UNIT - VIIIPower Cycles : Otto, Diesel, Dual Combustion cycles, Sterling Cycle, Atkinson Cycle,Ericcson Cycle, Lenoir Cycle – Description and representation on P–V and T-Sdiagram, Thermal Efficiency, Mean Effective Pressures on Air standard basis –comparison of Cycles.In gas power cycles gas is the working fluid. It does not undergo any phase change . Engines operating on gas cycles may be either cyclic or non-cyclic. Hot air engines using air as the working fluid operate on a closed cycle. Internal combustion engines where the combustion of fuel takes place inside the engine cylinder are non-cyclic heat engines.CARNOT CYCLE :

Two reversible isotherms and two reversible adiabatics. If an ideal gas isassumed as the working fluid, then for 1 kg of gas

STERLING CYCLE:

IT consists of Two reversible isotherms and two reversible isochores. For Ikg of ideal gas

Due to heat transfers at constant volume processes, the efficiency of theStirling cycle is less than that of the Carnot cycle. However, if a regenerativearrangement is used such that

So, the regenerative Stirling cycle has the same efficiency as the Carnot cycle.

ERICSSON CYCLE :

IT CONSISTS of Two reversible isotherms and two reversible isobars.For 1 kg of ideal gas

Since part of the heat is transferred at constant pressure and part at constanttemperature, the efficiency of the Ericsson cycle is less than that of the Camotcycle. But with ideal regeneration, so that all the heat is added fromthe external source at TI and all the heat is rejected to an external sink at T2• theefficiency of the cycle becomes equal to the Camot cycle efficiency, since

Air Standard Cycles

Internal combustion engines in which the combustion offuel occurs inthe engine cylinder itself are non-cyclic heat engines. The temperature due to the evolution of heat because of the combustion of fuel inside the cylinder is so high that the cylinder is cooled by water circulation around it to avoid rapiddeterioration. The working fluid, the fuel-air mixture, undergoes permanentchemical change due to combustion, and the products of combustion after doingwork are thrown out of the engine, and a fresh charge is taken. So the workingfluid does not undergo a complete thermodynamic cycle. To simplify the analysis of 'I.C. engines, air standard cycles are conceived. In an air standard cycle, a certain mass of air operates in a complete thermodynamic cycle, where heat is added and rejected with external heat reservoirs, and all the processes in the cycle are reversible. Air is assumed to behave as an ideal gas, and its specific heats are assumed to be constant. These air standard cycles are so conceived that they correspond to the operations ofintemal combustion engines.OTTO CYCLE

The Otto cycle is the air standard cycle of Spark Ignition (S.l.)engine used in automobiles. The sequence of processes in the elementary operation of the S.1.engine is given below, with reference to Fig. where the sketches of the engine and the indicator diagram are given.

Process 1-2, Intake. The inlet valve is open; the piston moves to the right,admitting fuel-air mixture into the cylinder at constant pressure.Process 2-3, Compression. Both the valves are closed, the piston compresses the combustible mixture to the minimum volume.Process 3-4, Combustion. The mixture is then ignited by means of a spark,combustion takes place, and there is an increase in temperature and pressure.Process 4-5, Expansion. The products of combustion do work on the pistonwhich moves to the right, and the pressure and temperature of the gases decrease.Process 5-6, Blow-down. The exhaust valve opens, and the pressure drops to theinitial pressure.Process 6-1, Exhaust. With the exhaust valve open, the piston moves inwards toexpel the combustion products from the cylinder at constant pressure.The series of processes as described above constitute a mechanical cycle, andnot a thermodynamic cycle. The cycle is completed in four strokes of the piston.Figure (c) shows the air standard cycle (Otto cycle) corresponding to theabove engine. It consists of: .Two reversible adiabatics and two reversible isochores.

Air is compressed in process 1-2 reversibly and adiabatically. Heat is thenadded to air reversibly at constant volume in process 2-3. Work is done by air in. expanding reversibly and adiabatically in process 3-4. Heat is then rejected byair reversibly at constant volume in process 4-1 , and the system (air) comes back to its initial state. Heat transfer processes have been substituted for thecombustion and blow-down processes of the engine. The intake and exhaustprocesses of the engine cancel each other.

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ηotto

It is clear from the above expression that efficiency increases with the increase in the value of r, which means we can have maximum efficiency by increasing r to a considerable extent, but due to practical difficulties its value is limited to about 8.The net work done per kg in the Otto cycle can also be expressed in terms of p, v. If p is expressed in bar i.e.,

CONSTANT PRESSURE OR DIESEL CYCLE This cycle was introduced by Dr. R. Diesel in 1897. It differs from Otto cycle in that heat is supplied at constant pressure instead of at constant volume. Fig. (a and b) shows the p-vand T-s diagrams of this cycle respectively.This cycle comprises of the following operations :(i) 1-2......Adiabatic compression.(ii) 2-3......Addition of heat at constant pressure.(iii) 3-4......Adiabatic expansion.(iv) 4-1......Rejection of heat at constant volume.

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It may be observed that eqn2 for the efficiency of diesel cycle is different from that of the Otto cycle only in the bracketed factor. This factor is always greater than unity because ρ>1. Hence for a given compression ratio the Otto cycle is more efficient.