The Thermodynamics of Internal Combustion Engines: Examples of Insights
A major goal of the development of internal combustion (IC) engines continues to be higher performance and efficiencies. A major aspect of achieving higher performance and efficiencies is based on fundamental thermodynamics. Both the first and second laws of thermodynamics provide strategies for and limits to the thermal efficiencies of engines. The current work provides three examples of the insights that thermodynamics provides to the performance and efficiencies of an IC engine. The first
... ngine. The first example evaluates low heat rejection engine concepts, and, based on thermodynamics, demonstrates the difficulty of this concept for increasing efficiencies. The second example compares and contrasts the thermodynamics associated with external and internal exhaust gas dilution. Finally, the third example starts with a discussion of the Otto cycle analysis and explains why this is an incorrect model for the IC engine. An important thermodynamic property that is responsible for many of the observed effects is specific heat. One of the major current goals of IC engine development is to achieve higher performance and thermal efficiencies. Perhaps the most important aspects of the performance and efficiency of engines is related to the fundamental thermodynamics. An engine's performance and efficiency is limited by the first and second laws of thermodynamics. To provide quantitative examples of the thermodynamics of engines, a cycle simulation is used in this work. This cycle simulation incorporates all features of the first and second laws of thermodynamics. Although many examples exist of the importance of thermodynamics relative to engine efficiency and performance, this paper will select three examples to illustrate the insights available. These three examples are the thermodynamics of low heat rejection engines, the thermodynamics of exhaust gas dilution, and the thermodynamics of the ideal Otto cycle. This paper includes subsections on the laws of thermodynamics and a description of the engine cycle simulation. The paper ends with a summary, and a list of conclusions and findings. Brief Review of the Laws of Thermodynamics The primary laws of thermodynamics that are relevant to this discussion are the first and second laws. These laws are well described in numerous thermodynamic (e.g., [5, 6] ) and engine text books [1,2,7]. The following is a brief summary. The first law of thermodynamics is often attributed to James Joule and Julius Mayer  . In the early 1840s, they individually recognized that heat transfer and work were different forms of the same quantity. This observation led quickly to the idea that energy remained constant, but could change form. The first law of thermodynamics is often called the conservation of energy law. Basically, energy cannot be created or destroyed. This means that for a control volume, the change of energy must be equal to the net result of all inputs and outputs of energy. The second law of thermodynamics is perhaps a much richer and more complex law than the first law. This law is often attributed to the original work of Carnot  in 1824. The second law is based on a number of related physical observations that have a wide range of implications with respect to engineering design and operation of thermal systems. For example, the second law can be used to determine the direction of processes, to establish the conditions of equilibrium, to specify the maximum possible performance of thermal systems, and to identify those aspects of processes that are detrimental to overall performance. Because of this large number of related observations, there is no simple, single statement that captures the full extent of the second law of thermodynamics. Although thermodynamic text books commonly use the Kelvin-Planck and Clausius observations as statements of the second law, these are but two aspects of the second law. The second law is basically a set of observations that share a common fundamental principle that relates to the quality of energy. As described below, one way to quantify this quality of energy is to introduce a property (exergy or availability) that captures this concept. This will be described in the following subsection on the second law. Description of the Engine Cycle Simulation Basics of the Simulation The engine cycle simulation used in this work is a zero-dimension formulation that includes all four strokes, uses three-zones for the combustion process, and requires a number of empirical sub-models. The details of this cycle simulation are available in a book  and in numerous technical articles (e.g.,     ). The following is a brief description of the simulation. This will be followed by subsections on the required items for solutions, descriptions of the engine and operating conditions, and strategies for the following computations. Figure 1 is a schematic of the control volume for the three-zone simulation. The three-zones are the unburned zone, the adiabatic core, and the boundary layer. The latter two regions comprise the burned zone. Cylinder heat transfer is based on one of several heat transfer correlations (described Inventions 2018, 3, 33 3 of 30 below). The cylinder heat transfer is allocated to the unburned zone and the boundary layer (of the burned zone) based on the temperatures and volumes of the zones . Inventions 2018, 3, x FOR PEER REVIEW 3 of 30 Formulations The first law of thermodynamics is used to derive expressions for the time (crank angle) derivatives of the pressure, the three temperatures, the three volumes, and the three masses in terms of engine design variables, operating parameters, and sub-model constants. For this development, only steady state operation is considered. The fuel is assumed to be completely vaporized and mixed with the incoming air. Any blow-by is neglected, and combustion is assumed 100% complete (Note that due to the Wiebe function used for combustion a small amount (0.7%) of fuel is not consumed.). The set of ordinary differential equations is solved numerically as a function of crank angle. The items needed for the solutions include the thermodynamic properties, piston-cylinder kinematics, combustion process descriptions, cylinder heat transfer, mass flow rates, and algorithms for the mechanical friction. Each of these is briefly described next. Items Needed for Solutions Properties The properties of the working fluid are based on air, fuel vapor, and combustion properties. For the thermodynamic conditions of IC engines, the working fluid may be assumed to obey the ideal gas model. The concentrations of combustion products may be "frozen" for the lower temperatures, or based on instantaneous (shifting) chemical equilibrium for the higher temperatures. The unburned mixture during the compression stroke prior to combustion consists of the inlet mixture of air, fuel vapor, exhaust gas recirculation (if specified), and the residual gases (combustion products). The burned gases are determined from the stoichiometry and either the frozen or equilibrium composition. Once the composition of the gas mixtures is known, the thermodynamic properties may be determined. Properties for each species are determined from polynomial curve-fits to the property data for the species [1, 9] . Kinematics The kinematics and geometric parameters are based on standard reciprocating engines  . The required inputs include the cylinder bore, stroke, connecting rod length, and compression ratio. With these inputs, the instantaneous cylinder volume, the rate of change of the cylinder volume, and the instantaneous surface area may be determined. Inventions 2018, 3, 33 5 of 30 components: rubbing (crankshaft, bearings, pistons, valve train, etc.), pumping, and auxiliaries (oil and water pumps and alternators). Description of the Engine All the results reported below are for the same engine: a 5.7 L, V-8 configuration with a bore and stroke of 101.6 and 88.4 mm, respectively. Most of the following results, however, are not too sensitive to the actual engine. Table 1 lists some of the engine specifications, and Table 2 lists some of the parameters (and "how obtained") that are needed to complete the computations. Note that the open and close crank angles for the valve events represent a valve lift of zero.