Читать книгу Internal Combustion Engines - Allan T. Kirkpatrick - Страница 45

Studying ideal gas engine cycles as simplified models of internal combustion engine processes is very useful for illustrating the important parameters influencing engine performance. Ideal gas engine cycle analysis treats the combustion process as an equivalent energy addition to an ideal gas. By modeling the combustion process as an energy addition, the analysis is simplified since the details of the physics and chemistry of combustion are not required. The various combustion processes are modeled either as constant volume, constant pressure, or finite energy release processes.

The internal combustion engine is not a heat engine, since it relies on internal combustion processes to produce work, and it is an open system with the working fluid flowing through the cylinder. However, gas engine models are useful for introducing the cycle parameters that are also used in more complex combustion cycle models, specifically the fuel–air cycle, to be introduced in Chapter 4. The fuel–air cycle accounts for the change in composition of the fuel–air mixture during the combustion process.

This chapter also provides a review of closed‐system and open‐system thermodynamics. This chapter first uses a first‐law closed‐system analysis to model the compression and expansion strokes and then incorporates open‐system control volume analysis of the intake and exhaust strokes. An important parameter in the open‐system analysis is the residual fraction of combustion gas, , remaining in the cylinder at the end of the exhaust stroke.

Let us assume, to reduce the complexity of the mathematics, that the gas cycles analyzed in this chapter are modeled with an ideal gas that has a constant specific heat ratio and gas constant . This assumption results in simple analytical expressions for the efficiency as a function of the compression ratio. Chosen values of for internal combustion engine gas cycle calculations typically range between 1.2 and 1.4, and values of the gas constant typically vary between 0.28 and 0.31 kJ/kg‐K. An unburned stoichiometric iso‐octane/air mixture at a compression temperature of 650 K has = 1.31 and = 0.28 kJ/kg‐K, and after combustion at an expansion temperature of 2250 K the equilibrium combustion product mixture has = 1.19 and = 0.30 kJ/kg‐K.

The scientific theory of heat engine cycles was first developed by Sadi Carnot (1796–1832), a French engineer, in 1824. His theory has two main axioms. The first axiom is that in order to to use a flow of energy to generate power, there needs to be two bodies at different temperatures, a hot body and a cold body. Work is extracted from the flow of energy from the hot to the cold body or reservoir. The second axiom is that there must be at no point a useless flow of energy, so heat transfer at a constant temperature is needed. Carnot developed an ideal heat engine cycle, which is reversible, i.e., if the balance of pressures is altered, the cycle of operation is reversed. The efficiency of this cycle, known as the Carnot cycle, is a function only of the reservoir temperatures, and the efficiency is increased as the temperature of the high temperature reservoir is increased. The Carnot cycle, since it is reversible, is the most efficient possible, and it is the standard to which all real engines are compared.