Gas Power Cycleslibvolume6.xyz/mechanical/btech/semester4/applied...Power Cycles Ideal Cycles,...
Transcript of Gas Power Cycleslibvolume6.xyz/mechanical/btech/semester4/applied...Power Cycles Ideal Cycles,...
Power Cycles
� Ideal Cycles, Internal Combustion� Otto cycle, spark ignition
� Diesel cycle, compression ignition
� Sterling & Ericsson cycles
� Brayton cycles
� Jet-propulsion cycle
� Ideal Cycles, External Combustion� Rankine cycle
Ideal Cycles
� Idealizations & Simplifications
� Cycle does not involve any friction� All expansion and compression processes are
quasi-equilibrium processes
� Pipes connecting components have no heat loss
� Neglecting changes in kinetic and potential energy (except in nozzles & diffusers)
Gas Power Cycles
� Working fluid remains a gas for the entire cycle
� Examples:
� Spark-ignition engines
� Diesel engines
� Gas turbines
Air-Standard Assumptions
� Air is the working fluid, circulated in a closed loop, is an ideal gas
� All cycles, processes are internally reversible
� Combustion process replaced by heat-addition
from external source
� Exhaust is replaced by heat rejection process
which restores working fluid to initial state
Cold-Air-Standard Assumption
� Air has constant specific heats, values are for room temperature (25°C or 77°F)
Otto Cycle
� Processes of Otto Cycle:
� Isentropic compression
� Constant-volume heat addition
� Isentropic expansion
� Constant-volume heat rejection
Otto Cycle
� Ideal Otto Cycle
� Four internally reversible processes
� 1-2 Isentropic compression
� 2-3 Constant-volume heat addition
� 3-4 Isentropic expansion
� 4-1 Constant-volume heat rejection
Otto Cycle
� Thermal efficiency of ideal Otto cycle:
� Since V2= V3 and V4 = V1
� Where r is compression ratio
k is ratio of specific heats
Spark or Compression Ignition
� Spark (Otto), air-fuel mixture compressed (constant-volume heat addition)
� Compression (Diesel), air compressed, then fuel added (constant-pressure heat addition)
Diesel Cycle
� Processes of Diesel cycle:
� Isentropic compression
� Constant-pressure heat addition
� Isentropic expansion
� Constant-volume heat rejection
Brayton Cycle
� Four internally reversible processes
� 1-2 Isentropic Compression (compressor)
� 2-3 Constant-pressure heat addition
� 3-4 Isentropic expansion (turbine)
� 4-1 Constant-pressure heat rejection
Brayton Cycle
� Back work ratio � Improvements in gas turbines
� Combustion temp
� Machinery component efficiencies
� Adding modifications to basic cycle
Turbojet Engines
� Turbofan: for same power, large volume of slower-moving air produces more thrust
than a small volume of fast-moving air (bypass ratio 5-6)
� Turboprop: by pass ratio of 100
Jets
� Afterburner: addition to turbojet
� Ramjet: use diffusers and nozzles
� Scramjet: supersonic ramjet
� Rocket: carries own oxidizer
Second Law Issues
� Ideal Otto, Diesel, and Brayton cycles are internally reversible
� 2nd Law analysis identifies where losses
are so improvements can be made
� Look at closed, steady-flow systems