Thermodynamics

Chapter 9 | 537pistons, and prevents the engine from rusting. Therefore, oil and oil filter shouldbe changed as recommended by the vehicle manufacturer. Fuel-efficient oils(indicated by “Energy Efficient API” label) contain certain additives that reducefriction and increase a vehicle’s fuel economy by 3 percent or more.In summary, a person can save fuel, money, and the environment by purchasingan energy-efficient vehicle, minimizing the amount of driving, beingfuel-conscious while driving, and maintaining the car properly. These measureshave the added benefits of enhanced safety, reduced maintenance costs,and extended vehicle life.SUMMARYA cycle during which a net amount of work is produced iscalled a power cycle, and a power cycle during which theworking fluid remains a gas throughout is called a gas powercycle. The most efficient cycle operating between a heatsource at temperature T H and a sink at temperature T L is theCarnot cycle, and its thermal efficiency is given byh th,Carnot 1 T LT HThe actual gas cycles are rather complex. The approximationsused to simplify the analysis are known as the airstandardassumptions. Under these assumptions, all theprocesses are assumed to be internally reversible; the workingfluid is assumed to be air, which behaves as an ideal gas;and the combustion and exhaust processes are replaced byheat-addition and heat-rejection processes, respectively. Theair-standard assumptions are called cold-air-standard assumptionsif air is also assumed to have constant specific heats atroom temperature.In reciprocating engines, the compression ratio r and themean effective pressure MEP are defined asr V maxV min V BDCV TDCw netMEP v max v minThe Otto cycle is the ideal cycle for the spark-ignition reciprocatingengines, and it consists of four internally reversibleprocesses: isentropic compression, constant-volume heat addition,isentropic expansion, and constant-volume heat rejection.Under cold-air-standard assumptions, the thermal efficiencyof the ideal Otto cycle ish th,Otto 1 1r k1where r is the compression ratio and k is the specific heatratio c p /c v .The Diesel cycle is the ideal cycle for the compressionignitionreciprocating engines. It is very similar to the Ottocycle, except that the constant-volume heat-addition processis replaced by a constant-pressure heat-addition process. Itsthermal efficiency under cold-air-standard assumptions ish th,Diesel 1 1r k1c r k c 1k 1r c 12 dwhere r c is the cutoff ratio, defined as the ratio of the cylindervolumes after and before the combustion process.Stirling and Ericsson cycles are two totally reversiblecycles that involve an isothermal heat-addition process at T Hand an isothermal heat-rejection process at T L . They differfrom the Carnot cycle in that the two isentropic processes arereplaced by two constant-volume regeneration processes inthe Stirling cycle and by two constant-pressure regenerationprocesses in the Ericsson cycle. Both cycles utilize regeneration,a process during which heat is transferred to a thermalenergy storage device (called a regenerator) during one partof the cycle that is then transferred back to the working fluidduring another part of the cycle.The ideal cycle for modern gas-turbine engines is the Braytoncycle, which is made up of four internally reversibleprocesses: isentropic compression, constant-pressure heat addition,isentropic expansion, and constant-pressure heat rejection.Under cold-air-standard assumptions, its thermal efficiency ish th,Brayton 1 1r 1k12>kpwhere r p P max /P min is the pressure ratio and k is the specificheat ratio. The thermal efficiency of the simple Brayton cycleincreases with the pressure ratio.The deviation of the actual compressor and the turbine fromthe idealized isentropic ones can be accurately accounted forby utilizing their isentropic efficiencies, defined ash C w sw a h 2s h 1h 2a h 1