Question

engine performance experiment (otto cycle)

III.4 Isentropic Polytropic Processes The air-standard Otto cycle models both the compression and expansion phases of the combustion gases as isentropic. How well does this assumption hold up? From the same set of data used in III.1, plot a graph of log p versus log V (or ln p versus In V) From visual inspection of the graph obtained, are expansion and compression processes polytropic? If so, determine the polytropic indices. By comparing these values with k for air, is the assumption these processes are isentropic valid?

Answer 1

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Assumptions 1. The gas in the engine cylinder is a perfect gus ie., it obeys the gas laws and has constant specific heats. 2. The physical constants of the gas in the cylinder are the same as those of air at moderate temperatures i.e., the molecular weight of cylinder gas is 29. G, 1.005 kJ/kg K, c, - 0.718 kJkg K 3. The compression and expansion processes are adiabatic and they take place without interna ction, Le., these processes are isentropic. 4. No chemical reaction takes place in the cylindet. Heat is supplied or rejected by bringing a hot bodl or a cold body in contact with cylinder appropriate points during the process. 5. The cycl e is considered closed with the same always remaining in the cylinder to cycle. air repeat