Internal Combustion Engines. Allan T. Kirkpatrick

Figure 2.2 Otto cycle thermal efficiency and imep as a function of compression ratio and energy addition ().

      Example 2.1 Otto Gas Cycle Analysis

      The operation of a single‐cylinder engine with a compression ratio = 8 is to be modeled using an Otto cycle. The engine is fueled with octane and operates at a lean air–fuel ratio of 17:1. At the beginning of compression at bottom dead center, the cylinder pressure = 100 kPa and temperature = 320 K. Assume the cycle average = 0.29 kJ/kg‐K and = 1.26. (a) What is the nondimensional energy addition and the cycle average specific heats and , (b) What is the thermal efficiency and imep/, and (c) What is the maximum temperature and pressure ?

      Solution

Comment: The efficiency we have computed, , is about twice as large as measured for actual engines. There are a number of reasons for this. We have not accounted for internal friction, heat transfer losses, and the incomplete combustion of fuel within the engine cylinder.

2.4 Constant Pressure Energy Addition

      This cycle is often referred to as the Diesel cycle and models a gas engine cycle in which energy is added at a constant pressure. The Diesel cycle is named after Rudolph Diesel (1858–1913), who in 1897 developed an engine designed for the direct injection, mixing, and autoignition of liquid fuel into the combustion chamber. The Diesel cycle engine is also called a compression ignition engine. As we will see, actual diesel engines do not have a constant pressure combustion process.

The cycle for analysis is shown in Figure 2.3. The four basic processes are:

Figure 2.3 The Diesel cycle (

,

).

Again assuming constant specific heats, the student should recognize the following equations:

       Compression stroke

      (2.21)

       Energy addition

      (2.22)

       Expansion stroke

      (2.23)

      where we have defined the parameter , a measure of the combustion duration, as

(2.24)

      In this case, the indicated efficiency is

(2.25)

      The term in brackets in Equation (2.25) is greater than one, so that for the same compression ratio,

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