4 – Gas Power Cycles: Otto, Diesel, and Brayton
Gas power cycles are the backbone of modern energy conversion systems. Otto, Diesel, and Brayton cycles power our vehicles, ships, and aircraft, converting thermal energy into mechanical work. These cycles differ in their compression, heat addition, and expansion processes, leading to unique efficiency characteristics.
Understanding these cycles is crucial for engineers and thermodynamics students. By analyzing the ideal cycles and their real-world applications, we can optimize engine design, improve fuel efficiency, and reduce emissions in various transportation and power generation systems.
Similar to the Otto cycle but with key differences in the heat addition process
Process 1-2: Isentropic compression of air only (no fuel)
Compression ratios are higher than in the Otto cycle, leading to higher temperatures
Process 2-3: Isobaric heat addition (constant pressure)
Fuel is injected into the compressed hot air, causing it to ignite and burn at nearly constant pressure
Process 3-4: Isentropic expansion
High-pressure gases push the piston down, performing work on the piston
Process 4-1: Isochoric heat rejection (constant volume)
Exhaust valve opens, and the burnt gases are expelled from the cylinder
Thermal efficiency depends on the compression ratio, specific heat ratio, and the cutoff ratio (ratio of volumes at the end and start of the heat addition process)
Higher compression ratios and lower cutoff ratios lead to higher efficiencies
Brayton Cycle Analysis
Open gas turbine cycle used in jet engines and power generation
Consists of four processes: isentropic compression, isobaric heat addition, isentropic expansion, and isobaric heat rejection
Process 1-2: Isentropic compression in the compressor
Ambient air is drawn into the compressor and compressed to a higher pressure
Process 2-3: Isobaric heat addition in the combustion chamber
Fuel is injected and burned at constant pressure, increasing the temperature of the gas
Process 3-4: Isentropic expansion in the turbine
Hot, high-pressure gases expand through the turbine, generating power to drive the compressor and provide useful work
Process 4-1: Isobaric heat rejection (exhaust)
Exhaust gases are released to the atmosphere at constant pressure
Thermal efficiency depends on the pressure ratio (ratio of compressor outlet to inlet pressures) and the specific heat ratio of the working fluid
Higher pressure ratios lead to higher efficiencies
Efficiency Comparisons
Otto, Diesel, and Brayton cycles have different ideal thermal efficiencies due to their unique operating principles
Otto cycle efficiency: η=1−rγ−11, where r is the compression ratio and γ is the specific heat ratio
Efficiency increases with higher compression ratios and specific heat ratios
Diesel cycle efficiency: η=1−rγ−11(γ(rc−1)rcγ−1), where rc is the cutoff ratio
Efficiency increases with higher compression ratios and lower cutoff ratios
Brayton cycle efficiency: η=1−rp(γ−1)/γ1, where rp is the pressure ratio
Efficiency increases with higher pressure ratios and specific heat ratios
In general, Diesel engines have the highest efficiency, followed by Otto engines and then Brayton engines
However, actual efficiencies are lower than ideal due to various irreversibilities and losses
Real-World Applications
Otto cycle: Used in gasoline-powered vehicles, small engines (lawnmowers, generators), and some aircraft piston engines
Diesel cycle: Used in diesel-powered vehicles (trucks, buses, trains), heavy machinery, and marine propulsion systems
Preferred for high-torque, low-speed applications due to their high compression ratios and fuel efficiency
Brayton cycle: Used in jet engines for aircraft propulsion and in gas turbines for power generation
Combined cycle power plants use a gas turbine (Brayton cycle) in conjunction with a steam turbine (Rankine cycle) to achieve higher overall efficiencies
Modifications to ideal cycles: Real engines incorporate additional features to improve efficiency and performance
Turbochargers (Otto and Diesel) use exhaust gases to compress intake air, increasing power output
Regenerative Brayton cycles use heat exchangers to preheat the compressed air before combustion, improving efficiency
Problem-Solving Strategies
Identify the type of cycle (Otto, Diesel, or Brayton) and the given parameters
Determine the process paths and their corresponding thermodynamic relations
Use isentropic, isochoric, and isobaric process equations as appropriate
Apply the First Law of Thermodynamics to each process, considering heat transfer and work interactions
Calculate the heat input, heat rejection, and net work output for the cycle
Use specific heat capacities, temperature changes, and pressure ratios as needed
Determine the thermal efficiency using the appropriate formula for the cycle
Consider the effects of varying parameters (compression ratio, cutoff ratio, pressure ratio) on efficiency and performance
Analyze the results and compare them to the ideal cycle efficiencies and real-world expectations
Discuss potential sources of irreversibilities and losses that reduce actual efficiency