Thermodynamic analysis of CI engines:
Cylinder pressure versus crank angle data over the compression and expansion strokes of the engine operating cycle can be used to obtain quantitative information on the progress of combustion. Suitable methods of analysis which yield the rate of release of the fuel's chemical energy (often called heat release), or rate of fuel burning, through the diesel engine combustion process will now be described. The method of analysis starts with the first law of thermodynamics for an open system which is quasi static (i.e., uniform in pressure and temperature). The first law for such a system
where dQ/dt is the heat-transfer rate across the system boundary into the system, p(dV/dt) is the rate of work transfer done by the system due to system boundaq mass flows across the system boundary displacement, ṁ, is the mass flow rate into the system across the system boundary at location i (flow out of the system would be negative), hi is the enthalpy of flux i entering or leaving the system, and U is, the energy of the material contained inside the system boundary.
The following problems make the application of this equation to diesel combustion difficult:
1. Fuel is injected into the cylinder. Liquid fuel is added to the cylinder which vaporizes and mixes with air to produce a fuel/lair ratio distribution which is nonunifom and varies with time. The process is not quasi static.
2. The composition of the burned gases (also nonuniform) is not known.
3. The accuracy of available correlations for predicting heat transfer in diesel which cannot be determined exactly.
4. Crevice regions (such as the volumes between the piston, rings, and cylinder wall) constitute a few percent of the clearance volume. The gas in the regions is cooled to close to the wall temperature, increasing its density and, therefore, the relative importance of these crevices. Thus crevices increase heat transfer and contain a non negligible fraction of the cylinder charge at conditions that are different from the rest of the combustion chamber.
Due to difficulties in dealing with these problems, both sophisticated method of analysis and more simple methods give only approximate answers.