Air-standard analysis treats the fluid flow through the entire engine as air and approxi¬mates air as an ideal gas. In a real engine inlet flow may be all air, or it may be mixed, up with 7% fuel, either gaseous or as liquid droplets, or both. In air-standard analysis, even if all fluid in an engine cycle were air, some error would be introduced by assuming it to be an ideal gas with constant specific heats.
At the low pressures of inlet and exhaust, air can accurately be treated as an ideal gas, but at the higher pressures during combustion, air will deviate from ideal gas behavior.
A more serious error is introduced by assuming constant specific heats for the analysis. Specific heats of a gas have a fairly strong dependency on temperature and can vary as much as 30% in the temperature range of an engine. During the cycle of a real engine there are heat losses which are neglected in air-standard analysis.
Loss of heat during combustion lowers actual peak temperature and pressure from what is predicted. The actual power stroke, therefore, starts at a lower pressure, and work output during expansion is decreased. A detailed study of the performance of a reciprocating internal combustion engine would take into account many features. These would include the combustion process occurring within the cylinder and the effects of irreversibility have associated with friction and with pressure and temperature gradients.
Heat transfer between the gases in the cylinder and the cylinder walls and the work required to charge the cylinder and exhaust the products of combustion also would be considered.
Owing to these complexities, accurate modeling of reciprocating internal combustion engines normally involves computer simulation. To conduct elementary thermodynamic analyses of internal combustion engines, considerable simplification is required. One procedure is to employ an air-standard analysis having the following elements:
In addition, in a cold air-standard analysis, the specific heats are assumed constant at their ambient temperature values. With an air-standard analysis, we avoid dealing with the complexities of the combustion process and the change of composition during combustion.
Although an air-standard analysis simplifies the study of internal combustion engines considerably, values for the mean effective pressure and operating temperatures and pressures calculated on this basis may depart significantly from those of actual engines. Accordingly, air-standard analysis allows internal combustion engines to be examined only qualitatively. Still, insights concerning actual performance can result with such an approach.
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