Internal Combustion Engines. Allan T. Kirkpatrick

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Figure 2.4 Diesel cycle characteristics as a function of compression ratio and energy addition ().

Although Equation (2.25) is correct, its utility suffers somewhat in that is not a natural choice of independent variable. Rather, in engine operation, we think more in terms of the energy transferred in. The two are related according to Equation (2.26).

(2.26)

      The indicated mean effective pressure (imep) is represented by the same equation, Equation (2.20), as the Otto cycle:

      (2.27)

      Maximizing the mean effective pressure is important in engine design so that one can build a smaller, lighter engine to produce a given amount of work. As shown in Equation (2.20), there are evidently two ways to do this: (1) increase the compression ratio , and (2) increase the energy input . However, there are practical limitations to these approaches. For spark‐ignition engines of conventional design, the compression ratio must be low enough to avoid engine knock; whereas for diesel engines, increasing compression ratio increases engine friction. Other more complicated factors influence the selection of compression ratio, especially constraints imposed by emission standards and, for some diesel engines, problems of startability.

      One might expect that we can increase by increasing the fuel flowrate delivered to an engine. As we shall see in our studies of fuel–air cycles in Chapter 4, this is not always correct. With fuel‐rich mixtures not all of the fuel energy is used, since there is not enough oxygen to burn the carbon monoxide to carbon dioxide nor the hydrogen to water. The fuel–air cycle predicts that the efficiency decreases as the mixture is made richer beyond stoichiometric.

      According to the gas cycles, and to the fuel–air cycles to be discussed later, the efficiency is greatest if energy can be added at constant volume:

      (2.28)

Why then do we build engines that resemble constant pressure energy addition when we recognize that constant volume energy addition would be better? To illustrate how difficult that question is let us ask the following: Suppose that the maximum pressure in the cycle must be less than some value . How should the energy be added to produce the required work? The answer is now

      (2.29)

      This can be demonstrated with the aid of a temperature‐entropy diagram. If the Otto cycle and the Diesel cycle are drawn on such a diagram so that the work done in each cycle is the same, it can then be shown that the Diesel cycle is rejecting less energy and must therefore be the most efficient.

2.5 Limited Pressure Cycle

      Modern compression ignition engines resemble neither the constant‐volume nor the constant‐pressure cycle, but rather a cycle in which some of the energy is added at constant volume and then the remaining energy is added at constant pressure. This limited pressure or 'dual' cycle is a gas cycle model that can be used to model combustion processes that are slower than constant volume, but more rapid than constant pressure. The limited pressure cycle also can provide algebraic equations for performance parameters such as the thermal efficiency and imep. The distribution of energy added in the two processes is something an engine designer can specify approximately by choice of fuel, the fuel injection system, and the engine geometry to limit the peak pressure in the cycle.

The cycle notation is illustrated in Figure 2.5. In this case we have the following Equation (2.30) for :

       Energy addition

(2.30)

Figure 2.5 The limited pressure cycle (

,

).

The expansion stroke is still described by Equation (2.24), provided we write . If we let , a pressure rise parameter, it can be shown that

      (2.31)

The constant‐volume and constant‐pressure cycles can be considered as special cases of the limited‐pressure cycle in which and , respectively. The use of the limited‐pressure cycle model requires information about either the fractions of constant volume and constant pressure energy addition or the maximum pressure, СКАЧАТЬ