Ideal Cycles AirStandard Assumptions and The Otto Cycle

Ideal Cycles, Air-Standard Assumptions, and The Otto Cycle

Thermodynamic Cycles • Power Cycles vs Refrigeration Cycles • Power Cycles – Gas vs vapor – Closed vs Open – Internal Combustion vs External Combustion

Ideal cycles are simplified

Be careful how you interpret results from ideal cycles

Ideal Cycles • More realistic than Carnot cycle. • Internally reversible but not totally. • Idealizations: – No friction – Expansions and compressions – quasiequilibrium – Heat transfer is negligible

Area inside the cycle represents net work out for Ts or Pv diagrams.

Diagrams for a Carnot Cycle

Equipment for a Carnot cycle. Changes have to be slow, with very large heat exchangers, so not practical.

Example 8 -1 Proof of thermal efficiency of a Carnot cycle qin and qout areas under lines so TH(s 2 – s 1), etc.

Air Standard Assumptions

Air Standard Assumptions • Air in closed loop – ideal gas • Internally reversible • Combustion replaced by heat addition • Exhaust replaced by heat rejection • Properties at room temp. – cold air standard assumptions.

Nomenclature

MEP = mean effective pressure, same amount of work Compression ratio?

Otto Cycle, ideal for spark ignition engines

Two-stroke engine

Ts plot of the Otto Cycle

Efficiency vs Compression Ratio Higher ratios produce autoignition and knocking

Efficiency vs ratio of specific heats He Air Combustion mixture

Example 8 -2 The Ideal Otto Cycle